Why are some stars different colors?
Published on Nov 27, 2005 at 7:58 pm.
1 Comment.
Filed under stars.
This is a question that I get asked a lot in my public talks, school talks, etc. Most any astronomer knows the answer. But of course, everyone has to hear it the first time somewhere, right? Basically, the answer is that different stars are different colors because they are different temperatures. The cooler stars are red (cool still being thousands of degrees!), then hotter ones are sort of yellow hot, and hotter is white hot, with the hottest being bluish.
Stars form from basically the same material. They are just about all 90% hydrogen (by number of atoms, closer to 75% by mass), and about 9% helium (also by number of atoms). There is very little difference in composition — certainly not enough to change the color of the star. The biggest difference is that they form with different masses. Stars hold themselves up against the inward pull of gravity by thermal energy. This energy is primarily generated through nuclear fusion in the core of the star, when everything is ballanced, that is. The more massive the star, the more thermal energy it takes to ballance gravity, and so the hotter the star will be. The higher thermal energy requires more fusion, and so ironically, the more massive the star then the quicker it will use up its fuel (it fuses hydrogen into helium). When the star runs out of hydrogen fuel in its core, it goes through a series of changes. Most of these changes involve the core of the star either getting smaller and hotter (compressing gasses makes them hotter) or bigger and cooler. The changing temperatures of the core affect the mid layers of the star, and they will expand or contract based on the change in temperature to them as a result of the core’s temperature change. As the mid layers expand when they are heated, that causes the outer layers of the star to also expand. Expanding gas cools, and so the outer layers of the star get cooler, and redder. So, strangely enough, the core collapses to a very small, very hot cinder, heating the mid layers of the star, which expand, causing the outer layers to expand and cool. All the result of the core getting hotter. At any rate, the star is big and red — a red giant. Several other changes occur, the particular ones depend upon the mass of the star, and the star either expands and cools, contracts and warms, or goes through cycles. When large, the stars are very bright.
So, stars are different colors due to temperture. When you see a red star in the sky, it is likely an old, expanded, dying star. The stable red stars (the ones that are fusing, but very small, needing little thermal energy to ballance against gravity, and thus fusing slowly) are all too small and dim to see without a telescope. The blue stars that you see are the ones that are normally still fusing hydrogen, but are very massive, and thus very hot. A few massive dying stars will turn bluer during some of the later stages of their deaths, and they also make up some of the blue or white stars that you see, but very few.
OK, for those that really know what is going on, I realize that this is not very complete,but I didn’t want to give my two weeks of lectures on stellar evolution here. Those that want to learn more can read up on it, or sign up for my class!
Now, this applies to stars, not planets. Mars is reddish not due to temperature, but due to the fact that the surface is covered in iron oxides — basically rust.
-Astroprof
matt on September 18, 2008 at 11:09 am: 1
this was a vary helpful account thx for your help