Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the Universe, part two

Marcus' post about the Herschel satellite and infrared telescope is a reminder of the fact that much of the groundbreaking science in astronomy today is being done with infrared instruments. I'll return to a few things that you could read in the link he gave us.

First, though, I'll pick up where I left. I said before that when stars are on the main sequence, they are brighter the hotter they are. And the hotter they are, the bluer they are. The Pleiades is a young cluster, about a hundred million years (which is young for stars!) and all the stars in it are still on the main sequence.



So the stars of the Pleiades are young. But how do we know that? Astronomers determine the age of a cluster by plotting the brightness of the stars of a cluster versus the color, the so-called B-V index, of the stars. The smaller the B-V index is, the bluer is the star. Conversely, the larger the B-V index is, the redder is the star. This is the "color-magnitude diagram" of the Pleiades:



The stars are lined up along a nice line going steadily upward and to the left. The higher up we get, the brighter is the star, and the farther to the left we get, the bluer is the star. In the Pleiades cluster, the stars are bluer the brighter they are.

The apparent magnitude of the brightest stars of the Pleiades cluster, combined with the estimated distance to the cluster of about 400 lightyears, shows us the that the brightest blue stars of the Pleiades are generally a few hundred times brighter than the Sun. That's a lot, but it also means that they don't belong to the hottest spectral class, class O. There are no class O stars in the Pleiades cluster. This is also obvious from the color of the stars. The color is "normal", and the stars aren't half hidden behind a lot of dust that would dim and redden the stars. We have found that the stars of the Pleiades are "unreddened". The cluster is also old enough for even the faintest stars to have emerged from their "stellar cocoons", their "wombs". Since the most lightweight stars take the longest to be born, we can say that this cluster is several dozen million years old. But since the brightest stars are still blue, and since their spectra show them to be of class B, we can say that this cluster can't be very much older than a hundred million years, because if it was, the bright blue stars in it would have begun to change.

To see how, take a look at this diagram which show how "all stars" line up on a color-magnitude diagram:



The vertical, slightly curved line running from lower right to upper left is the so-called "Zero age main sequence" color and brightness for stars of all masses. The more massive they are, the brighter and bluer they are. You can find stars that I have talked about on this "main sequence" line, such as Procyon (which has admittedly slightly left the main sequence), Vega and Regulus. And you can see the Sun, of course. And you can see that this curved vertical line going up and to the left looks almost identical to the color-magnitude diagram of the Pleiades.

But there are other clusters whose color-magnitude diagram look anything but the one of the Pleiades. If the Pleiades trace the main sequence of stars, the largest clusters of the Milky Way, the globular clusters, trace much of the region to the upper right of the main sequence. One such great cluster is called M55. This is the cluster:



And this is the color magnitude diagram of globular cluster M55:



In this cluster you can still see the bottom part of the main sequence. The "turnoff point" is the point where the stars in this cluster have run out of hydrogen in their cores and evolved off the main sequence. The more massive a star is, the more quickly will it deplete its central hydrogen and evolve off the main sequence. There are probably still stars like the Sun on the main sequence in this cluster, but no stars like Procyon, Sirius and Vega.

After the stars have left the main sequence, they start other processes in their centers and grow redder and brighter. The redder they are, the more energy they generate, and the brighter, larger and more "puffed-up" they are.

In this diagram you can also see some very blue "horizontal branch stars". Such stars, with this particular color and brightness, have also left the main sequence, but they can only form if the gas that gave birth to them was very pristine and contained extremely small quantities of trace elements like oxygen, carbon, calcium, iron etcetera. The Sun was born out of a cloud rich in such trace elements, and the Sun can never become a "blue horizontal branch star".

You can see that the horizontal branch stars are fainter the bluer they are. That is because they are smaller the hotter they are.

