A self-luminous body that during its life generates (or will generate) energy and support by thermonuclear fusion.
Names
Over 6000 stars can be seen with the unaided eye. The brightest carry proper names from ancient times, most of Arabic origin. A more general system names stars within constellations by Greek letters roughly in accord with apparent brightness, followed by the Latin genitive of the constellation name (for example, Vega, in Lyra, is also Alpha Lyrae). More generally yet, brighter stars carry numbers in easterly order within a constellation (Vega also 3 Lyrae). All naked-eye stars also have HR (Harvard Revised) numbers assigned in order east of the vernal equinox. A variety of catalogs list millions of telescopic stars. See also Astronomical catalogs; Constellation.
Magnitudes and colors
About 130 B.C., Hipparchus assigned naked-eye stars to six brightness groups or “apparent magnitudes” (m), with first magnitude the brightest. This scheme is now quantified as a logarithmic system such that five magnitudes correspond to a factor of 100 in brightness, rendering first magnitude 2.512··· times brighter than second, and so on.
Stars assume subtle colors from red to blue-white, reflecting different spectral energy distributions that result from temperatures ranging from under 2000 K (3100°F) to over 100,000 K (180,000°F). The magnitude of a star therefore depends on the detector's color sensitivity. Numerous magnitude systems range from the ultraviolet into the infrared, though the apparent visual magnitude (mv = V) is still standard. The differences among the systems allow measures of stellar color and temperature. See also Color index; Magnitude (astronomy).
Distances
The fundamental means of finding stellar distances is parallax. As the Earth moves in orbit around the Sun, a nearby star will appear to shift its location against the background. The parallax (p in arcseconds) is defined as one half the total shift, and is the angle subtended by the Earth's orbital radius as seen from the star. Distance in parsecs (pc) is 1/pc, where 1 pc = 206,265 AU = 3.26 light-years. (A light-year is the distance that a ray of light will travel in a year.) The nearest star, a telescopic companion to Alpha Centauri, is 1.29 pc = 4.22 light-years away. See also Light-year; Parallax (astronomy); Parsec.
Distribution and motions
All the unaided-eye stars and 200 billion more are collected into the Milky Way Galaxy (or simply, the Galaxy), 98% concentrated into a thin disk over 100,000 light-years across. From the Sun, inside the disk and 27,000 light-years from the Galaxy's center in Sagittarius, the disk appears as the Milky Way. Surrounding the disk is a vast but sparsely populated halo.
Angular proper motions across the line depend on velocities across the line of sight and distances. Statistical analysis of these motions shows the Sun to be moving through the local stars at a speed of 20 km/s (12 mi/s), roughly toward Vega. From radial velocities of sources outside the Galaxy, it is found that the Sun moves in a roughly circular orbit at 220 km/s (137 mi/s), which when combined with the space motions of other stars allows their galactic orbits to be determined. Stars in the disk have closely circular orbits; those in the halo have elliptical ones. See also Doppler effect; Milky Way Galaxy.
Absolute magnitudes
The apparent visual magnitude of a star depends on its intrinsic visual luminosity and on the inverse square of the distance. Knowledge of the distance allows the determination of the true visual luminosity, expressed as the absolute visual magnitude, Mv. This quantity is defined as the apparent visual magnitude that the star would have at a distance of 10 pc. Absolute visual magnitudes range from Mv = −10 to +20 (a factor of 1012). The Sun's absolute visual magnitude is in the middle of this range, +4.83.
Spectral classes
Stars exhibit a variety of absorption-line spectra. The absorptions, narrow cuts in the spectra, are produced by atoms, ions, and molecules in the stars' thin, semitransparent outer layers, or atmospheres. Over a century ago, Edward C. Pickering lettered them according to the strengths of their hydrogen lines. After he and his assistants dropped some letters and rearranged others for greater continuity, they arrived at the standard spectral sequence, OBAFGKM, which was later decimalized (the Sun is in class G2). Classes L and T were added in 1999. Since the sequence correlates with color, it must also correlate with temperature, which ranges from 55,000 K (100,000°F) for hot class O, through about 6000 K (10,000°F) for solar-type class G, to under 2000 K (3100°F) for class L, and below 1000 K (1300°F) for class T (all of which are brown dwarf substars).
The majority of stars in the galactic disk have chemical compositions like that of the Sun. Differences in spectra result primarily from changes in molecular and ionic composition and in the efficiencies of absorption, all of which correlate with temperature. See also Astronomical spectroscopy; Spectral type.
Hertzsprung-Russell diagram
Shortly after the invention of the spectral sequence, H. N. Russell and E. Hertzsprung showed that luminosity correlates with spectral class. A graph of absolute visual (or bolometric) magnitude (luminosity increasing upward) plotted against spectral class (or temperature, decreasing toward the right) is called the Hertzsprung-Russell (HR) diagram (see illustration). The majority of stars lie in a band in which luminosity climbs up and to the left with temperature (with the Sun in the middle). But another band begins near the location of the Sun and proceeds up and to the right (to lower temperature), luminosities increasing to thousands solar as temperature drops to class M. To be bright and cool, such stars must have large radiating areas and radii. To distinguish between the bands, the larger stars are called giants, while those of the main band are termed dwarfs (or main-sequence stars). Yet brighter stars to the cool side of the main sequence, with luminosities approaching 106 solar, are called supergiants. In between the giants and the dwarfs lie a few subgiants. At the top, superior to the supergiants, are the very rare hypergiants. See also Dwarf star; Giant star; Subgiant star; Supergiant star.
Hertzsprung-Russell (HR) diagram, showing the positions of the major kinds of stars.
