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.
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.