The Astronomy Mega Thread (More than 50 pages of info!)

[VoijaRisa]

Stellar Evolution - The Effects of Convection

In my first post in this series on stellar evolution, I set up the basic model of how stars live, and die. The next two posts looked at theoretical models and showed that they can reproduce where stars should and should not be on the H-R diagram reasonably well.

One thing that, until now, I haven’t mentioned is another important process that occurs in stars and allows for another important set of tests for stellar evolution. That process is convection. For various masses of stars, convection occurs at different places in the star. In our own sun, we know this occurs on the surface. We can see this in the form of granules.

Figure 1
Source

But convection doesn’t always have to occur at the surface. In other stars, convection may occur in the cores, or somewhere in between. In some cases, the whole star may be convective! Convection tends to occur when radiative processes (photons traveling through space) are insufficient to carry the energy of the star towards the surface. This would be due to things such as the density getting sufficiently high that the photons just can’t get anywhere before being reabsorbed or scattered off in a random direction (possibly back towards the center of the star). Another thing models tell us is that the properties which determine how effective radiative transport is, will change over time. This, in turn, means that convection will start and stop at various points throughout the stars lifetime.

When observing stars, it should go without saying that the only thing that we directly observe is whatever is on the surface. Convection is important because it can dig down into the star and bring up material from deeper down, or carry fresh material down closer to the core where temperatures are hotter. In the case of some special elements, this can destroy them.

By comparing abundances of elements in various phases of a stars life to observations, we can get another test of on stellar evolution.

The first case, in which different material is brought to the surface, occurs when convection digs deep enough into a star that it encounters a layer in the star’s interior that had previously been subject to fusion. Since fusion creates heavier elements than what would be present on the surface, the convection would make this visible as it brings it up. This process is known as “dredging”.

Dredging may occur at several points after a star leaves the main sequence. The first occurs shortly before the onset of helium fusion in the core of stars while the star is in the red giant phase.

So what we can do is look at stars on the evolutionary tracks I mentioned in the previous post, and compare the ratios of abundances before and after this dredge up phase. What we should see, if the theory is correct, is an increase in certain heavier elements. And not only that, models should also be able to predict, fairly well, how much those ratios should change.

In this first dredge up, one of the most notable changes will be the ratio of C-13 to C-12 (note: The -N notation is being used to denote subscripts). In the second dredge up phase, models predict that there will be increases in He and N content. Sufficiently massive stars should also undergo a third dredge up, in which carbon and oxygen will be brought up.

All of these changes have been confirmed observationally, and within reasonably amounts, they conform to the predicted ratios. As should be expected, some variation is inevitable due to difference in initial abundances.

But, as I mentioned earlier, increasing surface abundances isn’t the only effect that periods of convection should have. Certain elements, if subjected to convection should be destroyed. Lithium is the most prominent in this case.

Since lithium isn’t something that’s readily created, the amount stars start off with is pretty much all they’re ever going to get. Lithium is also fairly easily destroyed at higher temperatures. The surfaces of most stars aren’t quite hot enough to do so, but if lithium is pulled down to depths where temperatures are hotter through convection, then the surface abundance of lithium will be depleted. Again, models predict at what phases in evolution we should begin seeing lithium depletion, and again, they’re reasonably accurate.

So let’s quickly what we’ve discovered in this series thus far: First off, evolutionary models predict where we should and should not see stars on the H-R diagram. They match where they will speed along, and when they’ll clump. In this post, we’ve explored how models also predict and match chemical abundances throughout the evolutionary process.

[VoijaRisa]

Stellar Evolution - Synthesis of Elements in Stars

I've promised to discuss the synthesis of heavy elements in stars, where "heavy" to astronomers means anything beyond helium.

This month (October 2007) marks the 50 year anniversary of one of the most important papers ever published: "Synthesis of Elements in Stars", by Burbidge, Burbidge, Fowler, and Hoyle (frequently abbreviated BBFH or B^(2)FH).