The so-called "blue stragglers" are fascinating. Astronomers believe that they are the products of stellar collisions inside the crowded globular cluster. Two small stars have merged into one larger star, fresh hydrogen has sunk into the newly formed larger star's interior, and the star can get its internal hydrogen fusion going again.

Finally, the white dwarfs are burnt-out stellar cinders, the remains of more massive stars that have cast off their gaseous "envelopes" and revealed their naked stellar cores, where all energy-generating processes have ceased. These stellar corpses just radiate their remaining heat out into space.

I said before that the Pleiades is a young cluster, about a hundred million years. Well, the globular clusters are old. Most of them are considered to be ten to twelve billion years old. The color-magnitude diagram shows us several clues to the age of these clusters: the truncated main sequence (because all stars more massive then the Sun have already used up the hydrogen in their cores and evolved off the main sequence), the fact that the brightest stars are red, the fact that even the brightest stars are not enormously bright (because the very brightest stars of the globular clusters exploded as supernovae long ago) and the fact that the stars of the globular clusters are made of very pristine gas. Later generations of stars were born out of gas that was a lot more "contaminated".



The Pleiades are like the young pups of the clusters, but the globulars clusters are like the tired old dogs - ah, but they are still going strong! But even yonger than the Pleiades is the NGC 2264 cluster, where young class O star S Monocerotis is stirring up its natal gas cloud and its younger, smaller siblings:





Talk about a boisterous baby!

Let's return to the "color-magnitude diagram of all stars", the one which showed you, among other stars, the Sun. In this diagram you can see stars that on the main sequence, stars on the red giant branch (Arcturus, Aldebaran, Pollux and Mira). You can see white dwarf stars, Sirius B and Procyon B, which are tiny little embers.

At the top of the diagram you find the supergiants. They are all very big stars and humongously bright, and they all started their lives as very bright blue stars, usually of class O. Since then they have swelled prodigiously and cooled enormously, at least on the surface (their interiors are unimaginably hot). The redder a supergiant is, the bigger it is. Often the biggest red supergiants are also the brightest. In the red supergiants many different fusion processes are taking place at different depths of the humongous star. The various fusion processes "puff the star up", making it swell to monstrous dimensions.



I was going to show you the size of a red supergiant compared with the size of the Sun, but Aldebaran is really just a red giant, not a supergiant. A red supergiant is much bigger!

Let's return to Marcus' post and the link he gave us. Here you can read about the largest known star, VY Canis Majoris. These are some of the things that we are told about VY Canis Majoris:

This stellar behemoth is surrounded by large quantities of hot steam!



Can you imagine a star doing this? It is possible for VY Canis Majoris because it is so cool and because there are so many energy-generating processes going on inside it. And it is so cool and "steamy" because it is so big.

So the star is estimated to be 4900 light years away. With today's technology there is no way of directly measuring such a large distance, and astronomers must estimate the distance by scrutinizing the spectrum of the star and comparing its estimated distance with the estimated distances of other bright massive stars which are undoubtedly found relatively close to this behemoth. That is because large massive stars are always born in clusters, and they rarely have time to leave their clusters before they explode as supernovae.

And the size of the star is estimated to be 2600 solar radii! That's amazing! The distance from the Earth to the Sun is about 200 solar radii. VY Canis Majoris is thirteen times bigger! If this stellar monster were to replace the Sun at the center of our solar system, it would swallow not only Mercury, Venus, the Earth and Mars, but Jupiter and Saturn too!!!



VY Canis Majoris would eat our solar system all the way out to Saturn!

To sum it up, stellar behemoths aside, the point I wanted to make with this post is that there is no clear connection between stellar brightness and stellar color. Stellar spectra give us many clues about the brightness and general nature of a star, but it is still very hard to know exactly what kind of star you are looking at when you pick a point of light in the sky and find that it is too far away for triangulation.

Twinkle, twinkle, little star, how I wonder where you are! And until we get a moderately good handle on that, we won't know much about the universe beyond our own galaxy, and then we won't know much about the larger universe at all.

Ann