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December 26 Off into the wild black yonder...Well, tomorrow, I go on my trip. I have absolutely no idea
whether or not I'll have internet access, so for those who regular read
the blog, I don't know when I'll get to post again. It might not
be until early January. We are past the winter solstice, so the days are getting longer again. For those of us in the central US, December 21 marked the winter solstice, the day with the shortest length of daylight. This is when the Earth's northern hemisphere is most tilted away from the Sun. It marks the start of the calendar winter in North America. I always thought that the solstices and equinoxes were silly markers for the division between seasons. Really, I think that the seasons should start about a month before each of those times. It is the tilt of the Earth that causes the seasons. Many of my students seem to believe that the seasons are caused by the varying distance between the Earth and the Sun. Well, that obviously can't be what is making it winter here, since the Earth is actually closest to the Sun (perihelion) on January 4! Earth will be at its farthest from the Sun (aphelion) on July 3, 2006. So, if I don't get to post between now and the perihelion data, then Happy New Year! -Astroprof December 24 Constellations and AsterismsWell, here it is Christmas eve, and family have arrived. It is sad that I have such low expectation, but high hopes, and it is always the expectations that come about. I know that I’d be happier if I had someone special to spend the holidays with, but sometimes I think that with the family dramas and slights and so forth that goes on around here that in lieu of what I want, then spending the day completely alone might be better. Well, next week I am going to be out of town, and perhaps things will be much smoother. The holidays seem to be almost set up to make me feel worse.
Anyway, I figured that I’d say a few words about constellations and asterisms. I can’t remember if I did this already or not. In the distant past, people imagined patterns in the sky, and these led to the first constellations. Many of our constellations that we use today go back to the Romans. They had 66 constellations. Parts of the sky had no particularly bright stars useful for making patterns, so these were considered regions of unformed stars. Many of the constellations were figures in stories of classical mythology. So, we have Hercules, Pegasus, Perseus, Andromeda, etc. Of particular interest are twelve constellations along the ecliptic. If you imagine dimming the sun to the point that you could see the stars behind it, then it would appear to be in different constellations on different days as the Earth orbits. The apparent path of the Sun through the sky is called the ecliptic. The planets and the Moon also move through the sky within a few degrees of the ecliptic, so these are also the constellations that they move through. (Actually, there are 13 constellations along the ecliptic now.) These constellations along the ecliptic are known as the zodiac.
From Rome, not all of the sky could be seen. So, when explorers ventured far enough south, they saw stars and patterns not seen from Rome. Different people made different patterns. Later astronomers made more pattern from the dim stars ignored by the Romans, filling all parts of the sky with constellations. This was in part due to a desire to designate in what constellations new discoveries, such as nebulae, were found. Also, naming stars required that the stars be in a constellation for both Bayer designations and Flamsteed numbers (see my post on Star Names).
Well, there was little agreement between astronomers, particularly astronomers from different nations, as to what constellations existed, and what they were called. So, in 1933, the International Astronomical Union declared 88 official constellations covering the entire sky. Under this definition, the constellations are no longer patterns in the sky, but rather they are regions of the sky. Many of the old patterns are contained within the new regions, but those patterns are no longer of anything but historical interest to astronomers. Now, you can look on a map and see the borders of the constellations drawn. Embarrassingly, some stars with Bayer designations or Flamsteed numbers associated with one particular constellation are now technically located in another constellation. Oh, well. For historical reasons, we won’t rename them.
Now, the patterns are not important to professional astronomers, but they are to the amateur or the layman. People like to see the patterns, and of course if you have a small telescope that you point yourself, you need to know which stars are which, and how to find things in the sky. To do this, you rather need to recognize some patterns in order to know what star chart to use. Many really easy patterns, though, do not belong to a constellation, or perhaps they are composed of only part of the stars in a constellation. The most famous example are the seven stars that in North America are called the Big Dipper. Really, these are stars that make up part of the constellation Ursa Major, the big bear. Another famous pattern is the Summer Triangle, composed of the stars Altair, Vega, and Deneb, each of which is in a different constellation! These unofficial, but useful, patterns are called asterisms.
So, there is your Christmas astronomy lesson.
-Astroprof
December 22 A hole in the calendarSo, here it is, late in December. Decem means tenth in Latin, so this is the tenth month, right? Well, of course that depends on how you count months. At one time this was the tenth month. The original Roman calendar started near the vernal equinox, which is in March. That made December the tenth month. The original calendar only had ten months. So, how can you get only ten months and still start each year at the equinox? Were the months different lengths. Well, yes, but not like you’d expect. Rather than being longer, the months averaged a day or so shorter than the current months! So, how did this work? Well, quite simply, they quit counting days at the end of the tenth month, and they did not start the new calendar year until it was time to do so near the equinox. This sounds pretty bizarre, but think about the early Rome. It was agrarian in nature, and nothing much happens with farming between the winter solstice and the vernal equinox. You just wait out the winter until it is time to plant again in the spring. So, the calendar worked until the Romans started to do things other than farm. Then, they added two new months. The calendar still had major issues, but the Romans lived with it until the days of Julius Caesar, who made major changes. In his honor, the then fifth month was renamed July. A few years later, another minor revision to the calendar came, and the month after Julius’ month was renamed after Augustus Caesar. The calendar remained this way until Pope Gregory XIII made the final changes to the calendar in 1582 in a revision of the way that leap years are computed, and this is the calendar that we use today. We don’t really have leap years every four years. It is every four years, skipping the leap years in centennial years (1800, 1900, etc), unless they are evenly divisible by 400 (1600, 2000, etc). We call this the Gregorian calendar. I might do a blog on that later, since it is really an interesting story. I have a whole lecture in my first semester astronomy class dedicated to just telling time. I really find it interesting.
This blog, though, just goes back to thinking about the earliest Roman calendar --- the one that just ended, and then picked up two or three lunar cycles later. There was this gap in which the days were not counted. These days were not in any year, really. They were sort of a temporal limbo between the years. Again, not much happened then. Someone might be born or might die between the years, but most people in the ancient world did not celebrate, or even really recognize, birthdays such as we do today, and death was so commonplace, that the date of a death was not really notable unless it was of some very important figure. For this intercalary period between the years, folks just holed up and waited for time to plant crops again in the spring.
This seems sort of strange, but I have a feeling like this right now. For those of us in academia, we run ourselves ragged for most of the year. Then, there is this sudden break after finals in the fall semester, and before things start up in January. Colleges and universities close, students go away, and pretty much our normal lives halt until we go back after the Christmas break. I feel like the ancient Romans, waiting to begin the days again. This year seems more of a disconnect for me than normal, and I don’t really know why. Maybe that is why I am even less in a Christmas sort of mood than normal.
