Therefore, if we can determine how one cluster of stars formed, we can generalize our findings to apply to all clusters. This idea of a relationship between the number of stars formed in a star forming region and their mass is referred to as the stellar initial mass function. Let us follow the evolution of an entire cluster of stars through several stages of its lifetime. You will see a series of schematic HR diagrams for the stars in a cluster. In each frame, as the stars age, their luminosities and temperatures evolve, changing the overall appearance of the diagram with age.
What we see in the sequence is that as a cluster of stars ages , the top of the Main Sequence disappears first. The analogy you often hear is that it is like the wick of a candle—as the cluster stars burn out, the Main Sequence gets shorter. Therefore, if you can identify exactly what type of star is just now undergoing the transition from Main Sequence to red giant called the Main Sequence Turn-Off , and if you know how long theoretically that it takes stars of that type to use up all of their hydrogen, you can estimate the age of that star.
Now, because we assume that all of the stars in the cluster formed simultaneously, we can assume that all stars in the cluster have the same age as the most massive star left on the Main Sequence. Astronomers frequently use this technique of Main Sequence Turn-Off fitting to estimate the age of star clusters.
The way this is done in practice is the following:. If you compare the HR diagrams for stages , these are very similar to the HR diagrams for open clusters.
The HR diagram for stage 6 appears to be very similar to that of a globular cluster. Thus, we can conclude that open clusters are young usually a few tens of millions or hundreds of millions of years old , while globular clusters are very old typically about billion years old.
In the image above, you can see a schematic HR diagram with plots of lines that represent the Main Sequence for a number of open clusters. This realization explains several of the other observations that we made of the differences between these two types of clusters. Since open clusters are young, they have not had a chance to move very far from the location where they were born. Thus, there is likely to be leftover material from the molecular clouds in which they formed nearby which creates the reflection nebulae seen in the Pleiades.
If a globular cluster is more than 2 billion years old, then there will be no hydrogen-burning star more massive than 2 solar masses. The oldest globular clusters contain only stars less massive than 0. These low mass stars are much dimmer than the Sun.
This suggests that the oldest globular clusters are between 11 and 18 billion years old. The uncertainty in this estimate is due to the difficulty in determining the exact distance to a globular cluster hence, an uncertainty in the brightness and mass of the stars in the cluster.
Another source of uncertainty in this estimate lies in our ignorance of some of the finer details of stellar evolution. Another way to estimate the age of the Universe is to measure the "Hubble constant". The Hubble constant H 0 is a measure of the current expansion rate of the Universe. Cosmologists use this measurement to extrapolate back to the Big Bang. This extrapolation depends upon the current density of the Universe and on the composition of the Universe.
If the theory of general relativity is modified to include a cosmological constant, then the inferred age can be even larger. Star clusters are groups of hundreds to millions of stars that provide astronomers crucial insight into stellar evolution through comparisons of stars' ages and compositions. Geller, an astronomer at Northwestern University, told Space. The densest areas of those molecular clouds collapse into themselves to form stars.
In some cases, the stars disperse after their creation, Geller said. But star clusters are not galaxies , which, confusingly, are also gravitationally bound groups of stars. If there's dark matter in the mix, then that group of stars is likely a galaxy. Related: How to spot the Pleiades, Hyades and other star clusters in the winter night sky.
Galaxies are generally larger than star clusters, too. Another factor that differentiates star clusters from galaxies is that within each cluster, stars are roughly the same age and made from roughly the same materials, given that they formed from the same molecular cloud, according to the Australia Telescope National Facility ATNF.
Stars within a galaxy, on the other hand, can be a variety of ages and have a variety of compositions, Steffen said. That's why stellar clusters are so important to astronomers who are studying stellar evolution. Comparing the two "helps you understand what's going on on the insides of the stars and how they evolved throughout their lifetimes," according to Steffen. There are three main types of star clusters: globular clusters, open clusters, and stellar associations, each of which has different properties that provide different information to astronomers.
If viewed with the naked eye, globular clusters look like faint smudges of light against the darkness of space. But a telescope reveals their true form: thousands to millions of stars form a spherical shape with a bright, dense core.
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