Stars are huge spinning balls of hot, luminous gas. Each one is unique
because stars differ in colour, temperature, size, brightness, and mass.
Over time, characteristics change and the star evolves from one stage
of star life to another. The key to a star’s life is its mass – the amount
of gas it is made from. Mass determines the star’s life span, as well
as its other characteristics and how these change.Stars
◀ YOUNG STARS
A protostar gets denser and hotter as
its gas becomes more squashed. When
its core reaches about 10 billion°C
(18 billion°F), nuclear reactions start.
Hydrogen is converted to helium,
energy is produced, and the star
shines. Stars born at the same time
and from the same region of cloud
exist as a cluster. Some clusters, like
the two shown here, remain together
for millions of years, but eventually
their stars drift apart.132
STAR BIRTH ▼
Stars are formed inside
vast clouds of hydrogen
gas, such as the Eagle
Nebula. A small fragment
of cloud collapses under
gravity. It becomes
increasingly squashed
and eventually forms a
spinning ball of gas
termed a protostar – the
first sign of a new star.Eagle NebulaMID-LIFE STARS ▼
Most stars glow steadily for
most of their lives. During
this stage they are known
as main sequence stars – the
Sun is one. Planets orbit
around some stars; others,
such as Fomalhaut, have
discs of dusty material
that may form planets.Detail of the Eagle NebulaSirius, the brightest
star in Earth’s night skyFomalhautAlbireo, a double
star systemJewel Box clusterTrapezium cluster132_133_Stars.indd 132 03/01/19 12:10 PM