A
blindingly bright star bursts into view in a corner of the night sky — it
wasn't there just a few hours ago, but now it burns like a beacon.
That bright
star isn't actually a star, at least not anymore. The brilliant point of light
is the explosion of a star that has reached the end of its life, otherwise
known as a
supernova.
Supernovas
can briefly outshine entire galaxies and radiate more energy than our sun will
in its entire lifetime. They're also the primary source of heavy elements in
the universe.
On average,
a supernova will occur about once every 50 years in a galaxy the size of the
Milky Way. Put another way, a star explodes every second or so somewhere in the
universe.
Exactly how
a star dies depends in part on its mass. Our sun, for example, doesn't have
enough mass to explode as a supernova (though the news for Earth still isn't
good, because once the sun runs out of its nuclear fuel, perhaps in a couple
billion years, it will swell into a red giant that will likely vaporize our
world, before gradually cooling into a white
dwarf).
A star can
go supernova in one of two ways:
Type I supernova:
star accumulates matter from a nearby neighbor until a runaway nuclear reaction
ignites.
Type II
supernova: star runs out of nuclear fuel and collapses under its own gravity.
Let's
look at the more exciting Type II first:
For a star
to explode as a Type
II supernova, it must be at several times more massive than the sun
(estimates run from eight to 15 solar masses). Like the sun, it will eventually
run out of hydrogen and then helium fuel at its core. However, it will have
enough mass and pressure to fuse carbon. Here's what happens next:
- Gradually
heavier elements build up at the center, and it becomes layered like an
onion, with elements becoming lighter towards the outside of the star.
- Once the
star's core surpasses a certain mass (the Chandrasekhar limit), the star
begins to implode (for this reason, these supernovas are also known as
core-collapse supernovas).
- The core
heats up and becomes denser.
- Eventually
the implosion bounces back off the core, expelling the stellar material
into space — the supernova.
What's left
is an ultradense object called a neutron star.
There are
sub-categories of Type II supernovas, classified based on their light curves.
The light of Type II-L supernovas declines steadily after the explosion, while
Type II-P's light stays steady for a time before diminishing. Both types have
the signature of hydrogen in their spectra.
Stars much
more massive than the sun (around 20 to 30 solar masses) might not explode as a
supernova, astronomers think. Instead they collapse to form
black holes.
Type I
Type 1
supernovas lack a hydrogen signature in their light spectra.
Type Ia supernovae are generally thought to originate from
white dwarf stars in a close binary system. As the gas of the companion star
accumulates onto the white dwarf, the white dwarf is progressively compressed,
and eventually sets off a runaway nuclear reaction inside that eventually leads
to a cataclysmic supernova outburst.
Astronomers
use Type 1a supernovas as "standard candles" to measure
cosmic distances because all are thought to blaze with equal brightness at
their peaks.
Type 1b and
1c supernovas also undergo core-collapse just as Type II supernovas do, but
they have lost most of their outer hydrogen envelopes.
Recent
studies have found that supernovas vibrate like giant speakers and emit an
audible hum before exploding.
Last year,
scientists caught a supernova in the act of
exploding for the first time.
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