Hot Star Winds are highly supersonic outflows from massive,
hot stars. The most massive stars in the galaxy--spectral type O and early-B--have roughly
10 to 50 times the mass of the sun. Due to their larger central pressure caused by this greater
mass and thus their greater gravitational compression, they burn their nuclear fuel much more rapidly
than does the sun. Thus, massive stars give off much more energy than the sun (10,000 to 1,000,000
times more) and live much shorter lives (tens of millions of years, compared to the sun's
The enormous amount of light given off by hot, massive stars can actually drive the material on the surfaces of these stars to incredible velocities (up to 3000 km/s or more; 100 times the local sound speed and 1% the speed of light). These radiation-driven stellar winds are an important source of energy and momentum in the galaxy's interstellar medium. And along with the supernovae explosions that end the lives of massive stars, hot star winds help to recycle material processed in the interiors of stars back into the interstellar medium where it is incorporated into the next generation of stars and, perhaps, planets.
In addition to these important implications for the evolution of the galaxy, the winds of hot stars involve some interesting physics in their own right.
The solar wind and the winds of other "cool" stars (the sun has a surface temperature of "only" about 6000 degrees Kelvin, while hot stars range from 15,000 K to 50,000 K) are accelerated by the deposition of heat near the stellar surface; the solar wind is driven by temperature gradients and associated pressure gradients due to the heat supplied by the solar corona. Hot star winds are driven by a completely different mechanism. The star's enormous luminosity actually pushes on the stellar wind, transferring momentum directly from the star's light to its wind, with no associated heating.
Light has momentum, just like any other moving object (think of photons as little particles of light--they don't have mass but they do have energy). Just as for matter, the momentum of light is proportional to its energy divided by its velocity. So photons are an inefficient carrier of momentum for a given amount of energy, since their velocities have the highest possible value. However they do carry some momentum, and when a photon is absorbed or scattered by matter, it imparts not only its energy to that matter, but also its momentum. Remember, while energy is a scalar quantity, momentum is a vector quantity. Almost all the star light is coming from the same direction, so it tends to push the wind outward.
The sun's light carries much more momentum than does the solar wind (by about a factor of 10,000). However, the solar wind is not driven by radiation pressure. This is because the solar wind does not have significant opacity at those wavelengths where most of the sun's light is. The sunlight simply streams through the wind material, not interacting very much and taking its momentum away with it.
There is a minimum radiation force on any stellar plasma which is that due to scattering radiation off of free electrons. This force is frequency independent, and if the force of radiation pressure on free electrons in a star exceeds gravity (a limit referred to as the Eddington limit), the surface of a star cannot remain bound and the star would basically blow itself apart.
...but a stellar wind is not this, rather it is a steady loss of material from a star that is otherwise stable.
A hot star wind does not rely on free electrons to absorb the momentum from the star's radiation field. Instead, it uses electrons bound in atoms to absorb the radiation: Radiation pressure is transferred to the wind material via spectral lines. This has two important consequences that contrast with electron scattering:
1. the radiation force can exceed gravity with much lower flux levels because bound electrons provide much more opacity than do free electrons (see Ken Gayley's paper on the Q-factor approach to calculating the line-force in hot star winds, in which he discusses line opacity in terms of the high quality of the resonance between photons and bound electrons); and
2. as the wind is accelerated, the line profiles absorbing the radiation become broader due to the doppler shift, which desaturates optically thick lines, generating more overall opacity. In this way, a line-driven stellar wind bootstraps its own acceleration.
Hot star winds, unlike the solar wind, have plenty of line opacity in the ultraviolet where most of the photospheric radiation is. This good matching between the radiation and the opacity allows for the efficient driving of winds in hot stars by radiation pressure. And the stronger ultraviolet lines of hot stars show direct evidence for wind acceleration: P Cygni profiles, which are blue shifted absorption features in spectral lines (from the column of gas in front of the stellar disk that is moving toward the observer) accompanied by red shifted emission features (from the scattering of radiation off of fast moving wind material on the sides and back of the wind). Here's an example of a P Cygni line profile and a cartoon demonstrating how these features get their distinctive shapes (taken from Lamers and Cassinelli's textbook):
These models do an excellent job of reproducing the time average properties of hot star winds.
However, hot star winds are not in steady state. Here are some line profile time-series of hot star winds.
Fluctuations that appear random in the line-profiles of hot star winds, as well as these large-scale, coherent, and periodically variable profiles are seen in UV and optical spectra. One can only conclude that the radiation-driven winds of hot stars are variable on time scales and spatial scales both large and small.
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