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Moon and Spica

May 24, 2010

The bright star Spica huddles close to the Moon tonight. It's above the Moon as night falls. There are no other bright stars or planets around it, so Spica is easy to pick out.

Spica is 260 light-years away, which means that the light you see from Spica tonight actually left the star 260 years ago. During that time, the light crossed more than a million billion miles of space. But it took far more time for the light to cross the first few million miles -- the distance from the star's core to its surface: up to several million years.

A star creates its energy through nuclear reactions in its core -- it combines lighter-weight elements to make heavier ones. Spica, for example, is fusing together hydrogen to make helium. Each reaction produces a gamma ray -- the most powerful form of energy.

But the gamma ray doesn't just zip out into space. Spica is several times bigger than the Sun, so it's about a three-and-a-half-million-mile journey from the core to the surface, through dense layers of gas.

So as the gamma ray heads outward, it quickly rams into an atom of gas, where it's absorbed, then re-radiated. As it moves outward, energy goes through this process over and over again. As it gets closer to the surface, the energy level drops -- to X-rays, then to ultraviolet and visible light. Spica's surface is quite hot, so it emits a mixture of ultraviolet and visible light -- which then races into space at the speed of light.

Script by Damond Benningfield, Copyright 2010

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