Accretion powers the brightest stars

Abstract

The brightest known stars in external galaxies are the Hubble–Sandage variables. These are normally assumed to be hot, massive, single stars evolving either towards or off the main-sequence. An alternative model is developed here involving binary mass transfer and accretion on to main-sequence stars. In this case the Hubble–Sandage variables are a third, and presumably final, class of binary in which accretion generates the bulk of the radiation observed. The other two classes are binaries containing an accreting white dwarf (cataclysmic variables)1,2 and an accreting neutron star or black hole (binary X-ray sources)3,4. Accretion disk models developed in these two cases can be scaled directly to treat the main-sequence case. One of the most complex objects in our own Galaxy is the η Carina nebula, rivalling in this respect the Orion Nebula and the Crab. I suggest below that η Can itself is an accreting main sequence star whose explosive outburst in the nineteenth century was caused by runaway mass accretion.