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

I enjoyed the astronomy information but I have to admit I enjoyed learning a new expression just as much. I find languages and their idioms fascinating.

Very cool!

I finally saw "Watchmen" this week, Ann, and I thought of you when Dr. Manhattan said, "I am looking at the stars. They're so far away and their light takes so long to reach us. All we ever see of stars are their old photographs."

Hi Ann,

This was a good overview and I loved the pictures.

Two points:
1. You didn't use the old mnemonic that I was taught back in the 70's:
"Oh Be A Fine Girl Kiss Me." -> O B A F G K M for the 7 classic spectral classes. A bit sexist but it's always worked.

2. No green stars. I've always thought green stars would look great. It's not fair that the physics of blackbody radiation mean that there are no green stars. Somebody should fix that.

Thanks for the diversion.

Bob

Ann,
I know you're a teacher (and a wonderful one at that!)... I'm just wondering what subject you teach?? Because you SHOULD be teaching Astronomy if you're not! I find space, etc fascinating in and of itself but you can make my head spin (in a good way) with your vast knowledge of the cosmos. Well done.

I love your posts. I always learn something new. Plus, I like the pretty pictures!

Thank you all for your kind words!

Cookiesmom, the correct expression is actually "The club of interest" or "The club of interested people", but I thought "The support club" sounded better in English.

Sue, I'm so flattered that you were thinking of me when you heard Dr. Manhattan talk about the stars!

Bob, thank you!

As for that famous mnemonic, there is actually a less sexist version of it, "Oh, Be A Fine Guy, Kiss Me!"

There are, as you said, seven "classic" spectral classes, but now astronomers have added two more, spectral classes L and T. The L and T stars are brown dwarfs. There are two things you need to know about brown dwarfs: one, they are so lightweight (they weigh less than 10% of the Sun) that they can't get their "stellar engines going", which means that they can't fuse hydrogen to helium in their cores. And two, they are not brown! When they are young they do produce a tiny bit of light, because they were "born hot" and they produce a bit of heat as they settle into their proper spherical shapes. They briefly manage to do a little bit of fusion, too, though I can't remember what element they fuse and what that element turns into when they are done fusing it! So they do produce a bit of light, but this light is not brown, but red. Indeed, brown dwarfs are the reddest and also the faintest of all known stars!

To understand what the light of brown dwarfs looks like, you may be able to repeat an experiment I carried out for myself. I have a stove which looks somthing like this:



As you can see, it has got four hot plates or hobs, and the biggest one is the hottest. One night when it was quite dark outside I made my kitchen as pitch dark as I possibly could, by turning off the lights, closing the door and closing the blinds. Then I turned on the biggest hot plate on maximum heat. Would I be able to make the hot plate glow visibly red?

Actually, yes! After several minutes, the hot plate started glowing a very, very dull, dim red color. Wowzers! It was almost eerie, like the evil eye of Sauron was suddenly glowering at me from the hot plate of my stove!



It should be pointed out that this picture shows a modern hot plate on a modern stove, which glows red automatically as soon as you turn it on because of some built-in illumination under red glass, or something. But my hot plate has nothing like that, and it was really glowing red because it became hot enough.

And that's what a brown dwarf will look like in space, pretty much - like a dull, dull, dim dark redness in the pitch blackness of space. An L or T brown dwarf, in fact!

Bob, you said:

I'm sure you are right about what you say about blackbody radiation, but I've always thought that we can't see green stars because the Sun is green, and it is not a good idea for us to see it as green! Surely it would be impractical for us if the sky always looked like this, and daylight always was this color?



This picture actually shows a nightly skyscape with stars, lit by the northern lights, which are often green. Okay, but surely the Sun isn't green?

Yes, it can be argued that it is. No, it isn't as green as this, of course:



No, but the Sun emits most strongly in the green part of the spectrum, as you can see in these two pictures of the spectrum of the Sun:





The upper picture says that the Sun emits most strongly in the yellow-green part of the spectrum, while the lower one says that it reaches its maximum in the blue-green part of the spectrum. I don't know which of these spectra is the most correct one, but I do know that the Sun emits a lot of green light, really a lot! But because we see the Sun as white (or at least, we see daylight as white, and daylight is sunlight), we will see all other green stars as white, too! What a pity! Some stars really are green, but they look white to us!



Twinkle, twinkle, green star, how I wonder where you are!

Steph, thank you so much for your kind words! No, I don't teach astronomy, because it's not a subject at Swedish high schools. I teach Swedish and (I'm blushing a bit now) English. I definitely prefer teaching Swedish, because I make no grammar errors there!

Iolanthealias, thank you! I like pictures too, as I'm sure you've noticed!

Ann

Oh, and... with all this talk about how it's not easy being green, I thought Kermit the Frog should probably be a part of this post, singing his famous song:

It\'s not easy being green

Ann wrote:
How is that less sexist?

Oh, I think the astronomy lessons are pretty cool, too. I always learn a lot from these threads.

Terry, you are right. That is not less sexist. The non-sexist version goes like this: "Oh, Be A Fine Guy/Girl, Kiss Me!"

Ann