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What or why are globular clusters


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Having imaged M13 a few times now I can;t help wondering what exactly has given rise to these globular clusters. I did here one theory a while ago that they may be remnants of smaller galaxies that have been "swallowed up" at some time in the past by our Milky Way. Another one is that there is a black hole at the centre which is causing a higher than usual gravitationla force which is causing the cluster to form.

Any thoughts or comments would be interesting to read.

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I've always thought of these as mini elliptical galaxies, so wouldn't be surprised by either of those theories

..or are they future elliptical galaxies just starting to form?

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This is from the net:

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin globulus—a small sphere. A globular cluster is sometimes known more simply as a globular.

Globular clusters, which are found in the halo of a galaxy, contain considerably more stars and are much older than the less dense galactic, or open clusters, which are found in the disk. Globular clusters are fairly common; there are about 150[2] to 158[3] currently known globular clusters in the Milky Way, with perhaps 10 to 20 more still undiscovered.[4] Large galaxies can have more: Andromeda, for instance, may have as many as 500.[5] Some giant elliptical galaxies, such as M87,[6] may have as many as 10,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 40 kiloparsecs (approximately 131,000 light-years) or more.[7]

Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, and almost every large galaxy surveyed has been found to possess a system of globular clusters.[8] The Sagittarius Dwarf and Canis Major Dwarf galaxies appear to be in the process of donating their associated globular clusters (such as Palomar 12) to the Milky Way.[9] This demonstrates how many of this galaxy's globular clusters were acquired in the past.

Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear. It does appear clear that globular clusters are significantly different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy rather than as a separate galaxy.[10] However, recent conjectures by astronomers suggest that globular clusters and dwarf spheroidals may not be clearly separate and distinct types of objects.[11]

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They are a different order of magnitude from galaxies. (100,000 to 1,000,000 stars in a glob?) Harlow Shapely produced a 3D map of globular distribution in the early 20th century and found that they had a spherical distribution, as well as being spherical themselves. His eminently reasonable conclusion was that the galactic centre lay at the centre of this distribution. Observation of globulars in other galaxies confirms this. To the best of my knowledge (which is not great) they are not elliptical but always spherical to a fairly high order of precision but, like elliptical galaxies, there is no systematic rotation. The stars orbit randomly. I think they are great, with their ancient low metal content stars going back incredibly close to the BB. They look pretty decent, too!

But here's a question; do we ever detect globulars in ellipticals? I have no idea.

Olly

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Yes, ellipticals have lots of globulars too.

Most globulars formed very early on in the galaxies evolution -- the stars are nearly as old as the Universe itself. In some galaxies (inc Andromeda I believe) we see young globulars, and it's a bit of a puzzle why all the milky way globulars are old.

Whether there are black holes in the centre globular clusters is an open question. You'd expected something like a 1000-10000 solar mass black hole in the middle maybe; which is beyond the range of current telescopes to detect.

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Yes, ellipticals have lots of globulars too.

Most globulars formed very early on in the galaxies evolution -- the stars are nearly as old as the Universe itself. In some galaxies (inc Andromeda I believe) we see young globulars, and it's a bit of a puzzle why all the milky way globulars are old.

Whether there are black holes in the centre globular clusters is an open question. You'd expected something like a 1000-10000 solar mass black hole in the middle maybe; which is beyond the range of current telescopes to detect.

Contrary to that are the "Blue Straglers", young blue stars that shouldn't really be there! It's still quite a mystery, I reckon it's due to colliding stars, given the density it must happen quite often

Blue straggler - Wikipedia, the free encyclopedia

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it's a bit of a puzzle why all the milky way globulars are old.

Contrary to theory that these globs are old are the "Blue Stragglers", young blue stars that shouldn't really be there! It's still quite a mystery, I reckon it's due to colliding stars, given the density it must happen quite often

Blue straggler - Wikipedia, the free encyclopedia

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Yes, blue stragglers are odd (and probably due to a coalescence/collision of two older stars as you say) -- but they are only a few stars out of the many tens of thousands in the globular cluster. I don't think there is any suggestion that they call into question the age of the GC overall. One could argue they are evidence for on-going star formation; but that wouldn't sit too well with the overall population, so it seems unlikely.

In some galaxies, you do see globulars where the entire population is young (by this I mean ~5Gyr rather than 13Gyr).

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