APOD: Millions of Stars in Omega Centauri (2010 Mar 31)

[APOD Robot]

Millions of Stars in Omega Centauri

Explanation: Featured in the sharp telescopic image, globular star cluster Omega Centauri (NGC 5139) is some 15,000 light-years away and 150 light-years in diameter. Packed with about 10 million stars much older than the Sun, Omega Cen is the largest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way.

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[BMAONE23]

Is the Bubble feature in the bottom left corner an artifact of processing or part of a nebula?

[tesla]

A million stars in a spherical group. So if gravity did not form the group, what did?

[workgazer]

ref- what formed this the answer is gravity and time nothing else.
what would life / the view be like living on a planet in that cluster?

[DaveBone]

If you lived on a planet towards the center of this cluster, what would the night sky be like. Would it be uniformly bright or would the entire sky have the hazy appearance of our own milky way. With so many stars so close together, there would be hundreds of stars as bright as our brightest. Star gazing would be awesome, but would you ever find out about other galaxies and clusters?

[Glenn]

An interesting twist to some of these pictures would be to generate an image of what might the earth's nightsky would look like if our solar systems was in the middle Omega Centaur.

[Axel]

ref- what formed this the answer is gravity and time nothing else.
what would life / the view be like living on a planet in that cluster?

Like several posters I am curious too - what would it be like if I looked at the night sky from a planet with an Earth-like atmosphere abouy 50 l.y. from the cluster's centre? Does anyone know of a (well-informed) artist's impression of this that can be seen online?

[htom]

An average of 0.13 cubic light year per star? Eight stars per cubic light year? Did I make a mistake? What's the density as we approach the center? Presumably they would rotate a common center of mass ... but that wouldn't yield a globe. Are they being attracted to (repelled from!) a center?

[jando35]

It seems eminently possible for someone with appropriate computer software to simulate the impression of the night sky from the centre of a globular cluster. Anyone have access to such?

[bystander]

An average of 0.13 cubic light year per star?

I get 1.4137 ly³ per star (0.45*π), using the standard formula for the volume of a sphere.

[Chris Peterson]

An average of 0.13 cubic light year per star? Eight stars per cubic light year? Did I make a mistake? What's the density as we approach the center? Presumably they would rotate a common center of mass ... but that wouldn't yield a globe. Are they being attracted to (repelled from!) a center?

Central densities in globulars are very high- 100 stars per cubic light year, or more. While the cluster has a center of mass, it really just forms a sort of "average" locus that the member stars orbit. In practice, "orbit" is a much more complex idea. The stars in a globular cluster are perturbed by other stars (and at the edges, by the parent galaxy). So stellar paths loop and twist. Since most stars are binaries, interactions can result in energy being added or removed from the binary systems, and transferred between different star systems. This results in a sort of mass sorting, where more massive stars tend to settle towards the core. It's a very complex system- one that can only be studied closely using computer models. And even then, the complexity of the system requires making certain assumptions and simplifications.

[BMAONE23]

an interesting You Tube Vid of a Globulr Cluster's orbital chaos

[owlice]

Loved the picture and the write-up, and got a particular kick out of this:

Tomorrow's picture: wait for it ...

[DonAVP]

what would life / the view be like living on a planet in that cluster?

In the writeup regarding this image was a link from the word 'Packed'. The link takes you to an earlier APOD post showing what it might look like.
http://antwrp.gsfc.nasa.gov/apod/ap080906.html

[Utah gazer]

Central densities in globulars are very high- 100 stars per cubic light year, or more.

0.01 cyl per star equates to interstellar distance of about 0.25 light years. But that's only an average. At the short end of that distribution, couldn't we talking about instellar distances of 0.1 ly or less?!? How close can one star be to another?

[htom]

Looking at some of the simulations, yes, they are attracted to the center -- and in doing so, their velocity shoots them through the center, so they act as if they are repelled! Along the way, depending on how close they come to the other stars, their flight will be diverted, from minor bends to three-dimensional do-si-dos with a number of partners; they may embark on a journey to or through the center, but there is no way of knowing if they will even retain a starting partner.

Planets in such a place may not have anything like an orbit at all.

[Axel]

Planets in such a place may not have anything like an orbit at all.

And therefore no Kepler or Newton, even if the temperature is stable.

