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Тhe star clusters must be regions of incredible beauty. Imagine living on a planet inside a star cluster. Your night sky would be densely spangled with bright points of light. Glowing bands of cosmic gas might arch overhead. The spectacle could make Earth's clearest winter skies seem dull.
For terrestrial astronomers, star clusters also have a practical side. They are enormous astrophysical laboratories in which we verify our theories of how stars are born, live and die. The largest densest, and most symmetrical are globular clusters. Many are nearly spherical. Others are distorted into ellipsoids by the motion of the hundreds of thousands of stars swirling within them.
Types of Star Clusters
Globular clusters surround our galaxy most of them far above or below the disc that contains nearly all of the Milky Way's stars. In these clusters we find the oldest stars—stars that condensed from primordial gas before most of the rest of the galaxy had formed.
Open, or galactic, clusters lie within the Milky Way's disc and are less dense than the globular variety. Their forms are irregular with few symmetrical features. Open clusters contain younger stars, often entangled in a web of cosmic gas and dust left over from their formation. Such a cluster may have a few dozen stars or a few thousand.
The loosest groups of all are the "associations," swarms of very young, hot stars, still evolving and often flying slowly away from one another. They are subdivided according to their characteristic stars; T associations contain the low-mass, still-forming T-Tauri stars, named after the first star of their type that astronomers recognized. OB associations harbor the extremely luminous OB stars among the most brilliant in the universe. R associations show much gas and dust– the afterbirth of stellar formation.
A good example of a globular cluster can be seen on very clear nights as a small, fuzzy patch in the constellation Hercules. The Pleiades, an open cluster in the constellation Taurus, are the most famous of this type, celebrated in poetry by writers from Sappho to Tennyson. An association of very young stars lies within the Orion Nebula, found in Orion's sword. You can locate any of these clusters on a star map.
Importance of Star Clusters
To see why clusters are so important, consider what astronomers cannot do in studying stars: They can't breed them, feed them, or run them through mazes to see how they behave. For the most part, they can't even watch an individual star age. Stellar astronomy is strictly a hands-off affair. But by examining a star cluster we can find the answers to many significant questions.
We can't tell how old a star is just by looking at it, but we can usually say that it is no older than a given maximum. We know that a star behaves normally as long as it is still "burning" hydrogen to form helium and energy–most of its life. Such stars, like our Sun, are said to be on the main sequence.
When the hydrogen in a star's central furnace is exhausted, the star swells and becomes red. Eventually this red giant ends its days as a white dwarf star a neutron star or even a black hole—if such a thing exists.
Studies tell us that mass determines how long a star remains on the main sequence. The larger the star the shorter its life. In the main sequence a star's mass also determines its color. So if we see a yellow star, like our sun, we know that it must be less than 10 billion years old. A hot blue star must be less than 1 billion years old. And a main-sequence red star could be as old as the universe, a red dwarf's lifetime is longer than the 15 billion or so years the universe has been around.
But any single star could be much younger than its maximum possible age. There is no way we can be sure just by looking at it.
Studying Star Clusters
In a cluster however, we can safely assume that all the stars were formed at about the same time. We can tell how old the cluster is by examining the color of the stars just leaving the main sequence, just beginning to bloat and cool. It is roughly as old as the number of years it takes those stars to begin turning into red giants.
Similarly all the stars in a cluster have the same composition when they are formed. Most stars are made of hydrogen and helium and very little else. Spectroscopic analysis shows that elements heavier than helium are relatively rare. Even in stars rich in them, heavier elements make up no more than a minute fraction of the star's material. The slight differences in stellar evolution caused by the minor variations in a star cluster's composition are insignificant.
From cluster to cluster, however there are marked chemical differences. The stars of a globular cluster consist almost entirely of hydrogen and helium, with almost none of the heavier elements present. Stars in an open cluster are much richer in heavy elements. The study of stellar evolution helps explain this contrast.
We think that the heavy elements are created by supernovas, the cataclysmic explosions that end the lives of some stars. While the normal nuclear fusion that goes on in all stars produces elements other than hydrogen and helium, only supernovas can form elements heavier than iron. The explosion spews these elements out into the interstellar gas, enriching it for the next generation of stellar birth. In this way each generation of stars is richer in heavy elements than the one before it. The globular-cluster stars, which formed first, were made of primordial material, unenriched by supernovas since its creation at the beginning of the universe. Open clusters formed later on. The sun is a second-or third-generation star. We know this from the abundance of heavy elements detected in its spectrum.
A study of meteorites has shed some light on the sun's birth. In the meteorites are the remains of a radioactive form of aluminum. It is an isotope that is radioactive for only a short time, cosmically speaking, and should not occur in such material as meteorites unless it was formed only a little while before the meteorites themselves were.
Astronomers think that this aluminum was formed in a supernova that preceded the birth of our solar system several billion years ago. The cataclysm both enriched the gas that was to make up our sun—it contributed about a tenth of 1 percent of our material—and started the contraction of the gas cloud. This supernova caused the formation of our solar system. Some of the material from that stellar explosion is now part of your body.
Future space probes will collect pristine meteoroids, perhaps even pieces of comets, and will reveal more details of the early solar system. Much further in the future, interstellar probes may visit the Pleiades, one of our most beautiful astrophysical laboratories. From these will come a fuller picture of how we fit into the 15-billion-year evolution of the universe.