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Blue Straggler Stars in Globular Cluster M53

Explanation: If our Sun were part of M53, the night sky would glow like a jewel box of bright stars. M53, also known as NGC 5024, is one of about 250 globular clusters that survive in our Galaxy. Most of the stars in M53 are older and redder than our Sun, but some enigmatic stars appear to be bluer and younger. These young stars might contradict the hypothesis that all the stars in M53 formed at nearly the same time. These unusual stars are known as blue stragglers and are unusually common in M53. After much debate, blue stragglers are now thought to be stars rejuvenated by fresh matter falling in from a binary star companion. By analyzing pictures of globular clusters like the above image taken by the Hubble Space Telescope, astronomers use the abundance of stars like blue stragglers to help determine the age of the globular cluster and hence a limit on the age of the universe. M53, visible with a binoculars towards the constellation of Bernice's Hair (Coma Berenices), contains over 250,000 stars and is one of the furthest globulars from the center of our Galaxy.

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Ah, back to the cosmological jewel box again. Neat

As a newby here, are there any theories why these globular clusters haven't formed into galaxies (yet?). Or why all galaxies haven't formed into globular clusters ?

Or are they galaxies, and not discernable as such... ?

APOD Robot wrote:

Most of the stars in M53 are older and redder than our Sun

Presumably all 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.

Ann

I'm slightly surprised at the statement "astronomers use the abundance of stars like blue stragglers to help determine the age of the globular cluster". Certainly they use the abundance of stars of different colours to determine the age of the cluster, as shown by Ann's diagram and also the slight metallicity colour shift she describes, but the blue stragglers are anomalies that don't seem to contribute to that. Is there something more subtle, not explained by the text or any of the links, that these blue stragglers can contribute? Based on the speed at which they might form? Limits on their numbers due to metal content?

Beyond wrote:Ah, back to the cosmological jewel box again. Neat

Pretty much what I was going to say, Beyond: another pile of dazzling diamonds!

Need - - to look at this!

Lovely!

Ron wrote:As a newby here, are there any theories why these globular clusters haven't formed into galaxies (yet?). Or why all galaxies haven't formed into globular clusters ?

Or are they galaxies, and not discernable as such... ?

Ron, they're too small. Globular clusters are thought to form as part of larger galaxies, while superficially similar small elliptical galaxies form on their own. The distinction is getting blurred with Omega Centauri now thought to be the remnant of a small galaxy that was captured and stripped by our own galaxy, and with super-large globular clusters (called Ultra-Compact Dwarf galaxies) now being studied.

a galactic Lagrange point ....

I forgot to say, in my previous post, anything about the integrated colors of clusters M53, the subject of today's APOD, and M55, the cluster of the diagram I posted.

It turns out that both clusters have integrated colors that are bluer than the Sun. M53, the cluster of today's APOD, has an integrated unreddened B-V index of 0.62, according to Sky Catalogue 2000.0, Volume 2. M55 is even bluer, with an unreddened B-V index of 0.59. The B-V index of the Sun is 0.656 ± 0.005.

The brightest stars of all globular clusters are red, due to the fact that the massive blue O- and B-type stars that these clusters once contained burnt out and disappeared - probably by exploding as supernovae - billions of years ago. The brightest stars that remain in the globulars are red giants on the verge of shedding their atmospheres to become white dwarfs. These red giants are quite bright and contribute a lot of red light, so in order to have an integrated B-V index that is bluer than the Sun both M53 and M55 must contain a large number of stars that are bluer than the Sun. They do, too.

My point is that stars aren't necessarily red just because they are old.

Ann

APOD is one of my most visited sites because of the information provided and (equally) because of the wonderful images like this one.

I have read in several places (links below) that the number of globular clusters in the Milky Way galaxy is around the 150 mark yet the text accompanying this image says there is around 250. This is quite a difference. Which is the correct figure?

http://hyperphysics.phy-astr.gsu.edu/hb ... bular.html
http://spider.seds.org/spider/MWGC/mwgc.html
http://ned.ipac.caltech.edu/level5/Harris2/Harris1.html
http://en.wikipedia.org/wiki/Globular_cluster (Reference No.4)

Wakaleo wrote:APOD is one of my most visited sites because of the information provided and (equally) because of the wonderful images like this one.

I have read in several places (links below) that the number of globular clusters in the Milky Way galaxy is around the 150 mark yet the text accompanying this image says there is around 250. This is quite a difference. Which is the correct figure?

http://hyperphysics.phy-astr.gsu.edu/hb ... bular.html
http://spider.seds.org/spider/MWGC/mwgc.html
http://ned.ipac.caltech.edu/level5/Harris2/Harris1.html
http://en.wikipedia.org/wiki/Globular_cluster (Reference No.4)

I think that the number 250 is wrong, or at least there is little support for it. You have quoted several sources, which seem to agree that the probable total number of globulars in the Milky Way is less than 200, even less than 180.

The reason why the number of globulars in the Milky Way is uncertain is because much of our view of our galaxy is hidden by dust. On the other hand, more and more of these "hidden parts" are becoming known thanks to advanced infrared-sensitive telescopes, which makes it unlikely that ~100 globulars can be hiding in the still unexplored parts of out galaxy.

Ann

I read somewhere that Blue Stragglers may form as a result from collision.
Here is a link to a recent submission: http://arxiv.org/abs/1302.7284

Ann wrote:
I think that the number 250 is wrong, or at least there is little support for it. You have quoted several sources, which seem to agree that the probable total number of globulars in the Milky Way is less than 200, even less than 180.

The reason why the number of globulars in the Milky Way is uncertain is because much of our view of our galaxy is hidden by dust. On the other hand, more and more of these "hidden parts" are becoming known thanks to advanced infrared-sensitive telescopes, which makes it unlikely that ~100 globulars can be hiding in the still unexplored parts of out galaxy.

Ann

Thanks Ann. I had assumed that it was a typo - let's hope that if it's not we get more info! =D