Clues revealed about crucial cosmic markers.

• Position: Supernovas

The origin of a famous supernova has been found using data from NASA's Chandra X-ray Observatory at the Marshall Space Flight Center, Huntsville, Ala. This supernova, discovered in 1604 by astronomer Johannes Kepler, belongs to an important class of objects that are used to measure the rate of expansion of the universe.

Astronomers have used a very long Chandra observation of the remnant of Kepler's supernova to deduce that the supernova was triggered by an interaction between a white dwarf and a red giant star. This is significant because another study already has shown that a so-called Type la supernova caused the Kepler supernova remnant.

The thermonuclear explosion of a white dwarf star produces such supernovas. Because they explode with nearly uniform brightness, astronomers have used them as cosmic distance markers to track the accelerated expansion of the universe. However, there is an ongoing controversy about Type la supernovas. Are they caused by a white dwarf pulling so much material from a companion star that it becomes unstable and explodes, or do they result from the merger of two white dwarfs?

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