Skywatch

  • September 14, 2006

    Episode 56: Faintest Stars

    Download this episode

    Shownotes

    Scientists have a theory about the size of stars. Stars must be about 80 times the mass of Jupiter in order to shine. It's the limit at which fusion can occur. Below that limit, scientists think, fusion occurs briefly before the object turns into a cool cinder called brown dwarf. New observations of nearby globular cluster NGC 6397 are helping confirm the theory. Globular clusters — compact groups of stars — are especially useful for this test since they contain hundreds to thousands of stars and are billions of years old. By studying the cluster until they found all the hydrogen-burning stars, scientists found the dimmest stars in the cluster, including the lowest-mass stars capable of sustaining fusion. The scientists also examined the burned out remains of stars that died long ago, called white dwarfs. In doing so, they discovered an effect that was predicted, but never witnessed. As a rule of thumb, the hottest stars are known to burn blue, while cooler stars burn red. The dimmest of the cluster's white dwarf stars have such low temperatures that they are undergoing a chemical change in their atmospheres that makes them appear bluer rather than redder as they cool.