Someone asked, in rather rude terms, how we know the age of globular clusters. I'm anything but an expert in that field, but a few things can be said:
For globular clusters, RR Lyrae variable stars are often used to measure the distances to these clusters:
RR Lyrae variables pulsate in a certain way and have known luminosities. When they are found in globular clusters, their pulsations and apparent brightness show astronomers how far away the cluster is.
Stellar isochrones are used to estimate the ages of clusters. The brighter stars the cluster contains, the more massive are the stars, and the younger is the cluster.
Let's look at some of the clusters that are shown in the illustration above. The most massive one in the illustration is actually two clusters, the Double Cluster of Perseus, h and chi Per or NGC 869 and NGC 884.
I picked an obvious member star of one of the clusters, HD 14143, and checked its Gaia parallax, which put the star at a distance of ~8,000 light-years. Coupled with this star's apparent V magnitude of 6.7 (and disregarding a considerable amount of dust reddening, which makes stars look fainter), this star's absolute V magnitude is about 10,000 solar luminosities. To be that bright, the star must indeed be massive. According to Wikipedia, both NGC 869 and NGC 884 are about 14 million years old.
I have not found any really reliable estimates of the mass of the the brightest stars of the Double Cluster. According to Focus Nordic, the masses of the brightest stars in the Double Cluster are 10-100 solar masses. Personally I doubt that stars of solar masses can live for 14 million years, which is to say that I don't believe that the brightest stars of the Double Cluster are that massive.
Let's look at the Pleiades:
The distance to the Pleiades is still somewhat controversial, but a good estimate is about 430 light-years. The brightest star is Alcyone, whose apparent V magnitude of 2.87 corresponds, at a distance of about 430 light-years, to about a thousand solar luminosities. Alcyone is much less bright than the blue giant in NGC 869, but it is still bright. According to Wikipedia, the mass of Alcyone is about 6 solar masses, and its age is about 70 million years. Note that the Pleiades Cluster as a whole is believed to be older, 75 to 150 million years old.
Let's look at the oldest cluster that was mentioned in the illustration that I posted, M67:
The brightest red star at lower right is a foreground star and not a member, so I checked one of the brightest red giants that belong to M67, TYC 814-2331-1. Its Gaia parallax puts it a distance of some 2,800 light-years, and coupled with its apparent V luminosity of 9.6, this corresponds to a true V luminosity of about a hundred solar luminosities. The age of cluster M67 that TYC 814-2331-1 belongs to is estimated to be between 3.2 and 5 billion years, according to Wikipedia.
My point is that rich massive clusters always produce big bright massive stars. But the more massive a star is, the sooner does it die. The turnoff point of a color-magnitude diagram for a particular cluster shows the age of the most massive stars that belong to the cluster and still fuse hydrogen to helium in their cores.
Ann
For globular clusters, RR Lyrae variable stars are often used to measure the distances to these clusters:
Wikipedia wrote:
RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype and brightest example, RR Lyrae.
In contemporary astronomy, a period-luminosity relation makes them good standard candles for relatively nearby targets, especially within the Milky Way and Local Group.
RR Lyrae variables pulsate in a certain way and have known luminosities. When they are found in globular clusters, their pulsations and apparent brightness show astronomers how far away the cluster is.
Stellar isochrones are used to estimate the ages of clusters. The brighter stars the cluster contains, the more massive are the stars, and the younger is the cluster.
Let's look at some of the clusters that are shown in the illustration above. The most massive one in the illustration is actually two clusters, the Double Cluster of Perseus, h and chi Per or NGC 869 and NGC 884.
I picked an obvious member star of one of the clusters, HD 14143, and checked its Gaia parallax, which put the star at a distance of ~8,000 light-years. Coupled with this star's apparent V magnitude of 6.7 (and disregarding a considerable amount of dust reddening, which makes stars look fainter), this star's absolute V magnitude is about 10,000 solar luminosities. To be that bright, the star must indeed be massive. According to Wikipedia, both NGC 869 and NGC 884 are about 14 million years old.
I have not found any really reliable estimates of the mass of the the brightest stars of the Double Cluster. According to Focus Nordic, the masses of the brightest stars in the Double Cluster are 10-100 solar masses. Personally I doubt that stars of solar masses can live for 14 million years, which is to say that I don't believe that the brightest stars of the Double Cluster are that massive.
Let's look at the Pleiades:
The distance to the Pleiades is still somewhat controversial, but a good estimate is about 430 light-years. The brightest star is Alcyone, whose apparent V magnitude of 2.87 corresponds, at a distance of about 430 light-years, to about a thousand solar luminosities. Alcyone is much less bright than the blue giant in NGC 869, but it is still bright. According to Wikipedia, the mass of Alcyone is about 6 solar masses, and its age is about 70 million years. Note that the Pleiades Cluster as a whole is believed to be older, 75 to 150 million years old.
Let's look at the oldest cluster that was mentioned in the illustration that I posted, M67:
The brightest red star at lower right is a foreground star and not a member, so I checked one of the brightest red giants that belong to M67, TYC 814-2331-1. Its Gaia parallax puts it a distance of some 2,800 light-years, and coupled with its apparent V luminosity of 9.6, this corresponds to a true V luminosity of about a hundred solar luminosities. The age of cluster M67 that TYC 814-2331-1 belongs to is estimated to be between 3.2 and 5 billion years, according to Wikipedia.
My point is that rich massive clusters always produce big bright massive stars. But the more massive a star is, the sooner does it die. The turnoff point of a color-magnitude diagram for a particular cluster shows the age of the most massive stars that belong to the cluster and still fuse hydrogen to helium in their cores.
Statistics: Posted by Ann — Thu Dec 19, 2024 5:48 pm — Replies 4 — Views 275