X-Ray Astronomy

Although the temperature of the sun's surface is about 6,000K (10,341°F; 5,727°C), by the 1930s there was evidence that the outer regions of the solar atmosphere were much hotter, meaning that they could be a source of x rays. At that time there was no way to verify this prediction, however. After World War II, when captured V-2 rockets allowed scientists to place instruments outside the protective atmosphere for the first time, a number of experiments were able to show that the Sun did indeed produce x rays.

The strongest early evidence came in 1948, when xray detectors registered x rays were coming from the direction of the Sun. Further investigations showed that the total x-ray output of the Sun was only a tiny fraction of the total energy generated. Because the total x-ray output was so small, despite the fact that the Sun is so close in terms of interstellar distances, many believed that no other sources would be found.

In 1962, a rocket was sent up to look for x rays from the Moon, which was theorized to generate x rays due to solar wind bombardment. No emission was detected from the Moon, but in a surprising discovery, the detector registered an x-ray source in the direction of the constellation Scorpio, along with a diffuse background coming from all directions; the source was called Scorpius X-1.

Since that time, a large number of rocket and Earth-orbiting satellites have discovered tens of thousands of x-ray sources in the sky, many of which are many orders of magnitude brighter than the Sun. The Crab Nebula, for instance, produces approximately 2,000 times more energy in the x-ray region of the spectrum than the Sun does over all wavelengths. Thus we now know that the Sun is relatively quiet as far as x-ray sources go.