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by Fraser Cain
20 October 2016
from
UniverseToday Website
There are times when I really wish astronomers could take their
advanced modern knowledge of the cosmos and then go back and rewrite
all the terminology so that they make more sense.
For example,
dark matter and dark energy seem
like they're linked, and maybe they are, but really, they're just
mysteries.
Is dark matter actually matter, or just a different way that gravity
works over long distances? Is dark energy really energy, or is it
part of the expansion of space itself.
Black holes are neither black, nor
holes, but that doesn't stop people from imagining them as dark
tunnels to another Universe. Or the Big Bang, which makes you think
of an explosion.
Another category that could really use a re-organizing is the term 'nova',
and all the related objects that share that term:
Okay, I made those last couple up.
I guess if you go back to the basics, a nova is a star that
momentarily brightens up. And a supernova is a star that momentarily
brightens up… to death.
But the underlying scenario is totally
different.
New research
shows that some old stars known as white dwarfs
might be held up by their rapid spins, and when
they slow down, they explode as Type Ia
supernovae.
Thousands of these
"time bombs" could be scattered throughout our
Galaxy.
In this
artist's conception,
a supernova explosion
is about to obliterate
an orbiting
Saturn-like planet.
Credit: David A.
Aguilar (CfA)
As we've mentioned in many articles already, a supernova commonly
occurs when a massive star runs out of fuel in its core, implodes,
and then detonates with an enormous explosion.
There's another kind of supernova, but
we'll get to that later.
A plain old regular nova, on the other hand, happens when a
white
dwarf - the dead remnant of a Sun-like star - absorbs a little too
much material from a binary companion.
This borrowed hydrogen undergoes fusion,
which causes it to brighten up significantly, pumping up to 100,000
times more energy off into space.
Imagine a situation where you've got two main sequence stars like
our Sun orbiting one another in a tight binary system. Over the
course of billions of years, one of the stars runs out of fuel in
its core, expands as a red giant, and then contracts back down into
a white dwarf. It's dead...
Some time later, the second star dies, and it expands as a red
giant.
So now you've got a red dwarf and a
white dwarf in this binary system, orbiting around and around each
other, and material is streaming off the red giant and onto the
smaller white dwarf.
Illustration
of a white dwarf
feeding off its
companion star
Credit: ESO / M.
Kornmesser
This material piles up on the surface of the white dwarf forming a
cozy blanket of stolen hydrogen.
When the surface temperature reaches 20
million Kelvin, the hydrogen begins to fuse, as if it was the core
of a star. Metaphorically speaking, its skin catches fire. No, wait,
even better. Its skin catches fire and then blasts off into space.
Over the course of a few months, the star brightens significantly in
the sky. Sometimes a star that required a telescope before suddenly
becomes visible with the unaided eye. And then it slowly fades
again, back to its original brightness.
Some stars do this on a regular basis, brightening a few times a
century. Others must clearly be on a longer cycle, we've only seen
them do it once.
Astronomers think there are about 40 novae a year across the Milky
Way, and we often see them in other galaxies.
Tycho Brahe:
He lived like a sage
and died like a fool.
He also created his
own cosmological model,
the Tychonic system.
The term "nova" was first coined by the Danish astronomer Tycho
Brahe in 1572, when he observed a supernova with his telescope.
He called it the "nova stella", or new
star, and the name stuck. Other astronomers used the term to
describe any star that brightened up in the sky, before they even
really understood the causes.
During a nova event, only about 5% of the material gathered on the
white dwarf is actually consumed in the flash of fusion. Some is
blasted off into space, and some of the byproducts of fusion pile up
on its surface.
Tycho's Supernova
Remnant.
Credit: Spitzer,
Chandra and Calar Alto Telescopes.
Over millions of years, the white dwarf can collect enough material
that carbon fusion can occur.
At 1.4 times the mass of the Sun, a
runaway fusion reaction overtakes the entire white dwarf star,
releasing enough energy to detonate it in a matter of seconds.
If a regular nova is a quick flare-up of fusion on the surface of a
white dwarf star, then this event is a super nova, where the entire
star explodes from a runaway fusion reaction.
You might have guessed, this is known as a Type 1a supernova, and
astronomers use these explosions as a way to measure distance in the
Universe, because they always explode with the same amount of
energy.
Hmm, I guess the terminology isn't so bad after all: nova is a flare
up, and a supernova is a catastrophic flare up to death… that works.
Now you know. A nova occurs when a dead star steals material from a
binary companion, and undergoes a momentary return to the good old
days of fusion.
A
Type Ia supernova is that final
explosion when a
white dwarf has gathered its last
meal.
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