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

Well, I've been kind of exhausted and not much good for anything, but now I'll try to write part two of my intended "Big Bang" series!

In part one of this, I described how you can actually measure the distance to nearby stars by triangulation or parallax. Unfortunately, this method only works for relatively nearby stars. For more distant stars, the "Earth orbit baseline" needed for measuring parallax becomes too tiny compared with the forbidding distance to the stars.

The first star that had its parallax measured, 61 Cygni, is one of the most nearby stars, "only" eleven light years away, and yet it is one of the faintest stars that you can see with the naked eye. 61 Cygni is actually a double star, two stars orbiting each other, and it turns out that the combined light of these two stars is less than a tenth of the luminosity of the Sun! But many, indeed almost all bright stars in the sky are farther away from us than 61 Cygni. Yes, most bright stars are farther away, but not all. Take a look at this image:



This picture shows "the Winter Triangle" with Orion. The "Winter Triangle" is made up of bright Sirius at the bottom of this picture, orange Betelgeuse at top right here and whitish Procyon at top left. It so happens that Sirius and Procyon are two of the brightest-looking stars in the sky, but they are also two of the most nearby stars. Sirius is even closer to us than 61 Cygni, only 8.6 light years away, easily close enough for us to know the distance to it very well. Sirius is also 21.79 times brighter than the Sun. And Procyon, interestingly, is almost exactly as far away as 61 Cygni, 11.41 light years for Procyon compared with 11.36 light years 61 Cygni. But the luminosity of Procyon is about seven times that of the Sun, which makes Procyon about eighty times brighter than 61 Cygni.

Now take a look at the picture of the Winter Triangle again. At the far right, you can see a blue-white star whose brightness seems to be more or less the same as Procyon's. This blue-white star is Rigel. But Rigel is estimated to be almost 800 light years away, much too far away to have its distance reliably determined by parallax. But if it is indeed that far away, its luminosity would be about 40,000 times that of the Sun. But how do we know how far away Rigel is, if we can't measure its distance with the help of triangulation?

I'm sure you have heard about Newton. Back in the 17th century, Newton discovered that white light from the Sun could be separated into all the colors of the rainbow:



It basically works like this:



Astronomers then discovered that if they magnified these "colored bands of sunlight", thin black lines appeared in the bands of light. It looked like this:



What were those little black lines? Astronomers were able to identify them as signatures of various elements, for example sodium. By heating sodium into a gas and then sending light through this gas, astronomers found that they could prefectly reproduce some of those black lines that had been found when sunlight was separated into colors. It appeared as if some of the black lines in the color of sunlight had been produced when sunlight passed through a sodium gas. Later on, other lines were identified as signatures of gaseous calcium, magnesium, iron, oxygen and other gases. Okay, but where did that sodium, calcium, magnesium, iron, oxygen and other gases come from, and why did sunlight pass through them?

Astronomers gradually realized that the black lines appeared when the light produced by the Sun was passing through one of the outermost layers of the Sun, the chromosphere. (To learn more about the chromosphere and the photosphere of the Sun, look here .) The chromosphere is gaseous, and it is mostly made up of hydrogen and helium, but it also contains traces of many other elements, for example all those elements that had been identified in the "colored bands of sunlight", or, as these criss-crossed bands of sunlight are called, the solar spectrum. The dark lines appear when sunlight passes through the Sun's chromosphere and some of the sunlight is absorbed by various elements in the chromosphere, leaving dark lines behind.

Okay! But then somebody got the bright idea that you might try to see if the light from the stars in the sky could be separated into colored bands just like the light from the Sun, and if the colored bands of light from the stars (if the stars could produce such colored bands of light) would show dark lines too, just like the colored bands of light from the Sun. And guess what they found? Remember Sirius?



Look! Colored bands of light from Sirius, and there are dark lines in them. But the dark lines in the spectrum of Sirius aren't the same as the dark lines in the spectrum of the Sun. Or, wait a minute. All the dark lines in the spectrum of Sirius are present in the spectrum of the Sun. The really big difference is that Sirius hasn't got all the dark lines that the Sun has. Why is that? Does the chromosphere of Sirius contain fewer gaseous elements than the chromosphere of the Sun?

That could have been the answer, but astronomers, who soon got very good at reading spectra, found that that was not the case. Instead, the difference is the temperature of the chromospheres of the Sun and Sirius. The chromopshere of Sirius is hotter, more than 9,000 degrees Kelvin for Sirius compared with a little less than 6,000 degrees Kelvin for the Sun. Astronomers soon found that the spectrum of a star is extremely strongly affected by temperature:



You can see that a star whose chromosphere and photosphere have a temperature of around 6,000 degrees Kelvin (like the Sun) will have many more "spectral lines" than a star whose chromosphere and photosphere have a tempereature of around 10,000 degrees Kelvin, like Sirius. But you can see from this chart, too, that you can identify many other stellar temperatures from the stars' spectra.

Let's consider some of the stars I have talked about so far. 61 Cygni is a star of spectral class K, which means that the temperature of its photosphere and chromosphere is about 4,000 degrees Kelvin. We can clearly tell from the spectrum of 61 Cygni that it does indeed belong to spectral class K and that it has a temperature of about 4,000 degrees Kelvin. But we can also see, by measuring the distance to 61 Cygni by means of triangulation and parallax, that 61 Cygni is much fainter than the Sun, since the two components of this star (both of spectral class K) together produce less than a tenth as much light as the Sun.