-Astroprof
December 21 GRBsNow, for another astrophysical mystery. OK, I know that I haven't
finished with the dark matter thing, but here is one that we are
perhaps further along in figuring out. In about three weeks, the American Astronomical Society (the professional astronomer organization for North America) will have a meeting. One of the sessions will include papers about Gamma Ray Bursters (GRBs). So, I thought that I'd mention something about them. In the early 1960's, treaties were signed to end atmospheric nuclear testing. This did not put an end to testing nuclear devices, but rather just confined such tests to underground explosions. Weapons engineers did not like this, since the dynamics of an underground explosion are quite different from those of a surface or above surface blast. Thus, these tests did not give a true indication of what a weapon would actually do if it were used for real. So, there was real concern that nations might cheat on the provisions of the treaty. To monitor against such cheating, satellites were deployed with powerful gamma ray detectors. The only known sudden burst of gamma rays was from detonating nuclear weapons, so this seemed to be a good way of finding any cheaters. Much to the surprise of the intelligence agencies monitoring the satellites, a great number of gamma ray bursts were recorded from random directions in the sky! Naturally, this information was classified for a long time. Why weren't these gamma ray bursts discovered from ground based measurements? Well, it turns out that gamma rays and X-rays don't really pass through all that much air before they are absorbed. Most of the radiation is absorbed in only feet of air. Only really massive bursts, like those from nuclear explosions, could travel the miles needed to be detected through the atmosphere. Cosmic gamma ray bursts were simply too weak for their radiation to penetrate the atmosphere (thankfully). By the 1990's, the existence of the gamma ray bursts from the early spy satellites had been declassified. Since gamma rays don't penetrate the atmosphere well, special astronomical gamm ray telescopes were placed in orbit around. Hundreds, and then thousands, of gamma ray bursts (GRBs) were discovered. However, no one knew what these things were. Two types of GRBs were found: long duration and short duration bursters. Long duration is a matter of perspective, since this meant from a couple seconds up to a few minutes. The short duration bursts were tens of milliseconds up to about a second or so. All kinds of wild ideas were proposed. But nobody even knew how far away these events were happening! After all, it makes a difference if they are nearby and fairly weak, or far away and very powerful. Both would appear the same brightness to us on Earth. The problem was that the events lasted such a short time that by the time astronomers monitoring the gamma ray detectors had contacted other astronomers to point their instruments towards the GRB to see what was there, it had already faded! Eventually, in the late 1990's, though, astronomers managed to capture the afterglow of a GRB. This turned out to be in the outer parts of a galaxy nearly 7,000,000,000 lightyears distant! This was an amazing discovery, since this was WAY farther than anyone had ever suspected. This meant that the GRBs had to have many times more energy than the entire energy output of a galaxy!!!! Now, we had a major problem. How do we account for such energy production. Even wilder explanations came about to try to explain how such energy could be liberated in such a short time in such a confince space. Ideas were floated about such as exploding black holes, merging neutron stars, matter-antimatter annihilation, flareups of active galaxy nuclei (still not understood at that time, though we now know more about these events, too), collisions of cosmic strings, etc. Part of the difficulty was that GRBs were always detected far away, so study was difficult. It became apparent, though, that we really didn't want a GRB nearby. In 2003, a GRB "only" 2.6 billion lightyears away was powerful enough to partially ionize the Earth's upper atmosphere. If a GRB were to occur anywhere within tens of thousands of lightyears, then it would be energetic enough to fry Earth, and perhaps to sterilize the surface of the planet of life (including us). It would be pretty, of course, since the air would glow a pretty blue color all over the side of the planet facing the GRB for a few seconds. Then, we'd die of radiation poisoning. Darn! Probably, we'd die long before we even figured out what caused the GRB. Well, at least it would be pretty. ;) A few other GRBs were identified quickly enough for their afterglow to be seen and studied. Then, a mystery surfaced, because some parts of the spectrum of the afterglow seemed to be similar to those of a supernova. A supernova occurs when the core of a very massive star collapses into a neutron star or a black hole. The sudden release of energy of this collapse is sufficient to blow the rest of the star apart in a massive explosion that temporarily makes the exploding star as bright as the entire light ouput of the galaxy in which it resides. Yet, GRBs were brighter still. It was suggested that perhaps an even bigger form of supernova might occur from the collapse of stars of over 100 solar masses, a sort of super-supernova. The term for coined for this event was hypernova. A hint came when some supernovae were found that appeared to be slightly under luminous. This led to the suggestion that perhaps supernovae are not symmetric. Up to that point, all the equations and theoretical work had been with spherically symmetric explosions. Spherical symmetry makes solving the equations involved much easier. MUCH, MUCH, easier. Extremely easier! But, if the explosion isn't symmetric, then excess energy would be channeled in certain directions. A suggestion was made that perhaps a very massive star collapsing into a black hole might develop a fast rotating accretion disk around the black hole for a moment (or longer). Such an accretion disk would have a powerful magnetic field, and this field would channel the bulk of the ionized particles from the explosion into two jets heading out the magnetic north and south poles. This is much the idea of how shaped explosive charges work --- most of the explosion is directed in one direction. So, this would make the explosion of the supernova more powerful in the direction of the poles. So, rather than a hypernova, the GRB would be what you would see if one of these very large assymetric supernovae were to occur with the jet pointed towards Earth. The assymetric supernova model seems to work fairly well to explain long duration GRBs, but it fails to fully explain short duration GRBs, which still remain a mystery (and still have lots of wild theories running around about them). More observations are needed, though, to fully support this model. At any rate, we are finally getting an idea of what is going on. So, I hope that this wasn't too much or too involved for y'all. I know that there is a wide range of readership out there. -Astroprof December 19 Hold off on the giant meteorite ...Well, now that I have grades turned in, then it would be just plain
annoying to have a giant meteorite come and wipe out life on
Earth. Of course, I am still dealing with the holidays, ..., but
still it can hold off a bit. It isn't a question, though, of "if" a giant impact happens. It is a matter of "when" it will happen. We have ample evidence of impacts. You can look at the Moon, of course, but Earth is an even bigger target, so we get hit at least four or five times more often than the Moon. So, where are all the craters? They are all over the place! Most have been filled in by erosion, so they are hard to recognize as craters. A lot of people think that the atmosphere protects us from impacts. Well, it does protect us from all of the small meteorites. The big ones, though, won't even notice it when they come in. So, where are the impactors coming from? There are thousands of asteroids whose orbit ranges from closer to the Sun than Earth to farther from the Sun that Earth. Over 700 of these can be classified as potentially hazardous. They pass quite close to Earth's orbit. These orbits are not fixed. With all the planets in the Solar System moving around, the gravitational influences on these asteroids gradually shift the orbits around (sometimes they quickly alter orbits). So, an asteroid that misses Earth might one day be on a collision course. About 20 years ago, it was also shown that asteroids who have nice neat orbits within the asteroid belt can be kicked out of their orbits to become Earth crossing asteroids. So there is a supply of fresh asteroids to replace the ones that run into the inner planets. Wait long enough, and one will hit us. Asteroids are not the only concern, though. Comets also pass through the inner solar system. It does not matter at all if you are hit by a mountain of rock or a mountain of ice. Both will have the same effect. Comets originate in the outer solar system. When their orbits are disturbed in the right (or wrong) way, they come into the inner solar system. New ones are coming all the time. We had a very near miss within the last decade by Comet Hyakutake. It was spotted just weeks before it passed us. So, if we spot an asteroid or comet on a collision course, what can we do? Well, not much. Despite what you see in the movies, we can't shoot it down or blow it up. That just isn't possible. However, we might deflect it so that it misses us. How is this possible? If we catch the comet or asteroid while it is years, or better decades, from impact, we can give it a gentle nudge. When the orbit is changed by a nearly imperceptible amount far from the Sun, it can have a big effect when the body gets close to the Sun. So, a fairly gentle nudge might make it miss us. So, how do you nudge something that is many miles across? Well, that is the hard part. First, we don't have rockets lying around to do this with. We can't just launch a missile, since our ICBMs are aimed at Earth. They just toss weapons high enough to fall on someone else. We'd have to build the rocket. That takes time. Secondly, we need to deploy something that will push the asteroid or comet. A common proposal is to detonate a nuclear or thermonuclear