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SUMMARY OF KEY IDEAS

Stars differ in size, luminosity, temperature, color, mass, and chemical composition—facts that help astronomers understand stellar structure and evolution.

Magnitude Scales

Determining stellar distances from Earth is the first step to understanding the nature of the stars. Distances to the nearer stars can be determined by stellar parallax, which is the apparent shift of a star’s location against the background stars while Earth moves along its orbit around the Sun. The apparent magnitude of a star, denoted m, is a measure of how bright the star appears to Earth-based observers. The absolute magnitude of a star, denoted M, is a measure of the star’s true brightness and is directly related to the star’s energy output, or luminosity.
The luminosity of a star is the amount of energy it emits each second.
The absolute magnitude of a star is the apparent magnitude it would have if viewed from a distance of 10 pc (32.6 ly). Absolute magnitudes can be calculated from the star’s apparent magnitude and distance from Earth.

The Temperatures of Stars

Stellar temperatures can be determined from stars’ colors or stellar spectra.
Stars are classified into spectral types (O, B, A, F, G, K, and M) based on their spectra or, equivalently, their surface temperatures.

Types of Stars

The Hertzsprung-Russell (H-R) diagram is a graph on which luminosities of stars are plotted against their spectral types (or, equivalently, their absolute magnitudes are plotted against surface temperatures).
The H-R diagram reveals the existence of four major groupings of stars: main-sequence stars, giants, supergiants, and white dwarfs.
The mass–luminosity relationship expresses a direct correlation between a main-sequence star’s mass and the total energy it emits.
Distances to stars can be determined using their spectral types and luminosity classes, a method called spectroscopic parallax.

Stellar Masses

Binary stars are fairly common. Those that can be resolved into two distinct star images (even if it takes a telescope to do this) are called visual binaries.
The masses of the two stars in a binary system can be computed from measurements of the orbital period and orbital dimensions of the system.
Some binaries can be detected and analyzed, even though the system may be so distant (or the two stars so close together) that the two star images cannot be resolved with a telescope.
A spectroscopic binary is a system detected from the periodic shift of its spectral lines. This shift is caused by the Doppler effect as the orbits of the stars carry them alternately toward and away from Earth.
An eclipsing binary is a system whose orbits are viewed nearly edge-on from Earth, so that one star periodically eclipses the other. Detailed information about the stars in an eclipsing binary can be obtained by studying the binary’s light curve.

How near to us is the closest star other than the Sun? The closest star, Proxima Centauri, is about 40 trillion km (25 trillion mi) away. Light from there takes about 4 years to reach Earth.
How luminous is the Sun compared with other stars? The most luminous stars are about a million times brighter, and the least luminous stars are about a hundred thousand times dimmer than the Sun.
What colors are stars, and why do they have these colors? Stars are found in a wide range of colors, from red through violet as well as white. They have these colors because they have different surface temperatures.
Are brighter stars hotter than dimmer stars? Not necessarily. Many brighter stars (such as red giants) are cooler but larger than hotter, dimmer stars (such as white dwarfs).
Compared to the Sun, what sizes are other stars? Stars range from more than 1000 times the Sun’s diameter to less than 1/100 of the Sun’s diameter.
Are most stars isolated from other stars, as the Sun is? Yes. In the vicinity of the Sun, one-third of stars are found in gravitationally bound pairs or larger groups.