The Big Bang model is only able to explain the existence of very light elements, such as hydrogen, helium, and trace amounts of lithium, and beryllium. Even if the model didn't predict this, it was apparent that older clusters of stars had less of these heavy elements present in their spectra. As such, it became clear that what astronomy needed was some mechanism to generate heavier elements from lighter ones. This is known as nucleosynthesis.

Thus, the paper asks,

BBFH

What has been the history of the matter, on which we can make observations, which produce the elements and isotopes of that matter in the abundance distribution which observation yields?

In other words, the goal of the paper was to explain how nature could possibly generate this distribution of elements:

Figure 1

The story we tell in most introductory classes is horribly oversimplified, in some ways, to the point of being outright wrong. What's usually explained is that fusion builds up heavier and heavier elements in the cores of stars. When the star dies, either in a catastrophic supernova or in the slow death into a planetary nebula, those cooked up elements are released into the universe at large.

But there's a few problems with this. One of the most obvious is that either way, the real heavy elements are cooked up in the cores. Unless there's convection, they pretty much stay put. Whether the star explodes or quietly gives off it's out layers, those heavy elements don't do a lot of moving unless convection has brought them to the outer layers of the star.

Even if we assume that some of those heavy elements made it to the outer layers of the stars to be exploded off, or just gently released, there's still another major problem with the case in the former: Supernovae release a lot of energy. Enough energy that they can break up all those heavier nuclei that they just spent all that work making. This is known as photodisintegration.

So as it stands at that point, the only mechanism we have to really get more heavy elements is to build it up in stars, get it to the surface via convection, and then slowly release it after the red giant phase. But since the massive stars (the ones that can build elements much past carbon) explode, this can't account for the chemical abundances we observe! And even if astronomers dishonestly ignored all these little problems, there's still one more: Even in the cores of massive stars, you still don't cook up elements past iron. So where do all the elements heavier than those come from?

What the BBFH paper did, was to emphasize some alternative forms of heavy element generation. And it did so at great length. The BBFH paper is a book in its own right, taking over 100 pages to establish the novel processes that were being put together. (So the Creationist canard that they go to books to get their ideas across because laying the foundations for a good theory takes up too much space is nonsense. If you actually have a good theory, you can get space in journals.)

The two big ones are known as the s process and the r process, which stand for the slow process and the rapid process respectively. As the names suggest, one of the main differences is the time scales on which they occur.

The idea behind both processes is that in the right conditions, you can occasionally have a neutron smash its way into the nucleus of an atom. Sometimes that's not a big deal; It just creates a different isotope of the same element. However, some isotopes are more stable than others. The life of particular isotopes is frequently shown in the wonderfully scary looking chart of the nuclides.

Since I'm not sure how many people are familiar with this chart, I'd better do some explaining. Each column in this table is a particular element. Elements are uniquely defined by the number of protons the specific nucleus contains. The number of neutrons can change without changing the type of element it is. Rather, it just changes the isotope. But as I mentioned before, some isotopes are more stable than others.

In that particular chart, the gray isotopes running pretty much along the center are the stable forms. The further you get from this stable center, the less stable the atom is. If you're not really familiar with all this, you should probably be wondering what I mean by "stable". You can probably figure out that if something is stable, nothing really changes, but if it's unstable, what happens?

The answer is that one of the extra neutrons will undergo what's known as a "beta minus decay" (β−). This just means it gives off an electron. But if you're paying attention to your conservation of charge, you'll notice that we had a neutral particle, that just gave off a negatively charged particle, so we're going to need a positive one here somewhere. The answer is that the neutron became a proton (we see the reverse of this when we form neutron stars).

Since we now have another proton, that means instead of just having a different isotope, we now have an entirely different element; one that's one atomic number higher than the previous one.

Depending on how many neutrons are forcing their way in determines which process you're looking at. If the neutron flux is low (say one proton being captured per nuclei per ~10,000 years), then the chemical evolution is driven by the s-process. If it's high, then it's the r process.