In the lower left corner of the diagram, beneath the main sequence, are stars so dim that they must be very small. Since the first ones found were hot and white, they became known as white dwarfs in spite of their actual temperatures or colors. White dwarfs must be positioned on the HR diagram according to their temperatures (rather than their spectral classes). See also White dwarf star.
Stars in the galactic halo are deficient in heavy elements. Low metal content makes halo dwarfs bluer than those of the standard main sequence, shifting them to the left and seemingly downward on the HR diagram, where they are known as subdwarfs. See also Stellar population.
Double and multiple stars
Most stars are members of some sort of community, from doubles through multiples (double-doubles, and so forth) to clusters, which themselves contain doubles. Separations between components of double stars range from thousands of astronomical units (with orbital periods of a million years) to stars that touch each other (and orbit in hours). See also Binary star.
Masses and main-sequence properties
Observations of hundreds of binary stars show that mass (M) increases upward along the main sequence from about 8% solar at cool class M to over 20 solar in the cooler end of class O. Extrapolation by theory suggests masses of 120 solar at the extreme hot end (class O3) of the main sequence. Fainter than the Sun, luminosity is proportional to M3; brighter than solar, luminosity goes as M4. This mass-luminosity relation is the result of higher internal temperatures and pressures in more massive stars caused by gravitational compression. See also Mass-luminosity relation.
As internal temperature climbs above the 8 × 106 K limit, hydrogen fuses to helium via the proton-proton cycle at an ever-increasing rate. Above about 15 × 106 K (27 × 106 °F), so does fusion by the carbon cycle, in which carbon acts as a nuclear catalyst. See also Carbon-nitrogen-oxygen cycles.
The onset of carbon-cycle dominance coincides with a change in stellar structure. The Sun has a radiatively stable core surrounded by an envelope whose outer parts are in a state of convection that helps produce a magnetic field and magnetic sunspots. Hotter dwarfs have shallower convection layers, and above class F, envelope convection disappears, the cores becoming convective. The convective layers of cooler dwarfs deepen, until below about 0.3 solar mass (class M4), convection takes over completely and the stars are thoroughly mixed. See also Convection (heat).
Below a lower mass limit of 8% solar, internal temperatures and densities are not great enough to allow the proton-proton chain to operate. Such stars, called brown dwarfs, glow dimly and redly from gravitational contraction and from the fusion of natural deuterium (2H) into helium. See also Brown dwarf.
Clusters
Doubles and multiples are highly structured. Clusters are not, the member stars orbiting a common center of mass. Open clusters are fairly small collections in which a few hundred or a thousand stars are scattered across a few tens of light-years. About 150 globular clusters occupy the Galaxy's halo, the poorest about as good as a rich open cluster, the best containing over a million stars within a volume 100 light-years across.
The HR diagrams of clusters are radically different from the HR diagrams of the general stellar field. Those of open clusters differ among themselves in having various portions of the upper main sequence removed. The effect is related to the cluster's age, since high-mass stars die first. Globular clusters, which lack an upper main sequence and are therefore all old, contain a distinctive “horizontal branch” composed of modest giants.
Variable stars
Dwarfs are generally stable. Giants and supergiants, however, can have structures that allow them to pulsate. Cepheids (named after the star Delta Cephei) are F and G supergiants and bright giants that occupy a somewhat vertical instability strip in the middle of the HR diagram. They vary regularly by about a magnitude over periods ranging from 1 to 50 days. The pulsation is driven by a deep layer of gas in which helium is becoming ionized. Larger and more luminous Cepheids take longer to pulsate. Once this period-luminosity relationship is calibrated through parallax and main-sequence cluster fitting, the period of a Cepheid gives its absolute magnitude, which in turn gives its distance. Cepheids are vital in finding distances to other galaxies. See also Cepheids; Hubble constant.
Miras (after the star Mira, Omicron Ceti), or long-period variables, are luminous giants that can vary visually by more than 10 magnitudes over periods that range from 50 to 1000 days, the pulsations again driven by deep ionization layers. See also Mira.
Duplicity produces its own set of intrinsic variations. If the members of a binary system are close enough together, one of them can transfer mass to the other, and instabilities in the transfer process cause the binary to flicker. If one companion is a white dwarf, infalling compressed hydrogen can erupt in a thermonuclear runaway, producing a sudden nova that can reach absolute magnitude −10. If the white dwarf gains enough matter such that it exceeds its allowed limit (the Chandrasekhar limit) of 1.4 solar masses, it can even explode as a supernova. See also Cataclysmic variable; Nova; Variable star.
Off the HR diagram
Various kinds of stars are not placeable on the classical HR diagram. The most common examples are the central stars of planetary nebulae, which are complex shells and rings of ionized gas that surround hot blue stars. See also Planetary nebula.
Even hotter neutron stars are found associated with the exploded remains of supernovae (supernova remnants). Only 25 or so kilometers (15 mi) across, the visible ones spin rapidly, and are highly magnetic, with fields 1012 times the strength of Earth's field. Radiation beamed from tilted, wobbling magnetic axes can strike the Earth to produce seeming pulses of radiation. See also Neutron star; Pulsar.
Evolution
The different kinds of stars can be linked through theories of stellar evolution. Stars are born by the gravitational collapse of dense knots within cold dusty molecular clouds found only in the Galaxy's disk. When the new stars are hot enough inside to initiate fusion, their contraction halts and they settle onto the main sequence. The higher the stellar mass, the greater the internal compression and temperature, and the more luminous the star. But the higher the internal temperature, the greater the rate at which hydrogen is fused, and the shorter the star's life. See also Molecular cloud; Protostar; Stellar evolution.
