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Brightness
Brightness is also referred to as a star's magnitude or luminosity. There are seven levels of brightness that stars are classified as, with first magnitude stars being the brightest. Second magnitude stars are the second brightest, and the pattern continues up to the sixth magnitude. Instead of a seventh magnitude, the faintest stars are called visible stars.
Color and Temperature
These two characteristics are discussed together because a star's surface temperature determines its color. Stars actually emit several colors, but they appear as various shades of red, yellow or blue when viewed with most conventional equipment.
A star's temperature is measured on the Kelvin scale. One kelvin (K) equals one Celsius degree, but the temperature scales begin at different points. For example, 0 degrees Celsius equals 273 K. Red stars have a temperature of around 2500 K. Yellow stars, like the sun, have a temperature of around 5500 K. Blue stars have a temperature of more than 10,000 K.
Size
Astronomers use the size of Earth's sun to describe the size of stars. The unit is known as a solar radii. The star known as Antares is 776 times the size of Earth's sun, and its size is described as 776 solar radii. The size of a star has an impact on its brightness. If two stars have the same temperature but one is four times as large as the other, the larger star will be eight times as bright.
Mass
To describe a star's mass, astronomers again use Earth's sun as a guide. The unit is known as a solar mass. Considering the mass of the Earth's sun in kilograms would be written as 2 times 10 to the 30th power (2 followed by 30 zeroes), a smaller unit just makes things easier. A star with twice as much mas as Earth's sun would 2 solar masses. The size and mass of a star are not necessarily related because densities between the stars can vary.