Champagne supernovae in the sky

Not all stars end their lives as white dwarfs. In fact, if a star is
more than a few times the mass of the Sun it is destined for a much
more spectacular end. Once the nuclear processes inside it cease,
its extra mass means that it will exert more gravitational pressure
on its core. This causes the core to become so dense and hot it
sends a shock wave of matter back out through the star causing it
to explode as a supernova. Briefly it will be the most spectacular
object in the whole galaxy. For a few days it will shine a hundred
million times more brightly; brighter than all the other stars in the
galaxy put together.
One property of stars I have not mentioned is that most of
them come in pairs, called binary systems, in which the two stars
orbit around each other. In fact, single, isolated stars such as the
Sun are in the minority.
The above scenario of a single massive star exploding is
known as a type II supernova. These have varying degrees of
brightness and do not depend on whether the star was part of
a binary or not. There is a more common way a star can go
supernova. It is known as type I, and occurs in binary systems.
Even if a star is not initially massive enough and ends up as a white
dwarf, it may still be able to suck material from its companion star
and put on weight. It can thus gain the critical mass this way.
One of the most celebrated supernovae in recent years was
seen in 1987. All the stars we see in the night sky are in our own
MilkyWay Galaxy. Other galaxies are so far a way that we cannot
see individual stars. The star that exploded in 1987 was not in our
galaxy but in a neighbouring one known as the Large Magellanic
Cloud. Yet, at its brightest, it could be seen clearly in the night sky.At the centre of many supernovae remnants resides a small
dense core, the remains of the original star. This object has the
same diameter as a large city such as London or New York, so it
is much smaller than a white dwarf star. It is therefore far denser
since it still contains a significant fraction of the material of the
original star that exploded. Atiny piece of this dense core the size
of a pea would weigh, on Earth, as much as Mount Everest! Such
an object is called a neutron star, and is one of the most fascinating
objects in astrophysics. In fact, neutron stars are the subject of
much current research activity. You may also have come across
the term ‘pulsar’. All neutron stars spin very rapidly and sweep
a beam of radiation out into space as they do so. If the Earth
happens to be in the path of this sweeping beam, the neutron star
will look to us as though the light is pulsing on and off, hence
the name pulsar. Some pulsars spin many times per second and I
will be coming back to them later in the book when I consider the
possibility of using them to make a time machine.
Despite all these exotic sounding astronomical objects, we
have yet to meet a black hole. Let us consider what happens when
an even bigger star, say twenty or thirty times the mass of the
Sun, stops shining. Such a star will not be able to resist its own
gravitational collapse. It will keep on collapsing until it has been
squashed to such a density that even its own light cannot escape its
gravitational pull. To someone watching from a distance the star
will suddenly disappear from view. It has become a black hole.
But there is much more to the story than that and I will come
back to black holes in Chapter 4. In the next chapter we will put
to use some of the ideas about the curvature and stretching of
space to look at the Universe as a whole. A lot of what we have
learned about the Universe has only become known over many
years of astronomical measurements and observations. Some
theoretical ideas have yet to be confirmed while others remain
highly speculative. One thing is for sure: there are still many
unanswered questions. Over the next few pages I will review
some of the latest ideas about the origin, shape, size and fate of
our Universe.