A couple weeks ago, Tom posted an entry about galaxy NGC 1309.  Here he mentioned the expansion of the universe, and sort of suggested that I say something about it.  Now, I am doing just that.

So, to set the story, let’s go back to the early 20^th Century.  Alexander Friedmann, a Russian mathematician, was working with Einstein’s equations of general relativity.  Now general relativity is often misunderstood.  Yes, it is a description of the warping of space and time that leads to black holes, but it is more than that.  Gravity is doing the distortion of spacetime, so general relativity is a theory of gravity.  The distortion of spacetime due to mass is at the heart of the theory.  Working with these equations, Friedmann devised an equation describing a metric, or a measure of spacetime in the universe.  He found that his equation had two stable classes of solutions:  either the metric is increasing or it is decreasing.  The only way that it could be constant would be if there were no mass in the universe to distort spacetime.  The implication of this is that the universe must either be expanding or contracting.  This was a surprising finding.  Einstein himself was so philosophically opposed to these results that he went back and arbitrarily added a term to the Friedmann equation that allowed for a class of stable solutions --- neither expanding nor contracting.  The strength of this term is given by a term called the cosmological constant.

In 1929, just a few years after Friedmann’s work was published, Edwin Hubble announced his findings that the more distant a galaxy were located from the Milky Way, the more quickly it was moving away from us.  The relationship appeared linear.  The recession velocity equals the distance times a number called the Hubble Constant.  This could only be explained if the universe were expanding.  The finding was so important that Einstein traveled all the way from Germany to California to meet with Hubble.  After that meeting, he pronounced that his cosmological constant was the greatest blunder of his scientific career.  So, the universe is expanding.  Georges Lemaitre pointed out that the expansion of the universe, when run backwards far enough, resulted in all matter in the universe being squeezed into a very small volume.  He proposed that running the universe backwards would even give the age of the universe.  Such and age is given by the reciprocal of the Hubble Constant.  His original idea was that all matter would be crammed into a single object, a sort of “primordial atom.”  This idea, though, was quickly abandoned and no cosmologist has believed that in almost 60 years, though some of the pre-college textbooks still teach the primordial atom. At any rate, Lemaitre’s model required that the universe have a beginning event.  Being a priest, Lemaitre jumped on this idea as being consistent with his faith. Astrophysicist Fred Hoyle was unwilling to accept that the universe had a beginning, and he came up with his own model called the Steady State Model, in which the expansion of the universe opened up empty spaces in which new matter and galaxies came into being.  Thus the universe would be infinite in extent and eternal in character.  Hoyle was so opposed to the Lemaitre model that the even coined a derisive name for it:  The Big Bang.  He pointed out that it was ludicrous to expect that some sort of big bang could happen that would tear apart a primordial atom and hurl the bits out into space.  Indeed physics does not support the concept of a primordial atom, and even Lemaitre agreed, adjusting his model not to require a single massive object at its beginning.

Along comes George Gamow and solved the problem of the initial condition of the universre.  He showed that at high enough densities and temperatures, the particles that carry the forces of nature break down.  At high enough energy density, for example, the electromagnetic force and the weak nuclear force become a single force called the electroweak force.  Running the expansion of the universe backwards, you find that eventually all the forces merge into one force.  Without the four forces, matter as we know it does not have the same properties.  This permits the entire universe to be squeezed down to a single point, called a singularity.  As the singularity expands, then the forces “freeze out,” particles begin to form, and then nuclei begin to form.  Eventually, at the universe expands, it cools and atoms begin to form.  The universe is like a giant cloud of gas at this point.  This cloud then collapse into clumps that form the seeds for stars and galaxies.  It is at this point that some of the details get fuzzy.  It is important to note that a major difference exists here between Gamow’s model and Lemaitre’s model.  Lemaitre at first proposed that material was hurled outward from a central object.  Gamow proposes that space itself is expanding.  Rather than things hurling outward, they are sitting still and space is expanding carrying objects farther from each other.  This is really a significantly different model.  This model is called the Big Bang model to distinguish it from Lemaitre’s primordial atom model which is called the Big Bang model.  Hmm.  Maybe that is why some of the pre-college textbook authors are confused.  Actually, the modern view of this event is slightly different from Gamow’s model, and we call the modern model the Big Bang model.  Yeah, right.  A decade ago, Sky and Telescope had a contest in which votes were taken on what to name the newest version of the description of the beginning of the universe, and the winning name is ….. the Big Bang.  Obviously, cosmologists don’t really have a flare for naming things.  Try teaching this stuff.  I get to say in class that “No one still believes in the big bang.  Instead we believe that the proper description of the event is the big bang, which is a revision of the big bang, which replaced the earlier view of the big bang, which supplanted the big bang as the description of this event.”

