Star formation starts when gravity starts to pull gas and dust in the nebula together. As
the gas and dust falls together, it forms into one or more spheres. As one of these spheres
collapses further, the pressure inside increases. As the pressure increases, the temperature
of the gas also increases. Eventually, the pressure and temperature become great enough to
cause nuclear fusion to start in the center. At this point, the ball of gas has become a star.
Figure 26.3: The Eagle Nebula and the Pillars of Creation. The pillars of gas and dust
shown here are in the Eagle Nebula. ( 16 )
The Main Sequence
For most of a star’s life, the nuclear fusion in the core combines hydrogen atoms to form
helium atoms. A star in this stage is said to be amain sequence star, or to be on
the main sequence. This term comes from the Hertzsprung-Russell diagram, that plots a
star’s surface temperature against its true brightness or magnitude. For stars on the main
sequence, the hotter they are, the brighter they are. The length of time a star is on the main
sequence depends on how long a star is able to balance the inward force of gravity with the
outward force provided by the nuclear fusion going on in its core. More massive stars have
higher pressure in the core, so they have to burn more of their hydrogen “fuel” to prevent
gravitational collapse. Because of this, more massive stars have higher temperatures, and
also run out of hydrogen sooner than smaller stars do.