Although our solar system only has one star, most stars like our sun are not solitary, but are binaries where two stars orbit each other, or multiples involving even more stars. In fact, just one-third of stars like our sun are single, while two-thirds are multiples — for instance, the closest neighbor to our solar system, Proxima Centauri, is part of a multiple system that also includes Alpha Centauri A and Alpha Centauri B. Still, class G stars like our sun only make up some 7 percent of all stars we see — when it comes to systems in general, about 30 percent in our galaxy are multiple, while the rest are single, according to Charles J. Lada of the Harvard-Smithsonian Center for Astrophysics.
Binary stars develop when two protostars form near each other. One member of this pair can influence its companion if they are close enough together, stripping away matter in a process called mass transfer. If one of the members is a giant star that leaves behind a neutron star or a black hole, an X-ray binary can form, where matter pulled from the stellar remnant's companion can get extremely hot — more than 1 million F (555,500 C) and emit X-rays. If a binary includes a white dwarf, gas pulled from a companion onto the white dwarf's surface can fuse violently in a flash called a nova. At times, enough gas builds up for the dwarf to collapse, leading its carbon to fuse nearly instantly and the dwarf to explode in a Type I supernova, which can outshine a galaxy for a few months.
Brightness
Astronomers describe star brightness in terms of magnitude and luminosity.
The magnitude of a star is based on a scale more than 2,000 years old, devised by Greek astronomer Hipparchus around 125 BC. He numbered groups of stars based on their brightness as seen from Earth — the brightest ones were called first magnitude stars, the next brightest were second magnitude, and so on up to sixth magnitude, the faintest visible ones. Nowadays astronomers refer to a star's brightness as viewed from Earth as its apparent magnitude, but since the distance between Earth and the star can affect the light one sees from it, they now also describe the actual brightness of a star using the term absolute magnitude, which is defined by what its apparent magnitude would be if it were 10 parsecs or 32.6 light years from Earth. The magnitude scale now runs to more than six and less than one, even descending into negative numbers — the brightest star in the night sky is Sirius, with an apparent magnitude of -1.46.
Luminosity is the power of a star — the rate at which it emits energy. Although power is generally measured in watts — for instance, the sun's luminosity is 400 trillion trillion watts— the luminosity of a star is usually measured in terms of the luminosity of the sun. For example, Alpha Centauri A is about 1.3 times as luminous as the sun. To figure out luminosity from absolute magnitude, one must calculate that a difference of five on the absolute magnitude scale is equivalent to a factor of 100 on the luminosity scale — for instance, a star with an absolute magnitude of 1 is 100 times as luminous as a star with an absolute magnitude of 6.
The brightness of a star depends on its surface temperature and size.
Color
Stars come in a range of colors, from reddish to yellowish to blue. The color of a star depends on surface temperature.
A star might appear to have a single color, but actually emits a broad spectrum of colors, potentially including everything from radio waves and infrared rays to ultraviolet beams and gamma rays. Different elements or compounds absorb and emit different colors or wavelengths of light, and by studying a star's spectrum, one can divine what its composition might be.
Surface temperature
Astronomers measure star temperatures in a unit known as the kelvin, with a temperature of zero K ("absolute zero") equaling minus 273.15 degrees C, or minus 459.67 degrees F. A dark red star has a surface temperature of about 2,500 K (2,225 C and 4,040 F); a bright red star, about 3,500 K (3,225 C and 5,840 F); the sun and other yellow stars, about 5,500 K (5,225 C and 9,440 F); a blue star, about 10,000 K (9,725 C and 17,540 F) to 50,000 K (49,725 C and 89,540 F).
The surface temperature of a star depends in part on its mass and affects its brightness and color. Specifically, the luminosity of a star is proportional to temperature to the fourth power. For instance, if two stars are the same size but one is twice as hot as the other in kelvin, the former would be 16 times as luminous as the latter.
Size
Astronomers generally measure the size of stars in terms of the radius of our sun. For instance, Alpha Centauri A has a radius of 1.05 solar radii (the plural of radius). Stars range in size from neutron stars, which can be only 12 miles (20 kilometers) wide, to supergiants roughly 1,000 times the diameter of the sun.
The size of a star affects its brightness. Specifically, luminosity is proportional to radius squared. For instance, if two stars had the same temperature, if one star was twice as wide as the other one, the former would be four times as bright as the latter.
Mass
Astronomers represent the mass of a star in terms of the solar mass, the mass of our sun. For instance, Alpha Centauri A is 1.08 solar masses.
Stars with similar masses might not be similar in size because they have different densities. For instance, Sirius B is roughly the same mass as the sun, but is 90,000 times as dense, and so is only a fiftieth its diameter.
The mass of a star affects surface temperature.
Magnetic field
Stars are spinning balls of roiling, electrically charged gas, and thus typically generate magnetic fields. When it comes to the sun, researchers have discovered its magnetic field can become highly concentrated in small areas, creating features ranging from sunspots to spectacular eruptions known as flares and coronal mass ejections. A recent survey at the Harvard-Smithsonian Center for Astrophysics found that the average stellar magnetic field increaseswith the star's rate of rotation and decreases as the star ages.
Metallicity
The metallicity of a star measures the amount of "metals" it has — that is, any element heavier than helium.
Three generations of stars may exist based on metallicity. Astronomers have not yet discovered any of what should be the oldest generation, Population III stars born in a universe without "metals." When these stars died, they released heavy elements into the cosmos, which Population II stars incorporated relatively small amounts of. When a number of these died, they released more heavy elements, and the youngest Population I stars like our sun contain the largest amounts of heavy elements.