Mass Difference Between Brown Dwarfs and Full-Fledged Stars Finally Determined

Like so many things in the cosmos, some stars practically die a-borning. These “failed stars” are known as “brown dwarfs,” and are simply not large enough to sustain a long-term hydrogen fusion reaction.

Before now, scientists believed the minimum mass required to sustain a long-term hydrogen reaction is equal to that of 75 times that of Jupiter. Although Jupiter’s chemical makeup is almost identical to that of a star, it’s mass is far too small to even begin a hydrogen reaction. As far as brown dwarfs are concerned, it has been difficult to confirm theories because brown dwarfs look a lot like red dwarfs – which are able to sustain the reaction required for full “starhood.”

Harvey Richer of the University of British Columbia has used Hubble to observe the faintest red dwarf stars yet observed. These are located in a very old cluster identified as NGC 6397, about 8500 light years away. At 13.5 billion years, this cluster dates back to the beginning of the universe. According to another team member, Jay Anderson of Rice University in Texas, “…the brown dwarfs have by now faded off into obscurity so there is a very stark contrast between the stars that could burn hydrogen and the ones that couldn’t.”

Theories predict that the low-end mass for true stars should be determined by how many metals are present.  Stellar objects with a metal content similar our own sun would need a minimum mass no less than 7.5% that of Sol. The NGC 6397 cluster has a metallic content that is less than 1% of the Sun’s, so this dividing line is expected to be slightly greater – 8.3% that of Sol, or 83 times the mass of Jupiter.

The new Hubble observations are able to identify starts 10 to 20 times fainter than earlier ones. “This kind of observation is probably the best kind of constraint one can get,” says Gibor Basri of UC Berkeley. He added that the hydrogen burning limits observed so far are in line with current theory.

Via New Scientist

Like so many things in the cosmos, some stars practically die a-borning. These “failed stars” are known as “brown dwarfs,” and are simply not large enough to sustain a long-term hydrogen fusion reaction.

Before now, scientists believed the minimum mass required to sustain a long-term hydrogen reaction is equal to that of 75 times that of Jupiter. Although Jupiter’s chemical makeup is almost identical to that of a star, it’s mass is far too small to even begin a hydrogen reaction. As far as brown dwarfs are concerned, it has been difficult to confirm theories because brown dwarfs look a lot like red dwarfs – which are able to sustain the reaction required for full “starhood.”

Harvey Richer of the University of British Columbia has used Hubble to observe the faintest red dwarf stars yet observed. These are located in a very old cluster identified as NGC 6397, about 8500 light years away. At 13.5 billion years, this cluster dates back to the beginning of the universe. According to another team member, Jay Anderson of Rice University in Texas, “…the brown dwarfs have by now faded off into obscurity so there is a very stark contrast between the stars that could burn hydrogen and the ones that couldn’t.”

Theories predict that the low-end mass for true stars should be determined by how many metals are present.  Stellar objects with a metal content similar our own sun would need a minimum mass no less than 7.5% that of Sol. The NGC 6397 cluster has a metallic content that is less than 1% of the Sun’s, so this dividing line is expected to be slightly greater – 8.3% that of Sol, or 83 times the mass of Jupiter.

The new Hubble observations are able to identify starts 10 to 20 times fainter than earlier ones. “This kind of observation is probably the best kind of constraint one can get,” says Gibor Basri of UC Berkeley. He added that the hydrogen burning limits observed so far are in line with current theory.

Via New Scientist