Most stars in the Universe are small and insignificant,
and they will – eventually – fizzle out
without much drama.
But a few light up the sky when they die.
And in the process, they don’t just tell
us about the lives of stars: they create the
building blocks of life, and help us to unravel
the whole history of the Universe.
There are perhaps 200 billion stars in our
galaxy, the Milky Way, although nobody really
knows exactly how many.
One thing that is known, though, is that a
tiny fraction of these stars has a disproportionate
effect on the rest of the galaxy.
Similar stars in other galaxies have taught
us much of what we know about the evolution
of the Universe.
They are the stars that end their lives as
supernovae – a topic in astronomy to which
Hubble has made great contributions since
it was launched in 1990.
Supernovae come in two broad categories.
Now to understand what’s going on in the
first category, you have to realize that a
star is actually a very finely balanced thing.
The pressure from the nuclear reactions at
the centre of the star is balanced by the
star’s gravity.
Now when a really massive star runs out of
nuclear fuel, the pressure in the centre drops
dramatically and the star collapses in on
itself, and then explodes.
The other type of supernova involves white
dwarf stars, which are remnants of stars like
our own Sun.
Now normally, a white dwarf is a pretty stable
thing.
But, if one lies close to another star it
can actually pull matter off its neighbor,
thereby gradually increasing its mass, until
finally it reaches the critical mass for a
thermonuclear explosion!
Supernovae are rare — a galaxy like ours
can only expect a few every century.
The last one to be seen in the Milky Way was
in 1604, observed by the great astronomer
Johannes Kepler, a few years before the invention
of the telescope.
Now we know that since then, a number of supernovae
events have taken place in the Milky Way,
because we can see the debris left behind
by the explosions.
But we never got to see the explosions themselves,
because they were shrouded by dust at the
time.
And so the fact remains that no supernova
inside the Milky Way has been directly observed
since the invention of the telescope.
Instead of just sitting around and waiting
for one, astronomers have decided to increase
their odds by widening the search far beyond
our own galaxy.
And since we’re talking about a very small
and distant phenomenon, we need a telescope
that can deliver extremely precise images
— a telescope like Hubble.
The most famous supernova that Hubble has
directly observed came with the death of a
giant star in the Large Magellanic Cloud.
The light from the initial blast first reached
Earth in 1987, a few years before Hubble’s
launch.
But Hubble’s images of the evolving supernova
over the quarter of a century since then have
become the gold standard for understanding
this event.
Astronomers have been able to study the complex
explosion in great detail, showing how the
shock from the exploding star is interacting
with the gas that surrounds the star, making
it light up.
More distant supernovae can’t be observed
in the same kind of detail as 1987A, but Hubble
is still a great help.
For example, because Hubble has been in orbit
for more than 20 years now, astronomers have
been able to make before and after images
of galaxies which allows them to search for
the progenitors of supernovae.
Now these kinds of observations potentially
tell us a lot about the conditions of the
progenitors just prior to the explosion.
As well as telling us about the star that
has just died, supernovae are powerful tools
for probing the cosmos.
The supernovae that come from exploding white
dwarfs have a peculiar property: they all
have the same intrinsic brightness.
This means that how bright they appear to
a telescope is a measure of how distant they
are, much like a street light looks bright
when you are near it, and dim when you are
far away.
Supernovae are extremely bright.
In fact, they are so bright that they usually
outshine their entire host galaxies.
And that is why it’s relatively easy to
detect them, even out to large cosmological
distances.
In 2011, the Nobel Prize in Physics was awarded
to two teams that measured the brightness
of many supernovae to map out their distances.
And what they found was that the faraway supernovae
were surprisingly faint, which could only
mean that they were even more distant than
expected.
Now we already knew that the Universe was
expanding, but what this research proved was
that the expansion is in fact accelerating
— and that came as a complete surprise.
Now this is really cutting-edge science, and
astronomers continue to study distant supernovae
to better understand the expansion of the
cosmos.
And Hubble plays a big part in this game.
It just recently hit another milestone when
it spotted the most distant supernova yet
discovered of this type.
It is so far away that its light has taken
more than 9 billion years to reach us — that’s
about two thirds the age of the Universe.
Closer to home, Hubble has played a big role
in imaging the wreckage left behind by supernovae.
Even though a supernova is only bright for
a short period of time, and its shockwaves
only propagate visibly for a few years, the
dusty clouds left over can last for millennia.
Their effect on the surrounding interstellar
gas lasts even longer.
And that means that although no supernovae
in our galaxy have ever been observed with
any telescope, plenty of supernova remnants
have been.
Hubble’s sharp images of their complex structures
help to chart the processes involved in their
violent formation.
What’s more, the clouds of debris are an
important reminder of the huge role that supernovae
play in shaping everything around us.
Nuclear reactions inside stars and in these
explosions are the source of most of the elements
found in nature, including the carbon in our
bodies, the oxygen we breathe, and the iron
and silicon in the planet we live on.
And so although they tell us a lot about the
past and future expansion of the Universe,
supernovae also teach us something even more
profound: they literally tell us where we
come from.
This is Dr J signing off for the Hubblecast.
Once again, nature has surprised us beyond
our wildest imagination.
