APOD Robot wrote:
Most of the stars in M53 are older and redder than our Sun
Diagram: B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope
Explanations and markings: CSIRO Australia 2004
of the stars in M53 are older than the Sun. Certainly most of the stars in M53 are redder than the Sun, if only because most of the stars in M53 are tiny red dwarfs which spend all their lives (at least the part of their very long lives that we can know anything about) being cooler and redder than the Sun.
But the tiny red dwarfs are so faint that they don't contribute much to the total light output of a large globular cluster. When it comes to the stars that are bright and numerous enough to contribute significant light, it is not necessarily true that most of them are redder than the Sun.
Take a look at the color-magnitude diagram of globular cluster M53 and let's compare it with some "vital statistics" of the Sun. What is the temperature of the Sun? What is the B-V color index of the Sun?
According to Jim Kaler, the temperature of the Sun
is not 6,000 Kelvin, but 5,800 Kelvin. Compare that temperature with the temperature scale of the color-magnitude diagram. Note that the temperature scale is "drawn out" on the cool red side on the right and compressed on the hot blue side on the left, which means that the cool red side appears broader and more richly populated than it really is. Note that the turnoff point, the temperature at which the stars in M55 have used up the hydrogen in their cores and evolve off the main sequence, is located at a hotter temperature than the Sun's 5,800 Kelvin. It appears to be situated at around 6,400 Kelvin.
Now let's look at the B-V index of the stars of M55 and compare them with the B-V index of the Sun. The B-V index of a star is a measure of its color. The higher the B-V is, the redder is the star. The lower the B-V is, the bluer is the star.
So what is the B-V index of the Sun? According to http://en.wikipedia.org/wiki/Color_index
, the Sun has a B–V index of 0.656 ± 0.005. Let's compare that value with the spread of B-V values of the stars of M55. Fascinatingly, a large number of the stars of M55 appear to have lower, bluer B-V indexes than 0.65 or 0.66. Judging from the color-magnitude diagram, it would appear as if stars of M55 as cool as about 5,400 Kelvin have the same B-V index index as the Sun, around 0.65 or 0.66. A large chunk of the main sequence of M55 appears to be bluer than the Sun, even though several of the "bluer-than-the-Sun" stars are the same temperature as the Sun or even cooler. So there must be thousands of stars in a globular cluster like M55 that are bluer than the Sun.
Why are the stars of M55 so systematically blue? The reason is probably that these stars are extremely metal-poor compared with the Sun. This means that they contain extremely small amounts of elements heavier than hydrogen and helium. Certainly very metal-poor stars are the only ones that spend part of their lives as blue horizontal branch stars. Our Sun, which is metal-rich, will never become so hot and blue as it ages, not until it becomes a white dwarf.
The point I've been trying to make is that globular clusters aren't very red. According to Sky Catalogue 2000.0, Volume 2, the typical integrated B-V index of globular clusters is around 0.70. Their overall light is a little bit redder than the the light of the Sun, but certainly not much redder. The B-V index of Omega Centauri is 0.68, only about 0.02 or 0.03 redder than the Sun.
Speaking of blue stragglers, I'm a bit impressed that Hubble can spot them so easily in an image like this one, where color is such a low priority.
Last edited by Ann on Mon Apr 09, 2012 1:18 pm, edited 1 time in total.