I don’t really have time to go into all of the observational evidence supporting this model.  This blog is already going to be a long entry.  Of course, I have my own observations of the expansion of the universe.  I used to wear size 32 pants, but now wear size 34.  That’s expansion of the universe, right, not just expansion of me?  There are a couple of really good books, though, which explain the Big Bang very well.  One, by Simon Singh, gives the history of the big bang models without any of the math common in detailed descriptions of the model.  It describes things in layman terms and is a very easy read.  Amir Aczel has written a very good book that gives a couple of the equations, but does not derive anything.  It is written at about the level of Sky and Telescope, and it is very readable.   I have links to these books on my booklist.

Now for the strange parts of the modern big bang models.  We know that the universe is expanding, but does it do so at a constant rate?  The answer is NO.  We know that gravity pulls on the universe, and that this should slow the expansion.  For years, the holy grail of cosmologists has been to find the deceleration parameter, which would tell how quickly gravity is slowing the expansion of the universe.  This is determined in large part by the density of matter in the universe.  If the density is too low, then the universe will continue to expand forever, just going slower.  If the density is too high, then the universe will collapse in on itself, in something called the big crunch.  If the density is just right, exactly at some critical value, then it will expand forever, but will gradually come to a stop after an infinite length of time.  There is only one critical value, but there are basically an infinite number of possible values of greater or lower densities.  The chances of the universe randomly being at the critical density are basically zero.  If we look out into space, we see only about 10% of the material needed to slow the expansion of the universe.  However, when we look at the masses of galaxies or galaxy clusters we find that they have a mass about ten times larger than we get from looking at the material that we can see.  We call this extra matter dark matter.  Including dark matter in the equations gives a total mass and density VERY close to the critical value.  So, how can the universe randomly wind up with a value that has almost no chance of happening randomly?  For years, cosmologists have been trying to find extra mass which would result in the universe eventually collapsing in upon itself.  Personally, I have never really understood this fixation with that particular solution.  I have never had a problem with it expanding forever, but then I am not a cosmologist.  I suspect that such solution, though, introduces some questions that many cosmologists are not comfortable with, so they try to avoid them.  That isn’t really good science.  Another problem facing cosmologists, though, was that the universe is too flat and homogeneous.  In other words, you see the same thing, and make the same measurements, no matter what direction that you look.  In particular, the universe is the same temperature in all directions.  In order for two objects to be the same temperature, they must interact long enough to exchange thermal energy and reach equilibrium.  The problem is that if you look in opposite directions in the universe, you see the same temperature.  Yet, these parts of the universe are too far apart to have ever interacted.  Thus, they can NOT be the same temperature.  Yet they are.  Hmm.  A problem exists here.

Never fear, though, because my story isn’t over.  Along comes Alan Guth.  In 1979, Guth proposed that perhaps there might be a cosmological constant after all!  In particular, he proposes that it isn’t really constant, but that it was large at one point, and then dropped to zero.  When it was large, the universe expanded much faster than it does today.  This period of rapid expansion is called “inflation.”  According to inflation theory, one of the forces didn’t freeze out quite as expected in the original Gamow model.  The freezing out of the forces is much like a phase transition.  It is known that you can actually cool water below its freezing point without it freezing if you do so carefully.  This super cooled water will then freeze suddenly at the least little disturbance, with a sudden release in thermal energy.   Inflation theory proposes that something like that happened at one of the phase transitions in the early universe.  The universe overshot for a fraction of a second the point at which the grand unified force came apart.  When it finally did come apart, it did so with a sudden release of energy that the cosmological constant was briefly very large.  Space itself suddenly expanded by a factor of perhaps 10 to the 50 power times.  This would expand something the size of a hydrogen atom out to a distance of lightyears.  In fact, it would carry much of the universe so far away that it would take longer than the age of the universe for light to reach us from that distance.  So, all that we see would have essentially been at about one spot in the early universe, and so it would certainly have had time to reach thermal equilibrium.  Inflation would stop at about the critical density of the universe.  So, we now have an explanation for what we see.

