Different Light Waves, Same Object
Astrophysicists learn different things about cosmic phenomena by studying them at different wavelengths of light. As shown here with the supernova remnant known as Cassiopeia A, the energy of the light emitted by various parts of the remnant reveals the physical processes occurring there, not all of which are apparent in a familiar optical image. In the x-ray image at right, for example, one of the first images from the Chandra X-ray Observatory, scientists discern two shock waves. Cas A was named in 1948 by radio astronomers who "rediscovered" it as the strongest radio source in the constellation of Cassiopeia. Optical astronomers later found faint wisps at the same location (below) and determined that Cas A is the remnant of an explosion that occurred about 300 years ago.
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X-ray
Visible here are shock waves that arise as ejected material from the supernova explosion plows into space and encounters bands of gas and dust. The bright point near the center may be the neutron star or hot material around the black hole remaining after the explosion that produced Cas A. |
Optical
Matter with a temperature of about 10,000 degrees shows up at visible wavelengths. Some of these wisps are concentrations of heavy elements, dense clumps of ejected stellar material.
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Infrared
Dust grains that have been swept up and heated to several hundred degrees by the expanding hot gas emit infrared radiation. |
Radio
Long-wavelength radio emission comes from high-energy electrons moving in large spirals around magnetic field lines of force. |
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and explosions, including the solar flares that periodically erupt at active regions on the Sun. They also signal the presence of energetic but invisible objects such as neutron stars or black holes. Gas that wanders near one of these compact objects spirals into a whirling disk. Intense frictional forces make temperatures within the disk soar to millions of degrees as the gas funnels into the object. The hot gas radiates x-rays into space before it strikes the surface of the neutron star or, in the case of the black hole, vanishes from sight.
X-ray astronomy also reveals more mass in the universe than the stars alone expose. X-rays flow copiously from clusters of hundreds of galaxies. The photons come not from the galaxies themselves but from diffuse gas that fills and cocoons the clusters. This blazingly hot gas, up to 100 million degrees, exists within most thickly populated clusters of galaxies. The gas weighs about as much as all the stars in the universe combined.
Ultraviolet (UV) light lies just beyond the range of our eyes' sensitivity, although honeybees and many other insects see it quite well. Even though we can't see UV light, we encounter it every day as the most energetic light that reaches Earth's surface from the Sun. The ozone layer in the atmosphere absorbs most UV light, but enough gets through to cook any skin that's left unprotected by sunscreen. This type of light also breaks down molecules in plants and animals. Dwindling levels of ozone over parts of the globe may cause higher rates of skin cancer and other harmful biological effects.
The hottest stars, young monsters called blue giants, shine most energetically in UV light. These short-lived stars blaze thousands of times more luminously than our Sun. When blue giants clump closely together in space, they betray regions of vigorous star formation in galaxies. The Orion Nebula is one such hotbed of starbirth. Blue giants often remain together in beautiful clusters until they consume their nuclear fuel within a few hundred million years. The most obvious such cluster is the Pleiades, a lovely grouping of hundreds of stars that we also call the Seven Sisters.
Visible light falls within a narrow band, as we have noted. Within that band the colors that compose "white" light also have a range of energies. From least to most energetic, we know the colors as red, orange, yellow, green, blue, indigo, and violet. Students still learn a classic acronym for the order of the colors: "Roy G. Biv." However, you won't find indigo in most crayon boxes. |
