Novas

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A star can be old and new at the same time. In fact, it can even be dead and new. The dead star can gain a bit of new life from a companion. That turns the star into a nova — a Latin word that means “new.”

The term was first used more than four centuries ago. At first, it applied to stars that suddenly became visible where no star had been seen before. Such stars were “new” to the night sky.

As astronomers developed the tools to study the stars in detail, though, they discovered that the stars weren’t new at all. Instead, they were dead stars that gained a brief bit of glory from a close companion.

The dead star in such a system is a white dwarf — the hot, dense core of a once-normal star. The white dwarf’s powerful gravity pulls hydrogen off the surface of its companion — usually a bloated star that soon will become a white dwarf as well.

Some of the gas piles up on the surface of the white dwarf. This new layer gets hotter and hotter. Eventually, it gets hot enough to explode like a giant H-bomb. That blasts the new material out into space. In a few hours, the system can flare to thousands of times its normal brightness — creating a nova.

The blast interrupts the flow of hydrogen from the companion. Before long, though, it starts again. And that can produce another explosion — usually thousands of years later. But some novas can pop off again in just a few decades. And we’ll talk about the best-known example tomorrow.

Script by Damond Benningfield

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