Bodies so massive that nuclear fusion occurs within them, emitting vast quantities
of energy into space.
This starscape belongs to a world in a denser region of the Milky Way galaxy than our
own. The glittering sky is crowded with stars of all types.
There is a minimum mass for a star: roughly 10 × 1028 kg.
Below this mass (about a twentieth that of the Sun, or twenty times that of Jupiter), fusion
cannot occur spontaneously. Most stars are much more massive than this: the most massive
and luminous known are more than 2,000 times more so than this minimum value.
Stellar fusion, in most cases, generates energy by fusing hydrogen into helium, though
more massive stars may in the later stages of their existence produce heavier elements.
Nuclear fusion of this kind is responsible for producing most of the heavier elements of the
The Evolution of a Typical Star
Stars begin their lives among the clouds of dust and gas that form a galaxy's
Interstellar Medium. Gravity causes regions of this medium, which consists mainly of hydrogen, to collapse inward upon
themselves. These regions of increasing density are known as protostars, and as gravity drives the collapse of the
protostar material, the intense forces at its core begin to produce nuclear reactions. At this point, a true star is born.
Because the Interstellar Medium is composed mainly of hydrogen, so is a typical young star. The fusion reactions in its core,
though, overcome the internal bonds of the hydrogen atoms, releasing enormous amounts of energy and forming the element
helium. A star in this phase of its life, when it belongs to the 'main sequence', runs on
hydrogen 'fuel', which it will eventually convert entirely into helium.
The lifetime of a main sequence star is heavily dependent on its mass. Our own
Sun - a fairly typical main sequence star - has
been burning hydrogen for about 5,000 million years, and will probably continue to do so for another 5,000 million. More
massive stars, though, have much shorter lifespans, often measured in mere hundreds of millions of years.
As the star's hydrogen supplies run out, its form changes significantly. Its core, now composed almost entirely of helium,
begins to collapse upon itself, releasing further energy. This is sufficient to power an expansion of the matter around the
decaying core, and the outer layers of the star swell to many times their original size. Meanwhile, the collapsing helium core
reaches a point where fusion can proceed once again, this time fusing atoms of helium to produce carbon and oxygen.
In this new phase, the temperature of the outer layers of the swollen star has cooled to give it a
red light, and the resulting star type is known as a red giant.
Just as the star's original hydrogen fuel was eventually exhausted, so the star's new supply of helium
fuel will also eventually be exhausted too. The next developments in the star's life will depend on its mass.
If it is sufficiently massive, it can once again 'recycle' its fuel into heavier and heavier elements, until
eventually its core is composed of iron. But beyond this limit no further processing is possible: the core
collapses completely to form a superdense neutron star, while the outer shells of material are blasted
away in a catastrophic event known as a supernova.
Our Sun, though, has too little mass to pass through this process. Once its helium core is depleted,
nuclear reactions will cease, and the core will shrink to roughly the size of the Earth. The result
will be an inert and highly dense stellar remnant, known as a white dwarf. Meanwhile, the outer shells of
the giant phase will drift away from the dead core, eventually forming a bubble or ring
of matter known as a planetary nebula.
However a star dies, much of its matter is spread back into the Interstellar Medium, whether through the supernova
explosion or the formation of a nebula, and in that medium the processes that formed
the original protostar are always at work. The matter cast away by one dying star may eventually find itself taking part in the
birth of another.
The sky's best known star cluster, the Pleiades in
Taurus, is made up of hot blue
stars. These stars are newborns by
stellar standards: just 50 million years old, and still surrounded by the
nebulous material from which they formed.
Anatomy of a Star: The structure of a typical, Sun-like star. The powerhouse of the star
is nuclear fusion at its core, from where energy is radiated out to the outer 'surface' or photosphere.