Jump-start from an exploded sun
In space, the birth of a star begins with a giant gas cloud. The mass of this cloud must be so great that gravity prevails against the internal pressure and the turbulence that would drive the filigree structure apart. For its birth to proceed, the star presumably needs a little gentle help from outside, such as the pressure wave of a nearby supernova, that is, an exploded sun (see the box “Furious Finale”).
At some point, the cloud breaks up into smaller lumps, each of which collapses. Shackled by gravitation, the particles within such a fragment bunch up. “If this were to continue indefinitely, the star’s birth would end in a black hole,” says Achim Weiss. How does the inside of the emerging gas sphere withstand the growing gravitational pressure? What stops the stellar embryo from breaking up?
The compressive work of gravity generates heat and pressure. The heat causes the electrons to separate from the cores of their atoms – a plasma is produced. And the pressure enables the gas to build up a “counter-force” against the gravitation: at any given distance from the sphere’s center, the pressure is exactly equal to the weight of the gas masses lying above it. The star has become a stable structure. Or as an astrophysicist would put it: it is in a state of hydrostatic equilibrium.
Such a state can be reproduced by a simple experiment: carefully press in a bicycle pump, then block off the outlet with your finger. Since air in the pump is no longer able to flow out, pressure builds up in the tube and prevents the piston from moving. If the right amount of pressure is applied to the piston, it remains stationary in the tube of the pump and a form of equilibrium is produced.
“What happens next in the star’s life depends entirely on its mass,” says Achim Weiss. The mass is therefore the decisive parameter in the model calculations. In a perfectly normal, average star like our own Sun (mass: 1.989 x 1030 kg), an event with far-reaching consequences occurs after its birth, which lasts a few hundred thousand years. In the center, the gas – primarily hydrogen – heats up to a temperature of over ten million degrees Celsius. At this astronomically high temperature, a fusion reactor ignites, and nucleosynthesis begins: four hydrogen nuclei (protons) combine to form a nucleus of helium-4.
Only now has the cosmic gas sphere become a full member of the star family. The reason is that stars have another property that differentiates them crucially from planets: they shine, because they derive energy from nucleosynthesis. The fusion reactor also ensures that the gas remains hot and delivers sufficient pressure to maintain the hydrostatic equilibrium.
Some stars, however, do not possess sufficient substance at birth. If their mass is less than 75 times that of the planet Jupiter, or in other words less than 8 percent of the mass of the Sun, fusion reactions may still occur on a limited scale within them; a proton, for example, may fuse with deuterium nucleus, consisting of one proton and one neutron, to form a helium-3 nucleus. However, lightweights such as these among the stars never reach the stage of steady hydrogen burning. (The term “burning” is used for historical reasons and is usual in astrophysics; it actually refers to “fusion” and is unrelated to chemical combustion.)
These “black sheep” of the star family are called brown dwarfs. Their lives are fairly unspectacular: owing to their low core temperature, the gas pressure is not sufficient to keep the gas spheres in equilibrium in the long term. Ultimately, gravity gains the upper hand. The brown dwarfs shrink and convert their gravitational energy into heat. Incidentally, this process, known as the Kelvin-Helmholtz contraction, was discussed by astronomers as one of the possible sources of stars’ energy, before they solved the riddle in the 20th century with the aid of nuclear fusion.