However efforts continued to be made to determine the density of the universe and whether or not it will expand forever or collapse in on itself.  Inflation might stop at about the critical density, but it would unlikely stop right at critical density.  By the 1990s, technology had allowed measurements of the expansion of the universe to be made much farther than ever done before.  This was, in fact, one of the goals of the Hubble telescope when it was placed in orbit --- to measure the Hubble constant of the early universe.  The slowing due to gravity actually means that the Hubble constant isn’t really constant.  Over time, it will change.  In 1998, I was at a meeting of the American Astronomical Society in which results from two teams of astronomers making just these sorts of measurements were reported.  The fist of these speakers that I heard, Saul Perlmutter, gave surprising evidence that the universe, rather than slowing down, was accelerating its expansion!  I had heard tidbits and hints of this finding, but I had not read any preprints, and had not heard details until then.  Aczel’s book gives a better description.  And Donald Goldsmith’s book is all about this accelerating universe.  So, how does this acceleration of the universe’s expansion happen???

At present, we don’t really know why the universe is accelerating.  Theorists are working overtime to explain it.  The best explanation so far seems to be to look at the energy of space itself.  We know that the universe is not empty.  Even in a vacuum, there are particles coming into being and then annihilating each other all the time.  These are called virtual particles.  They have an interesting effect.  If you put two metal plates next to each other in a vacuum, there will be a repulsive force pushing them apart due to these virtual particles.  These particles are responsible for the evaporation of black holes.  Could they have something to do with the repulsion that is accelerating the universe?  Another possibility, somewhat related, is that we have not only dark matter but dark energy in the universe.  This dark energy could manifest itself in a repulsive force, like a non-zero cosmological constant, that would create an accelerating universe.  Other more bizarre theories include energy leaking to or from higher dimensions of space, making it appear as if there were a repulsive force.  Really, I don’t know if there is any agreement here as to why the universe is accelerating its expansion.  Dark energy seems to be the best explanation.  Again, though, I should point out that nothing is really moving in all of this.  Rather space itself is expanding, and it is that expansion that is accelerating.  The matter in the universe is just going along for the ride.  Now if that isn’t enough to give you a headache --- nothing is moving, but everything is getting farther away from everything else. 

So, I am sorry that I can’t explain why the universe is accelerating, but it does seem to be doing so.  If y’all want more, I can blog about this in more detail, but this seemed like a long post already.

Now, I’d be remiss if I didn’t point out that not everyone is on board with this accelerating universe thing.  A few astronomers say that perhaps there is a problem with the distance measurements that Perlmutter made.  He was measuring distances using Type Ia supernovae, which have a pretty uniform brightness.  Material between here and these supernovae change their apparent brightness, which could throw off the measurements.  Also, if stars in the early universe, which had slightly different compositions from stars today, exploded differently, then that, too, could throw off the results.  The leaking energy people, who propose energy leaking to and from other dimensions of spacetime than the normal four that we see, say that perhaps this leaking energy can mimic acceleration.  Other stranger ideas abound.  However, various teams of astronomers have been reproducing the accelerating universe measurements using other means, so I think that we are safe to accept that interpretation of the measurements.  Why?  Dark energy seems the best guess so far.

-Astroprof

Look in the back of any astronomy textbook, and you’ll see a table of planetary data.  Among the data are the masses of the planets.  So, just how do you go about weighing a planet?  After all, you can’t just carry it into the lab and plop it down on a laboratory balance, right?  Hmm.

