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