by Ann » Thu Mar 19, 2020 8:05 pm
sillyworm 2 wrote: ↑Thu Mar 19, 2020 2:01 pm
Thanks..I always confuse color with age.Color is eventually more a size element?
As Chris said, color means temperature. We are talking about the temperature of the photosphere, the outermost visible layer of a star.
- The Herzsprung-Russell diagram of stars. When stars are on the main sequence, they fuse hydrogen to helium in their cores. The more massive the stars are, they faster they fuse their hydrogen, and the hotter and bluer they are. The more lightweight the stars are, the slower they fuse their hydrogen, and the cooler and redder they are. When stars have used up their core hydrogen, they swell into giants. Red supergiants like Betelgeuse become tremendously big, whereas blue, white and even yellow supergiants are somewhat smaller. Medium-mass stars like the Sun will eventually turn into red giants, but they will never be as big as Betelgeuse. In many globular clusters, red giants can shrink and become blue horizontal branch stars before swelling into red asymtotic branch giants.
Illustration: Astronomy magazine.
Massive young stars are blue (Chris would say blue-white), because they fuse hydrogen to helium in their cores at a furious rate. Medium-mass stars like the Sun have never been blue, because they have never produced enough energy in their cores to look blue. They don't need that much energy to keep themselves from collapsing, unlike the stellar heavyweights. And small stars like Proxima Centauri need even less energy to "hold themselves up", and they are yellow-orange in color.
When massive stars have used up their supply of energy in their cores, they start expanding. At first it doesn't affect their color that much. An example of a star that has used up its core hydrogen and started expanding without losing its blue color is
Alnilam, the middle star in Orion's Belt. But another star in Orion,
Betelgeuse, started out as a hot blue star, and it has since expanded so mightily that its outer layers have cooled to a yellow-orange color. Note that the core of Betelgeuse is still very hot, but we can't see it.
In globular clusters the brightest stars are red giants. The outer layers of the red giants have expanded mightily (though not as much as Betelgeuse) and they have cooled to an orange-yellow color. But there are blue stars in many globular clusters, too. These stars are always fainter than the brightest red giants, but they are relatively bright anyway.
These blue stars, called horizontal branch stars, are former red giants which have "turned on" their helium fusion. As they did so, they shrunk a lot and turned blue precisely because they became relatively small. Their outer layers are relatively close to their hot interiors, and the outer layers are therefore hot enough to be blue.
Ann
[quote="sillyworm 2" post_id=300458 time=1584626493]
Thanks..I always confuse color with age.Color is eventually more a size element?
[/quote]
As Chris said, color means temperature. We are talking about the temperature of the photosphere, the outermost visible layer of a star.
[float=right][img3="The Herzsprung-Russell diagram of stars. When stars are on the main sequence, they fuse hydrogen to helium in their cores. The more massive the stars are, they faster they fuse their hydrogen, and the hotter and bluer they are. The more lightweight the stars are, the slower they fuse their hydrogen, and the cooler and redder they are. When stars have used up their core hydrogen, they swell into giants. Red supergiants like Betelgeuse become tremendously big, whereas blue, white and even yellow supergiants are somewhat smaller. Medium-mass stars like the Sun will eventually turn into red giants, but they will never be as big as Betelgeuse. In many globular clusters, red giants can shrink and become blue horizontal branch stars before swelling into red asymtotic branch giants.
Illustration: Astronomy magazine."]https://i.stack.imgur.com/CGzowl.jpg[/img3][/float]Massive young stars are blue (Chris would say blue-white), because they fuse hydrogen to helium in their cores at a furious rate. Medium-mass stars like the Sun have never been blue, because they have never produced enough energy in their cores to look blue. They don't need that much energy to keep themselves from collapsing, unlike the stellar heavyweights. And small stars like Proxima Centauri need even less energy to "hold themselves up", and they are yellow-orange in color.
When massive stars have used up their supply of energy in their cores, they start expanding. At first it doesn't affect their color that much. An example of a star that has used up its core hydrogen and started expanding without losing its blue color is [url=https://i.pinimg.com/originals/f0/4e/73/f04e73f72bd4fb60cf89e94d37741764.jpg]Alnilam[/url], the middle star in Orion's Belt. But another star in Orion, [url=https://static01.nyt.com/images/2020/02/14/science/14betelgeuse-video-image/14betelgeuse-video-image-videoSixteenByNine768.jpg]Betelgeuse[/url], started out as a hot blue star, and it has since expanded so mightily that its outer layers have cooled to a yellow-orange color. Note that the core of Betelgeuse is still very hot, but we can't see it.
In globular clusters the brightest stars are red giants. The outer layers of the red giants have expanded mightily (though not as much as Betelgeuse) and they have cooled to an orange-yellow color. But there are blue stars in many globular clusters, too. These stars are always fainter than the brightest red giants, but they are relatively bright anyway.
These blue stars, called horizontal branch stars, are former red giants which have "turned on" their helium fusion. As they did so, they shrunk a lot and turned blue precisely because they became relatively small. Their outer layers are relatively close to their hot interiors, and the outer layers are therefore hot enough to be blue.
Ann