You can’t just assume that larger means more massive, either.  Not all planets have the same densities.  For example, Earth has an overall density of 5.5 grams per cubic centimeter, while Saturn has an overall density of only 0.69 grams per cubic centimeter.  Thus, while Saturn is big enough to fit nearly 725 Earths inside of it, Saturn’s mass is only 95 times that of Earth. 

Oh, and interesting note, since water has a density of 1 gram per cubic centimeter, then anything with lower density that that will float, so if you had a bathtub filled with water big enough Saturn would float in it!  Well, actually, Saturn is mostly made of gas, so really it would just dissolve, but it would leave rings.  ;)

So, if size doesn’t tell you the mass, how do you find it?  The clue came in the early 17^th Century when Johannes Kepler came up with his laws of planetary motion.  His third law was a relationship between the distance that a planet orbits the Sun and the time that it takes to complete that orbit.  Years later, Isaac Newton was able to derive Kepler’s third law.  Kepler, himself, never knew why his law worked, only that it did.  Newton, though, figured out a formula for the force due to gravity.  Very simplistic mathematics shows that the ratio of the orbital distance cubed to the period squared is proportional to total mass orbiting.  So, if you can see something orbiting a planet (ie: a moon) and you can determine how far the orbit is from the planet, then you can determine the mass of the planet.  Thus, since Jupiter had been known to have four large moons since the time of Galileo, the mass of Jupiter could be determined by watching these moons orbit the planet.  Just a bit before Newton came up with this revision of Kepler’s third law, Christiaan Huygens discovered a moon orbiting Saturn, which we call Titan.  This meant that the mass of Saturn could then be determined.

Actually, all that could be done for a century or so was to measure the relative masses of the planets in terms of solar masses, since the exact distances of the planets were not known, nor the exact distance between the Earth and the Sun.  However, since all the terms are proportional, we could determine the relative spacing of the planets and the relative masses.

Eventually, Uranus and Neptune were discovered, and shortly after the discovery of each planet, moons were discovered orbiting of those planets.  In the mid 19^th Century, Asaph Hall discovered moons orbiting Mars, so the mass of Mars was able to be measured.  However, Venus and Mercury have no moons.  So, all we could do was guess at their masses.  Venus was just a shade smaller than Earth, so it was assumed to have a little less mass.  Mercury was a bit larger than the Moon, so it was expected to have a bit more mass than the Moon.  However, all we could do was guess until spacecraft were sent to these planets to actually measure their gravity up close.  Then their mass could be calculated based upon their effect on the orbit of the spacecraft sent to investigate them.  Venus turned out to, sure enough, have a little less mass than Earth.  Mercury, though, was found to have quite a bit more mass than the Moon.  This led us to realize that Mercury is composed of a gigantic iron core, with only a comparatively small amount of rocky material in its mantle and crust.  Once Pluto was discovered, we again could only guess at its mass.  In fact, it was so far away that determining its size was a problem.  However, once Pluto’s moon Charon was found, it was possible to determine both the size and mass of Pluto. This is when we realized that it was little more than a giant chunk of ice and rock.

In fact, this technique of using the gravitational effects on orbital motion is pretty much how we measure all astrophysical masses.  We determine the masses of stars by observing the orbits of binary stars.  We determine the masses of galaxies by observing the orbits of stars in those galaxies, or of satellite galaxies orbiting the parent galaxies.  We determine the masses of galaxy clusters by measuring the motion of galaxies in those clusters, and so forth.

So, that is how you weigh a planet or a star.

-Astroprof

Back when I was an undergraduate, I took a course entitled “The Philosophy of Science.”  As a physics major, this seemed an interesting elective.  I wasn’t alone in that feeling, as a bit over half of the class was composed of science and engineering students.  There were some interesting things that we talked about in that class.  I came to realize that some of the philosophers that we covered knew less science about how science worked than they did philosophy.  However, others seemed to really be on to something.  I think that some of the things we talked about were oversimplifications of how science works, but then what do you expect of such courses?  In our introductory science courses, we talk about “the scientific method,” as if there were only one method used by scientists.  In reality, science uses all sorts of methods to understand the universe --- some methods being better than others.  However, that is a bit complicated to get across to freshmen who are looking for a cookbook way of doing their laboratory exercises.  We get them used to the traditional way, and then we move on in later classes to the variations used by scientists in the real world.