Let's look at an example of how this works for the s process first:

Figure 2

First, we'll start with 109-Ag (silver). Smack it with an extra neutron and it changes to 110-Ag. But, you'll notice that the 110-Ag doesn't last very long. According to the color key, it will decay in sometime less than a few years. Unless it's in some pretty special circumstance, it's not going to pick up another neutron before it decays. So it undergoes a beta decay and gets bumped up to 110-Cd. Most of the isotopes up from there are pretty stable until it hits 115-Cd. Then, again, it undergoes a beta decay and becomes 115-In. The process continues on, building heavier elements as it goes.

So the next question is, where can things like this happen? Where are protons energetic enough and prevalent enough to force their way into a nucleus?

It turns out that during the red giant phase of a star's life, the conditions are just right in the outer layers of stars for this process to occur. Additionally, nebulae can be bombarded with neutrons as well, allowing this process to occur there as well. Slowly but surely, heavy elements are built up.

But there's some elements that the s process can't account for. For the sake of clarity, let's look at another part of that nuclide chart.

Figure 3

On this, you'll notice that in the column for Pd, there's a particular isotope (109-Pd) that doesn't last very long. If we got there by the s process, it would decay to that 109-Ag we started with earlier before being able to be transformed into the stable 110-Pd. Thus, if there's any 110-Pd in nature, the s process certainly can't account for it.

This is where the r process comes in. For the s process, the flux of neutrons is relatively low (<10^11 neutrons per cm^2 per second). Meanwhile, for the r process, we're talking about instances when the flux is more like 10^22 neutrons per cm^2 per second!

Thus, even for isotopes that don't last for more than a fraction of a second, the rate at which they're being bombarded is so high, that they can get stuck full of neutrons before they can decay. Thus, the element runs all the way down the column until it's so full of neutrons that it physically can't hold any more and any additional ones "drip" off.

Since it's impossible to keep up that high of a flux of neutrons forever, the rate will eventually die off and the decay process can begin. It takes elements diagonally up and to the right (the way this diagram is drawn) until it finds a stable element. The diagram for that looks something like this:

Figure 4

Thus, the r process can build up the heavier isotopes that the s process can't account for. The s process can build up the abundances near the inner part of the nuclide chart whereas the r process takes care of the outer, heavier isotopes.

So where in the universe do we find ridiculously high neutron fluxes? In supernovae! While the initial energy release can destroy heavy elements, the neutron flux remains sufficiently high for a good while longer and can build them right back up to even higher atomic numbers than the core was able to fuse in the first place.

But do these models hold up to observation? Sure they do. Both processes predict specific relative abundances. The distributions predicted match the observed abundances extremely well:

Figure 5
Sneden et al [ApJ. <b>467</b>, 819 1996]

The BBFH paper laid the foundation for stellar nucleosynthesis 50 years ago at this point and has been well established for a long time now. But of course, dishonest creationists like Kent Hovind love to show that they're more closely related to ostriches than apes when they bury their heads in the sand and ask questions like this:

Figure 6

[VoijaRisa]

Stellar Evolution - Variables and Astroseismology

So far in this series on stellar evolution, we’ve talked about things that we don’t really observe happening in real time. Rather, models show that things happen over time, and we have a series of stills to compare it to because, typically, things take a very long time.

But not always.

Sometimes things can happen very quickly. And if our models are to really be any good, they must be able to explain these events too.

Perhaps the most common set of events that we can observe on short timescales is that of variable stars. This is a very large grouping, including stars that change their brightness in less than an hour to a year or so. These wonderful stars are extremely important to astronomy. Certain types are used to measure distances and many can be used to test our models of stellar evolution.

In astronomy, there are three main types of variables that are often discussed. Regular variables are stars that change their brightness in a regular fashion. Irregular ones are, well, irregular. The third type is known as cataclysmic variables. This group includes stars which explode as novae or supernovae.

For the first two groups, there are a very large number of sub classes, typically named for the first star of the type identified as variable. For example, it was long ago known that the star Delta Cephei varied in a regular manner. Thus, stars that were discovered later which varied in a similar manner were labeled Cepheids.

