Science Briefs

Radiative Instability of a Star

A star goes through most of its life cycle in a placid, sedate way, radiating its luminous smile for eons. But the devil has his violent moments, too. Not only are birth and death pangs traumatic for the star, but there are muscular contractions and expansions that shake up its body during middle age as well. Although not catastrophic, these violent pulsations are easily visible to the telescopic observer on Earth.

Photo of WR124

Age 1: Outflowing wind of the Wolf-Rayet star WR124. (Photo: NASA/Hubble Space Telescope)

Photo of AG Carinae

Age 2: Young nebula ejected by the Luminous Blue Variable AG Carinae. (Photo: Australia Telescope Compact Array)

Photo of HD 192765

Age 3: Old nebula surrounding the Wolf-Rayet star HD 192765. (Photo: Martin Altmann, Bonn University)

Among the most common stars, the emitted stellar light at such times rises and falls periodically, yet no damage is done to the star's body. The star evolves inexorably onward, and the ever-present wind blowing off the surface suffers little modification.

The most massive stars tell another story, though. These supergiants, with initial masses of 30 to 100 times the Sun's mass, are so luminous and so hot that radiation pours in a flood through their interiors. Pressure exerted by the myriad photons vastly exceeds the pressure arising from the whizzing gas particles, and the star resembles a mushrooming balloon of dazzling white light. Although the star's central core remains compact under the influence of a strong force of gravity, the outer envelope of matter becomes more and more loosely bound to the core as the star grows brighter during the course of its evolution. The fierce central thermonuclear fires that convert hydrogen into helium and, after that, helium into carbon account for the astonishingly large power output, which can exceed a million times that of the Sun.

By the same token, the extraordinary luminosity at the star's surface creates a terrific outflowing wind, because the photons push very hard against the more sluggish gas particles. The inevitable outcome is a rapid wasting away of the star's mass. A vicious cycle gets set up in which the lowering of the star's mass weakens gravity at the surface, so that gas particles can be all the more easily sloughed off. The outward acceleration of the wind in itself helps to counter the inward pull of gravity.

As this intricate process goes on, a certain time arrives when gravity is no longer able to restrain the bulk of the outer envelope. When this happens, the supergiant is said to be "radiatively unstable", a condition conjectured as unlikely by Sir Arthur Eddington in 1921, who first studied the problem. The star may also be out of hydrostatic equilibrium, a condition known as "dynamical instability." Then entire chunks of the outer envelope come streaming off in clumpy spherical slabs, forming visible gossamer shells around the compact core with its tenuous remains of the outer envelope.

Such a star, near the end of its active life but before its core finally collapses as a supernova, is called a Luminous Blue Variable. Approaching closer to the end, its remaining envelope becomes thinner and thinner and its instability ever greater, so that it begins to look different and is then called a Wolf-Rayet star, named after two famous nineteenth-century astronomers. The cause of the observed instability has long been a mystery, but GISS scientist Richard Stothers on the basis of new calculations believes that radiative instability is the culprit for the brightest objects.

In a spiral galaxy like ours, the brightest nonexplosive stars are always found to be remarkable examples of Luminous Blue Variables and Wolf-Rayet stars. These are undoubtedly the super-supergiants of them all.


Stothers, R.B. 2003. Radiative instability in stellar envelopes. Astrophys. J. 599, L103-L105.