We can also see that Sirius belongs to spectral class A, that it is hotter than the Sun, and that it is about 22 times brighter than the Sun (since we can easily measure the distance to it with the help of triangulation and parallax). So Sirius is hotter than the Sun and much brighter than the Sun, and 61 Cygni is cooler than the Sun and much fainter than the Sun. Is stellar brightness linked to stellar temperature, then? Are stars brighter the hotter they are?

I have already included so many pictures in this post that I don't think I will be allowed to post any more here, so I will end this post here and start another one just below it!

Ann

Arghhh, I just happened to erase everything I had written!!!!

Okay.... I'll try to write something now anyway. I asked in my latest post if stars are brighter the hotter they are, and the answer is... yes, usually! The hotter they are the brighter they are, as long as they are "on the main sequence". They are on the main sequence as long as they shine by fusing hydrogen into helium in their cores. In this thread , you can read about how stars fuse hydrogen into helium in their cores.

As long as stars are on the main sequence, they are indeed brighter the hotter and bluer they are, and they are fainter the cooler and redder they are. Take a look at this sequence of pictures of Alpha and Proxima Centauri:



The most nearby of all stars apart from the Sun is Proxima Centauri, about 4.3 lightyears away, and it is a tiny little runt indeed! Its temperature is about 3,000 degrees Kelvin, and its luminosity is only 0,000055 times that of the Sun, can you imagine? Here you can admire its red color:



Proxima belongs to spectral class M, the coolest class. The next spectral class, K, is represented by 61 Cygni, whose tempereature is about 4,000 Kelvin and whose luminosity is about 0.08 that of the Sun - but now we are talking about the combined light of two small stars!



Orangish 61 Cygni isn't that impressive.

After M and K follows G (what, didn't you know that?). Alpha Centauri is a star of class G2, just like the Sun. It is only slightly farther away from us than Proxima Centauri, about 4.4 lightyears. Alpha Centauri consists of two components, and it is the brightest one that is of spectral class G2. The temperature of this component is almost identical to that of the Sun, but Alpha Centauri A (as the brighter component of Alpha Centauri is called) is slightly brighter than the Sun, about 1.5 times brighter.



Alpha Centauri sits in a photogenic part of the sky, close to the Southern Cross.

If we raise the stellar temperature a notch from class G we get to spectral class F, which is well represented by Procyon, one of the bright stars of the Winter Triangle. The temperature of Procyon is about 7,000 degrees Kelvin, about a thousand degrees hotter than the Sun. Procyon is about seven times brighter than the Sun. You can see whitish Procyon in the Winter Triangle in my first post in this thread.

The next spectral class is A, and Vega is a fine representative of it. Vega is about 25 light years away, its temperature is about 10,000 degrees Kelvin, its luminosity is about 49 times that of the Sun, and in this link it looks quite shockingly blue (you need to scroll down a bit to see it):

Blue Vega

Well, in the past when I belonged to an astronomy club I showed Vega to quite a lot of people, and when they saw it through a telescope they would often exclaim, "How blue it is!".

After A follows B, and the most nearby star of class B in the sky is Regulus. It is about 77 light years away, a bit far, but close enough for triangulation and a parallax. The temperature of Regulus is about 13,000 degrees Kelvin and its luminosity is about 134 times the Sun. Here Comet Lulin is passing close to Regulus:



Finally, a star belonging to the hottest, brightest spectral class, class O, is a star called S Monocerotis. This star is very young and still on the main sequence. Its temperature is between 25,000 and 30,000 degrees Kelvin. The distance to the star is uncertain and too great for triangulation and a parallax, but it is estimated to be around a thousand light years, and the luminosity of the star is estimated to be around a thousand times that of the Sun in visual light. But the star puts out most of its energy as invisible ultraviolet light.

S Monocerotis, surrounded by the gas that birth to it.

S Monocerotis belongs to a sparse young cluster, NGC 2264. A very rich young cluster can be found much farther away, in a small satellite galaxy of ours, the Small Magellanic Cloud. The rich young cluster is called NGC 602:



Isn't this a splendid picture? You can see the remnants of the gas nebula that gave birth to the cluster. The tattered shreds of nebulosity make the gas cloud look like a gaping maw in the process of swallowing an entire school of tiny little fish, intermingled with a few larger individuals.

You can see that the cluster is young because it is surrounded by gas, and because it contains both several very bright stars and a large number of faint ones. The star cluster on the left is obviously older. It is farther away from the nebulosity, and it has lost very many of its faintest stars. That is because the smallest stars are quickly scattered away as the large bright stars begin their brutal game of stellar pin-ball.

The stars I have shown you so far are all on the main sequence, that is, they all shine by fusing hydrogen to helium in their cores. But not all stars do that, and this is where things get a bit messy.

Ann

A friend is an astronomer working on Herschel, the REALLY big infra-red astronomy satellite with a 3+ metre mirror the European Space Agency launched last year. He gave a talk at the SF convention I was at this weekend, showed some of the results they're getting - a lot of really new stuff on how stars form, and what's really going on inside nebulae etc.

http://sci.esa.int/science-e/www/area/index.cfm?fareaid=16

It's already pretty interesting, but apparently they are going to be making some major announcements in May, presumably on the first anniversary of the launch. From what I've seen already it's going to be pretty spectacular.

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