StarDate 7/17/2014: The Night Of The Living Dead Stars

FROM THE UNIVERSITY OF TEXAS McDONALD OBSERVATORY, AS HEARD ON KCBS RADIO WEEKDAYS @ 9:52 A.M., 7:35 P.M. & 2:52 A.M.

STARDATE 7/17/2014:  A white dwarf sounds harmless enough. It's the small, dead core of a once-normal star like the Sun. But to a close companion star, a white dwarf is anything but harmless. It's the living dead.

Although a typical white dwarf is only about as big as Earth, it's half as massive as the Sun or heavier, so its surface gravity is quite strong. So if a companion is close enough, the white dwarf can pull hot gas off its surface — like a vampire drinking the blood of the living.

Over time — anywhere from a few decades to a few centuries — a layer of gas many miles thick can build up atop the white dwarf. This gas gets hotter and hotter until it triggers a nuclear explosion, like a giant hydrogen bomb.

For a while, the white dwarf shines thousands of times brighter than normal. As seen from Earth, a "new" star suddenly appears where none was seen before, so such an outburst is known as a nova, which is the Latin word for "new."

In a few weeks or months, though, the nova once again fades from sight as the exploded material dissipates. So the white dwarf "dies" once again. This cycle of death, rebirth, and re-death has led some astronomers to give novae another name: zombie stars.

And the cycle can repeat. After an explosion, the white dwarf can once again begin sucking the gas from its companion. Over the decades, that can lead to another nova explosion — once again reviving a stellar zombie.

We'll have more about novae tomorrow.