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Basic Astronomy Concepts


Cosmic Background Explorer, radio region, Wilkinson Microwave Anisotropy Probe, WMAP, sky map

Deeper web pages:

>  Early History of the Study of Galaxies

>  Classification of Galaxies

>  Determination of Extragalactic Distances

>  Distribution of Galaxies

>  Rotation of Spiral Galaxies

Galaxy, a massive ensemble of hundreds of millions of stars, all gravitationally interacting, and orbiting about a common center. Astronomers estimate that there are about 125 billion galaxies in the universe. All the stars visible to the unaided eye from Earth belong to Earthís galaxy, the Milky Way. The Sun, with its associated planets, is just one star in this galaxy. Besides stars and planets, galaxies contain clusters of stars; atomic hydrogen gas; molecular hydrogen; complex molecules composed of hydrogen, nitrogen, carbon, and silicon, among others; and cosmic rays.

Radiation from a Galaxy

Knowledge of the appearance of a galaxy is based on optical observations. Knowledge of the composition and motions of the individual stars comes from spectral studies in the optical region also. Because the hydrogen gas in the spiral arms of a galaxy radiates in the radio portion of the electromagnetic spectrum, many details of galactic structure are learned from studies in the radio region. The warm dust in the nucleus and spiral arms of a galaxy radiates in the infrared portion of the spectrum. Some galaxies radiate more energy in the optical region.

Recent X-ray observations have confirmed that galactic halos contain hot gas, gas with temperatures of millions of degrees. X-ray emission is also observed from objects as varied as globular clusters, supernova remnants, and hot gas in clusters of galaxies. Observations in the ultraviolet region also reveal the properties of the gas in the halo, as well as details of the evolution of young stars in galaxies.

Origins of Galaxies

As the 21st century began, astronomers believed they were much closer to understanding the origins of galaxies. Observations made by the Cosmic Background Explorer (COBE) satellite, which was launched in 1989, confirmed predictions made by the big bang theory of the universeís origin. COBE also detected small irregularities, or ripples, in the background radiation that uniformly pervades the universe. These ripples were thought to be clumps of matter that formed soon after the big bang. The clumps became the seeds from which galaxies and clusters of galaxies developed. The ripples were studied in more detail in limited regions of the sky by a variety of ground-based and balloon-based experiments. A more recent spacecraft, NASAís Wilkinson Microwave Anisotropy Probe (WMAP), made even more accurate observations of these ripples across the entire sky. In 2003 WMAPís results confirmed the existence of these galactic seeds, providing a full-sky map of the universeís emerging galaxies.


Rubin, Vera C., M.A., Ph.D.

Staff Member, Department of Terrestrial Magnetism, Carnegie Institution of Washington. Associate Editor, "Astrophysical Journal Letters". Member, Editorial Board, "Science".

Article key phrases:

Cosmic Background Explorer, radio region, Wilkinson Microwave Anisotropy Probe, WMAP, sky map, supernova remnants, globular clusters, molecular hydrogen, COBE, astronomers, electromagnetic spectrum, background radiation, spiral arms, cosmic rays, hydrogen gas, hot gas, big bang theory, galaxy, Earth, galaxies, ripples, universe, nucleus, Milky Way, nitrogen, NASA, motions, composition, Radiation, silicon, predictions, existence, carbon, satellite, degrees, Sun, temperatures, century, properties, objects, appearance, Knowledge, results, energy, details

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