Star clusters are groups of stars that formed from the same giant cloud of gas and dust. Because the stars in a cluster are at nearly the same distance from Earth and began forming at about the same time, they are powerful natural laboratories for studying stellar life cycles. Astronomers compare open clusters and globular clusters to learn how stars change with age, mass, and chemical composition.
These clusters also help map the structure and history of our Milky Way galaxy.
Open clusters are loose groups of hundreds to thousands of young stars, often found in the spiral arms of galaxies where new stars are still forming. Globular clusters are dense, spherical collections of tens of thousands to millions of very old stars, usually found in a galaxy's halo. A color magnitude diagram can reveal a cluster's age because massive blue stars leave the main sequence first.
By measuring brightness, color, and distance, astronomers can estimate how far away a cluster is and how its stars have evolved.
Understanding Astronomy: Star Clusters
Gravity controls a cluster from its earliest stages. A cold cloud does not collapse as one smooth ball. It breaks into many smaller dense regions.
Each region can form a star. Leftover gas is important because young stars produce winds and radiation that push this gas away. If gas leaves quickly, the cluster loses much of the mass that was holding it together.
Many of its members then drift apart. This helps explain why loose groups of young stars often do not survive for very long on galactic timescales. A few clusters begin with enough stars and enough tightly bound mass to remain together.
Stars inside a cluster are not fixed in place. They orbit the cluster's common center of mass and repeatedly pass near one another. A close pass changes the paths and speeds of both stars by a small amount.
Across millions or billions of years, these tiny changes add up. Some stars gain enough speed to escape. Lower mass stars are often lost more easily, while heavier stars tend to settle nearer the crowded center.
This process is called relaxation. The gravity of the galaxy adds another effect.
As a cluster moves through the galaxy, tidal forces can pull at its outer stars. Passing near dense gas clouds can disturb a loose cluster even more.
Globular clusters are useful for studying crowded stellar environments. Their central regions can contain stars packed far closer together than stars near the Sun. Close encounters can disrupt planetary systems, exchange stars between binary pairs, or create unusual objects.
Binary stars matter because two orbiting stars can transfer material from one to the other. This can make a star appear hotter or younger than expected. Astronomers call some of these blue stragglers.
They stand out because they do not fit the simple pattern in which all stars have aged quietly since the cluster formed. Looking for these exceptions teaches astronomers that stellar evolution is affected by companions and collisions, not only by a star's starting mass.
Measurements of clusters need careful interpretation. Dust between Earth and a cluster can make starlight dimmer and redder. This effect can make a star seem farther away or cooler than it really is.
Astronomers correct for dust by comparing observations through different filters, such as blue and red light. They can then combine a cluster's apparent brightness with its known distance to estimate true brightness. Modern spacecraft measure tiny shifts in a star's apparent position as Earth travels around the Sun.
This shift is called parallax. Students should pay attention to the difference between brightness seen from Earth and a star's actual light output. They should also remember that a graph pattern describes a population, while individual stars can have unusual histories.
Key Facts
- Open clusters are young, loose, irregular groups of stars, usually found in a galaxy's disk or spiral arms.
- Globular clusters are old, dense, nearly spherical groups of stars, usually found in a galaxy's halo.
- Typical open cluster size: about 10 to 30 light years across, with hundreds to thousands of stars.
- Typical globular cluster size: about 50 to 200 light years across, with 10,000 to over 1,000,000 stars.
- Distance modulus: m - M = 5 log10(d) - 5, where d is distance in parsecs.
- Light travel relation: distance in light years = time in years for light to travel that distance.
Vocabulary
- Open cluster
- A loose group of relatively young stars that formed together and is usually found in the disk of a galaxy.
- Globular cluster
- A dense, spherical group of very old stars that orbits in the halo of a galaxy.
- Color magnitude diagram
- A graph of star color versus brightness that helps astronomers estimate a cluster's age and stellar evolution stage.
- Main sequence
- The long stable phase of a star's life when it fuses hydrogen into helium in its core.
- Galactic halo
- The roughly spherical region around a galaxy that contains old stars, globular clusters, and dark matter.
Common Mistakes to Avoid
- Calling every dense star field a cluster is wrong because stars can appear close together in the sky while being at very different distances from Earth.
- Assuming open clusters are older than globular clusters is wrong because open clusters usually contain young blue stars, while globular clusters are among the oldest objects in the galaxy.
- Judging a cluster's distance only by how bright it looks is wrong because apparent brightness depends on both distance and the actual luminosities of its stars.
- Thinking all stars in a cluster have the same mass is wrong because cluster stars formed from the same cloud but with a range of masses, which makes them evolve at different rates.
Practice Questions
- 1 An open cluster is 800 parsecs away. Using 1 parsec = 3.26 light years, how far away is it in light years?
- 2 A globular cluster has an apparent magnitude m = 15 and an absolute magnitude M = 0 for a reference star. Use m - M = 5 log10(d) - 5 to find the distance d in parsecs.
- 3 A cluster contains many bright blue stars and is located in a galaxy's spiral arm. Explain whether it is more likely an open cluster or a globular cluster, and give two reasons.