APOD: M13: The Great Globular Cluster in... (2017 May 12)

[Ann]

My very simplistic understanding of the formation of globular clusters and disc galaxies is that the globulars formed in the spherical halo, before the galaxy's gas had sufficient time to settle into a disc (assuming enough gas was present for a disc to form). The younger stars in the disc formed from the gas in the disc, which already had an angular momentum with a predominant direction. The presence of sufficient gas and time leads to the disc shaped formations.

Is this anywhere near the current consensus?

I don't have the energy to google it now, but I once read an ArXiv Astrophysics paper about the formation and duration of globular clusters that made a big impression on me. The authors of the paper argued that globular clusters formed "normally", inside the disks of very gas-rich galaxies in the young universe. But precisely because these early galaxies were so gas-rich they were also full of giant molecular clouds, and the newly formed enormously large globular clusters would repeatedly collide with gas clouds, until the clusters were disrupted. In other words, huge clusters would easily form inside these gas-rich galaxies, but the clusters would soon be disrupted due to collisions with giant molecular clouds, and their constituent stars would be scattered all over their parent galaxy.

However, according to the same paper, these young gas-rich galaxies often interacted with other galaxies. Such interactions could hurl a large, newly formed cluster into the halo of the galaxy where it was born. Once in the halo, the globular cluster was safe from disruptive collisions with giant molecular clouds. And once in the halo, many of these globulars could remain there until the present day.

To me, this seems like a compelling hypothesis.

Ann

[Chris Peterson]

I thought spirals formed from Mergers... they spiral material into the disc...it flattens from momentum.

Momentum doesn't flatten things.

Spiral galaxies are pristine. Mergers disrupt and destroy spirals, typically converting them to elliptical or irregular galaxies (which are higher entropy forms). Once a spiral is disrupted, there is no mechanism for it to reform.

[Chris Peterson]

My very simplistic understanding of the formation of globular clusters and disc galaxies is that the globulars formed in the spherical halo, before the galaxy's gas had sufficient time to settle into a disc (assuming enough gas was present for a disc to form). The younger stars in the disc formed from the gas in the disc, which already had an angular momentum with a predominant direction. The presence of sufficient gas and time leads to the disc shaped formations.

Is this anywhere near the current consensus?

There are several theories of globular cluster formation. I'm not sure any have broad enough support to really say there's a consensus at this point.

[neufer]

Click to play embedded YouTube video.

I thought spirals formed from Mergers... they spiral material into the disc...it flattens from momentum.

Momentum doesn't flatten things.

Spiral galaxies are pristine. Mergers disrupt and destroy spirals, typically converting them to elliptical or irregular galaxies (which are higher entropy forms). Once a spiral is disrupted, there is no mechanism for it to reform.

[Ann]

One of my favorite photo objects... and this image is a beaut!!!

One of my images....

:---[===] *

I somehow didn't see that until now! Great image, Boomer!

Ann

[RocketRon]

Eventually the cluster evaporates away, over a time scale of a few tens of billions of years.

If the universe itself isn't yet "a few tens of billions of years" old,
does that mean that no star cluster has actually evaporated away yet ?

[Ann]

Eventually the cluster evaporates away, over a time scale of a few tens of billions of years.

If the universe itself isn't yet "a few tens of billions of years" old,
does that mean that no star cluster has actually evaporated away yet ?

Click to view full size image

NGC 188. Credit: Digitized Sky Survey 2 (STScI/AURA,
Palomar/Caltech, and UKSTU/AAO)

Yes, we can be sure that huge numbers of open clusters have evaporated away. NGC 188 is one of the oldest known open clusters in the Milky Way.

Wikipedia wrote:
Unlike most open clusters that drift apart after a few million years because of the gravitational interaction of our Milky Way galaxy, NGC 188 lies far above the plane of the galaxy and is one of the most ancient of open clusters known, at approximately 6.8 billion years old.[4] NGC 188 is very close to the North Celestial Pole, under five degrees away, and in the constellation of Cepheus at an estimated 5,000 light years distance, this puts it slightly above the Milky Way's disc and further from the center of the galaxy than the Sun.

