The spaces between the stars are not completely empty, but
are filled with clouds of gas and dust called nebulas. We can see nebulas when
they glow or when they reflect light from nearby stars. Sometimes we cannot see
them, but we know they are there because they block out the light from stars
behind them. Dark nebulas include giant large molecular clouds, where new stars
are formed.
There are three main
types of nebula, which can all be seen here. Emission nebulas appear red or
pink. This is because they are mostly hydrogen gas, which glows red when it is
excited (given extra energy) by nearby stars. Reflection nebulas appear blue,
because they reflect light from nearby stars. Dark nebulas are regions where
dust is blotting out distant stars.
Stars are born in dark molecular clouds. Within these clouds, matter
clumps together as it collapses under gravity. Within these clumps, even denser
masses are formed, called cores. In the centre of a core, the matter becomes
increasingly compressed and heats up. It begins to give off heat and light as a
protostar. When the temperature of the protostar reaches 10 million°C
(18 million°F) or so, nuclear fusion reactions begin, and the star
begins to shine. It will shine steadily for millions or billions of years, but
eventually it will start to die. Whether a star becomes a red giant or a
supergiant depends on its mass.
