by Ann » Mon May 15, 2017 9:54 am
RocketRon wrote:Chris Peterson wrote: 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 ?
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!
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
[quote="RocketRon"][quote="Chris Peterson"] Eventually the cluster evaporates away, over a time scale of a few tens of billions of years.[/quote]
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 ?[/quote]
[float=left][img2]https://mcdonaldobservatory.org/sites/default/files/images/news/gallery/NGC_188.jpg[/img2][c][size=80]NGC 188. Credit: Digitized Sky Survey 2 (STScI/AURA,
Palomar/Caltech, and UKSTU/AAO)[/size][/c][/float]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.
[quote][url=https://en.wikipedia.org/wiki/NGC_188]Wikipedia[/url] 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.[/quote]
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!
[float=right][img2]http://bf-astro.com/m53fsq/m53.jpg[/img2][c][size=85]Globular clusters M53 (top right) and NGC 5053 (bottom left).
Photo: Bob Franke.[/size][/c][/float] 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 [i]may[/i] have grown that way due to evaporation, and that is [url=https://en.wikipedia.org/wiki/NGC_5053]NGC 5053[/url].
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