When a pulsar, a rapidly-rotating neutron star, finds itself in a tight orbit with another star, it will siphon mass from it, leading to electromagnetic emissions from the pulsar-star system. When the star is low in mass, forming an X-ray binary, the pulsar will heat and slowly evaporate the companion star, leading to the creation of “black widow” systems when the companion star loses enough mass. (Credit: ESA)
In astronomy, a star’s initial mass determines its ultimate outcome in life. Unless, that is, a stellar companion alters the deal.
When isolated stars form, their fates are pre-determined.
This region of space shows a portion of the plane of the Milky Way, with three extended star-forming regions all side-by-side next to one another. The Omega Nebula (left), the Eagle Nebula (center), and Sharpless 2–54 (right), compose just a small fraction of a vast complex of gas and dust found all through the galactic plane that continuously lead to the formation of newborn stars. (Credit: European Southern Observatory)
Supernovae types as a function of initial star mass and initial content of elements heavier than Helium (metallicity). Note that the first stars occupy the bottom row of the chart, being metal-free, and that the black areas correspond to direct collapse black holes. For modern stars, we are uncertain as to whether the supernovae that create neutron stars are fundamentally the same or different than the ones that create black holes, and whether there is a ‘mass gap’ present between them in nature. We must also consider that effects other than mass and metallicity (such as the presence of a companion) may indeed play major roles in determining the fate of massive stars, including in whether they can contribute to enriching the interstellar medium. (Credit: Fulvio314 / Wikimedia Commons)
Below 7.5% of the Sun’s mass, you’re only a failed star: a brown dwarf.
The exoplanet Kepler-39b is one of the most massive ones known, at 18 times the mass of Jupiter, placing it right on the border between planet and brown dwarf. In terms of radius, however, it’s only 22% larger than Jupiter, as deuterium fusion doesn’t substantially change the self-compressed object’s size. Objects up to ~80 times the mass of Jupiter are still approximately the same size, with only higher-mass objects initiating nuclear fusion in their cores and becoming true stars. (Credit: ESO)
Above that but below 0.4 solar masses, you’re a red dwarf.
Energy produced in a star’s core must pass through large amounts of ionized material before reaching the photosphere, where it’s radiated away. Inside the Sun, there’s a large, non-convective radiative zone surrounding the core, but in lower-mass stars such as red dwarfs, the entire star can convect on timescales of tens or hundreds of billions of years (or longer), enabling red dwarf stars to fuse 100% of the hydrogen within them. Red dwarfs cannot fuse heavier elements than hydrogen, so when all their hydrogen has fused, they simply contract down to a helium white dwarf. (Credit: APS/Alan Stonebraker)
Fully convective, its ultimate fate is a helium white dwarf.
