Lois & Clark Forums Forums General Discussion Off Topic The Support club is taking notes (or, the color of stars)

There is a Swedish saying, "The support club is taking notes", which means "Who cares anyway? Only your support club does, if you have one." So, yes, I realize that I probably don't have a support club which will be taking notes now, but I am sufficiently fascinated by star colors to post this little "lecture" here. It is absolutely necessary to look at an accompanying picture to understand what I mean, and I have found the perfect picture, but the picture is so large that I can only post it here as a link:

Star field with colored stars

Undoubtedly you will notice a startlingly reddish-orange star in the middle of the picture. That star is called T Lyrae, but I will return to that one. Instead I'll ask you to look at several other stars in the field, stars that have all got a so called "SAO number" and a spectral class (A0, A2, K0, K5 and M0). A star's spectral class is directly related to its temperature. (I should clarify that we are talking about the surface temperature of the star.)

Here you can see typical spectra of stars of various spectral classes:



The topmost spectrum is from a star whose surface temperature is about 30,000 degrees Kelvin (which is almost the same thing as degrees Celsius). 30,000 degrees Kelvin is very hot for a star, certainly hot enough for this star to "ionize" any gas clouds close to this star and make the gas glow red. The fact that this star is so hot means that most of the light it emits leaves the star unhindered, and only a few specific wavelengths are absorbed by the hot atmosphere and show up as black "gaps" or "spectral lines" when you "spread out" the light of the star into long bands of continuous color. This particular star belongs to spectral class O. These stars look blue to us, although their color can sometimes be "washed out" to the human eye by the red nebulae they will produce if they are close to a gas cloud.



AE Aurigae is an O star that is speeding through space. It has recently entered a gas cloud and ionized it and made it glow red.

The next star belongs to spectral class B, and it is also very hot, though not as hot as a star of spectral class O. A temperature of 20,000 degrees Kelvin is hot for a class B star. The bright stars in the lovely and famous cluster, the Pleiades, have temperatures around 13,000 degrees Kelvin.



Comet Machholz passing in front of the Pleiades. Comets have greenish "comas" due to various ionization processes as ices from the comet head sublimate and are ionized by ultraviolet radiation from the Sun, but the bright stars of the Pleiades are blue in themselves, because their temperature is around 13,000 degrees Kelvin.

The next spectral class is A. In the figure above the star of class A has a temperature of 10,000 degrees Kelvin. That is hot for a class A star. More normal temperatures for A stars are between 7,500 and 9,000 degrees Kelvin.



The five middle stars of the Big Dipper are all class A, and their temperatures are between 7,500 and 9,000 degrees Kelvin. They do look slightly "too blue" in this picture, but they would be bluish for real.

The next spectral class is class F. In the diagram it is represented by a star whose temperature i 7,000 degrees Kelvin, which is a fairly normal temperature for a star of class F.

[img]http://farm3.static.flickr.com/2004/2301944607_a071f17a69.jpg?v=0[/img]

This is an unfortunately rather bad picture of the Alpha Persei cluster as it was visited by Comet Holmes. (Hi, Sherlock!)

Another Holmes...

Couldn't resist.

Let's return to the Alpha Persei cluster. The comet is of course the fuzzy grey balloon-shaped thing. The cluster is to the left of it. The stars of the cluster look like little bright wiggly lines because the camera was probably stationary (or else it made an unfortunate movement) instead of following the stars' slow apparent motion across the sky, as the Earth turns on its axis. Can you see that most of the stars of the cluster appear to be blue? That's because they are generally of spectral class B. But the brightest star, Alpha Persei itself, looks white. That's because it is not much hotter than the Sun, so it tends to look white to human observers. It belongs to spectral class F.

The next spectral class is G, which is what our own Sun belongs to. The surface temperature of the Sun is, if I remember correctly, about 5,500 degrees Kelvin.



This starscape shows the most nearby of all stars, Alpha Centauri. It is on the far left in this picture, and it looks rather white, perhaps with a tinge of yellow. Alpha Centauri is four light-years away, about one and a half times as bright as the Sun, and very slightly cooler than the Sun, perhaps 5,400 degrees Kelvin. The blue star to the right of it is Beta Centauri, of spectral class B, whose temperature is about 17,000 degrees Kelvin. Beta Centauri is also dozens of times farther away than nearby Alpha Centauri.

The next spectral class i K, where the stars have a suface temperature of about 4,000 degrees Kelvin. Maybe the most famous of all K stars is the "topmost" star in the Big Dipper, Dubhe. This is really not a very good picture, but maybe you can see that the star to the upper right is yellowish:



Finally we come to spectral class M. Here the star temperatures are typically around 3,000 degrees Kelvin, and the stars are noticably yellowish. Take a look at the picture that showed you Alpha and Beta Centauri again. To the right of Alpha and Beta Centauri you can see the famous Southern Cross. The topmost star of the Cross is a star of spectral class M. As you can see, this star is noticably orangish.

Okay! Let's return to the picture that I only gave you a link to, showing a starfield where some of the stars had "SAO numbers". Two of the stars belong to spectral class A, and you can probably see in the picture that they look noticably bluish. One is spectral class K0, and you may be able to see that it is a very pale yellow.One is spectral class K5 and one is M0, and both are yellowish, but still rather pale.

And then we have the star in the middle, T Lyrae! What is this star, and how did it come by its color?

T Lyrae is a rare so-called "carbon star". Believe it or not, but this star has actually "polluted" its own atmosphere with soot! That is not because it produces carbon in its interior, as such. Many stars produce carbon in their interiors, but so very few manage to "dump" this carbon as "soot" in their atmospheres! But T Lyrae has done just that. Compared with normal stars, T Lyrae is actually shockingly red.

On the Earth, the sky may turn red even in the daytime if a volcano has had a very big eruption. This is a local and temporary phenomenon, because the sky will clear and turn blue again. But T Lyrae exists in a state of - for now - "permanent self-pollution". Hence its startling red color.



Well, I hope the support club has been taking notes!

Ann