An exploding star had
been sighted in M82, one of the nearest big galaxies. The "supernova"
(as such stellar explosions are called) was a special, rare "Type Ia" --
the kind that led to the Nobel-worthy discovery of dark energy.
Type Ia supernovae have
happened in our galactic neighborhood only three times in the last 80
years. Like astronomers around the world, we were excited to be at a
world-leading telescope, where we could collect new information about
this rare event.
Meg Urry
The new supernova,
SN2014J, is 11.4 million light-years from Earth, a mere stone's throw in
cosmic terms. The previous record-holders were found in 1937 (14
million light-years away) and 1972 (16 million light-years away).
Supernovae occur at the
end of a star's life when its furnace runs out of fuel. Because gravity
then overcomes the star's ability to remain puffed up, there is a
violent collapse, followed by an explosion that produces radioactive
elements such as nickel and cobalt. Most of the light we see from a
supernova is emitted as those radioactive elements decay, so the
brightness falls sharply over a period of weeks.
Incidentally, all the
iron in your blood came from the decay of radioactive nickel
manufactured in a stellar explosion. So most of the atoms in your body
were once in the interiors of stars.
Because supernovae change
brightness very quickly compared with galaxies (months compared with
billions of years for galaxies), we quickly diverted the Keck telescope
from our intended targets to SN2014J. Our galaxies will look the same
another night, but the supernova won't.
Amazingly, a professor
and his students at the University of London Observatory had discovered
SN2014J well before it reached its peak brightness.
"The weather was closing
in, with increasing cloud," said professor Steve Fossey. "So instead of
the planned practical astronomy class, I gave the students an
introductory demonstration of how to use the CCD camera on one of the
observatory's automated 0.35-meter telescopes."
Deciding to look at M82
was almost pure luck -- there were fewer clouds in that direction, and
the galaxy was pretty and bright. But Fossey quickly noticed M82 didn't
look right: It seemed to contain a bright new star. He realized this
might be a supernova and, together with his students, worked feverishly
to rule out other explanations (such as a flaw in the camera or an
asteroid in our galaxy appearing to pass by M82). In short order, the
discovery of the new supernova was confirmed, and the e-mail alerts and
notifications began.
Tom Wright, one of the
students, said, "One minute we're eating pizza, then five minutes later
we've helped to discover a supernova. I couldn't believe it."
The only supernova
that's more extraordinary is SN1987A, which was discovered in 1987. It
is in a tiny satellite galaxy orbiting the Milky Way, about 160,000
light-years away and 70 times closer than SN2014J.
Here's an analogy for
the vast distances across which we see supernovae: Imagine if 1987A were
on the back porch. Then SN2014J would be just down the street; a
garden-variety nearby supernovae would be in the next town, and the most
distant supernovae would be over in the next state.
Space is very empty --
there are only a few big galaxies near us. On average, the distance to
the nearest big galaxy is about 100 times its size. But space is also
huge, and there are billions of galaxies.
In an ordinary big
galaxy such as the Milky Way, one supernova -- of any type -- happens
only every hundred years or so. But since there are so many galaxies in
the universe, millions of supernovae go off every year. From Earth, we
can see hundreds of these.
One recipe we can use to
find a supernova is to take pictures of a few hundred galaxies, repeat a
few weeks later and look for the difference. The supernova will look
like an overly bright star compared with the galaxy in which it lies.
Finding one supernova in several hundred galaxies is equivalent to staring at one galaxy for several hundred years.
M82 is an unusual galaxy
because it has a high rate of star formation. That's why two other
supernovae were found in M82 as recently as 2008 and 2004. But they were
not the special Type Ia supernovae. Because we have a good idea of how
much light is emitted by Type Ia supernovae, the brightness we observe
is a direct indicator of the distance to the host galaxy. This makes
Type Ia supernovae incredibly valuable for measuring cosmic distances.
Careful observations of
Type Ia supernovae across the universe were essential to measuring the
expansion history of the universe over billions of years. This led
directly to the discovery of dark energy, a sort of fifth fundamental
force that is now one of the most important unknowns in physics or
astronomy.
The reason Type Ia
supernovae are special is their uniformity. Basically, they all explode
at about the same mass, so they are all roughly equally luminous. Better
understanding the physics of that explosion and the effect of local
galactic environment will make Type Ia supernovae even better "standard
candles" and improve our understanding of the properties of dark energy
and the cosmological evolution of the universe. The more data we can
get, and the closer the supernova, the better the calibration.
We are lucky that
SN2014J was discovered about two weeks before it will reach its maximum
brightness, rather than after. For the next week or so, the data will
get better as the supernova gets brighter.
On Tuesday night, it was
reported to be 11th magnitude (astronomer units), which is about 100
times fainter than can be seen with the naked eye. But it should get at
least 10 times brighter, with a maximum around 8th magnitude -- not
visible to the naked eye but certainly discernible with a good pair of
binoculars.
M82 lies far north in
the sky, in the constellation Ursa Major, near the Big Dipper, above the
Dipper's bowl and about a third of the way over toward Polaris, the
North Star. So if you can see the Big Dipper and the North Star, take a
shot at seeing one of the most unusual supernovae in your lifetime.
Of course, you can't get
too excited about the timing. We see this light 11.4 million years
after the explosion happened, because of the time light takes to reach
our galaxy. So it was a really special time in M82 11.4 million years ago.
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