geoffrey.landis wrote:It would be nice if APOD images had at least a little discussion of what the colors mean.
In RGB images (taken through red, green and blue filters), cool stars look yellow. Hot stars look blue.
To human eyes, most stars are very faintly colored. The yellow-colored stars are easiest to spot. You should easily be able to pick out
Betelgeuse in Orion,
Aldebaran in the Hydades,
Arcturus in Bootes,
Dubhe in the Big Dipper and
Antares in Scorpius in the night sky because of their color.
One reason why the yellow stars stand out is because the bright yellow ones, which belong to spectral classes K or M, are relatively rare. Most bright stars in the sky are hotter than the Sun and belong to spectral classes A or B. They typically look very white in the sky. Some stars, like Vega, often look quite blue when observed through a telescope.
The stars whose colors are easiest to spot are the strongly colored binaries. The most famous such binary is probably Albireo in Cygnus. The primary star is a K-type giant star, cooler than the Sun, and the secondary is a B-type star, hotter than the Sun.
To us humans, the Sun is "white". A better way of putting it is to say that daylight is "white", or neutral. To our eyes, therefore, stars cooler than the Sun look yellowish, whereas stars hotter than the Sun look blue-white. But when the night sky is full of blue-white stars, we typically think they all look white.
Although blue-white stars dominate the night sky, such stars are really quite rare. The huge majority of stars in the Milky Way are small cool stars, much smaller and cooler than the Sun and yellow in color. But they are so faint that we simply don't see them, certainly not with the naked eye.
Small cool stars are yellow and faint because their "fusion engines" run so slowly and produce so relatively little energy. Large cool stars are yellow and bright because their "fusion engines" run furiously, but since they have used up most of their prime fuel they now run on reserve fuel, farther and farther from their own centers. In the process, they puff up and become simply enormous in size. Their "surfaces" or photospheres are yellow and cool because they are so far from where the energy that lights them up is produced.
Hot young stars are bright because they are very massive, and they are still running on "prime fuel", and they are large but not stupendously huge in size. Their "surfaces" or photospheres are so hot that they are blue-white.
NGC 7129. Photo: Adam Block.
NGC 604 in galaxy M33. Photo: ESA/Hubble.
When several stars are born from the same molecular cloud, the most massive and brightest ones are the bluest. That can be seen even in NGC 7129 at left, which is a site of low-mass star formation. Smaller yellow stars are also born here, but the blue stars will dominate.
In a region of high-mass star formation, such as NGC 604 in galaxy M33, the small cool yellow stars are too faint to be seen. The near invisibility of the small stars is because the bright massive young stars are
so bright - it's like putting a candle in front of the mid-day Sun and trying to spot the flame. (Well, Hubble can spot the small yellow stars.) And you can indeed see a few bright orange stars among the blue ones in NGC 604. They are red giants or supergiants, which have run out of their "prime fuel" and swollen to gigantic sizes. These stars will die soon, which in astronomical terms means that they have only, at best, a few million more years to live.
So the massive stars die young, but the small cool yellow stars remain. In a galaxy where no more bright young stars remain, only the small cool stars (and a few moderate yellow giants) will persist. That means that if you see an all-yellow galaxy, like NGC 5195, then basically all the stars in it are old. (Note that when you see a bright yellow galaxy, or a bright yellow part of a galaxy, then that galaxy or that part of a galaxy must contain huge numbers of old stars. That's because old stars are mostly faint, and it takes huge numbers of them to produce a bright galaxy or a bright central part of a galaxy.)
But when you see a multi-colored galaxy whose photo has been taken through RGB filters, then yellow means (huge numbers of) old stars, blue means (fewer but brighter) young stars, pink means gas clouds which have been ionized (usually by hot bright stars) and made to emit a lot of red and some blue-green light, dark brown means dust and light brown might mean a faint population of (not so many) old stars, perhaps mixed with dust.
Hope that helped!
Ann
The Whirlpool Galaxy and Beyond
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