Cepheids are especially important in astronomy because they’re wonderful distance indicators (their period is related to the average brightness). However, as with everything else I’ve discussed in this series of posts, models better be able to account for why these stars are changing like this.

It turns out that there’s a region on the H-R diagram where models tell us stars should become somewhat prone to instabilities. It’s aptly named the instability strip. Cepheids fall right smack in the middle of it and many of the other types do as well.

As the star evolves across the H-R diagram heading towards the Red Giant Branch, some layers of the star become more opaque than normal (less light is getting through). Since light is what carries the energy out of the star, this creates a dam, blocking the energy which causes an increase in temperature and pressure. This buildup pushes these layers outwards. When it expands, it cools, and the primary cause of the blockage (ionized electrons) is removed. But now the star is overextended and begins to collapse again. The collapse causes the temperature to increase again, reionizing the electrons and beginning the process again.

That’s the simple description but works pretty well for the better-behaved Cepheids. For the rest, many patterns can be accounted for by adding more layers of ionization causing different pulsations as well as shock fronts and other mechanisms.

So for at least one type of variable star we do a pretty good job modeling what’s going on. And when our models can jive with observations, that means it’s a good model.

Meanwhile, some variable stars can be used the other way around. Instead of matching the model to the star as we’ve been talking about here, we can use the stars to make the model.

Here on Earth we’re all quite familiar with the practice of seismology which uses shock waves traveling through the Earth and bouncing off the various layers to figure out what sort of stuff is on the inside (sadly, it’s not a cream filling). This works because seismic activity such as the crashing of tectonic plates can create strong shock waves of various sorts.

It turns out that stars do the same thing although through different mechanisms. I’m not going to bother discussion the mechanisms because that’s beside the point. The point is that stars vibrate and by studying those vibrations, we can look inside stars.

And guess what! What we find matches pretty well with our models. One particular class of variables that is commonly used for this practice (known as astroseismology) is known as Delta Scuti stars. But really any star that vibrates and we can get accurate data on it’s radial velocity to an accuracy of a few meters per second is susceptible to this method. In fact, it was originally performed on the sun via helioseismology.

So, yet again, models and observations fit hand in hand, giving us a clear picture of how stars work and evolve. In my next post on stellar evolution, I plan to look at the last stages of a star’s life, which is one of the most important to us because, as Carl Sagan put it, “We are star stuff” and the heavy materials that comprise our bodies had to be cooked up in these late phases. That’s really the last topic I planned to cover on stellar evolution so I’ll probably make one more post as a grand summary and conclusion.

[VoijaRisa]

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[VoijaRisa]

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[VoijaRisa]

Four Misconceptions About the Big Bang

Having been on Gaia quite awhile now, I've realized that, just like with evolution, there’s a lot of really uneducated people that are being very vocal about their strawman version of the Big Bang. I began to notice that their entire argument was founded on these common misconceptions and thus, I figured it was about time to make a list of the really big ones and attempt to clear them up.

This is in no way a comprehensive list, nor is it meant to present all the evidence supporting the Big Bang, but instead, only to hit the highlights.

1) The Big Bang was an explosion
This seems to be a really big one. I’ve been told that I contradict myself because I point out that the Big Bang wasn’t an explosion so I obviously don’t know what I’m talking about. The term “Big Bang” was originally given to the theory (originally called “primeval atom”) by Fred Hoyle on a radio program in which he was mocking the theory. However, the misnomer stuck and has been causing confusion ever since.

Let’s first look at what the Big Bang theory really states: “Our universe began in a hot dense state which began, and still is expanding. In this initial event, all the matter in our universe was created with approximately 80% hydrogen and 20% helium.”

That’s my personal paraphrase, but after reviewing a great number of sources, it seems to be the most comprehensive one I can come up with. So let’s analyze it. You’ll notice that nowhere do we find the word “explosion.” Instead we find the term “expansion.”

The frequent picture people seem to have is matter flying outwards from a single point (like an explosion). However, the matter is all actually standing still while space itself expands dragging the matter with it.

