by Ann » Tue May 11, 2021 5:39 am
snuggs28 wrote: ↑Mon May 10, 2021 3:17 pm
NGC 2158 looks more like a globular cluster than an open cluster. Or is it due to the perspective of the APOD photo?
Good question. The thing is that you can't always tell at a glance if a cluster is an open cluster or a globular.
Globular clusters M53 and NGC 5053. Photo: Bob Franke.
As you can see, both open and globular clusters can be either rich of sparse. Admittedly, open clusters are usually puny indeed compared with globulars. But globular clusters can evaporate too. The globular clusters of M53 and NGC 5053 are strikingly different, even though they are not only close together on the sky, but also located at similar distances from us, so that they might indeed form a physical pair.
A color-magnitude diagram of a cluster plots the brightness of its stars versus the color of its stars. As stars age, they run out of fuel (hydrogen) in their cores, and when that happens, they swell and become brighter and redder in color. The more massive a star is, the bluer it is when it still has hydrogen in its core, but the sooner it runs ut of hydrogen in its core and turns into a red giant.
A color-magnitude diagram shows how evolved (i.e., old) a cluster is. The older it is, the shorter is its main sequence, because more and more of its stars have run out of hydrogen in their cores and turned into red giants. In the picture at left, you can see diagrams of four globular clusters. All have short main sequences (the thick black part at bottom). All have long red giant branches (the long branches curving away to the right from the main sequence at bottom and reaching up). All have horizontal branches curving left, although some globulars lack them.
The NGC 2158 color-magnitude diagram looks different. The main sequence is longer and reaches higher, because a much larger portion of the stars of NGC 2158 are still fusing hydrogen in their cores. The red giant branch does not reach all that high above the main sequence. And NGC 2158 appears to have a rather large population of blue stragglers, stars that are located above the main sequence, to the left. The blue stragglers are stars that have gained extra mass through some means, probably by interaction with other stars.
So NGC 2158 is not a globular cluster. It is a (very) rich open cluster. It is old as open clusters go (2 billion years old), but it is much younger than most globulars (some 12 billion years old).
Ann
[quote=snuggs28 post_id=313127 time=1620659867 user_id=143429]
NGC 2158 looks more like a globular cluster than an open cluster. Or is it due to the perspective of the APOD photo?
[/quote]
Good question. The thing is that you can't always tell at a glance if a cluster is an open cluster or a globular.
[float=left][img3="Open clusters M35 (top right) and NGC 2158 (lower left). Photo: wc5q of Cloudy Nights."]https://www.cloudynights.com/uploads/gallery/category_321/med_gallery_256656_321_8589481.jpg[/img3][/float][float=right][attachment=0]M53 NGC 5053 Bob Franke.png[/attachment][c][size=85][color=#0040FF]Globular clusters M53 and NGC 5053. Photo: Bob Franke.[/color][/size][/c][/float]
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As you can see, both open and globular clusters can be either rich of sparse. Admittedly, open clusters are usually puny indeed compared with globulars. But globular clusters can evaporate too. The globular clusters of M53 and NGC 5053 are strikingly different, even though they are not only close together on the sky, but also located at similar distances from us, so that they might indeed form a physical pair.
[float=left][img3="Color-magnitude diagrams of four globular clusters, including NGC 5053 and M53. Credit: SDSS."]https://www.researchgate.net/profile/Jason-Smolinski/publication/231075624/figure/fig11/AS:346168673488901@1459544451269/SDSS-color-magnitude-diagrams-for-the-four-globular-clusters-in-our-sample-with-Fe-H.png[/img3][/float][float=right][img3="Color-magnitude diagram of open cluster NGC 2158. Source: https://academic.oup.com/mnras/article/332/3/705/1052332"]https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/mnras/332/3/10.1046/j.1365-8711.2002.05326.x/3/332-3-705-fig005.gif?Expires=1623732053&Signature=KWi09MrKcKE9QOU7Wz~0zLUB8nOfGl52JtrjLaoeOBDIvSpFkDJ81wFtzRTrRcwXfxHDPnMg1PynBkmZElxMBsXj3rYUhcQ-nMjCmLbANcwWCcvwkNXRgnK1aeCIUH~Q2xXpxNwlzjWiBbhlpZSKu5CYmZn9cyiffFuIRnpykMDxeebIpdWjRpTRq0r9OIAE6tcWZsutfpNaS-GJgt9-BwIbrvBI5O3YWQrCIDTaW0pXbGAmF64XPivPy3Ta~zp~3ycrngzE2B8szqjqZXFLjeco1XSImnWJBXHhCErQqa6UxCg8ctr1Q5gnnKtfH7plb-82olFplHtF33wnxaCq5Q__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA[/img3][/float]
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A color-magnitude diagram of a cluster plots the brightness of its stars versus the color of its stars. As stars age, they run out of fuel (hydrogen) in their cores, and when that happens, they swell and become brighter and redder in color. The more massive a star is, the bluer it is when it still has hydrogen in its core, but the sooner it runs ut of hydrogen in its core and turns into a red giant.
A color-magnitude diagram shows how evolved (i.e., old) a cluster is. The older it is, the shorter is its main sequence, because more and more of its stars have run out of hydrogen in their cores and turned into red giants. In the picture at left, you can see diagrams of four globular clusters. All have short main sequences (the thick black part at bottom). All have long red giant branches (the long branches curving away to the right from the main sequence at bottom and reaching up). All have horizontal branches curving left, although some globulars lack them.
The NGC 2158 color-magnitude diagram looks different. The main sequence is longer and reaches higher, because a much larger portion of the stars of NGC 2158 are still fusing hydrogen in their cores. The red giant branch does not reach all that high above the main sequence. And NGC 2158 appears to have a rather large population of blue stragglers, stars that are located above the main sequence, to the left. The blue stragglers are stars that have gained extra mass through some means, probably by interaction with other stars.
So NGC 2158 is not a globular cluster. It is a (very) rich open cluster. It is old as open clusters go (2 billion years old), but it is much younger than most globulars (some 12 billion years old).
Ann