Star Clusters Day 22

Star-Forming Clouds

  • Protostars can evolve into different types of stars based on their mass:
    • More than 25M⊙25M_{\odot}: Protostar evolves into a main-sequence star, then a supergiant, and eventually a supernova.
    • 8 to 25M⊙25M_{\odot}: Protostar evolves into a main-sequence star, then a giant star, a supergiant, and ends as a supernova, which can form a black hole or neutron star.
    • 0.4 to 8M⊙8M_{\odot}: Protostar becomes a main-sequence star, then a giant star, and forms a planetary nebula, eventually becoming a white dwarf.
    • Between 0.08 and 0.4M⊙0.4M_{\odot}: Protostar evolves into a main-sequence red dwarf star.
    • Less than 0.08M⊙0.08M_{\odot}: Protostar becomes a brown dwarf.

Star Clusters: Testing Our Theories

  • Star clusters are useful for testing stellar evolution theories because:

    • Stars within a cluster formed at approximately the same time (same age).
    • Stars are at the same distance from us.

  • The sole judge of scientific truth is experiments or observations. Theories must make accurate, testable predictions.

  • Color-magnitude (H-R) diagrams of star clusters provide confirmation of stellar evolution models.

  • Differences in stars within a cluster can be primarily explained by mass.

  • Stars in a cluster:

    • Form at about the same time, so age differences aren't a major factor.
    • Form from the same gas cloud, so have similar chemical composition.
    • Are at approximately the same distance from us, so differences in apparent brightness are due to luminosity.

  • More luminous main sequence stars are more massive than dimmer ones.

After the Main Sequence

  • High-mass stars have shorter lifetimes.

  • By observing which stars leave the main sequence first in a cluster, we can test this hypothesis.

Star Clusters: General Information

  • Two main types of star clusters:
    • Open clusters: Do not last long, young stars easily dispersed.
    • Globular clusters: Larger and much older.

Open Clusters

  • Found in the disk of the Galaxy.

  • Have a range of ages, some as old as or older than the Sun.

  • Youngest open clusters are associated with the interstellar matter from which they formed.

  • Smaller than globular clusters, typically less than 30 light-years in diameter.

  • Stars usually appear well-separated, hence the name "open."

  • Thousands in our Galaxy, but interstellar dust dims the light of distant ones, making them undetectable.

  • Individual stars can survive for billions of years, but clusters typically remain together for only a few million to a few hundred million years.

  • In small open clusters, stellar speeds may exceed the cluster's escape velocity, causing stars to "evaporate."

Globular Clusters

  • Nearly symmetrical round systems of hundreds of thousands of stars.

  • Brightest stars are red giants with surface temperatures around 4000 K.

  • Among the oldest parts of the Milky Way Galaxy.

  • About 150 known in our Galaxy, mostly in a spherical halo around the disk.

  • Far from the Sun, some at distances of 60,000 light-years or more.

  • Diameters range from 50 to over 450 light-years.

  • Oldest structures in our Galaxy and other galaxies.

  • Youngest are about 11 billion years old, providing a limit on the age of the universe.

Star Clusters: Characteristics

CharacteristicGlobular ClustersOpen Clusters
Number in the Galaxy150 (mostly in the halo)Thousands (in disk and arms)
Diameter (in light-years)50–450<30
Mass (M⊙M_{\odot})10,000 to 1,000,000100 to 1000
Number of stars10,000 to 1,000,00050 to 1000
Color of brightest starsRedRed or blue
Gas and Dust ContentVery low or noneA lot, and higher metallicity
Typical agesOver 10 billion yearsFew hundred million years

How Old is NGC 2264?

  • Located approximately 2600 light-years away.

Star Clusters: Testing Our Theories

  • H-R Diagram shape, specifically the main sequence turnoff point, estimates the age of the star cluster.

  • The turnoff point indicates which stars are running out of hydrogen in their cores.

Star Clusters - Evolution

  • All stars form at approximately the same time.

  • High mass stars evolve faster.

  • Cluster stars form a "turn-off" point that can be used to find the age of the cluster.

  • Lifetime is calculated by: T∝M−1T \propto M^{-1}

Observations of Stellar Changes

  • Very young clusters have massive stars on the main sequence and lighter stars collapsing toward it.

  • As clusters age, massive stars die, leaving less massive stars on the main sequence.

  • The main sequence turnoff can be used to find the age of the star cluster.

Star Clusters - Evolution over time

  • Zero-age main sequence (Time = 0)

  • Time = 10710^7 years

  • Time = 10810^8 years, Main-sequence turnoff

  • Time = 10910^9 years, Red-giant branch, Subgiant branch, Horizontal branch.

  • Time =101010^{10} years, White dwarfs.