Stars in a binary system may come so close together that matter can be exchanged between them. We term this an accreting binary. The artist's conception in the left frame shows an accreting binary at the center of the globular cluster NGC 6624, which is about 28,000 light years away in Sagittarius.
The star is a source of powerful X-ray bursts. One member of the binary is a neutron star (neutron stars are described in Chapter 22) and it has a less-massive white-dwarf star companion, seen at the lower left. Matter appears to be accreting from the white dwarf onto the neutron star.
Not all accreting binaries are this exotic, but mass accretion from one star onto another is a common and very important phenomenon in astronomy.