device near the body. We are not trying to hit it and blow it up. We positively can not blow it up. At best, we could break it into smaller parts, so that we get hit by multiple massive meteorites. In fact, we might make things worse this way. So, we want to detonate the device nearby, so that the heat from the explosion would vaporize part of the surface of the object. We are talking only a few millimeters of material. However, this material shooting off of only one side of the body (the side towards the explosion) might give it the push that we want. An alternate proposal, one that the Europeans are looking to test soon, would be to slam a projectile into the asteroid giving it a tiny shove. This might also make it miss Earth. The big question, though, would be if either proposal would even work. We really don't know a lot about asteroids and comets. What we have learned in the last few years suggests that they are not uniform in composition, nor homogeneous in makeup. What might work to deflect one might fragment another one, or have absolutely no effect at all on a third. So, this leaves us with tough choices if such a body is found to be on collision course. The flurry of spacecraft to comets and asteroids over the last few years is yielding data on these bodies, but we need far more information to decide what to do if one were to be heading towards us. For now, all we can do is duck. I tell my students that if one were coming right towards us, then half of the worlds astronomers would gather near the projected impact site so that the last thing that we saw would be a really cool fireworks show, and the other half would gather on the other side of the world where they'd live a few minutes longer. -Astroprof December 18 Dark Matter (Part One)A while back, Tom had suggested dark matter as a blog topic. So,
here goes with part one. It is a pretty intense subject, and no
one really understands all about dark matter, so it will be the subject
of more than one entry. The story goes back over 70 years. Fritz Zwicky and Walter Baade were studying the Coma galaxy cluster. This is a very large cluster of galaxies in the direction of the border between the constellations Coma Bernices and Virgo. By studying the number and distribution of galaxies, along with measurable motions, they found that the galaxies were moving too fast. What do I mean by too fast? Well, all matter attracts other matter by gravitational forces. With a lot of stars or galaxies moving around in a cluster, the gravitational attraction of the bodies will pull any body moving out of the cluster back into the cluster. Occasionally, some bodies will be going fast enough to achieve escape velocity, and they will leave. Enough of the members doing this leads to evaporation of the cluster. Open star clusters tend to evaporate, while globular clusters are compact enough and have enough mass that they last a long time. Zwicky and Baade used these principles to study the Coma Cluster, and they found that the galaxies were moving so fast that the entire cluster would evaporate in a very short time. In fact, the motions were fast enough that they didn't see any way that there could even be a galaxy cluster. Clearly there was a cluster, so something had to be going on. They proposed that there must be more matter in the cluster than we could see. Zwicky postulated that this "dark matter" was the source of the additional gravitational forces holding the cluster together. No one knew what the dark matter was composed of, though. Within a decade, another astronomer, Robert Trumpler, was studying star clusters. He found that the more distant clusters were dimmer than they should be. What was going on? Were distant stars somehow different than nearby ones? Did light get "tired" and fizzle out on the way to Earth? These and several other ideas were suggested and deemed unworkable. Eventually, Trumpler suggested that all of space was filled with a kind of "fog" that is now termed the interstellar medium. We have now found that the interstellar medium is composed mostly of hydrogen and helium. Also, we have found that in spiral galaxies such as the Milky Way, the interstellar medium has more mass than the stars themselves. Is this the source of the dark matter? Unfortunately, no. By observing the orbits of stars around the galaxy, we can devise a velocity profile that shows the orbital speed of stars at different distances from the center of the galaxy. Such a profile should show that the more distant stars orbit slower than the inner stars. This is not what the velocity profiles of galaxies, show, though. They show that for most of the galaxy, the orbital speeds are the same. This can only be true if there is substantially more mass being orbited. We can determine the amount of interstellar medium in the galaxy. This does not come even close to the required mass to account for velocity profiles that we see. More recently, we have found hot plasma between the galaxies in most galaxy clusters. This intergalactic medium is also mostly hydrogen and helium. It has substantial mass, but it, too, is far too little to account for the motions that we see among the members of galaxy clusters. Over the years, we have found gas, dust, and all sorts of other forms of matter that we could not see before. Much of this material does not shine with visual light. However, it can still be detected by its absorption of visual light, or its emission of non-visual forms of light, such as X-rays, infrared, etc. Astronomers don't consider this to be "dark matter," though. That term is reserved for the undetected mass that seems to exist in abundance in the universe. These dark forms of matter that we detect by other means are often lumped together as "dim matter," to differentiate it from the dark matter. All told, the luminous matter and the dim matter account for only about 10% or so of the mass that seems to exist in the universe. Galaxies and galaxy clusters seem to be composed of nearly 90% dark matter. What is this stuff? Well, that is still a mystery. Since we've been unable to detect it, we can only guess. Numerous suggestions have been made as to the source of the dark matter. These range from ordinary objects (things that technology may eventually detect, making them dim matter) to rather bizarre particles, or even strange substances. I'll save some of these suggestions for a later posting. There have even recently been suggestions that there is no dark matter at all, but rather that the gravitational effects that we see which lead us to speculate that there is dark may actually be artifacts of the nature of space --- a sort of leaking of gravitational energy into other dimensions. These are the more exotic theoretical proposals, and again I'll leave them for later. Though we don't know what dark matter is, we can make some guesses as to its nature, assuming that we don't hold to the most exotic explanations. It must be fairly weakly interacting with ordinary matter, other than through gravitational forces, or we'd have detected it already. Studies of the clumping of galaxies show that the dark matter must be slow moving enough to clump up to form galaxy clusters within the time frame of the early universe. However, studies of the cosmic background radiation show that the whole universe is uniform in temperature, and this suggests that the dark matter must move at near the speed of light in order to smooth out the universe during the few years available after the big bang. Both suggestions can't be right! Well, perhaps they can --- if there were more than one type of dark matter. We call the slow moving, low energy form cold dark matter, and we call the fast, high energy form hot dark matter. These must be different forms of dark matter. They can not be just the same stuff with two different energies, otherwise we'd expect a continuum of energies. Also, it would be hard to imagine how the processes at work in the creation of the universe would give two entirely different energies to groups of the same type of matter. So, to fit our observations, we postulate that there must be this unseen form of stuff out there that does not interact with anything else, except by gravity. Making it more exotic, we contend that the bulk of the universe is made of this unseen stuff. Then, we go one step farther, and we say that there must be at least two different kinds of stuff out there! This gets to be a bit much to accept for some people, so you can imagine the conundrum that dark matter causes. The fact is that we have no idea what dark matter is. There are suggestions (later post), but even our best suggestions seem to fall short of what we need. So, that sets the stage. Unfortunately it does not answer the question of what the dark matter may be. No one knows that. If anyone were to really figure it out, then there's a Nobel Prize waiting for them! -Astroprof The Lion, the Witch, and the WardrobeWell, I got to see the Narnia movie. In an earlier post, I said
that was the one that I was looking forward to seeing. I really
liked it. I think that I may have done some things slightly
differently, but that is because I already had mental pictures of
certain scenes from reading the books. Overall, though, I think
that they did a good job. I always worry when they make a movie
from something that I liked, since Hollywood's ideas and mine seldom
agree. I turned in grades yesterday, so today I was just trying to get ready for my Dad to come tomorrow. Everything is on hold during the last couple weeks of the semester. Now, I am catching up. -Astroprof December 16 Star NamesEvery year, I hear on the radio, TV, or elsewhere about "buying star names" or naming a star after a loved one, or something. As an astronomy professor, often people bring me an envelope with a certificate for some star that they bought, or someone bought for them, and they ask me more more information on it. For some people, it is just a novelty. For others, they are serious, as the star is supposed to commemorate a deceased loved one. So, I dutifully try to see if I can find anything about these stars for them. There have been more than one company selling star names, I have found. Some clearly are just a ripoff, and they name non-existant stars, or they reuse the same stars over and over again. Others seem to actually try to have a unique star per person.