The general analogy for this is having a series of paperclips on a rubber band. As the rubber band is stretched, the paperclips appear to move away from one another even though they are in fact holding still with regard to the rubber band. Similarly, galaxies hold still more or less (there are small movements due to gravitational interactions) while they are carried by the expanding universe.

So again, there was no “explosion” but instead, an expansion which is carrying all the rest of the universe away from us.

2) The Big Bang theory doesn’t explain what caused it

This is another big one I see a lot. If the Big Bang was the beginning, then what could have caused the Big Bang? You’ll notice my paraphrase above didn’t include anything about this. Pretty big hole eh?

Not really. The Big Bang theory doesn’t say anything about what caused it because, well, it doesn’t need to. Theories don’t try to explain everything, just what evidence is available and pertinent. Asking the Big Bang (and Evolution) to do more than this is a double standard. After all, the theory of Gravity doesn’t explain where mass came from. The Germ theory of disease transmission doesn’t explain where germs came from. Electro-magnetic theories don’t explain where charge comes from. Atomic theory doesn’t state where atoms come from.

So while it might seem like a piece of the puzzle is missing, as far as this single theory is concerned, it’s not really important. The origin of all these other pieces requires separate theories, with their own evidence, which are being worked on, but often times, are still in their infancy (ie, brane theory to explain the precursors to the Big Bang, Abiogenesis to explain the first life…)

Additionally, the Big Bang doesn’t go all the way back because it really can’t. As I pointed out earlier, when you start going back to far, things become fuzzy. The physical laws we’re all familiar with start to break down under such high energy densities. Really weird stuff starts to happen, like different fundamental forces ceasing to exist and merging with one another.

Thanks to work in particle accelerators, which can recreate such high energy densities for brief fractions of a second, we’re starting to get a feel for how physical laws operate under these conditions, and thus, are slowly working our way backwards. But there comes a point where we just don’t have a good enough handle on things to be able to say how things work back to pretty early (10^-35 seconds), but things were happening so fast and furiously, there’s still a long ways to go before we can uncover what happened to cause the whole mess.

Perhaps as better particle accelerators come on line, we’ll be able to work back even further, but this will require new theories about how matter and energy behave when shoved that close together, including a theory which has proved difficult for nearly a century, describing how gravity fits in with the other three fundamental forces into something known as the Grand Unified Theory (GUT).

3) There’s no evidence for the Big Bang

Sadly, yes, I have actually seen this one fairly often. I have no idea where people get the idea that scientists make things up without having good evidence behind it (oh wait… we’re out to disprove God because all scientists hate God or some crap like that).

The Big Bang theory does have a good amount of evidence behind it. So we’ll take a look at the three biggies.

a) Cosmological Redshift: As I explained in my earlier post, we can use spectroscopy to determine the rate at which galaxies are moving away from us. Additionally, since it takes light time to travel, the further away we look, the further back in time we are looking.

What we find, is that all galaxies in the universe are moving away from us. The further they are, the faster they’re moving away. So if we play the whole thing in reverse, all the galaxies will come back together at a single point in time. This point in time is what we call the Big Bang.

b) The Cosmic Microwave Background (CMB): Figuring that if you played everything back in time like this that all that energy would be crammed into a smaller space, that means the temperature would go up. And also since galaxies couldn’t have formed yet, we’d expect a gaseous sort of universe early on. As I discussed earlier, hot dense gasses emit photons at a peak wavelength corresponding to their temperature. Unfortunately, since things were so dense, photons couldn’t get very far.

However, with the available information, astronomers were able to determine at what density and time, photons would finally be able to get far enough that we could observe them. This is called the “surface of last scattering” and has a very specific temperature. So we should be able to look for photons with energy (wavelength) corresponding to that temperature.

But due to redshift, they will appear at a different wavelength. This radiation should appear from every direction. This was a prediction made by the Big Bang theory that was later confirmed by Penzias and Wilson who stumbled on it accidentally!

No other theory of the universe has ever been able to make such a profound prediction to the degree of accuracy the Big Bang did in this instance. Making such amazing predictions is one of the highlights of a good theory. None before or since have ever been able to pull off such a feat.