At any rate, during the philosophy of science class, I remember being introduced to the term Copernican principle.  Having an interest in astronomy at the time, it stuck.  Later, I ran into the term in some introductory astronomy and cosmology textbooks.  I think that the philosophers were the first to come up with this idea, and then it worked its way into astronomy and cosmology.  Certainly it was not something that Copernicus came up with.  So, what is the Copernican principle? 

To answer this, it helps to refresh ourselves with Nicholas Copernicus.  He’s an interesting person in his own right, and well worth reading up on.  Copernicus was born February 19, 1473 (Hey, he’s sort of got a birthday coming up in a couple of days, sort of, but not really, taking into account the calendar changes between then and now, …), and he was a lay worker in the Church.  It was in this capacity, in an attempt to understand the problems with the calendar (Easter was not being calculated correctly), he realized that you could much more easily calculate the positions of the planets if you assumed that they as well as the Earth all went around the Sun.  This was the heliocentric model of the Solar System.  At the time, the prevailing idea was Geocentric, or that the Sun, Moon, Planets, and stars all went around the Earth. But, Earth going around the Sun meant that Earth is not the center of all.  That is the heart of what we call the Copernican principle:  we do not have a special location in the cosmos.

As I said, though, the true significance of this did not come for a number of years.  It was Geordano Bruno who most vocally pointed out the significance of both Earth and planets going around the Sun.  It meant that Earth was a planet.  He went on to say that that meant that the planets were Earths, and that they might be inhabited.  He was ordered to quit saying such things;  however, he continued to popularize his arguments.  For this he was burned at the stake.  Then, of course, he quit saying such things.

However, old ideas die hard.  With the realization that the stars were bodies like our sun, only much farther away, came the notion of the galaxy.  Astronomers tried to map the galaxy by estimating stellar distances.  All maps looked similar:  the galaxy appeared as a sort of disk-like structure with the Sun at its center.  Even as early as the beginning of the 20^th Century, most astronomers thought that the Milky Way galaxy was the entire universe, and that it was only a few ten thousand lightyears across, with the Sun almost exactly at its center.  In 1920, though, Harlow Shapley showed that globular clusters formed a spherical halo around the galaxy, and that the center of that halo was NOT the Sun, but rather a point tens of thousands of lightyears away, in the general direction of Sagittarius, beyond where the edge of the universe was thought to be!  Not too many years after this, in 1924, Edwin Hubble presented conclusive evidence that some of the “spiral nebulae” that astronomers had puzzled over for nearly a century were in fact entire galaxies located millions of lightyears away.  We have now found galaxies many billions of lighyears away. 

So, the Earth orbits the Sun as its third planet, the Sun orbits the center of our galaxy about ¾ of the way out from the center, our galaxy and the Andromeda Galaxy are dancing about each other in a local group of galaxies (actually, the Milky Way and the Andromeda galaxy are in a death spiral and will crash into each other in a few billion years), the local group of galaxies are moving around the Virgo cluster of galaxies, which is the center of a supercluster of galaxies, which forms a great wall of galaxies running through the universe.  There is nothing particularly unique about any of that.  So, we are not really the center of everything.  Hence the Copernican Principle. 

So, all I have to do is convince my students that they are NOT the center of the universe.

-Astroprof

PS:

Oh, and just to confuse the issue, it turns out that since the universe is finite in age, and isotropic, and thus you can see the same distance in all direction, so in that sense, we are the center of the observable universe, or at least I am!  However, that could be said about every point in the universe, so again our vantage point is nothing special.

This past week, the Olympic Games started in Torino (Turin), Italy.  In years past, I used to really enjoy watching the Olympics.  You have athletes who have trained their whole lives to compete in the games.  They work hard, and they give their all in competition.  Unlike with professional sports, where money is the driving factor, the Olympics are athletic competition in its purest form.