However, you should not think that anyone other than the company selling the star names will ever use those names. Professional astronomers very seldom use star names, except for the dozen or so brightest stars. All other stars are referenced in the literature by a catalog designation. Besides, the company selling the star names does not have any authority to do so. The only names recognized by astronomers are those approved by the International Astronomical Union. All other names are more like nicknames, at best.
The so-called "proper name" of a star, the name that people usually think of such as Sirius, Rigel, Deneb, etc, are generally agreed upon names from antiquity. Most have Arabic origins. However, these stars have a multitude of other names. This means that such names are not unique, or clear. So, in the early 17th Century, Johann Bayer produced a catalog of stars in which each star is designated with a Greek letter and the constellation name in which the star is found, such as Alpha Cygni, Gamma Scorpii, Lambda Orionis, etc. There are 88 constellations, so there are 88 alpha stars, 88 beta stars, and so forth. However, Bayer only designated the brighter stars in the constellations, so when later astronomers became interested in the dimmer stars, they did not have designations for them. There are only so many letters in the Greek alphabet, so the Bayer designations could only extend so far. By the early 18th Century, John Flamsteed had come up with his own designations based on numerals, such as 32 Cygni, 88 Tauri, etc. This system is not limited as to how many stars per constellation could be designated. He numbered stars in order of increasing right ascension (basically right to left) across the constellation. However, he could only designate the stars that he saw. Later astronomers compiled much more extensive catalogs, such as F.W.A. Argelander's Bonner Durchmusterung of 1860. A star designation in this catalog sounds something like BD+25degrees2147. As telescopes got better, more complete catalogs were developed, with the star names being a designation within the catalog. Each catalog overlapped all the less complete ones that came before, so stars wound up with a multitude of designations. Variable stars have their own system of names (there are two types in routine usage), so this adds more names to stars. The ultimate catalog is the Hubble Guide Star catalog, developed for the HST. These are all the official names that astronomers use. So, any other names would be lost in the list of names that stars already have. Since these catalog designations are readily available, and referencing stars in the same manner as prior publications allows for easier literature searches, these are the designations that professionals use.
So, when people come to me to find information about their star, I can't access it by the name that they "purchased." I need to know one of the other designations. That is almost never given, though. One of the biggest star naming companies at least gives the coordinates of the star. Now, that I can use. There are professional databases that can be searched by position. However, almost never do I find a star at the position listed. This does not automatically mean that the whole thing is a sham, though. Stars move around slightly. A bigger factor is that the Earth is slowly precessing (ie: changing the direction that its north pole points). This changes the apparent positions of all the stars in the sky. Professional astronomers, therefore, when giving coordinates, also give the epoch (or year of validity) of the coordinates. If you know the epoch, then you can account for the effect of precession through simple calculations. However, I have never had one of these certificates that people bring me list an epoch. So, I put the coordinates into my search, and then look at what comes out. I try several standard epochs, and they seldom actually match, so I don't know if the companies are rounding, using a non-standard epoch, or just making something up. Often, for one of the biggest star naming companies, I find that there is at least a little dim star somewhere nearby. Is that the one? Well, for most people, I tell them that there was no star at the coordinates given, but this one is closest. Once in a while, though, I find someone who has named a star after a child that has passed away, or something equally emotional, so I would never tell them that there is no star there. I just say that this star appears to be the one, and tell them what I can find out about it.
But for the rest of you, then you should know that these star names are a novelty gift, and professional astronomers will never use them (especially if no one can figure unambiguously out what star is what!).
-Astroprof
December 15 More Student TalesWhat can I say? It's finals, so I am thinking of finals, grades, etc.
An update on the two students who palgiarized. One took her term paper and slinked off. The other tried to argue that she didn't really plagiarize. She "summarized" things she said. I asked, then, if she had summarized, how could she accidentally use the same words, phrases, etc for several sentences in a row? She had no answer. Anyway, she seemed shocked when I had attached to her term paper (which had absolutely no bibliography at all) a copy of the first page of the web page that she took most of the paper from. Of course, given that the source was linked from my web faculty page it didn't take a lot of work to find it.
OK, now for the other end of student stories. My astronomy class last night gave me a calender and a DVD with Flash Gordon and Rocky Jones, Space Ranger! Cool.
Also, I was grading physics finals today, and when I got done, I started adding up points, and I was shocked to find a student with a 98. That just doesn't happen on physics tests. Then, about eight papers later, a student got a perfect score! Wow. This sort of confirmed what I had thought for a while, that being that that particular class was one of my better ones in years. Granted, not everyone did well, or even passed, but the average was pretty high. Now, in my third class, it is the other way around. I have not finished computing grades, but it looks like that class won't have any A's in it at all. That doesn't happen all that often, either. Usually, at least somebody does really well.
Anyway, the grading goes on. And on. And on. Final grades are due tomorrow morning. So, I will finish soon.
-Astroprof December 14 Student StoriesWell, the end is upon us. This is final exams week, and I've given the first set of finals. The next batch is tonight. I am beset upon by students all in a tizzy with the end of the semester. This is when they get sort of more scatter brained and flakey than normal. I thought that I'd relate some student stories.
Student A:
This student writes on the last page of his final, "Astroprof, I know that there is absolutely no way for me to pass this class, but I wanted you to know that I really did learn a lot, and I really enjoy the way that you teach. I will make sure that I take you again when I retake the class." Now, this is a good story, I guess. I'd have preferred for the student to have done better in the class, but ... . See an earlier post about this.
Student B:
This student calls me earlier in the week. He wants to know when the final is going to be. I said that we have talked about that in class. He says that he missed that day. I recognized the voice, so I knew that he has missed just about every day since Thanksgiving. He also wanted to know what Scantron to bring to the final. Confused, I asked him, "Scantron?" He says, "Isn't the final multiple choice?" I ask him if any of the other tests were multiple choice. "No," he replies. As a study guide I put previous semester's exams on my web page so that they know how I ask questions and what sorts of things I emphasize. I asked if any of these were multiple choice. Again, he replies "No." "So," I ask, "what possess you to think that this final will be any different from any other physics test that I've given?" The best thing that he could come up with was that he had heard that someone had said that it was multiple choice.
Students C1 and C2:
They show up together Monday to turn in their term papers, due two weeks earlier. I said, tough. They whined and pleaded, and looked like little puppies. Finally, I said, "well you can give them to me, but I probably won't grade them, since I don't have much time this week, and grades are due Friday." They wanted to know if I can turn in their grades late. I told them that first of all, I met my deadlines, and secondly I am not given extensions. If I don't turn in their grades by the due time, then the computer converts their grades to Fs. They look shocked. Then, I told them that they really should think about turning in the papers. These are optional, and give the students who don't do well on exams a chance to show that they really do know something. If they do well on the papers, they can help them, but if they do badly, then they can hurt. I warn them that I might not grade them, but if I do, then I'll take off 30 points for being late after I assign a grade. So, unless the papers are really good, then they might actually do worse off turning them in. They still insist. Well, last night while giving my final then, I looked at their papers. As I expected, they sucked. They were disjointed, different paragraphs seemed to say things either completely different from what other paragraphs said, or the same things in different words. And, of course, no bibliography. So, a quick Google search, and sure enough ... they plagiarized. I can't really figure out where the plagiarized from, since I found sentences and even paragraphs identical to their papers on multiple web pages. Nothing like plagiarizing from someone plagiarizing someone else's plagiarism! Good grief. Anyway, I had been hoping that this semester, I would actually have a semester that I did not catch someone just cutting and pasting from the web. Wrong! So, by what I told them, I gave them zeros for plagiarizing, and then subtracted 30 points. So, the grade that averages into their scores is a -30. I'll pass back the papers at the final tonight. Won't they be pleased!
