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Rare ultramassive stars that exceed this limit – for example in the R136 star cluster – might be explained by the following proposal: Some of the pairs of massive stars in close orbit in young, unstable multiple-star systems must, on rare occasions, collide and merge when certain unusual circumstances hold that make a collision possible. [3]
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
Medium-sized, low-mass stars like the Sun have a core region that is stable against convection, with a convection zone near the surface that mixes the outer layers. This results in a steady buildup of a helium-rich core, surrounded by a hydrogen-rich outer region. By contrast, cool, very low-mass stars (below 0.4 M ☉) are convective ...
List of the largest known stars in Andromeda and Triangulum galaxies Star name Solar radii (Sun = 1) Galaxy Method [a] Notes Theoretical limit of star size (Andromeda Galaxy) ≳1,750 [11] L/T eff: Estimated by measuring the fraction of red supergiants at higher luminosities in a large sample of stars. Assumes an effective temperature of 3,625 K.
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.
At the same time, hydrogen may begin fusion in a shell just outside the burning helium shell. This puts the star onto the asymptotic giant branch, a second red-giant phase. [15] The helium fusion results in the build-up of a carbon–oxygen core. A star below about 8 M ☉ will never start fusion in its degenerate carbon–oxygen core. [13]
Using the Very Large Telescope to study the motions of about 300 planetary nebulae, astronomers have determined that M87 absorbed a medium-sized star-forming spiral galaxy over the last billion years. This has resulted in the addition of some younger, bluer stars to M87.
Rigel, the brightest star in the constellation Orion is a typical blue-white supergiant; the three stars of Orion's Belt are all blue supergiants; Deneb is the brightest star in Cygnus, another blue supergiant; and Delta Cephei (itself the prototype) and Polaris are Cepheid variables and yellow supergiants.