OK.  Now, back to reality.  That is what the Olympics are supposed to be, and is what the Olympics used to be.  Sadly, it is nowhere near the truth anymore.  Now, we have athletes who show Olympic potential, and they get sponsors so that they don’t need to work at a real job anymore.  All they do is compete and practice and compete.  Doesn’t that, then, make them professional athletes?  After all, if they make their living by being athletes, then they are not really amateurs, are they?  Then, there is the matter of drugs.  Athletes are caught doping all the time.  In fact, many try find all sorts of ways around the rules.  “This wasn’t on the proscribed list, so it’s OK.”  Well, if they are taking a drug, or eating something with the sole purpose of assisting their competition, then they are cheating.  That is pure and simple.  Just because they find something that hasn’t made it onto the list yet doesn’t mean that it is OK.  Also, look at the equipment that these athletes use.  This isn’t the stuff that you go to a sporting goods store and buy.  The skis are in the thousands of dollars range.  The gear that they wear is specially made to minimize air resistance.  Good grief!  When a skier goes down the slopes, he carries with him things worth more than most cars!  “But,” you might say, “this is his career not an avocation, which is why his equipment costs so much more than similar things that you might buy.”  Bingo.  That’s my point.  They are professional athletes.  Yeah, there will be occasional new records made in competition, but these don’t really mean anything.  You can’t really compare the record setting performance of today with the old record held decades ago by someone who competed on the side, did not have performance enhancing drugs, special diets, corporate sponsors, special trainers, specially designed equipment, and so forth. 

Then, there is the whole commercialization of the Olympics.  Everyone wants to host the Olympics.  They want to host it so much that virtually every location on Earth that is under consideration will try to find out how they can bribe their way into hosting the games.  Oh, they won’t call it a bribe, and they will carefully study the rules to find a loophole that won’t look like a bribe.  But, …  .  So, why such an effort to be host to the Olympics?  Is it the prestige of the games?  Is it a chance to showcase the city to the world?  No.  It is money.  The Olympics are seen as a cash cow.  You get all sorts of private money, sponsorships, advertising placements, etc.  You get the state or federal government to give you grants to build new venues for the games.  You get to have thousands of tourists arrive at the event, and you can set whatever prices for goods and services that you want.   Twenty dollars for a burger and fries?  Sure.  You can jack the price up, and no one will postpone their trip to some later date when it is cheaper.  And, of course, you’ll have all those cool venues afterwards.  You can create a resort community to keep attracting tourists for years.  It is all about money.  And, oh yeah, there will be some athletes there, too.

So, I am sort of disenchanted with the Olympics. 

OK, now I’ll get off of my soapbox and get on to astronomy.  There is an interesting connection between the Olympics host city and astronomy.  Some years ago, astronomers began to take seriously the possibility of a huge meteorite strike on Earth.  We have been finding more and more asteroids whose orbits cross that of Earth’s.  We have found more and more evidence on Earth itself of asteroid collisions.  And, in 1994, we saw the result of a small comet hitting Jupiter.  So, what do we do if we see an asteroid or comet heading towards us?  Well, if we find it is on a direct collision course anytime soon, … nothing.  There is nothing that we could do.  However, if we see that the object might hit in the future, then we can monitor it and perhaps devise some plan of action.  But which ones do we monitor?  Asteroid and comet orbits are not really stable.  They shift around some.  Which ones are the most important to monitor?

To answer this question, a conference was held in Torino, Italy.  Here, a rating system was devised to assess a hazard rating for objects whose orbits may pose a risk to Earth.  This is known as the Torino Impact Scale.  A numeric guide was devised.  NASA has a description of the scale here, and a more complete description here.  A sort of matrix can be used to determine the risk number assigned.  One dimension is the size of the object, and the amount of damage that the object could inflict.  For example, tiny objects would either burn up in the atmosphere, or else they’d just make a small crater a few yards across.  These we won’t worry about.  Bigger ones would wipe out a city, were they to hit, but the likelihood of striking a city, even if they hit Earth, is pretty slim.  Bigger impacts would do serious damage to a large region of the Earth’s surface, and even large impacts would threaten civilization across the whole planet, doing major damage everywhere.  Truly massive impacts would kill everyone.  However, do we really need to worry about the monster asteroids if we are certain that they are not going to hit us?  Wouldn’t we be more concerned with a city-killer if we knew for certain that it would hit in a few years?  So, the second dimension is the certainty of hitting, ranging from no chance in the foreseeable future all the way to definite with no chance of missing us.  As I said, these orbits are not entirely stable, and it takes a number of observations to even figure out what the orbit is doing right now.  The first few observations may not be accurate enough, so we might only be able to say that it will pass near Earth, with a large degree of possible error in the calculations.  The range of possibilities might even include an impact.  So, this would rate more attention than a certain miss.  Several times this has happened.  After further study, though, the range of errors narrowed, and we found that the asteroid would miss us after all. 

