What
is a star cluster and why does its age matter to us?
Stars
are formed in cool, dense molecular clouds due to gravitational collapse. Such
molecular clouds are massive and may have multiple stars forming at the same
time. Therefore, stars normally reside in clusters. We assume all stars in the
same cluster have the same age and are at the same distance from us. This allows us to compare the brightnesses of stars in the same cluster and infer other stellar properties without having to worry about the differences in their age and distance. The study
of star clusters and their properties are crucial to us in understanding more
about stars in general.
The
age of a star cluster is one of the essential properties that are of interest
to us. Astronomers usually cannot determine the age of an individual star by
itself. With a large cluster of stars, we are able to tell the age of the
cluster, thus inferring the ages of the stars within it. We could determine the
accurate age of a star cluster by comparing detailed models of stellar
evolution plotted in the Hertzsprung-Russel Diagram
to the data of the particular cluster of interest. In a star cluster, massive
blue stars with high luminosity and surface temperature fuse hydrogen in
their cores faster and die first, moving away from the main sequence into the
red giant phase of their lives. The death of these massive blue stars are followed by white, yellow, orange and red
stars. By locating the main sequence turn-off point of the star cluster on the HR diagram, we
are able to tell its age.
How do you find the main sequence turn-off point? Since
every single star spends most of its life on the main sequence, its brightness
and temperature remains the same for most of its life. Therefore, even it is very difficult to determine an individual star's age, it is possible to determine the age of a star cluster. With stars of all masses formed at about the same time, the bluer and hotter a star is in the cluster, the more massive it is, with a shorter lifetime on the main sequence. We could simply observe and determine the temperature and mass of the bluest star in a cluster, since anything more massive would have already evolved off the main sequence into the red giant phase. Therefore, we know that the age of the star cluster must be similar to the lifetime of the most massive star that is still on the main sequence. By knowing that particular star’s mass, we are able to infer
the amount of hydrogen fuel a star has in its core. The lifetime of the star can
thus be calculated by taking the ratio of the total fuel it had since the
beginning to the rate the fuel burns out.
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| Source: https://en.wikipedia.org/wiki/Hertzsprung–Russell_diagram#/media/File:Hertzsprung-Russel_StarData.png |
In
our galaxy, the Milky Way, star clusters can be classified into two
categories—open clusters and globular clusters. Open clusters (shown in the left-hand figure below) are loosely packed collection of a few hundred to a thousand stars. Globular clusters have up to a million or more stars and are packed together
into a dense ball due to gravitational attractions. Open clusters are younger and
exist in the disk of our galaxy while the older globular clusters are in the
“halo” region of our galaxy. Globular clusters were formed early in the history of our galaxy, when the universe was quite young. Open clusters, on the other hand, come in a wide
range of ages, as our Milky Way galaxy started to form these clusters soon after
it reached its present size. The formation of open clusters continues even today.
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| Credit: ESA/Hubble & NASA |
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| Credit: ESO Acknowledgement: ESO Imaging Survey |
Through studying star clusters, especially old globular clusters, astronomers are able to discover many secrets about stellar revolution and the universe itself. For example, after determine the age of different clusters, astronomers are able to compare the spectrum of stars that are of different ages to study the difference in stellar composition as the universe ages. The early stars in globular clusters are relatively metal-poor, while the open cluster stars are more metal-rich. The age of globular clusters also provides a hint for the age of the universe. Early estimates showed that the universe was about 10 billion years old, the discover of globular clusters with ages between 12 to 15 billion years old suggested that the estimation for the age of universe was wrong (Than, 2006). Until today, dating star clusters continues to assist astronomers in making more discoveries about our beautiful universe.
Reference:
Than, K. (2006, April 18). Star Clusters Hold Secret to Stellar Evolution. Retrieved from http://www.space.com/2306-star-clusters-hold-secrets-stellar-evolution.html
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