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Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution
Given the length of the main sequence in G-type stars, [26] the levels of ultraviolet radiation in their habitable zone, [4] the semi-major axis of the inner boundary of this region [19] and the distance to their tidal locking limit, [27] among other factors, yellow dwarfs are considered to be the most hospitable to life next to K-type stars. [1]
However, when one can observe a red giant star with a known mass, one can calculate the main-sequence lifetime, [4] and thus the minimum age of star is known given that it is in an advanced stage of its evolution. As the star spends only about 1% of its total lifetime as a red giant, [5] this is an accurate method of determining age.
The observed upper limit for a main-sequence star is 120–200 M ☉. [34] The theoretical explanation for this limit is that stars above this mass can not radiate energy fast enough to remain stable, so any additional mass will be ejected in a series of pulsations until the star reaches a stable limit. [35]
Indeed, F0 stars (7,400 K, 1.6 M ☉︎, 1.7 R ☉︎, ~7 L ☉︎) are considered by many scientists as the hottest and most massive stars capable of supporting habitable planets. A planet orbiting an F-type star at the Earth boundary within the HZ would receive 2.5 (F9 star) to 7.1 (F0 star) times the UV that Earth gets from the sun.
A star system 3,000 light-years away from Earth is predicted to become visible to the naked eye this year — likely a once-in-a-lifetime viewing opportunity, as the phenomenon only occurs roughly ...
For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 M J. [13] [14] When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 M J.
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