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A multiple star system consists of two or more stars that appear from Earth to be close to one another in the sky. [dubious – discuss] This may result from the stars actually being physically close and gravitationally bound to each other, in which case it is a physical multiple star, or this closeness may be merely apparent, in which case it is an optical multiple star [a] Physical multiple ...
Main-sequence stars vary in surface temperature from approximately 2,000 to 50,000 K, whereas more-evolved stars – in particular, newly-formed white dwarfs – can have surface temperatures above 100,000 K. [3] Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest.
K-type main-sequence stars are about three to four times as abundant as G-type main-sequence stars, making planet searches easier. [17] K-type stars emit less total ultraviolet and other ionizing radiation than G-type stars like the Sun (which can damage DNA and thus hamper the emergence of nucleic acid based life). In fact, many peak in the red.
A B-type main-sequence star (B V) is a main-sequence (hydrogen-burning) star of spectral type B and luminosity class V. These stars have from 2 to 16 times the mass of the Sun and surface temperatures between 10,000 and 30,000 K. [1] B-type stars are extremely luminous and blue.
Ordinary stars are composed mainly of the light elements hydrogen and helium.They also contain a small proportion of heavier elements, and this fraction is referred to as a star's metallicity (even if the elements are not metals in the traditional sense), [3] denoted [m/H] and expressed on a logarithmic scale where zero is the Sun's metallicity.
Morgan-Keenan spectral classification of stars. Most common star type in the universe are M-dwarfs, 76%. The Sun is a 4.6 billion year-old G-class (G2V) star and is more massive than 95% of all stars. Only 7.6% are G-class stars. The stars below are more similar to the Sun and having the following qualities: [1]
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha process) and by hydrogen fusion (via the CNO cycle) in a shell surrounding the core.
However, very massive stars can lose 10 −7 to 10 −5 M ☉ each year, significantly affecting their evolution. [79] Stars that begin with more than 50 M ☉ can lose over half their total mass while on the main sequence. [80] An example of a Hertzsprung–Russell diagram for a set of stars that includes the Sun (center) (see Classification)