• One method for determining the distance to a star is to use a measurement called stellar parallax, the extremely slight back-and-forth shifting in a nearby star's position due to the orbital motion of Earth. The farther away a star is, the less its parallax. A unit used to express stellar distance is the light-year, which is the distance light travels in a year–about 9.5 trillion kilometers (5.8 trillion miles).

  

• The intrinsic properties of stars include brightness, color, temperature, mass, and size. Three factors control the brightness of a star as seen from Earth: how big it is, how hot it is, and how far away it is. Magnitude is the measure of a star's brightness. Apparent magnitude is how bright a star appears when viewed from Earth. Absolute magnitude is the "true" brightness if a star were at a standard distance of about 32.6 light-years. The difference between the two magnitudes is directly related to a star's distance. Color is a manifestation of a star's temperature. Very hot stars (surface temperatures above 30,000 K) appear blue; red stars are much cooler (surface temperatures generally less than 3000 K). Stars with surface temperatures between 5000 and 6000 K appear yellow, like the sun. The center of mass of orbiting binary stars (two stars revolving around a common center of mass under their mutual gravitational attraction) is used to determine the mass of the individual stars in a binary system.

• A Hertzsprung-Russell diagram is constructed by plotting the absolute magnitudes and temperatures of stars on a graph. A great deal about the sizes of stars can be learned from H-R diagrams. Stars located in the upper-right position of an H-R diagram are called giants, luminous stars of large radius. Supergiants are very large. Very small white dwarf stars are located in the lower-central portion of an H-R diagram. Ninety percent of all stars, called main-sequence stars, are in a band that runs from the upper-left corner to the lower-right corner of an H-R diagram.

  

• New stars are born out of enormous accumulations of dust and gases, called nebula, that are scattered between existing stars. A bright nebula glows because the matter is close to a very hot (blue) star. The two main types of bright nebulae are emission nebulae (which derive their visible light from the fluorescence of the ultraviolet light from a star in or near the nebula) and reflection nebulae (relatively dense dust clouds in interstellar space that are illuminated by reflecting the light of nearby stars). When a nebula is not close enough to a bright star to be illuminated, it is referred to as a dark nebula.

• Stars are born when their nuclear furnaces are ignited by the unimaginable pressures and temperatures in collapsing nebulae. New stars not yet hot enough for nuclear fusion are called protostars. When collapse causes the core of a protostar to reach a temperature of at least 10 million K, the fusion of hydrogen nuclei into helium nuclei begins in a process called hydrogen burning. The opposing forces acting on a star are gravity trying to contract it and gas pressure (thermal nuclear energy) trying to expand it. When the two forces are balanced, the star becomes a stable main-sequence star. When the hydrogen in a star's core is consumed, its outer envelope expands enormously and a red giant star, hundreds to thousands of times larger than its main-sequence size, forms. When all the usable nuclear fuel in these giants is exhausted and gravity takes over, the stellar remnant collapses into a small dense body.

• The final fate of a star is determined by its mass. Stars with less than one-half the mass of the sun collapse into hot, dense white dwarf stars. Medium mass stars (between 0.5 and 3.0 times the mass of the sun) become red giants, collapse, and end up as white dwarf stars, often surrounded by expanding spherical clouds of glowing gas called planetary nebulae. Stars more than three times the mass of the sun terminate in a brilliant explosion called a supernova. Supernovae events can produce small, extremely dense neutron stars, composed entirely of subatomic particles called neutrons; or even smaller and more dense black holes, objects that have such immense gravity that light cannot escape their surface.

  

• The Milky Way galaxy is a large, disk-shaped, spiral galaxy about 100,000 light-years wide and about 10,000 light-years thick at the center. There are three distinct spiral arms of stars, with some showing splintering. The sun is positioned in one of these arms about two-thirds of the way from the galactic center, at a distance of about 30,000 light-years. Surrounding the galactic disk is a nearly spherical halo made of very tenuous gas and numerous globular clusters (nearly spherically shaped groups of densely packed stars).

• The various types of galaxies include 1) irregular galaxies, which lack symmetry and account for only 10 percent of the known galaxies; 2) spiral galaxies, which are typically disk-shaped with a somewhat greater concentration of stars near their centers, often containing arms of stars extending from their central nucleus; and 3) elliptical galaxies, the most abundant type, which have an ellipsoidal shape that ranges to nearly spherical, and lack spiral arms.

  

• By applying the Doppler effect (the apparent change in wavelength of radiation caused by the motions of the source and the observer) to the light of galaxies, galactic motion can be determined. Most galaxies have Doppler shifts toward the red end of the spectrum, indicating increasing distance. The amount of Doppler shift is dependent on the velocity at which the object is moving. Because the most distant galaxies have the greatest red shifts, Edwin Hubble concluded in the early 1900s that they were retreating from us with greater recessional velocities than more nearby galaxies. It was soon realized that an expanding universe can adequately account for the observed red shifts.

• The belief in the expanding universe led to the widely accepted Big Bang theory. According to this theory, the entire universe was at one time confined in a dense, hot, supermassive concentration. About 20 billion years ago, a cataclysmic explosion hurled this material in all directions, creating all matter and space. Eventually the ejected masses of gas cooled and condensed, forming the stellar systems we now observe fleeing from their place of origin.

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