[Chris Peterson]

0.01 cyl per star equates to interstellar distance of about 0.25 light years. But that's only an average. At the short end of that distribution, couldn't we talking about instellar distances of 0.1 ly or less?!? How close can one star be to another?

I don't think there's any limit. Double stars are separated by astronomical units or less. Of course, when you get stars this close together there's little chance of having a stable planetary system, so it's pretty certain that nobody is looking up at those amazing skies from inside clusters.

[neufer]

what would life / the view be like living on a planet in that cluster?

In the writeup regarding this image was a link from the word 'Packed'. The link takes you to an earlier APOD post showing what it might look like.
http://antwrp.gsfc.nasa.gov/apod/ap080906.html

If you go to APOD search and search for Wally you will find a plethora of skyscapes in time exposures where one can probably see 10th or 11th magnitude stars. That is about what you would see with your naked eyes in a globular cluster.

[Amir]

Is the Bubble feature in the bottom left corner an artifact of processing or part of a nebula?

anyone knows the answer?
seems to me all of the stars have the same bubble around them, but the one you mentioned is larger because the star has greater brightness.
so maybe we should blame filters, if there were any.

[Chris Peterson]

Is the Bubble feature in the bottom left corner an artifact of processing or part of a nebula?

anyone knows the answer?
seems to me all of the stars have the same bubble around them, but the one you mentioned is larger because the star has greater brightness.
so maybe we should blame filters, if there were any.

Not filters, I think. Just a processing artifact- bright stars have fairly bright tails (that is, the tails of their roughly Gaussian profile); those tails are as bright as dimmer parts of the image that had their contrast boosted. It's pretty common in images with high dynamic range to see halos like these around bright stars.

(While scatter off something in the optics is a possibility, the fact that the halos match the star colors leads me away from the idea this was introduced by filters- if that was the cause, I'd expect different color components to be scattered differently, resulting in color casts around the stars.)

[jando35]

It seems eminently possible for someone with appropriate computer software to simulate the impression of the night sky from the centre of a globular cluster. Anyone have access to such?

In case anyone is still following this discussion, great computer simulations of the night sky seen from a star within a globular cluster can be viewed on You Tube ['Sirius Stargazing - Globular Cluster M15'] another stunning video that fills out the Omega Centauri story is also there: 'Black Hole discovered in Globular Clusters'.

[dougettinger]

An average of 0.13 cubic light year per star? Eight stars per cubic light year? Did I make a mistake? What's the density as we approach the center? Presumably they would rotate a common center of mass ... but that wouldn't yield a globe. Are they being attracted to (repelled from!) a center?

Central densities in globulars are very high- 100 stars per cubic light year, or more. While the cluster has a center of mass, it really just forms a sort of "average" locus that the member stars orbit. In practice, "orbit" is a much more complex idea. The stars in a globular cluster are perturbed by other stars (and at the edges, by the parent galaxy). So stellar paths loop and twist. Since most stars are binaries, interactions can result in energy being added or removed from the binary systems, and transferred between different star systems. This results in a sort of mass sorting, where more massive stars tend to settle towards the core. It's a very complex system- one that can only be studied closely using computer models. And even then, the complexity of the system requires making certain assumptions and simplifications.

How does the nebula hypothesis work for a star field that is 100 stars per ly^3 or even 8 stars per ly^3? Does not the nebula hypothesis require a very high volume interstellar molecular cloud (IMC)?

[Chris Peterson]

How does the nebula hypothesis work for a star field that is 100 stars per ly^3 or even 8 stars per ly^3? Does not the nebula hypothesis require a very high volume interstellar molecular cloud (IMC)?

I don't think the nebular hypothesis is intended to explain the formation of stars in globular clusters. These are some of the first stars that formed- nearly contemporaneous with the formation of galaxies. The environment of the Universe was very different then- presumably just dark matter, hydrogen, and helium, and at much higher densities (and maybe large black holes, as well). This was well before the sort of clouds formed that produced later generation stars within mature galaxies (that is, clouds consisting of complex molecules as well as heavy elements in dust).

[dougettinger]

Thanks for sharing that information that stars can form in various ways. I have not heard that explanation before but suspected that was the case.

Then, I presume the stars in galaxial globular star clusters are consistently very old stars with very little higher metals. I believe astronomers call them Population I stars. I would also presume that these stars are all mid-size or smaller stars that would have long lives that compare with the age of the universe less 1 or 2 billion years. Are these presumptions proven by observational data?