Student D:
He shows up for his physics final, and wants to know if he can borrow a calculator. Apparently he didn't think to bring one. I go to my office and get a slide rule for him. Student D is mortified.
Student E:
Forgot to bring a pencil.
Student F:
Forgot to put her name on her final. Luckily I caught it while I could still figure out who had turned in the paper.
Student G:
Forgot to show up for the final.
I am sure that there will be others tonight.
-Astroprof
Update:
Student H:
The department secretary just told me about another prof's student who, after taking the final, called the department office saying that she wasn't doing well and wanting to know if she could drop the class. December 13 Where is Vulcan?Now, for a bit of astronomical trivia. I am giving a final at the moment, and am taking the time to blog a bit before going back to grading a mountain of papers.
I saw an interesting book at a used book store a few months back. It was entitled In Search of Planet Vulcan. Now, this is not the Star Trek Vulcan. Rather, it is a hypothetical planet that was postulated to lay between Mercury and the Sun. I had heard bits and pieces of the story over the years, but this book collects the odds and ends nicely.
The story is set in the Nineteenth Century. In the early part of the century, astronomers noticed that there was a discrepency in the orbit of Uranus. Uranus had been discovered less than a century earlier, but had already made it around the Sun once. However, in one part of the planet's orbit, there was a slight distortion of the orbit. Mathematical astronomers Adams and LeVerrier computed that this distortion must be due to another planet farther from the Sun than Uranus. A search for the new planet turned up Neptune right where it was predicted to be. Now, this discovery is a very interesting tale of hard work, politics, heartache, and hurt feelings that makes up another piece of astronomical history. This may be another entry in the future.
Well, LeVerrier had previously noted another discrepency in Mercury's orbit. As it turns out, Mercury has a very elliptical orbit. But, as the planet goes along its orbit, it does not follow precisely the same path around the Sun. Rather the orbit precesses. This means that if you were to look down on the Solar System and saw Mercury's elliptical orbit, then the orientation of the ellipse would gradually change. At first, it would be pointed to your left, then up, then to the right, etc. If you looked very carefully at Mercury's orbit, the path taken by the planet would look like a giant spirograph pattern. Newtonian physics actually predicts a slight precession, but the precession was too big to be accounted for using plain orbital dynamics. So, LeVerrier proposed that there must be another planet closer to the Sun than Mercury to account for this effect.
Throughout most of the rest of the Nineteenth Century astronomers searched for LeVerrier's missing planet. They even named it before they found it! The chosen name was Vulcan, after the Roman fire god. There were several mistaken reports of a discovery. Usually these turned out to be sunspots. Unfortunately there is no planet there. It took a long time for many astronomers of the Nineteenth Century to accept that, given the problem with Mercury's orbit. However, not finding the planet did not deter many from thinking that it was still there, but simply hard to see. It is actually very difficult to even see Mercury, since it is so close to the Sun. Seeing anything even closer is even tougher. We know that some comets and asteroids pass very close to the Sun in great elliptical orbits. There is speculation that there may be some asteroids in more circular orbits in there somewhere, but the proximity to the Sun makes them hard to find. Any time that there is a solar eclipse, astronomers hunt for vulcanoid asteroids.
So, if there is no Vulcan, how do we explain the excess precession in Mercury's orbit? Well, it turns out that we had to wait for Albert Einstein to come up with General Relativity to explain it. You see, mass distorts space and time. More mass distorts space and time more. The Sun has lots of mass, so it distorts space and time quite a bit --- enough, in fact, that the orbit of Mercury is skewed a bit with each passage close to the Sun (Remember that Mercury has a very elliptical orbit. At its farthest distance from the Sun, it is nearly 50% farther than at its nearest distance to the Sun.) Including the effects of general relativity, we can now calculate the precession of Mercury's orbit. The calculated value and the observed value agree to extremely high precission.
So, if there is a planet Vulcan, then it is not orbiting our Sun!
-Astroprof December 12 Subaru conjunction!There will be another lunar conjunction (see a couple posts ago). This time, it will be with the star cluster M45, called the Pleiades in the western world, and named Subaru in Japan. This is one of the nearer open star clusters, and hence has been very important historically in determining cosmological distances. We can only directly measure distances to stars by geometry out about this far. However, using the stars of the Pleiades as a guide, we can determine distances to other star clusters. These can then be used to calibrate our Cepheid variable distance scale, which then is a step towards calculating distances to galaxies.
The Pleiades looks sort of like a tiny version of the dippers, and often people who don't know better think that it is the Little Dipper. The cluster is named for the seven muses, and is often called the Seven Sisters. However, if you look at it you see six stars quite well. In darker skies, you see nine. It seems sort of silly to me, then, to call it the Seven Sisters, but no one asked me. As I said, in Japan, it is called Subaru, and yes, the cars do have this star cluster on them. It has been adopted as the logo for the automaker.
Tuesday morning, just before dawn in the US Central Time Zone, the Moon will be passing the Pleiades. The Moon's orbit around the Earth shifts slightly as it orbits, a wobble called precession. The path repeats every 18 years and 10 days. About every 9 years, the path of the Moon carries it through the Pleiades, instead of past it. This will begin to happen this next year. As the Moon passes through the Pleiades, it will cover one or two stars at a time. It is actually a lot of fun to watch the stars disappear on one side of the Moon and reappear on the other side about an hour or less later. The first time that I saw this event occur was back nearly 18 years ago, and it was a lot of fun. A small telescope or binoculars helps. You can look up the approximate times that the individual stars will occur, and then watch the occultations. For now, though, you'll have to settle for a near miss.
