Sometime in the next few months, a spectacle could light up the northern sky.
There, in the Corona Borealis constellation, at a distanceof more than 2,500 light-years, a star called T Coronae Borealis lurks,
building up to an explosion that will, temporarily, cause the star to become
one of the brightest objects in the night sky.
Astronomers are on tenterhooks waiting for this thing to
blow, not just because it will be amazing, but for the wealth of data we'll be
able to collect on a type of star explosion called a classical nova.
The reason we know T Coronae Borealis (T CrB for short) is
going to explode is because it has done so once every 80 years, for at least
eight centuries.
This means that it's very close to a once-in-a-lifetime
event – and that the technology we have to observe it now vastly outstrips what
we had during its last excursion, back in February 1946.
"There are a few recurrent novae with very short
cycles, but typically, we don't often see a repeated outburst in a human
lifetime, and rarely one so relatively close to our own system," says
astronomer Rebekah Hounsell of NASA's Goddard Space Flight Center.
"It's incredibly exciting to have this front-row
seat."
Not to be confused with the near obliteration of stars in
the cataclysmic explosions known as supernovae, classical novae are smaller
explosions that leave the star more or less intact. In fact, this is far from
the first time this particular cosmic object has gone through this experience.
The reason T CrB explodes repeatedly, and on schedule, is a
quirk of the type of star it is. It's a binary star system that contains the
remnant collapsed core of a Sun-like star called a white dwarf, and a puffy red
giant companion.
White dwarfs are very small and very dense, between the size
of Earth and the Moon, packing into that size as much mass as 1.4 Suns. That
means that they are pretty gravitationally intense; and if they have a binary
companion in a close enough orbit, they tend to siphon off material,
predominantly hydrogen.
Over time, this hydrogen accumulates on the surface of the
white dwarf, compressed down due to the gravitational pull. Eventually, the
pressure and heat on the bottom layer of hydrogen become so intense that the
whole thing ignites in a runaway thermonuclear explosion that violently expels
the excess hydrogen out into space in spectacular style.
That's the nova; and, for T CrB, the length of time this
process takes is about 80 years or so.
Over the last decade, astronomers have observed the binary
system exhibiting behavior similar to how it behaved leading up to the 1946
explosion; specifically, a dip in brightness that heralds the close approach of
the eruption. Their analysis suggests that it could occur very soon – as early
as before September 2024.
This means that astronomers are keeping a very close eye on
a little patch of sky clustered with constellations – Lyra, Hercules, Boötes –
and a little arc of stars sandwiched between them. That's Corona Borealis.
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Where you can find Corona Borealis in the sky. Watch out for
a 'new' star! (NASA) |
We expect that we'll hear about the nova pretty much as soon
as it happens. It will bloom in the sky to become visible to the naked eye,
then gradually fade from visibility over the course of a week. So you should
have time to get out there and look at it, if that strikes your fancy.
In fact, if you can, that would be amazing. Citizen
scientists are being called upon to collect data too. The more eyes there are
on T CrB, the better we'll be able to understand its flashy outbursts.
And of course there will be as many telescopes tuning in as
can be arranged, from the longest radio wavelengths, to the most powerful X-
and gamma radiation.
"Recurrent novae are unpredictable and contrarian," says astrophysicist Koji Mukai of NASA Goddard. "When you think there can't possibly be a reason they follow a certain set pattern, they do – and as soon as you start to rely on them repeating the same pattern, they deviate from it completely. We'll see how T CrB behaves."