But the successes of the CMB prediction don’t stop there. Another important piece of the puzzle lies in that the CMB couldn’t be completely even. If it were, then galaxies couldn’t form since there would be no “seeds” with higher mass and thus a stronger gravitational pull to form around.

Thus, the Big Bang theory had to predict that the CMB would not be completely homogeneous. It should have some variations to it, and those variations would have to be of a specific size in order to get the universe we see today.

Early results for the Big Bang didn’t look too good for this prediction and threatened to sink the whole ship. However, the devices used were not actually sensitive enough to pick up these minute variations. But recently, with the Wilkinson Microwave Anisotropy Probe (WMAP), these perturbations have been discovered precisely as predicted.

Score two strong predictions for the Big Bang. Zero for any others.

c) Distribution of Elements: With the conceptual framework intact thanks to the first point, it was also possible to calculate how much of each element should be formed in the initial event. It should be obvious that, given a bunch of protons, electrons, and neutrons, hydrogen should be the easiest to form. Indeed, stick a proton and an electron in a room together and they’ll automatically hook up due to their magnetic attractions.

Additionally, with such high energies, it would be possible to fuse some of this hydrogen into helium and even a little bit of heavier elements. Since astronomers had a good handle on the energies, it was possible to calculate how much of each there should be. If that number didn’t match up with observations, the Big Bang theory would be shot.

Fortunately, the predictions do match up pretty closely. I stated a value earlier of 80% hydrogen, 20% helium, and neglected the rest since it would be statistically insignificant. In the universe today, we observe 75% hydrogen, 24% helium, and 1% everything else. This discrepancy is easily accounted for by nearly 14 billion years of stars cooking hydrogen into helium and other heavier elements.

So there’s three major pieces of evidence for the Big Bang, any one of which, if it had turned out any other way, would completely discredit the theory. Fortunately for the Big Bang, it has passed all of those tests, and not a single other theory has yet been able to adequately explain such things, or many anywhere near as profound of predictions (or any successful predictions for that matter). This is why the Big Bang stands alone as the premiere theory in cosmology today.

4) The Big Bang doesn’t leave room for God

This isn’t a scientific argument, but rather a philosophical one which is completely beside the point. However, since I see it used frequently, I’ll go ahead and address it.

The Big Bang, like all science, doesn’t have any implications either for or against God. What it may do, it place constraints on how God did things and these may run contrary to scripture. However, there’s two important questions here:

First off, is the scripture right in the first place? And, second, assuming it is, are you interpreting it correctly?

The first one is really beside the point given that it would be folly to approach such a topic, but the second is worth addressing. Many Christians have absolutely no problem interpreting scripture in a manner that’s completely compatible with scientific observations like the Big Bang and Evolution. In regards to the Big Bang, many people choose to interpret the “7 days” as a rather metaphorical statement in which days are better understood as “phases” and could have, in reality, been billions of years. Such people also note that Genesis’ account (roughly) follows the order in which science says things happened (although the order does differ on some points).

I think it’s also important to note that the Catholic Church has affirmed the Big Bang and finds no problem reconciling the theological and scientific perspectives on this point. Both Pope John Paul II and the Vatican’s official astronomer, George Coyne, have given strong support for the Big Bang theory. Additionally, the theory itself was originated by a Belgian priest named Georges Lemaître.

So we see, the Big Bang can fit well with scripture so long as one is willing to look at things from the right point of view.

I hope that clears up a few of the misconceptions people have been having, and I’m pretty sure that most people reading this blog were already familiar with all that, but perhaps this has given you a bit more detailed information that you can use next time someone throws out their strawman Big Bang.

[VoijaRisa]

On the Demotion of Pluto's Planethood

Many people have been rather upset about the fact that Pluto is no longer classified as a planet. Reasons for the complaints range from things like "It's been one for a long time, so why change it" to "Sailor Pluto was my favorite Sailor Scout! gonk "

However, none of these criticisms are terribly reasonable. In this post, I intend to illustrate why it was changed, and the importance of proper classification.

