Issue 5.08

The Astrophysics Spectator

May 7, 2008

When a star is born, it is powered only by gravitational potential energy.  A star in this childhood stage is called a protostar.  With a simple source of energy and a simple mechanism—convection—that transports heat from the center to the photosphere, protostars have a simple, common internal structure.  Over tens of millions of years, a protostar shrinks in size until thermonuclear fusion commences at its center.

The evolution of a protostar to the main-sequence stage, however, is not controlled by what happens internally, but by what happens at the photosphere.  A protostar is very luminous when it begins its life, but it rapidly becomes less luminous as it loses energy and shrinks in size.  As it shrinks, the temperature of its photosphere varies very little.  This means that the luminosity of the protostar depends only on the area of the photosphere, which shrinks as the protostar shrinks.  Because of this decrease in luminosity, and because a protostar generates more gravitational potential energy when it is small than when it is large, the protostar shrinks more and more slowly as it ages.

The evolution of a protostar would be much different if the conditions at the photosphere were somewhat different.  For instance, if the photosphere became hotter as the protostar shrank in size, the protostar's luminosity could increase as the protostar shrank, and the protostar could shrink at a constant rate.

So the evolution of a protostar is not controlled by something hidden from view deep in the star, but by something that is directly in view: the photosphere.  This same point applies to some extend to the Sun and to the smaller main-sequence stars, where energy is transported from the core to the photosphere through convection.  It is not the ability of convection to transport energy out of the star that matters so much as how efficiently the star can radiate energy into space.  A star would have a shorter life if the radiation could escape from the hotter, deeper layers, rather than from the cooler, outer layers of the star.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for May 21.

Stars

Protostars.  A star begins its life as a brilliant protostar, powered only by its self-gravity, and outshining main-sequence stars of similar mass.  It is physically-large and luminous.  A one-solar-mass protostar can be 1,000 times as luminous and 70 times as large in radius as the Sun.  A protostar shrinks rapidly in size, causing its luminosity to fall rapidly, but throughout this evolution, its temperature changes very little.  A solar-mass star lives in the protostar stage for only about 10 million years, which is a brief prologue to its 9 billion year life on the main-sequence.  (continue)