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Just now, somewhere in the universe, a star exploded.
There goes another one.
In fact, a supernova occurs every second or so in the observable universe,
and there is one on average every 25 to 50 years
in a galaxy the size and age of the Milky Way.
Yet we've never actually been able to watch one happen
from its first violent moments.
Of course, how would we?
There are hundreds of billions of stars close enough
that we could watch the supernova explosion
break through the surface of the star.
But we'd have to have our best telescopes focused on the right one
at precisely the right time to get meaningful data.
Suffice it to say, the odds of that happening are astronomically low.
But what if we could anticipate a supernova before its light reached us?
That may seem impossible.
After all, nothing travels faster than the speed of light, right?
As far as we know, yes.
But in a race, fast doesn't matter if you take a detour
while someone else beelines it for the finish line.
For exactly that reason,
photons don't win the supernova race to Earth.
Neutrinos do.
Here's why.
There are two types of supernova.
Type 1 is when a star accumulates so much matter from a neighboring star,
that a runaway nuclear reaction ignites and causes it to explode.
In type 2, the star runs out of nuclear fuel,
so the gravitational forces pulling in
overwhelm the quantum mechanical forces pushing out,
and the stellar core collapses under its own weight in a hundredth of a second.
While the outer reaches of the star are unaffected by the collapsed core,
the inner edges accelerate through the void,
smash into the core, and rebound to launch the explosion.
In both of these scenarios,
the star expels an unparalleled amount of energy,
as well as a great deal of matter.
In fact, all atoms heavier than nickel, including elements like gold and silver,
only form in supernova reactions.
In type 2 supernovae,
about 1% of the energy consists of photons,
which we know of as light,
while 99% radiates out as neutrinos,
the elementary particles that are known for rarely interacting with anything.
Starting from the center of the star,
the exploding matter takes tens of minutes, or even hours,
or in rare cases, several days, to reach and break through the surface of the star.
However, the neutrinos, thanks to their non-interactivity,
take a much more direct route.
By the time there is any visible change in the star's surface,
the neutrinos typically have a several hour head start over the photons.
That's why astronomers and physicists
have been able to set up a project called SNEWS,
the Supernova Early Warning System.
When detectors around the world pick up bursts of neutrinos,
they send messages to a central computer in New York.
If multiple detectors receive similar signals within ten seconds,
SNEWS will trigger an alert warning that a supernova is imminent.
Aided by some distance and direction information from the neutrino detectors,
the amateur astronomers and scientists alike
will scan the skies and share information
to quickly identify the new galactic supernova
and turn the world's major telescopes in that direction.
The last supernova that sent detectable neutrinos to Earth was in 1987
on the edge of the Tarantula Nebula
in the large Magellanic Cloud, a nearby galaxy.
Its neutrinos reached Earth about three hours ahead of the visible light.
We're due for another one any day now, and when that happens,
SNEWS should give you the opportunity to be among the first to witness something
that no human has ever seen before.
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【Ted-Ed】是如何偵測到超新星的勒?? (How to detect a supernova?)

3021 分類 收藏
SylviaQQ 發佈於 2015 年 9 月 8 日    Michael Wu 翻譯    Janet Chang 審核
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