First off, Pluto was discovered back in the 1920's by Clyde Tombaugh an American amateur astronomer (who went on later to get a degree at the University of Kansas). Part of the reason it was immediately billed as a planet was because of the nationalistic pride of Americans. It was the first new object of this size ever discovered by an American.

However, by the late 1990's astronomers were discovering some new objects out past Pluto that were nearly as large. Until this point, there had never been enough intermediate objects between "planets" and "asteroids" to ever worry about the definition. But with the discovery of these new objects (Senda, Quaoar, Xena...), it became clear to astronomers that we needed to actually decide what the exact definition of a planet should be.

The decision of definitions of things in astronomy is determined by an international group of astronomers named the International Astronomers Union (IAU). Early this year, they decided that, under the new definition of planet, Pluto shouldn't make the cut.

The reasons: It was too small and has much more in common with a class of non-planetary objects known as Kupier Belt Objects (KBOs). It also did not have a nearly circular orbit like all other planets do. Additionally, it's orbital inclination is very noticeably out of the plane of the solar system.

Thus, it was decided that Pluto should not be allowed to retain the title of planet and needed to be reclassified. Many people are upset by this but fail to realize that this isn't the first time that this has happened. the largest asteroid in the solar system (Ceres) was originally classified as a planet when it was discovered. However, astronomers began to discover the presence of many more smaller objects in the same vicinity and realized that Ceres was just a large member of what we now call the asteroid field.

Similarly, when Pluto was first discovered, it was unique. But now we realize that Pluto isn't special. It's just one among a much larger collection of objects from the Kupier Belt. As such, it has been relabeled appropriately.

This of course, begs the question, to many, of why we should care what the label is. No matter what we call it, Pluto is still Pluto, so why not just let it retain the title for historical purposes? After all, there's a great deal in science (astronomy especially) that's funny because of historical purposes. For example, the system of measuring brightnesses of stars (known as magnitudes) runs backwards with brighter stars being negative.

But ultimately, labels are extremely important. The reason for this is that classification systems allow us to find common traits and understand systems of objects.

One example of this is the development of our entire understanding of stellar properties during the early 1900's. Originally, we classified stars based on their spectrum. But we had no idea how it progressed. As such, stars were placed in classifications rather randomly and no further progress was made. Since we had stars in classification systems with no meaning, we couldn't figure out the common trends.

But by the 1930's, a female astronomer named Annie Jump Cannon devised a new classification system and it was quickly realized based on this that the spectral classifications she derived would allow us to understand the progression of effects that temperatures has on stars.

Thus, with a proper classification system, we can learn things. Without, we stumble in the dark. This reveals the importance of classifying Pluto properly and getting over the historical impetuous to allow it to retain the title. Astronomers need accurate classifications. The layperson does not.

So as an astronomer, I ask those of you who are not, to get over your nostalgic view of things.

[VoijaRisa]

The M81 Group

M82 is one of my favorite galaxies. It's an exciting galaxy that had a collision with its neighbor M81, which is also chewing on NGC 3077, a while back. The results of this are having all sorts of exciting effects not only on M82, but also in the resulting mess from the collision.

These galaxies look like they're quite independent, but when astronomers Yun, Ho, and Lo started mapping atomic hydrogen in 1994, it turned out there were large bridges of gas between the galaxies. This sort of thing happens when galaxies interact due to tidal forces; the galaxies get stretched because the end closer to the other galaxy is being pulled on more than the far end. This is very well illustrated by galaxies that are very obviously interacting, such as the Mice or the Antennae. We can even find these long stretched out tails around our own galaxy where we've torn apart dwarf galaxies that have gotten too close.

But not to be content with just saying that the galaxies were related, astronomers went so far as to actually model the system and try to recreate the observed structure! This isn't an easy task with even two galaxies, but here we have three that are interacting. In this image, you can see just how closely their result matches with the actual observed morphology. The overall shapes, the angles, the relative sizes all match with amazing precision. Pretty nifty.