So NGC is located in a sparse part of the Milky Way, where it runs a low risk of being gravitationally disrupted by other star clusters or by molecular clouds. In this way it is similar to the globular clusters of the Milky Way, which are also located away from the crowded disk of our galaxy. Note, however, that NGC 188 is "only" about 6.8 billion years old, which makes it much younger than, I think, all known globular clusters of the Milky Way.

Because NGC 188 is possibly the oldest open cluster of the Milky Way even though it's only about 6.8 billion years old, we can be sure that huge numbers of open clusters have evaporated away during the billions of years that our galaxy has existed. One cluster that is definitely gone for good is the one that our Sun was originally a member of!

Click to view full size image

Globular clusters M53 (top right) and NGC 5053 (bottom left).
Photo: Bob Franke.

What about globular clusters? Have any of those evaporated away? Well, I believe it is certain that some of the globulars that formed some 10-12 billion years ago were disrupted not too long after their birth through collisions with huge molecular clouds, which were sure to exist in the young Milky Way at the era of peak star formation. But of those that survived the dangers of the rough and tumble infant Milky Way, have any of them evaporated away?

I don't think we can answer that particular question. However, we do see one example of a Milky Way globular that is really very "emaciated" and which may have grown that way due to evaporation, and that is NGC 5053.

We can't be sure that NGC 5053 was ever a rich globular cluster, but my guess is that it used to be richer than it is now. In fact, we can actually be sure that all old globulars are less rich now than they used to be, due to evaporation processes. But NGC 5053 has come farther along the path to complete evaporation than any other globular cluster that I know of in the Milky Way.

Ann

[Chris Peterson]

Eventually the cluster evaporates away, over a time scale of a few tens of billions of years.

If the universe itself isn't yet "a few tens of billions of years" old,
does that mean that no star cluster has actually evaporated away yet ?

As Ann notes, there's a big difference between globular clusters and open clusters. Both evaporate by the same process, but globular clusters are more tightly bound by gravity and contain a million stars, whereas open clusters are loosely bound and contain a thousand or less. So while the former take billions of years to evaporate, the latter last only a few million years at most.

[Cousin Ricky]

The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

How does a star cluster that old have so many blue stars?

[Ann]

The cluster's evolved red and blue giant stars show up in yellowish and blue tints.

How does a star cluster that old have so many blue stars?

Click to view full size image

The center of M13. ESA/Hubble and NASA.

There are many blue stars in M13 because this globular cluster is very old and very metal-poor, and very metal-poor stars of a certain mass turn blue during a certain part of their evolution, after they have left the main sequence.

Take a look at the picture of M13 at left. There are many blue stars here, but note that they are all of more or less the same brightness. They are clearly brighter than the wealth of faint white stars in the globular, but they are clearly fainter than the bright red giant stars.

Click to view full size image

Hertzsprung-Russell diagram of M13.
Source: https://www.princeton.edu/~rvdb/images/ ... atlab.html

At right you can see a Hertzsprung-Russell diagram, or HR diagram, for M13. A HR diagram plots the magnitude of stars against the color of the stars. Bluer stars are at left in a HR diagram, and redder stars are at right. The brighter the stars are, the higher they are in the diagram.

As you can see, the brightest stars in M13 are red giants. But as you can also see, there is a group of fairly bright blue stars at left. These are the Horizontal branch stars. I prefer to call them "blue horizontal branch stars", because not all horizontal branch stars are blue.

Click to view full size image

Hertsprung-Russell diagram for not-so-metal-poor 47 Tucanae.
Note the short horizontal branch at +1.
Source: http://ase.tufts.edu/cosmos/view_picture.asp?id=567

As I said, M13 contains many blue horizontal branch stars because the stars of M13 are very metal-poor, and they turn into blue horizontal branch stars during a part of their normal evolution. Only sufficiently metal-poor stars go through this particular evolutionary phase. According to Wikipedia, the metallicity of M13 is [Fe/H] = –1.33^[7] dex. Another famous Milky Way globular, 47 Tucanae, has no blue horizontal branch stars at all, because it is too metal-rich. The metallicity of 47 Tuc is [Fe/H] = –0.78^[7] dex. That's not sufficiently metal-poor for blue horizontal branch stars to form.

Click to view full size image

47 Tucanae lacks blue horizontal branch stars.

As you can see in the picture at right, 47 Tucanae lacks blue horizontal branch stars, because it is insufficiently metal-poor for them to form.

Ann