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

[Chris Peterson]

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?

Population I stars are high metal content, like the Sun. Globular clusters are made up of low metal, Population II stars, like those found in the galactic halo and bulge. The stars all appear to have formed at the same time, but they don't all have the same mass, so you find a range of luminosities. The stars in globulars are very old, as you suggest. Most seem to be about a billion years younger than the Universe- the same as galaxies. This is all well supported by observation. There are a small number of globulars that have different characteristics, such as split star populations. These aren't well understood, but might be the product of later evolution due to collisions or tidal effects.

[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?

I realize thae Omega Cen globular cluster is an exception and possible reasons are stated. My above question is concerned about the general trend of data observed in the typical globular clusters. I am very intriqued by the possibilities. Are the globular clusters and their stars that are a halo around the Milky Way possibly as old as the age of the universe? I would really appreciate, if not an educated guess, then an opinion. Thanks.

[Chris Peterson]

Are the globular clusters and their stars that are a halo around the Milky Way possibly as old as the age of the universe?

No. The oldest stars in globular clusters apparently formed about a billion years after the Big Bang (based on analysis of H-R diagrams).

[dougettinger]

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?

Population I stars are high metal content, like the Sun. Globular clusters are made up of low metal, Population II stars, like those found in the galactic halo and bulge. The stars all appear to have formed at the same time, but they don't all have the same mass, so you find a range of luminosities. The stars in globulars are very old, as you suggest. Most seem to be about a billion years younger than the Universe- the same as galaxies. This is all well supported by observation. There are a small number of globulars that have different characteristics, such as split star populations. These aren't well understood, but might be the product of later evolution due to collisions or tidal effects.

Chris, thanks for answering my question and correcting me about Population I and Population II stars. Is it correct to assume then that most subsequent generation stars, young massive short-lived blue stars, and Population I stars with high metal content are mostly made in spiral and barred galaxies and irregular galaxies like the LMC? Older, possibly original stars, are mostly found in elliptical galaxies and the bulges/globular clusters of spiral galaxies.

[Chris Peterson]

Chris, thanks for answering my question and correcting me about Population I and Population II stars. Is it correct to assume then that most subsequent generation stars, young massive short-lived blue stars, and Population I stars with high metal content are mostly made in spiral and barred galaxies and irregular galaxies like the LMC? Older, possibly original stars, are mostly found in elliptical galaxies and the bulges/globular clusters of spiral galaxies.

I think that is generally how most cosmologists see things.

[dougettinger]

Wow! That is for me like putting large pieces of the puzzle together.

[tesla]

If, as suggested, it is gravity and time that has formed Omega Centauri, and gravity alone that forms a normal galaxy, there seems to be two different types of gravity working!
Are the stars moving towards the center or moving outwards? If they are moving outwards, it is not gravity. If they are in a static position, it is not gravity. If they are moving inwards I guess it is a mythical black hole!

[rstevenson]

... Are the stars moving towards the center or moving outwards? If they are moving outwards, it is not gravity. If they are in a static position, it is not gravity. If they are moving inwards I guess it is a mythical black hole!

None of the above. They are orbiting their mutual center of mass.

Rob

[Chris Peterson]

None of the above. They are orbiting their mutual center of mass.

Except in the case of a globular cluster, there really isn't a mutual center of mass. Better to say that each star is orbiting (and these orbits tend to have odd shapes) its own source of gravitational attraction, which is different for each. Certainly the cluster as a whole has a center of mass, and that is an important component in determining the paths of individual stars, but the stellar density is such that local effects are very strong as well.

[BMAONE23]

The simulation
This visualization presents a globular cluster composed of 6,144 stars. The width of the frame represents more than a hundred trillion miles. As the movie unfolds, the evolution of the cluster is shown in this time-lapse movie, in which each second represents thousands of years passing by! As the stars orbit one other, several stars are ejected from the cluster through close gravitational encounters with more massive stars. The stars are shown in a scientific approximation of what the human eye would see: each star's brightness depends both on its intrinsic brightness and on its distance away from the “virtual camera,” while each star's color is only slightly exaggerated

http://www.haydenplanetarium.org/resour ... 03strclust courtest of Hayden Planetariun (it takes a while for the movie file to download in the IE viewer imbed. But accessing via the Download option is faster or download mpg file
And some other interesting Globular Cluster info

[rstevenson]

Thanks for the clarification Chris. And thanks for the link BMAONE23.

About that video... Near the end of it, are we suddenly zooming in? Or is the cluster suddenly breaking up?

Rob

[BMAONE23]

This is all that is stated about the simulation

The simulation
This visualization presents a globular cluster composed of 6,144 stars. The width of the frame represents more than a hundred trillion miles. As the movie unfolds, the evolution of the cluster is shown in this time-lapse movie, in which each second represents thousands of years passing by! As the stars orbit one other, several stars are ejected from the cluster through close gravitational encounters with more massive stars. The stars are shown in a scientific approximation of what the human eye would see: each star's brightness depends both on its intrinsic brightness and on its distance away from the virtual camera, while each star's color is only slightly exaggerated.

though I would imagine that it was intended as a zoom through at the end. I've never heard of one of these breaking up (though anything is poossible).

[Chris Peterson]

I've never heard of one of these breaking up (though anything is poossible).

They don't break up. They evaporate over tens of billions of years, but that's a very gradual process, and far gentler than anything I'd describe as "breaking up".

[dougettinger]

To use your term - everything in the known universe "evaporates" over tens of billions of years. Of course, that assumes there only is and only will be one Big Bang. And that is a big, big assumption.

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

To use your term - everything in the known universe "evaporates" over tens of billions of years.

"Evaporation" in this sense means that gravitational perturbations occasionally give a member star enough velocity to escape from the cluster. Over time, the cluster loses stars to this evaporation. Eventually the cluster ceases to exist, because none of its original members are gravitationally bound.

Not all objects evaporate in this way. For instance, stars at the end of their lives leave a remnant that is stable for trillions and trillions of years, and possibly forever. This describes many particles, as well.

[dougettinger]

Thank you for correcting my understanding of evaporation.

Doug Ettinger