Content area
Full text
PERSPECTIVES: ASTRONOMY
Stars are born in cold clouds of interstellar dust and gas. They first generate energy by thermonuclear fusion ("burning") of hydrogen (H) into helium (He) and, after several stages of thermonuclear evolution in their interior, end their life as compact remnants. In the absence of nuclear energy, they cool and fade to invisibility. More than 99% of the stars in our Galaxy finally become white dwarfs; the others end as neutron stars or occasionally as black hole remnants of stellar explosions.
Theory indicates that white dwarf remnants are about as small as Earth and may consist either of He or carbon and oxygen (C/O). CIO white dwarfs emerge from stars like our Sun, whereas He white dwarfs are expected to originate from stars with a mass less than half that of the Sun. The latter burn H for much longer than the present age of the universe. Because low-mass stars outnumber more massive ones, He white dwarfs should eventually take over as the Galaxy ages. But low-mass stars are not yet burnt out and we do not expect He white dwarfs to exist in our present-day Galaxy, where all white dwarfs should be of the C/O type.
Observations have shown, however, that a small fraction of white dwarfs are of the He type (1). We therefore have to consider additional formation scenarios. The most promising candidates are close binary stars. In this case, the He white dwarf results from an interaction of its progenitor with a close companion star. After exhausting its H fuel, the progenitor expands to giant dimensions and transfers its envelope to the companion before the He core has grown to the critical mass of half a solar mass: A He white dwarf is born.
Several He white dwarfs have faint companions that are white dwarfs themselves (1), either of the He or of the C/O type. According to model estimates, there should be as many as 250 million double white dwarf systems among the hundred billion stars of our Galaxy (2). Such double white dwarf systems cannot exist forever. Einstein's theory of general relativity predicts the emission of gravitational waves (3), leading to a loss of angular momentum and decay of the binary orbit. Finally, the components will coalesce....