APOD: Globular Star Cluster NGC 6934 (2010 Oct 09)

[APOD Robot]

Globular Star Cluster NGC 6934

Explanation: Globular star clusters roam the halo of our Milky Way Galaxy. Gravitationally bound, these spherical groupings of typically several hundred thousand stars are ancient, older than the stars of the galactic disk. In fact, measurements of globular cluster ages constrain the age of the Universe (it must be older than the stars in it!) and accurate cluster distance determinations provide a rung on the astronomical distance ladder. Globular star cluster NGC 6934 itself lies about 50,000 light-years away in the constellation Delphinus. At that distance, this sharp image from Hubble's Advanced Camera for Surveys spans about 50 light-years. The cluster stars are estimated to be some 10 billion years old.

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[Boomer12k]

To be that old, they must be very stable stars, right?

[Ann]

To be that old, they must be very stable stars, right?

Low-mass stars last for a very long time. The Sun may last for about 10 billion years, but the Sun is more massive than most stars in the Milky Way. The most "typical" star of the Milky contains about half as much mass as the Sun or less, and they may last for a trillion years or more. The question is, of course, how long our universe will last!

The reason why NGC 6934 is still a dense cluster after 10 billion years is that the cluster was so incredibly massive when it was born. The self-gravity of the cluster has prevented most of the stars from escaping. But you can be sure, nevertheless, that a number of small stars have been flung out of the cluster as the stars inside it have spent billions of years playing gravitational pin ball with each other.

Ann

[Ann]

Look at the color of the cluster, though. Almost all the stars look blue. As if. The B-V of this cluster is 0.74 according to Sky Catalogue 2000.0 Volume 2. A color index of 0.74 is yellower than the Sun, and yet this cluster looks blue. At best, a dozen stars here appear orangish.

The reason for the truly bad color balance is that the image was taken through two different filters, F814W (near infrared) and F606W (orange), coloured red and blue. So two long-wavelength, "red" filters were chosen to make the image, and then picture taken through the infrared filter was colored red and the picture taken through the not-quite-so-red filter was colored blue!!!

But NGC 6934 is a globular cluster with a fairly typical "globular cluster low metallicity", with a (Fe/H] of -1.54. That is definitely low enough for this globular to have a significant population of blue horizontal stars. Why, then, are two long-wavelength filters chosen to photograph a cluster which contains both "red" and "blue" stars (red giants and blue horizontal stars)? There is no way you can do the the different populations any sort of justice. Of course, you may decide that colors are of no importance, and that blue stars, in particular, are of no importance, so that you don't need to identify them by using a filter that will single them out. In fact, you will barely identify the red stars either, since even the reddest stars will be bright enough in orange light to prevent them from standing out in a picture taken through infrared and orange filters. Then, to make up for the fact that you photographed the cluster in "red" wavelengths, thereby blurring the "identity" and spectral classes of the constituent stars and particularly ignoring the blue stars, you color the whole cluster blue. Isn't that wonderful?

I don't think so!!

Ann

[León]

" The cluster stars are estimated to be some 10 billion years old. "

I stop at the age of the cluster, notes Apod that age is 10 billion years but Wikipedia in Spanish extends between 13 and 16,000 billion years more ancient than the estimate for the universe. This is a real paradox.

No other Wikipedia in another language stops at the age, but Apod point 15 billion in 1999 http://antwrp.gsfc.nasa.gov/apod/ap990630.html

Globular clusters indeed have secrets still undiscovered and are putting in distress generally accepted theories.

Bodies orbiting the galaxy as independent compact, solid, old with blue stars as "newly" born

[orin stepanek]

The cluster stars are estimated to be some 10 billion years old.

What would you do with 1billion pennies?

[biddie67]

It is amazing to me that the stars in the cluster haven't collapsed into one monster star - if it could even be called a star then. It makes me wonder about the possibilities of sub-BigBangs, wormholes, extra dimensions, who knows what - the science fiction writers could have a go at it.

[Chris Peterson]

It is amazing to me that the stars in the cluster haven't collapsed into one monster star - if it could even be called a star then. It makes me wonder about the possibilities of sub-BigBangs, wormholes, extra dimensions, who knows what - the science fiction writers could have a go at it.

There is no force present to cause such a collapse. The bodies are in orbit around each other. A multiple body system like this is chaotic, so the orbits are always changing. The result is that a star occasionally gets enough energy added that it has a velocity above the escape velocity of the cluster, and is ejected (this is called evaporation). The nature of optics makes these stars appear much larger than they actually are. In fact, even the core of the cluster is almost empty space. The likelihood of two stars actually colliding (which is what you need for a merger) is extremely small.

Globulars very slowly dissipate; they don't collapse.

[mexhunter]

Wonder if like other clusters that orbit our galaxy, they are small galaxies crashing with ours and they lost their material for the Milky Way, or they are a simply mini galaxies?
Certainly the picture is very well resolved and shows a very pretty galaxy.
Greetings
César

[orin stepanek]

It is amazing to me that the stars in the cluster haven't collapsed into one monster star - if it could even be called a star then. It makes me wonder about the possibilities of sub-BigBangs, wormholes, extra dimensions, who knows what - the science fiction writers could have a go at it.

There is no force present to cause such a collapse. The bodies are in orbit around each other. A multiple body system like this is chaotic, so the orbits are always changing. The result is that a star occasionally gets enough energy added that it has a velocity above the escape velocity of the cluster, and is ejected (this is called evaporation). The nature of optics makes these stars appear much larger than they actually are. In fact, even the core of the cluster is almost empty space. The likelihood of two stars actually colliding (which is what you need for a merger) is extremely small.

Globulars very slowly dissipate; they don't collapse.

That very interesting Chris! Thanks; I wasn't aware of the fact that they evaporate.

[BoatsBM1]

The cluster stars are estimated to be some 10 billion years old.

What would you do with 1billion pennies?

One Billion Pennies
http://www.kokogiak.com/megapenny/nine.asp

[biddie67]

......... a star occasionally gets enough energy added that it has a velocity above the escape velocity of the cluster, and is ejected (this is called evaporation). .....

I have to admit that this was an unexpected use of the word evaporation - I wasn't sure if Chris was kidding me or not - tried finding a reference to this usage on Google - no luck!

it conjours up an image of cosmic musical orbits - as in musical chairs - if a little star isn't careful, it will be left without an orbit in the cluster and will have to gracefully fade (e.g., evaporate) away ......

[Chris Peterson]

I have to admit that this was an unexpected use of the word evaporation - I wasn't sure if Chris was kidding me or not - tried finding a reference to this usage on Google - no luck!

Did you try searching on "globular cluster" AND evaporation? That should give enough hits to demonstrate that I didn't invent the term in this usage. <g>

[biddie67]

Thanks for the search link - there certainly was a lot of hits - such interesting stuff .... obviously globular clusters are a study all to themselves. I didn't appreciate the significance of globular clusters before -or- the difference between open and gobular clusters - thanks again for the help!