|From afar our galaxy looks like a flat pancake with prominent spiral arms. It contains about 100 billion stars, glowing hydrogen gas, and small, dense cold clouds which con-tain simple molecules and grains of dust. Most of the stars are similar to the sun but are too distant to be detected by existing radio telescopes. Strong radio signals in our galaxy do come from interacting binary star systems, huge clouds of gas, stars in their infancy or stars that explosively complete their evolution. Near the center of our galaxy there is something which emits a large amount of radio energy. Its nature is still unknown; it may be a black hole or a compact cluster of a million stars.
On a clear winter's night, the Orion Nebula can be seen just below Orion's belt. The red light is produced by a hydrogen cloud which is heated by four young stars within it. The cloud is shrinking under its own gravity, and it will continue to form stars over the next million years. Filaments and loops at the edge of the Nebula are caused by shock waves moving through the gas. These shocks are produced by strong winds from embryonic stars.
The radio photograph of the Nebula has a similar form to that of the optical photograph, because hot hydrogen gives off both light and radio waves. The colors represent the intensity of radio emission from red (brightest) to yellow, green, blue and black. From a comparison of the two images, astronomers can estimate the temperature in the cloud, its thickness, density, and dust content.