How does a star take mass from another star?
I heard that stars can take energy from one another, or that a larger star can take something away from a smaller star. Do you know what this is? Do you have any examples?
Stars in binary systems (two stars orbiting each other) can exchange mass. One of the stars (the donor) expands enough that some of its mass falls onto its companion (the acceptor). At this point, one of several things can happen. If the mass is transferred too fast, the acceptor star will not be able to accept it all, and some of the mass will form a cloud around both stars. Friction with this cloud will cause the stars to fall in toward each other, and the cloud will be kicked off into space.
If the mass transfer is slow enough for the acceptor (which is easier if the acceptor is a compact object like a neutron star or white dwarf), the matter will spiral into the acceptor in a flat disk, heating up and emitting X-rays as it goes (that is, unless the acceptor has a strong magnetic field, in which case the matter will fall directly onto the magnetic poles).
If the acceptor happens to be a white dwarf, which is a star that finished its life without enough mass to explode as a supernova, the mass falling on it may put it over the limit, and then it will explode. This is known as a Type Ia supernova. (There are also Type Ib and Ic supernovae, as well as Type II and III and subclasses thereof, but these are all thought to result from the collapse of a single massive star. They are classified by spectral characteristics and the behavior of their luminosity as a function of time.). Type Ia supernovae may also occur when two white dwarves collide, but this should be more rare than the scenario described above.
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