The three circles in this image of M13 indicate where three of the unexpected sources of lithium occur. This moderately shallow color picture was made from CCD images taken at the Burrell Schmidt telescope of the Warner and Swasey Observatory of the Case Western Reserve University, located on Kitt Peak in southwestern Arizona. At least 10 percent of the lithium in the Milky Way galaxy today was created in the Big Bang, but the origin of the remaining 90 percent is a major mystery. Astronomers have long suspected that the element is produced in stars, but the exact type of star that makes lithium was not known. Red giants that contain lithium are rare - only about a dozen red giants of the 700 surveyed contained detectable amounts.
Most stars destroy lithium during an early hydrogen-burning phase of their evolution.
When a star's supply of hydrogen is exhausted, the star expands to become a red giant. The upper layers of the star bloat and begin to roil like a boiling pot of water. Any remaining lithium in the star's outer layers is mixed with its deeper, lithium-depleted layers, and is quickly burned at the high temperatures found deep inside the star. The lithium detected in this study appears in red giants that are very bright, reaching more than 500 times the luminosity and 30 times the radius of the Sun, although their masses are slightly less than the Sun's. Such stars are almost ready to ignite helium in the phase known as the core-helium flash.