-Astroprof
December 11 Nearing the end of the semesterWell, here we are, near the end. I just finished a mountain of
papers that I grades. I have one mountain left. I give
finals Tuesday and Wednesday, and all grades are due in by Friday
morning. I still have to make up the exams. This is going
to be a busy week. My feeling is that I have a good crop of students this year. It varies, and it is sometimes interesting how you sometimes get a really good class, and sometimes a really poor class. The students sort of play off of each other. So, a handful of really outgoing good students sets the tone for the rest of them. It is too bad that I can't seed the classes with some of these students every semester. This is also the time of year that I reflect back on the classes and how they have come along. Retention, of course, is an issue. My astronomy classes do pretty good. They are at or above the college average on retention. The physics classes are a different matter. Attrition is terrible in my first semester classes. I lose over half of the students. There seems little that I can do about this. At least, I do better than the national average. Nationally, most of the students taking first semester physics are not really ready for it. They don't have the math or problem solving skills. We have the additional problem in that many of our students don't see the listed prerequisites as requirements for the course, but merely suggestions. Prerequisites are not really enforced. So, unqualified students take the classes, and they have no hope of keeping up. This is a problem. Another problem is that it is too easy to drop. The drop date for this semester was just before Thanksgiving, and the students don't have to ask for a faculty member's signature or anything to drop. They just go online and click a few buttons, and they have dropped the class. So, when the going gets tough, most of the students go away. I feel terrible, because by the end of the semester, it looks like someone had tossed a hand grenade into the class. I have only a fraction of the number of students that I started with. It isn't just physics, all the sciences have this problem. From talking with faculty in other disciplines, they have the same issues. This is even getting to be a problem at the bigger universities, even the large private university across town. The students don't want to take difficult classes, and so they just drop them. This makes teaching first semester physics very draining for me, since I really care that they make it through. Now, I should also point out, though, that my students that go on to second semester physics almost never drop. They know what they are getting into, and they come prepared. I know of several physics faculty that try to improve retention by cutting back on material, and taking things slower, and making sure that the students get the concepts. This is the "less is more" approach. The problem is that less is less. These faculty members have a bit better retention, but it is not a LOT better. They still lose over half the class. But, more telling, about 1/3 to 1/2 of their students going into second semester physics drop that class. Those of us who keep up the pace and cover all of the material that the catalog says that we will cover seldom have our students dropping the second semester. Of course, the second semester class uses concepts from the first semester, and if you have not covered all of the concepts, ... So, which is better, covering less material more thouroughly so that students are more likely to pass the first semester class, but may not be ready to take the second semester, or covering more material as best you can, leaving some students behind, so that the ones that finish are for sure ready for the second semester? This is the problem that you face when one course is a prerequisite for another. What to do ....? Fortunately, the astronomy courses don't seem to have this problem for me. It is mainly the introductory physics classes. Of course, each astronomy course is self contained, and is not a prerequisite for the others, so I can tweak them a bit more. Anyway, I may post more on this later. For now, I am grading papers and making up exams. -Astroprof PS: The class that I am talking about here is the non-calculus based physics. When I teach the calculus based physics, then there is a much lower attrition rate and the class is less draining on me. I think that the problem is that we are getting students that are not prepared to take the class. Many drop, go take math classes, and then come back. Most of the ones that do this try to take my sections, so they seem to think that I am doing a good job. I wish that we could enforce prerequisites, because that would be easier on the faculty, the students, and everyone. ConjunctionsLook up at the Moon tonight, if it is clear. Very near it, you
will see a bright star. That is actually the planet Mars.
Now, it isn't always next to the Moon. Both the Moon and the
planets move around the sky. The Moon takes about 27.3 days to orbit the Earth. This means that it moves about its own width towards the east every hour. You can actually watch it for a few hours as it passes a star or planet. When the Moon or a planet passes another celestial object, we call this a conjunction. The Moon will pass Mars tonight at about 11pm, Central Standard Time (the time here in Texas). For those of you elsewhere in the world, it will appear to pass Mars at about the same time, so you'll have to make the appropriate time zone conversion. Actually, to avoid such time zone issues, astronomers use a system of time called Universal Time, which is the based on the local mean time at the prime meridian. Here in Texas, we are UT-6, meaning that we are 6 hours behind Universal Time. So, the conjunction happens at about 5 UT on Monday morning. If you go out earlier in the evening, you will see the Moon just west of Mars, and then much later you will see it just east of Mars. If you were in just the right spot, (Siberia), then you would actually have the right geometry to see the Moon pass directly in front of Mars. When this happens, it is called an occultation. So, go look if you get a chance, and you'll see the Moon passing Mars. -Astroprof December 10 Globular ClustersOver at Tom's Astronomy Blog he has a posting about an image that he took of the globular cluster M71. It is actually a very good image. One of my favorite globular clusters is M13, probably because it was the first one that I saw. Globular clusters are really very interesting objects. They are often really not as appreciated as they should be as astrophysical objects. In amateur telescopes, they seldom appear as more than fuzzy balls to the eye. A camera (either film or CCD) will show some of the stars. Even so, you don't get the real picture.
They are roughly spherical balls of stars, often with hundreds of thousands of stars, bound gravitationally to one another. Globular clusters are generally much farther away than open clusters, such as the Pleiades, also known as M45 (I was originally going to explain all this M-stuff, but I changed my mind, so that I will save for a later blog). M13 is located about 25,000 lightyears distant, as opposed to only 440 lightyears for the Pleiades. This is because globular clusters lie outside the plane of the galaxy's disk. Visually, our galaxy looks like a giant pinwheel. The space between the spiral arms, though, is filled with material, so the galaxy is really more like a fat pancake. Most everything that you see in the sky with your naked eye is in that pancake. Globular clusters orbit around the pancake in a vast swarm. They are part of what we refer to as the halo of the galaxy. In fact, this fact allowed Harlow Shapley in the early part of the 20th Century to determine that the Sun is not located in the center of the galaxy, but is rather thousands of lightyears from the center! A little under 200 globular clusters are now known around our galaxy. We have found them around other galaxies, too. A few of the Andromeda Galaxy's globulars are even within the grasp of many amateur's telescopes.
Globular clusters are generally very ancient objects. In fact, globular clusters were among the first objects to form in the galaxy. Most globulars formed as the galaxy formed, it seemed. There was a second period of globular cluster formation a few billion years later, for reasons that are not clear --- perhaps a collision or near collision of the Milky Way with another galaxy? Interestingly, one of the satellite galaxies of our Milky Way Galaxy, the Large Magellanic Cloud, has globular clusters that are much younger than our own.
Given that globular clusters are relics of the days of the formation of our galaxy, the study of globulars can lead to an interesting insight into the early Milky Way. Which came first, the oldest globulars or the Milky Way? What are they doing in a big swarm around the disk of the galaxy?
How do we know that the globular clusters are so old? One way is to study the types of stars in the globular clusters. The high mass stars don't live long. The lower the mass, the longer the star lives (another blog topic here!). We can look at the highest mass stars in a star cluster, and this gives us a minimum age for the star cluster. Of course, we are assuming that the stars formed with a distribution of masses at the same time. There is strong evidence to suggest that this is what did happen. Another clue is the metallicity of the stars. Astronomers consider anything other than hydrogen and helium to be a "metal." Following the Big Bang, there were almost no other types of atoms but hydrogen and helium. All of the metals are formed in the nuclear fusions that occur in the cores of stars, or through the nuclear reactions that occur during a supernova explosion (which is how really massive stars die). Thus, the earliest stars would not contain metals. The later a star is formed, then the more metal enriched the gas that formed it would be. Globular clusters are very metal poor, suggesting great age. This is not as ironclad an argument as the stellar mass data that I mentioned, since there are few other stars in the halo to create metals. Presumably, a cloud of gas left over from the formation of the galaxy might remain fairly metal poor in the halo for much longer than a cloud of gas in the disk of the galaxy. Still, most globulars are very poor in metals because they are very old.
A rather startling finding, at least for me, was that one of the extrasolar planets that we've recently found is located in the globular cluster M4. What makes this remarkable is that the low metallicity of stars in globular clusters means that there was very little of the heavier elements to act as seeds for planets. One of the theories of planetary formation requires seeds of heavier elements in order to form gas giants. Apparently this is not how the planet in M4 formed. Until this planet was found, I had felt that the only stars that would have planets would be younger stars that had more metals. Now, the M4 planet is NOT a rocky planet like our Earth. It is a gas giant, similar to Jupiter. Except that it can't be just like Jupiter, because part of what makes Jupiter the way that it is is it's rocky core, which is over ten times the mass of our Earth. So, this suggests that there may be more than one mechanism by which planets form.