So, when an asteroid or comet is found whose orbit passes near Earth, an assessment is made as to how likely it is to hit us, and how much damage it would do if it did hit.  If the damage is minimal then we assign a hazard of 0 to it.  If the damage would be extreme, but we can say for certain that it will not hit, then it still rates a 0.  If the damage were moderate, and we were certain that it would hit, then we might give it a 3, but if we were not sure if it would hit, then we might give it a 1 or 2, depending on the level of uncertainty.  A truly massive impact might get a 7 if we knew it were going to hit, but only a 1 if we were pretty sure, but not certain, that it would miss.  This is the same sort of risk assessment that insurance companies do when setting insurance rates.  The Torino system was a bit cumbersome and confusing to people outside of the field, so it has recently been modified to make it simpler to understand, but I am not sure that the modifications make it any more clear. 

Anyway, I thought that it was sort of interesting when I heard a while back that the Olympic Games this year were going to be held at Torino, given the fact that this was where this impact rating scale had been developed.  A further interesting connection, for the science fiction buffs out there, is that Torino is in northern Italy.  In 1973, Arthur C. Clarke wrote a novel titled Rendezvous with Rama, in which astronomers detect a mysterious asteroid changing course.  Clearly, it was really a giant spaceship, and a team of explorers was sent to investigate.  The asteroid was detected due to a survey program called Spaceguard, set up to look for asteroids that may pose a hazard to Earth.  The first chapter explains that Spaceguard was established following a terrible meteorite impact in northern Italy which destroyed the cities of Padua, Verona, and Venice.  Interesting that this fictional setting isn’t all that far from where conference met in real life almost three decades later, in Torino, to establish a rating system for impact hazards.  Oh, and by the way, a number of astronomers in the 80’s began donating their time and efforts to form a voluntary program search for hazardous asteroids.  They named their project Spacewatch, and it still exixts  During its first week of operation, Spacewatch actually found an asteroid that had just made a near miss with Earth.  Now, there is a more formal international program, called Spaceguard, in honor of Clarke’s vision,  that is doing the same thing.  The list of potentially hazardous asteroids now numbers over 750 members.

-Astroprof

OK.  This one is for all you Trekkies out there!

Back a number of years ago,  (long enough to be depressing), I was teaching a second semester astronomy class.  We were covering star formation, and the lecture was on T Tauri stars.  Well, wouldn’t you know it, that week’s episode of Star Trek:  Next Generation entitled “Clues” had the Enterprise visiting a T Tauri star!  Out of a class of 100 students, several were Trekkies, and saw the episode (I did, too!), and they came to the next class all excited because they caught the reference.  It was particularly interesting because that episode got the concept of T Tauri stars right, but never actually said what they were!  There is no telling how many people watched that thinking that it was just a Star Trek term that they made up for the show.

Now, for the 95% of you reading this blog who don’t know what a T Tauri star is, I guess that I should explain.  First of all, let me explain the name.  Some time ago I did a blog entry on how stars got their names.  I left a few things out.  One was the convention for naming a certain class of stars called variable stars.  Despite the impression that the stars are unchanging, they do change over time.  Most of the time, these changes are pretty slow, so you never notice them.  However, at certain times in a star’s life, it can become unstable (in one fashion or another) and begin to change quickly.  The most obvious manifestation of these changes is that the star changes in brightness.  Such stars that change in brightness are called variable stars.  There are many types of variable stars.  Some change due to pulsations in the star, others due to rotational effects, others due to runaway nuclear reactions, and all sorts of other mechanisms.  There is even one class of variable stars that change in brightness as carbon crystallizes in their outer layers.  They expel these bits of crystalline carbon into space.  And yes, by crystalline carbon, I DO mean diamonds. 

