by Ann » Sun Feb 07, 2021 3:50 pm
VictorBorun wrote: ↑Sun Feb 07, 2021 12:43 pm
Ann wrote: ↑Sun Feb 07, 2021 6:39 am
But because the blue stragglers are still on the main sequence, they are not as bright as the blue horizontal branch stars, which have used up their core hydrogen and can be thought of as "giants".
I wonder why can there be comparable number of bright outlier stars of two sorts: quick burning massive blue giants (in a gasless place like globular stellar cluster they must come from binary mergers) and collapsing phase of an old smallish star that finally burnt out the hydrogen at its core?
Take a look at the chart at right that tracks the evolution of stars of different masses.
The gray diagonal line is the main sequence. Here you find all stars that shine by fusing hydrogen to helium in their cores. When a star is on the main sequence, it is brighter and bluer the more massive it is, and fainter and redder the more light-weight it is.
The chart tells you happens when a star has used up the hydrogen in its core and evolves off the main sequence. "Metal-rich solar-mass" stars like the Sun always become redder and brighter when they evolve off the main sequence. (Well, until they become white dwarfs, that is.) More massive stars than the Sun also become redder when they turn into giants.
But for metal-poor stars, like the ones we often find in globular clusters, the story is a bit different.
I want you to pay attention to the difference in the horizontal branch of these two globulars. In metal-poor cluster M92, the stars first rise in brightness on the red giant branch when they have used up the hydrogen in its core. At the top of the red giant branch, the stars undergo a "helium flash" and start fusing helium to carbon and oxygen. As they do so, they become considerably fainter, but still brighter than they were when they were on the main sequence. (Please note that this is true for low-and medium-mass stars.)
In a metal-poor cluster like M92, the stars move to the horizontal branch after they have undergone the helium flash. Here the stars become quite hot and blue. Can you see the horizontal branch of globular cluster M92? It is the curving line sloping downwards at around magnitude 16. Note the B-V color of this horizontal branch. Its B-V value is around 0.0, but the lower part of it has a negative B-V value. That means it's very blue.
Metal-rich clusters like 47 Tuc are different. They have "short and red" horizontal branches, so their stars never become blue after undergoing a helium flash. Can you see the horizontal branch of 47 Tuc? It is the short horizontal line at about magnitude 14. Note the B-V color of it. It is between +0.6 and +0.8, which means the stars here are the color of the Sun or yellower.
In short: The "medium-bright" blue stars that you can see in many globular clusters are horizontal branch stars. They are highly evolved, quite low in mass (because they were never very massive to begin with, and they have lost a lot of mass during their evolution) and they are quite hot and blue. Their presence means that the cluster is metal-poor.
So if you see a color picture of a globular cluster and note that it contains a relatively rich scattering of "medium-bright" blue stars, then you are seeing the horizontal branch stars of a metal-poor globular cluster.
By contrast, you will probably not be able to spot any blue stragglers in a typical photo of a globular cluster. The blue stragglers are much fainter and often less blue than the blue horizontal branch stars.
Ann
[quote=VictorBorun post_id=310623 time=1612701788 user_id=145500]
[quote=Ann post_id=310617 time=1612679940 user_id=129702]
But because the blue stragglers are still on the main sequence, they are not as bright as the blue horizontal branch stars, which have used up their core hydrogen and can be thought of as "giants".
[/quote]
I wonder why can there be comparable number of bright outlier stars of two sorts: quick burning massive blue giants (in a gasless place like globular stellar cluster they must come from binary mergers) and collapsing phase of an old smallish star that finally burnt out the hydrogen at its core?
[/quote]
[float=right][img3="Stellar evolutionary tracks off the main sequence. Source: Australian Telescope National Facility."]https://www.atnf.csiro.au/outreach//education/senior/astrophysics/images/stellarevolution/hrpostmainsuntrack.jpg[/img3][/float]Take a look at the chart at right that tracks the evolution of stars of different masses.
The gray diagonal line is the main sequence. Here you find all stars that shine by fusing hydrogen to helium in their cores. When a star is on the main sequence, it is brighter and bluer the more massive it is, and fainter and redder the more light-weight it is.
The chart tells you happens when a star has used up the hydrogen in its core and evolves off the main sequence. "Metal-rich solar-mass" stars like the Sun always become redder and brighter when they evolve off the main sequence. (Well, until they become white dwarfs, that is.) More massive stars than the Sun also become redder when they turn into giants.
But for metal-poor stars, like the ones we often find in globular clusters, the story is a bit different.
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[float=left][img3="Color-magnitude diagram of metal-poor globular cluster M92. Source: 'BVI Photometry and the Luminosity Functions of the Globular Cluster M92', by Nathaniel E. Q. Paust et. al."]https://www.researchgate.net/profile/Nathaniel_Paust/publication/242389736/figure/fig5/AS:668849259167780@1536477498681/The-V-B-V-and-V-V-I-color-magnitude-diagrams-for-the-groundbased-sample.ppm[/img3][/float][float=right][img3="Color-magnitude diagram of more metal-rich globular cluster 47 Tuc. Source: Chegg Study Textbook solutions."]https://d2vlcm61l7u1fs.cloudfront.net/media%2Facb%2Facb006e4-c478-4e25-bfb3-557f49655b3a%2FphpsTiIHV.png[/img3][/float]
[clear][/clear]
I want you to pay attention to the difference in the horizontal branch of these two globulars. In metal-poor cluster M92, the stars first rise in brightness on the red giant branch when they have used up the hydrogen in its core. At the top of the red giant branch, the stars undergo a "helium flash" and start fusing helium to carbon and oxygen. As they do so, they become considerably fainter, but still brighter than they were when they were on the main sequence. (Please note that this is true for low-and medium-mass stars.)
In a metal-poor cluster like M92, the stars move to the horizontal branch after they have undergone the helium flash. Here the stars become quite hot and blue. Can you see the horizontal branch of globular cluster M92? It is the curving line sloping downwards at around magnitude 16. Note the B-V color of this horizontal branch. Its B-V value is around 0.0, but the lower part of it has a negative B-V value. That means it's very blue.
Metal-rich clusters like 47 Tuc are different. They have "short and red" horizontal branches, so their stars never become blue after undergoing a helium flash. Can you see the horizontal branch of 47 Tuc? It is the short horizontal line at about magnitude 14. Note the B-V color of it. It is between +0.6 and +0.8, which means the stars here are the color of the Sun or yellower.
[float=right][img3="Metal-poor globular cluster M5. Note the blue horizontal branch stars in it. Photo: ESA, NASA and Hubble."]https://earthsky.org/upl/2009/06/M5-HST-e1497179043996.jpg[/img3][/float]In short: The "medium-bright" blue stars that you can see in many globular clusters are horizontal branch stars. They are highly evolved, quite low in mass (because they were never very massive to begin with, and they have lost a lot of mass during their evolution) and they are quite hot and blue. Their presence means that the cluster is metal-poor.
So if you see a color picture of a globular cluster and note that it contains a relatively rich scattering of "medium-bright" blue stars, then you are seeing the horizontal branch stars of a metal-poor globular cluster.
By contrast, you will probably not be able to spot any blue stragglers in a typical photo of a globular cluster. The blue stragglers are much fainter and often less blue than the blue horizontal branch stars.
Ann