Although all stars are born inside collapsing clouds of gas and dust, their masses, temperatures, luminosities, and final fates can be very different, and their lifespans can range from as little as a few million years to more than a thousand billion years.

STELLAR EVOLUTION AND THE H-R DIAGRAM

As described in Chapter 10, a collapsing gas cloud forms a protostar, which then continues slowly to contract while accreting more material from the surrounding nebula. As the protostar gains mass and continues to contract, its central temperature continues to rise until, when it reaches about 10 million K or so, nuclear fusion reactions commence in its core. Pressure exerted by the hot gas in the star's interior halts the contraction, and it then settles down to a long-lived stable phase of existence as a main-sequence star. The changing luminosity and surface temperature of a star, as it passes through its life cycle, can be plotted on a Hertzsprung-Russell (H-R) diagram, the resulting line of which is called an evolutionary track.

PRE–MAIN SEQUENCE

As the protostar continues to accrete more matter, its surface temperature increases and its luminosity rises for a time to a value much higher than its eventual main-sequence luminosity. Plotted on the H-R diagram, the newly forming protostar would appear above and to the right of the main sequence, moving, as it evolved, from right to left (in the direction of increasing temperature) and upward (in the direction of increasing luminosity).