Stellar Evolution

What Color is a Brown Dwarf?

Brown dwarfs are extremely low mass stars. They have masses only slightly larger than that of the planet Jupiter. Because a star's temperature depends on its mass (with smaller stars having cooler temperatures) brown dwarfs are extremely cool.

We know that as the temperature of a hot dense object decreases, it glows an ever duller red color. You can see this by watching an electric stove burner when you turn it off.

Are brown dwarfs such a dull red? Well, not quite. They are so cool that dust can condense in their atmospheres and mingled with the dust are sodium atoms. Sodium atoms absorb yellow light very strongly. If you take yellow light away from a dull red star, the star looks purple or magenta.

So brown dwarfs are actually purple!


Formation of High Mass Stars

Astronomers have known for many years that when a high mass star is born, the material falling on to the forming star heats it intensely. The resulting high temperature creates a very intense flood of radiation from the young star that pushes outward, creating a strong radiation pressure. The outward push of the radiation pressure makes it difficult for additional matter to fall on to the star. Thus, the star's mass can not increase indefinitely.

Computer studies of this process show that the radiation pressure is most intense near the star's poles and is relatively weak near the star's equator. Material falling inward, therefore, accumulates in orbit around the star and forms a rotating disk above the star's equator.

This rotating disk grows "spiral arms", much like the spiral arms of some galaxies, but on a much smaller scale. Gas in the disk flows inward toward the star along the arms and can gradually build the star's mass to larger values than were previously thought. Moreover, the disk may develop "lumps", much as we saw occurred in the Solar Nebula when our Solar System formed. In the case of the massive star, the lumps may be so big that they may merge into a second star. Thus, a forming high mass may create a partner star in orbit around it. In fact, a very high percentage of massive stars show exacvtly such a companion.