Clusters of Stars
Our Galaxy contains mostly loner stars like the Sun and paired (i.e. binary) stars such as brilliant Sirius and its dim companion. But from our Earthly vantage point, with our eyes unable to perceive the depth of deep space, the far-flung stars look bunched up, close to each other. Looking for order in this seeming randomness, we see patterns of stars – or "constellations" - such as Orion and Cassiopeia.
On the other hand, in a few groupings such as the Pleiades (in the Taurus constellation) or the hazy Beehive (in Cancer), the stars are indeed at about the same distance and actually do move in tandem. Four hundred years ago, Galileo used a telescope to observe these groupings and discovered them to be "star clusters."
Unlike constellations, star clusters are, in fact, real families of stars close together in space, having originated at the same time from a common cloud of gas and dust.
Open and Globular Star Clusters
Thousands of star clusters have been identified in our own Galaxy and other galaxies beyond. They are of two general types: open clusters and globular clusters.
Open clusters (e.g., the Pleiades and Beehive clusters) contain dozens to many hundreds of stars usually arranged loosely and asymmetrically in a region several light years across. They usually contain younger stars of practically every color and brightness, with their brightest stars being typically blue or whitish and hotter than our Sun. Several open clusters contain wisps of gas and dust surrounding their member stars.
In contrast, globular clusters (e.g., Messier 13 in Hercules) contain thousands to millions of stars packed compactly into a ball-like formation roughly 100 light years across. Globular clusters typically have older stars, and are dominated by the light of gigantic red stars cooler than our Sun. Few show signs of gas and dust.
What makes open- and globular clusters so different? It turns out that their stars have different mass ranges and are at different stages of their lives.
Star Clusters as Astronomical Tools
Because star clusters contain a population of stars born roughly at the same time and place, the different characteristics of their member stars must be a function of their mass.
Astronomers have long known that there is a relationship between a star’s surface temperature and intrinsic brightness (or power output). This relationship can in turn be used to determine a star’s mass and stage of life using our knowledge of how stars evolve. How do astronomers measure a star’s temperature and brightness in the first place? A star’s temperature can be determined by its color or from the lines in a spectrum, independently of its distance. But in order to know how intrinsically bright a star is we need to know how far away it is.
Star Clusters as Cosmic Yardsticks
The most direct way to measure a star’s distance is through parallax, a method of triangulations like the one used by surveyors to measure distances on Earth. Parallax measurements can be done accurately for stars less than 1,000 light years away, and only a few star clusters are this close. For these nearby clusters, astronomers can accurately measure not only the stars’ temperatures but also their actual brightness.
For clusters whose distance is not known, astronomers can measure the temperatures and how faint their stars appear to be in the sky. Then, the unknown distance can be estimated by comparing a diagram of the stars’ colors and apparent brightnesses with a cluster whose distance is known through parallax. Another way to find distances, using the fact that fainter means farther, is by comparing “variable stars” whose patterns of brightness variation is a clue to their intrinsic brightness.
Since globular clusters are bright enough to be visible at large distances, they can help us figure out our home galaxy’s structure. Astronomers have used variable stars as well as the relationship between stars’ temperature and brightness to calculate distances to globular clusters. Mapping the locations of clusters in three dimensions shows they are concentrated around a region of the Sagittarius constellation. This is a clear indication that the center of our Galaxy is in the direction of Sagittarius, at a distance of some 25,000 light years from the Sun.
Astronomers use star clusters as a step in building up a “distance ladder,” using ever more distant objects in comparison with nearby ones whose distances are known, ultimately probing the most distant reaches of the universe.
Star Clusters as Cosmic Clocks
From the estimated distance, the intrinsic brightness and therefore, masses of the members of the star cluster can be found. Astronomers are especially interested in figuring out the mass of the cluster’s heaviest stars that are still fusing hydrogen. Then, they apply our knowledge of stellar evolution to calculate the lifetime of these stars. Since the cluster hosts the stars, the cluster’s lifetime should be at least that of its oldest stars.
Some open clusters have been found to have ages as young as a few hundred thousand years, while others may predate the 4.5-billion year old Sun. Globular clusters, it turns out, are typically about 10 billion years old, ancient as the galaxies themselves. Since a daughter has to be younger than the mother, the oldest globular cluster stars also provide a lower limit to the age of the universe itself.
Burned out stars called white dwarfs in globular clusters have also been used to clock the universe, as a crosscheck on ages derived from stars that are still fusing hydrogen. Independent age estimates from the studies of the expansion of the universe and the pervasive cosmic background radiation have set the universe’s birthday to about 13.7 billion years ago.
How do stars form in isolation or in clusters? How do star clusters form in galaxy mergers? What are the exact ages of stars, galaxies and the universe? Astronomers are using a variety of observational tools including NASA’s orbiting telescopes, as well as advanced computational and analytical resources to pursue these still current mysteries.
The Spitzer Space Telescope shows the stars in the Pleiades open cluster are swathed in dust and gas.
Globular cluster 47 Tucanae imaged by the ground-based Very Large Telescope in Chile (left) and the Hubble Space Telescope (right).
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