So, that is probably more than you wanted to know about globular clusters, but I find them interesting.
-Astroprof December 09 Outside School?Hmm, Seeking Solace wants me to post what I do outside of school. Hmm. There is something outside of school? OK, I'm not that one dimensional. Still, it sort of all blends together. You see, if I had not become an astronomer, then I'd be an amateur astronomer on the side. I really like this stuff. So, I've been known to just set up a telescope and observe for the fun of it. I really like the subject matter, so I often read popular, or even higher level, books on various aspects of astronomy that are outside my own subfield. Now, since I teach astronomy, then often tidbits go back into my classes, or else the give me an idea for a little paper or something in a non peer reviewed publication, or some such. So, some would call that still working, even though it is fun. That is why I love my job --- I get paid to do what is fun for me. Now, that came at a cost of MANY years in graduate school. However, I think that it is that enthusiasm for the material that carries over into my class, and that is part of what makes me such a good professor. I'm not the only one, though, in the department like this. Quite a number of the faculty likewise have their vocation and avocation rather mixed together. This sort of gets further mixed since I got the student astronomy club started, and I am the faculty advisor. They meet once per week, and our college requires that a faculty member be present when a student organization meets. Well, that's OK, since this is sort of a social thing for me. It actually gives me a chance to interact with students other than my own and with former students in a way not quite like I do in the classroom. I don't have any children, so this is my chance to mentor and guide people. Besides, despite being an astronomer, I am also an amateur astronomer (that vocation/avocation duality thing, again), so this gives me an astronomy club. The regular one in the area meets on nights that I teach. Not being successful in finding a wife and starting a family, I sort turned my focus deeper and deeper into what I do, I guess as a sort of defense mechanism. I have no family, but I do have a cat. He is rather the ideal pet for an astronomer, since cats have a nocturnal tendency. So do astronomers. So he and I are up at the same time. OK, all that sounds pretty one dimensional, doesn't it? Well, I do other things. I read. I have always done a lot of reading. I was sick a lot as a child, so I stayed indoors while the other kids were out playing, and I would read. I've kept it up. I had a friend over for dinner once, and she took a look around my place and said that I really didn't need to worry about insulation since I could just put bookshelves against the walls and fill them with books! OK, that was a little apartment, and now that I am a homeowner, I have more walls. Still, ... Also, now that I am a homeowner, I have a garage. Since it is just me, then I only need room for one car, and much of the other half of the garage is given over to a workshop where I can do wookworking. That is fun and relaxing. For a few years there, I was constantly buying new tools. When some people are depressed, they buy stuff, and I bought tools. That has sort of stopped now, since I don't really need any more tools. I've already bought most of what I could realistically want! OK, I still need to get a router table to mount my router on, but that isn't really essential. I have sort of toyed with building one myself. Hmm... As for activities, I really like most things. I like them more if I have company, of course, but that is often rare. I like riding horses (when I get a chance), and of course target shooting (though, now that I think about it, I haven't been to the range in a while). Hey, I grew up here in Texas!!! I also like going to races (NASCAR, etc), hiking, etc. I really should like camping, but I am too accustomed to my nice bed at home, and I can't sleep if I am hot. In Texas, it is hot most of the year. Cold weather camping is nice, though. I occasionally do some things with people from church. Oh, and I am going on a ski trip right after Christmas. (So, Profgrrrrl, happy birthday, early! I will be out of touch, probably, when your's comes around and won't be able to wish it then!) I don't know. I can't think off of the top of my head what else I like to do, but it is really a lot. My therapist (I am overcoming depression) routinely points out that I have an amazing diversity of interests and talents. I don't really brag about them, and I forget them. They are just part of who I am. I have friends that are routinely amazed when these little things just pop out: oh, I like doing that, or I've done that before (like flown in an open cockpit biplane), or some such. Oh, and I am sort of active in politics here at the local level. I am the precinct chair for my party. They need someone with some sense! And then of course, being an astronomy professor, I never really get away from what I do. When at some social event, either through church or one of my other interests, or whatever, then I am always asked about something astronomical or space exploration related, or some such, whenever someone finds out what I do for a living. Again, it is really good that I actually enjoy what I do and talking about it and sharing my love of the sky. Otherwise, I would have real issues. And despite not having a spouse and children (the cat is similar to the latter, though), and that fact gets me down (and I get pretty despondant over the chances of that ever changing), then I need to remind myself that I have the perfect job. My avocation is my vocation, and I get paid to do what is fun for me. In fact, I get paid to do what I would otherwise likely pay to get to do! How great is that? OK, I get frustrated at some of the students sometimes, but that is because I really care. There are a few every semester that make it all worthwhile. -Astroprof December 08 Inconstant StarsI did Profgrrrrl’s request for a movie blog, so now I’ll tackle Tom’s variable star blog suggestion. As I said before, this is actually in my area of interest. I’ll try to keep this short and not go on and on and bore the non-astronomers reading this.
So, first of all, what is a variable star? Basically it is a star that changes brightness. Actually, we now know that all stars, including the Sun, vary somewhat. So, a variable star would be one that varies a lot (sort of subjective) or varies in a particular fashion (also those designations can be somewhat subjective, too).
Apparently ancient people observed that some stars sometimes get dimmer or brighter. One famous star, Algol (short for Al ras al Ghoul, meaning the head of the demon) is called the demon star. Just under every three days, it takes an hour or so to go from the second brightest star in the constellation Perseus to being quite a bit dimmer, and then an hour to return. Apparently, this was an evil omen to ancient skywatchers.
So, what’s up? Well, Algol is one of a category of stars that we call eclipsing binaries. These are binary stars which orbit close to one another whose orbits align such that one star occasionally passes in front of the other now and then. Thus, some of the light from one star is blocked, and the star appears dimmer. There are several sub-types of eclipsing binaries, and one of these is what I’ve done research on (W UMa type stars). I am sure that this designation probably means nothing to 99.9% of y’all, but I don’t want to spend too much time explaining it.
Another type of variable stars are pulsating variables. Deep inside the star is a layer of gas that is at just barely the right pressure to be opaque to the light most prevalent at that depth of the star. So, the gas absorbs the light energy and heats up. When it heats up, it expands. When it expands, the pressure drops, and the gas conditions change and it is no longer opaque, so it radiates away its heat energy, and that makes it cool and contract. Then, it becomes opaque again, and the cycle repeats. When this layer expands, though, it causes the outer layers of the star to expand and cool. The intensity of light radiated goes as the temperature to the fourth power, so cooling a bit means that the star gets dimmer, even though it is physically larger. When the driving layer contracts, the outer layers contract, compact, and heat up. So, the star, though smaller, gets brighter. There are several categories of pulsating variables, but they work basically in this fashion. Some are very regular, like RR Lyrae variables or Cepheid variables, and they vary with a periodicity that has a well defined relationship to the star’s average brightness. So, by observing how quickly the star varies, we know how bright it really is. Then, by comparing that to how bright the star appears, we can figure out how far away it is. This method was used to determine the size of our galaxy, the distance to nearby galaxies, and even the expansion of the universe, so historically it is a very important relationship. Interestingly enough, the relationship was determined at the beginning of the 20th Century by Henrietta Leavitt at a time when women were STRONGLY discouraged from studying science. In fact, she could her results were not accepted for publication until her supervisor, Edward Pickering, put his name on the paper. There’s actually a very interesting story here about women in science, and astronomy in particular, but that’s a different story for another day. I like to talk about this in my non-majors class, since even in this day and age I routinely get women students who tell me that “girls can’t do math and science.” I wonder who is teaching them that.