Meanwhile, tidal tails aren't just pretty. Clumps can form in them, containing thousands of times the mass of the sun worth of raw materials. In some of these knots, large numbers of young, blue stars have been discovered, suggesting that they can form new dwarf galaxies (Markova, 2002, Ciardullo, 2004). But this high rate of new star formation isn't limited to the tidal tails. M82 is undergoing such high star formation that it's blowing the galaxy apart.

I've talked a bit about how galactic interactions form new clusters, but M82 has them aplenty! Of the 650 clusters found in M82 by Chavez et al., 400 of them are in the area where starburst is no longer occurring, but that still leaves 250 brand new clusters in the area where the highest amount of star formation is taking place. And these clusters are massive.

Although these newly formed clusters have a distinct difference from what are typically considered to be globular clusters, it's possible that they may be the precursor to globular clusters. To determine this, the group is looking at how well such clusters can survive aging. To make it, clusters need to be able to survive three main processes.

The first is the pressure from early supernovae when the massive stars die. This is nicknamed the "infant mortality" stage and it lasts about 10^7 years. Next up is mass loss from stars decreasing the overall mass of the cluster and allowing it to drift apart. Another factor of 10 longer and if the cluster's still there, it should be OK. Last up are multibody interactions. There's two main forms to this. One is something I've discussed previously: tidal stripping. Just like the interaction of the galaxies in this association can draw out tidal tails, the same happens to clusters as they orbit the galaxy. As they get drawn out, the cluster disperses. The other is gravitational interactions within the cluster itself. Some stars will pass too close to another star and get a gravitational slingshot out of the cluster, again adding to loss. But if the cluster can make it past all these hurdles, and become a "relaxed system," it should be relatively stable.

The ones that survive become full fledged adult clusters, either globular or open depending on the number of stars. The question is whether or not these supermassive star clusters in M82 will survive to adulthood. Fortunately for the clusters in M82, they've already hit the 10^7 year mark, so many are well on their way. Based on their masses and sizes, the group expects that many of these clusters should survive to become brand new globular clusters.

And of course, where there's new stars, there's massive stars. And massive stars live fast, die young, and go out with a bang. More accurately, they go supernova. And as I've mentioned before, supernovae help seed the universe with heavy elements. Interestingly enough, the ratio of Silicon and Sulfur to that of Oxygen is unusually high. Too high, it seems, for the typical run of the mill supernovae to account for it. Thus, Umeda et al. (2002), have suggested that a good number of these massive stars were so massive, that they didn't end in just regular supernovae, but rather as the even more powerful hypernovae, which have a different metal output. Given that the lifetimes of these massive stars are nearly identical (~10^7 years) to the time that the rapid star formation occurred, this would seem like a plausible scenario since it would otherwise be a surprising coincidence.

It should go without saying that these huge supernovae are putting out some pretty intense stellar winds, which are whipping up the remaining gas, making huge bubbles. One group thinks that the shock front may be the cause of compact radio sources. The other leading hypothesis is that supernova remnants themselves may be the source.

Radio isn't the only non-visual regime in which there's some activity though. M82 is also highly active in the X-ray due to some supermassive black holes that are enjoying a feeding frenzy with all the activity. But black holes aren't something I've been much into, so I won't bother going into any detail on the current work.

Regardless, M82 and the rest of the M81 group is a pretty exciting set of galaxies in which a lot of fundamental astronomy is happening.

[krugerr]

Interesting thread,it s good for knowledge about it ,great

[Rayquazza]

VoijaRisa

Space Reserved

And that, Alanis, is ironic.

[The Astrobiologist]

Woo-hoo! Black body radiation!!!

[VoijaRisa]

Done some updating.

[ShadowFoxxxxx]

Rayquazza

VoijaRisa

Space Reserved

And that, Alanis, is ironic.

Well spotted. xd A star for the day. ...ah...no pun intended.

[VoijaRisa]

Updated with information on the 81 group of galaxies.

[CrimsonAimi2007]

I enjoyed reading this friend...

This is very intresting...I use to take astronomy in High School..

Please add more! biggrin