Once a star is identified as a variable, it can get a variable star designation (actually it is a bit more complicated than that).  By convention, the first variable star in a particular constellation is called R.  The second is S.  The third is T, and so forth.  So, T Tauri was the third variable star found in the constellation Taurus.  What do you do after you reach Z?  Then you go back and double up letters.  Not just any two letter combination works --- there are rules, but we won’t go into them here.  Eventually, you run out of the allowed double letter combinations, and so variables after than get a designation starting with V and a number, as in V395 Tauri.  But, I won’t bore you with all that.  Suffice it to say that T Tauri is the name for the third variable star in Taurus.  It turned out to have a rather characteristic type of variation and color, so any other star that looked and behaved like T Tauri was called a T Tauri star (even though those stars had their own separate variable star designation).  I hope that isn’t too confusing for y’all.  It’s a pretty awkward system, but I didn’t come up with it!

Many years later, we realized that T Tauri stars were not strictly speaking stars after all.  They are what we call protostars.  A protostar is a big ball of gas that is in its final stages of collapsing to form a star.  When stars form, they form out of a large cloud of interstellar gas called a nebula.  Portions of the gas clump up into small dark nodules called Bok globules.  The protostar forms inside the globule.  The gasses spiral into the protostar through a large disk-like feature surrounding the protostar that we call an accretion disk.  Eventually, the protostar gets hot enough to dissipate the remnants of the globule, and we get our first glimpse of the protostar, while it is still settling down to become a star.  These protostars vary in brightness, which is why they were initially classified as variable stars.  Often the remnants of the accretion disk remain for a while around the protostar, providing a place for planets to form.  Such a disk is called a proplyd.  The first planets to form would be gas giants, and then smaller rocky planets with rather unpleasant atmospheres.  It would take a long time for the atmosphere to evolve into something like our nitrogen-oxygen atmosphere. 

Anyway, I had just covered all of this in class.  Then the episode “Clues” aired.  It must have been a repeat, since I looked up the actual date that episode first aired, and that was too early in the semester for me to have gotten to all of this yet.  At any rate, for those Trekkies out there, I will give a brief synopsis of the episode.  The rest of you can skip past the next paragraph.  ;)  

“Clues”:  The Enterprise-D is passing the Ngame Nebula (made up name) and a T-Tauri star is observed.  Star formation being of some interest, a probe is dispatched to relay data.  Suddenly, the Enterprise is many lightyears away, and time has elapsed for the rest of the galaxy, even though the onboard chronometers show no passage of time and no one remembers any time passing.  The probe data shows an icy gas giant planet.  Captain Picard and his crew assume that a wormhole had tossed them instantaneously then and there, and so they go about their business.  Then, they start finding clues that time had actually passed between when the probe had been sent and when they realized that the ship was somewhere else.  The Enterprise returns to investigate the T Tauri star and to see what really happened, only to find that it is not a T Tauri star after all, and the icy giant planet was really an earth-like planet with oxygen-nitrogen atmosphere.  The planet is inhabited by advanced aliens who don’t like visitors.  So, they had the Enterprise sent far away, and the memories of the event erased from the crew.  The onboard instruments are reset, and the data faked to make it look like the ship had instantaneously jumped to the new location and time. 

OK, now back to astrophysics.  The interesting thing is that T Tauri stars, being protostars still forming, would not have earth-like planets.  Icy gas giant planets are likely what would be found.  The episode, though, did not really explain this, only hinting at the idea that an earth-like planet would be unexpected.  T Tauri stars are, in fact, often found near nebulae, as in this episode.  Wormholes are a theoretical construct of quantum physics whereby an object could take a shortcut to some other point in spacetime without passing through the spacetime in between.  In other words, falling into a wormhole could mean that you reappear somewhere else in the universe at any arbitrary time in the future or past.  None of this was really explained.  That macroscopic wormholes are improbable, of course, can be ignored in science fiction.  So, this was an episode that really appealed to us geeks that knew all the background information behind the story.  Hence, my students were all excited to understand what was going on in the story.

-Astroprof