Another category of variable stars would be cataclysmic variable stars. These are stars that undergo some major explosive event or other singular significant event that changes the brightness of the star. Some stars undergo superflares, in which the suddenly brighten by a factor of 100, 1000, or more. Some binary stars having a white dwarf can undergo a sort of runaway nuclear fusion event --- sort of a super-sized hydrogen bomb. The explosion blows away material that is falling from the primary into the white dwarf, and the system brightens by 1,000,000 or more in an explosive event that we call a nova. A supernova is really an entirely separate phenomenon in which a massive star simply explodes. Some stars, such as R CrB stars have sudden buildups of carbon in their outer layers causing a sudden drop in brightness. The trapped light heats the outer layers and ejects the carbon, and the star brightens up again (but with a shower of tiny diamonds shooting away from it). There are lots of other cataclysmic variables, but I won’t bore y’all. This sort of gets the idea across.
So, that is variable stars, short and sweet. I could go on, and on, and on, but most of my readers would likely be bored. So, any other requests for blogging topics?
-Astroprof
December 07 Movie Stars?Well, I got two blogging suggestions. Profgrrrrl wants to know about me and movies, and Tom wants me to talk about variable stars. If I combine the two, would that make variable movie stars? Hmm. Well, I will do my first blog on the movies. I will have to think about how to shorten my discourse on variables to something that won't bore y'all to bits and make for a lengthy read. Tom, it's interesting that you ask about variable stars. Variable stars are my research area!!!!! So, I will try to compress my two weeks worth of lectures for my stellar astronomy class into a short summary. That will be my next topic. But, for now, movies...
OK, Grrrrl, my favorite movies tend to be scifi movies. I guess that is a holdover from when I was growing up. I suppose that a distant future or galaxy or something always seemed better than my life here. Anyway, it combines multiple interests. As for my favorite, that is hard to pin down. I don't really have a favorite. It sort of depends upon my mood. I rather liked the Lord of the Rings movies (I know, that's really fantasy,not scifi, but I'll include it). Also, I liked "The Forbidden Planet". I introduced that to my students in the astronomy club. When it is too cloudy to go outside and observe, during meetings, we watch a movie. At first, it was educational films, then scifi movies. I've been introducing them to the films of the 50's and 60's, like "Forbidden Planet", "The Day the Earth Stood Still", "The Crawling Eye" (always fun), "Plan 9 From Outer Space" (hilarious because it is SO bad), "The War of the Worlds" (the 1953 one, not the new one), etc. I am having fun watching these old ones, and it is fun watching and laughing at the cheesy ones. I can sort of turn my brain off and just enjoy. I've seen most of them.
For Valentine's Day last year, I watched my copy of "On the Beach", another scifi classic. It fit my mood for the day, since everyone one Earth dies in the end. Sort of like my love life. I most definitely do not like romantic things, or movies with much of that in it, since that reminds me of what is missing from my life, and it just depresses me. So, I avoid them.
As for non scifi movies, one of my favorites is "Apollo 13". OK, so that's close to scifi in that it is about space. But, I also liked "The Hunt for Red October" and "Chariots of Fire", and they are not scifi at all (particularly "Chariots of Fire"). Each shares certain similarities, though, and so that shows the sort of things that I like to watch. I rather enjoy some of the old John Wayne movies, too.
As for movies coming out that I'd like to see, well that would have to be "The Lion, the Witch, and the Wardrobe." I really liked the Narnia Chronicles, so I hope that they did a good job with the movie. Again, it isn't really scifi, but it probably is not too far off in that it portrays another universe where things work out well for the heros of the story. That sort of thing always chears me up.
-Astroprof
December 06 Stardust, Haybusa, and the Andromeda StrainI am stressed by both end of semester stuff and personal issues, so y'all need to give me ideas to blog about, since I can't think. This one, though, I did manage to think up on my own.
Now, interestingly enough, two unmanned space probes are returning to Earth with samples from space. One, an American probe named Stardust, is returning samples from a comet. The other, a Japanese probe called Haybusa, is returning (hopefully) samples of an asteroid. Both of these bodies are quite different from the Moon. The Moon has no organic molecules making up its rocks. However, comets have been known for over a century to be composed of large amounts of organic molecules. Meteorites believed to have come from asteroids have been found to contain organic molecules, and the spectrum of many asteroids shows a composition that includes organic molecules. Now, we can not say that this means that these bodies have life on them. Here I am using the term "organic" in the way of chemists, in that it means carbon based molecules. However, life is carbon based. Under the harse radiation environment of space, these molecules break down and recombine into other forms. Some rather complex molecules can form, including even amino acids. These can be the building blocks needed for life. Again, there is nothing that would indicate the presence of live, and plenty to indicate its absence. However, this is interesting in that these probes are the closest that we have actually come to the scenario at the beginning of Crichton's book. It is particularly ironic in that in the book, the sample container is compromised upon return to Earth, which is exactly what happened to the samples of solar wind particles returned to Earth by the Genesis mission!
The samples returned will be handled with extreme care, as with the moon rocks, but more to protect them from contamination with Earth organisms than with their contaminating Earth. Besides, meteorites fall to Earth all the time from these bodies, so if the did contain organisms, then those organisms would already be here. We don't seriously expect to find life, and if we did it would be one of the biggest shocks that the field has ever had. But, I figured that it might be interesting to point out the connections of science fact and science fiction, remembering of course that science fiction is just that: fiction.
-Astroprof
December 05 EarthshineI'm worked up and can't sleep, so I'll blog. Have you ever gone out and looked at the little crescent moon and seen the rest of the moon dimly lit? Science@NASA has a nice article on the earthshine that you can read. In fact, this is a very good article, and there isn't much more that I can say. Basically, earthshine is sunlight reflected from Earth onto the Moon. Leonardo Da Vinci produced a very good explaination for the phenominon. Changes in weather patterns on Earth can change the amount of light reflected. The article that I reference talks about that. What it doesn't really talk about is when is the best time to see earthshine. Basically, you want the moon to be a small crescent. That means that it nearly lined up with the Sun, and so there is a nearly full Earth shining onto it. Now, you don't want the moon to be too small of a crescent, because that would be too close to new moon. The moon would then set during twilight, and you'd miss out on the full impact of the earthshine. If it is too big of a crescent, or near a quarter phase, then the Earth would not look as full as seen from the Moon, so the earthshine would be not as bright. Coupled with the Moon itself being brighter, this makes the earthshine either tough to see, or simply not very impressive. The best time is usually a few days after new moon. Right about now, in fact. Now, the moon's orbit around the Earth is not over the equator, so sometimes it is over the southern hemisphere and sometimes over the northern hemisphere. Right now, it is the former. This means that it is low in the sky for observers in the northern hemisphere, but higher in the sky for observers in the southern hemisphere. As the moon moves along its orbit, it will be too low to see earthshine as a small crescent for us in the northern hemisphere, so while the best time to observe the earthshine will come a day or so later than it will in the southern hemisphere. That makes it tonight as the optimal time to go look. The absolute best times to see earthshine would be when the moon is a small crescent but still far from the horizon. This happens about three days or so after new moon in March or April. You can see earthshine in the mornings, too, but the best time there will be a few days before new moon in September or October. (Reverse those months for the southern hemisphere). That doesn't mean, though, that it won't be cool to go look at tonight if you have clear skies. -Astroprof |
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