Within Our Galaxy
Looking up into the night sky, you have a visible-light view of objects within our galaxy. Optical telescopes show us countless stars and wonderful, detailed images of nebulae. Look within our galaxy in the infrared, however, and we get a completely different view. Areas which appear dark and empty in visible light reveal bright molecular clouds in which new stars are being formed. Infrared astronomy has revealed disks of material around other stars in which planets may be forming, wisps of warm dust throughout the galaxy, vast numbers of cooler stars, and the core of the Milky Way. X-rays tell us about the hot outer atmospheres of stars and the final phases of a star's life. When a star explodes it ejects hot shells of gas which radiate strongly in X-rays. This makes the X-ray region of the spectrum a valuable place to learn about supernovae, neutron stars, and black holes. X-ray observations have also led to the discovery of a black hole at the center of the Milky Way. Radio waves also bring us information about supernovae and neutron stars. In addition, radio observations are used to map the distribution of hydrogen gas in our galaxy and to find the signatures of interstellar molecules.
Orion (the Hunter) may be a familiar constellation in the winter night sky, but these dramatically different images at various wavelengths are anything but familiar!
The X-ray photograph reveals Sun-like stars, white dwarf stars, neutron stars, and supernova remnants in our own Galaxy, and (in the background) nearby galaxies, clusters of galaxies, and distant quasars. The ultraviolet image is a close-up view of the belt/sword region of Orion, including the famous Orion Nebula, and is dominated by emission from hot, young stars. The visible light image shows stars of all ages and temperatures. In the infrared, our view of Orion is dominated by emission from clouds of dust and gas, the materials from which new stars will be born. The radio image maps the distribution of hydrogen molecules in the interstellar medium, with red showing the areas of highest concentration.
Staying within the Milky Way, let us now look at the famous Hercules globular star cluster (Messier 13).
This cluster contains hundreds of thousands of gravitationally bound stars, and yet can be seen (clearly) only in visible light! This cluster is primarily made up of older and cooler stars, while X-rays are typically seen only from the hottest stars. Infrared light is produced by dust, and there is no significant amount of dust in the cluster; hence, the cluster vanishes in the infrared. It is essentially invisible at radio wavelengths because of the lack of any strong magnetic fields within the cluster. For additional images of the Hercules Cluster at various wavelengths, visit our Multiwavelength Gallery.
Now let's take a multiwavelength look at Cassiopeia A, a supernova remnant. This shell of expanding gas and dust is the result of a supernova explosion which was visible from the Earth in the 17th century.
The X-ray image shows gaseous clumps of silicon, sulfur, and iron ejected from the exploded star. The gases in the X-ray image are at a temperature of about 50 million degrees. The visible light image reveals the wispy filaments of gas at the edge of the spherical shell. The infrared photo shows bright knots of thermal emission produced by dust mixed with the gas in the expanding shell. The radio emission is primarily radiation generated by fast-moving electrons immersed in a magnetic field. For additional images of Cassiopeia A at various wavelengths, visit our Multiwavelength Gallery.
Finally, lets lets see how a supernova remnant, the Crab Nebula, appears at different wavelengths.
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