The complex workings of “windy stars”

Many astronomy books describe the Sun and other stars as “big balls of hot gas”. A moment’s inspection would show this not to be true. If this were the case, stars would be “big fuzzy blobs”.  The Sun, and apart from a few exceptions, stars in general do not look like that. They look like big glowing spheres, because almost all their energy is emitted into space by a thin layer, called the photosphere, which is so sharply defined it looks like a solid “surface”.  Simple balls of gas can’t do that; there has to be another ingredient, which we now know to be the presence of strong magnetic fields. The temperature in the middle of the Sun is between 10 and 20 million degrees Celsius. Then, as we move outwards towards the “surface”, the temperature falls, until at the photosphere it is about 6,000 Celsius. It is the temperature of the photosphere that dictates the colour of a star. A few thousand kilometres above the surface the temperature starts to rise again, rapidly reaching more than a million degrees. Once again we have the magnetic fields to thank for this.

In 1958 physicist Eugene Parker showed that the Sun’s atmosphere cannot be stable, and has to be rapidly flowing outward into space, with the loss being replenished by the evaporation of material from the photosphere.  He had come up with the idea of the “Solar Wind”. His work explained at last many of the Sun-Earth connections that had been known for more than a century, such as the occurrence of magnetic storms and displays of the aurora. Until then it was known that solar activity triggered these phenomena, but not how this happened, or what the connection was.

Stars form from collapsing clouds of cold gas. Eventually, in the core of the cloud, temperatures reach millions of degrees and the material is sufficiently compressed until nuclear fusion starts, and a star is born. This changes the game completely. Until that time the only force acting on the material forming the star was gravity, pulling the collapsing cloud inwards.  Now we have to take into account the radiation from the star too; that is the outward flow of energy.  At the surface of the Earth the Sun floods every square metre with about 1,400 Watts. That corresponds to every square metre of the Sun’s photosphere squirting out almost 14 million watts. Energy flows like this exert an outward force, which helps propel the solar wind.

How bright a star is depends upon its mass. A star twice the mass of the Sun will be about ten times as bright. One with four times as much mass would be 100 times brighter, and a big star, with about sixty times the mass of the Sun would shine a million times brighter. Imagine up to 14 trillion Watts being radiated by every square metre of a star’s surface. There will be regions where the outward “radiation pressure” will far exceed the star’s gravity, resulting in a stellar wind that is a tornado compared with our Sun’s solar breeze. This wind blasts into clouds of material in space, in some cases triggering their collapse and the birth of new stars, and can strip the atmospheres from planets. This is truly fascinating physics. Fortunately we have the observational tools to study it in detail, and also fortunately, it is taking place far away from us.

Saturn is well up in the east by dark. The Moon will reach first quarter on the 10th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.