Sometime soon, a pair of stars will blow their top.
Astronomers are running regular checks on a star system that
currently requires a telescope to see because they expect it will soon be
bright enough to view with the naked eye. According to one prediction, this
could happen any time from now until September, although there’s reason to
think we might need to wait another year.
One of the wonders of astronomy is the way, very
occasionally, stars suddenly brighten spectacularly. When the star in question
was previously too faint to see it can seem as if a new star has appeared, and
before the invention of the telescope these became known as novae from “De nova
stella”, Latin for new star.
Two and a half thousand light-years away in the direction of
the constellation Coronae Borealis, such a brightening is anticipated.
The event will occur because what is known as T Coronae
Borealis (T CrB) is not one star, but two. Although binaries are the most
common star arrangement, this pair are a dangerous combination of red giant and
white dwarf, both modestly more massive than the Sun. Over time the white
dwarf’s intense gravity is pulling material off the red giant, creating an
accretion disk a little like one circling a black hole.
When some of this material gets close enough to the white
dwarf it becomes heated to the point where fusion ignites, leading to a surge
in brightness. Most such brightenings are like those of an ordinary variable
star. For example, in 2016, the T CrB system roughly tripled in brightness.
However, since it was still barely visible in binoculars, few people cared.
In 1866 and 1946, on the other hand, the surge was something
quite different, increasing several thousand-fold so that it was easily visible
with the naked eye. It’s an event like this that astronomers are eagerly
awaiting.
Some white dwarfs that are pulling material off their
companions do so erratically, brightening only once in our observations.
Others, known as recurrent novae, keep to a regular schedule. You may not have
heard of them, because most recurrent novae are so far away that even when they
brighten we can’t see them unaided, which makes T CrB is almost unique. At its
peak in 1866 and 1946 it was almost as bright as Polaris, exceeding all but a
few hundred stars. Only one other recurrent nova is visible with the naked eye
at its brightest, and that one barely so.
The gap between T CrB’s two well-studied events was a little
under 80 years. If that were to be repeated precisely, we’d expect the next
event in late 2025. You can’t set your watch by a recurrent nova,
unfortunately, so a few years either side would not be surprising.
Last year however, Professor Bradley Schaefer of Louisiana
State University noted that prior to the 1946 event T CrB underwent some
notable dimming, and has now done something very similar. Based on the timing
from dip to peak, Schaefer predicted we should expect a show between February
and September this year. Schaefer also did some impressive sleuthing to find
evidence of what appear to be previous sightings in 1787 and 1217.
Although either previous report could be of something else
that happened to be in the same part of the sky, Schaefer makes a strong case
that these were most likely previous eruptions of T CrB, in which case we can
be more confident to expect it soon.
Nevertheless, Schaefer’s predictions on timing came with a
dose of uncertainty, which seems to have been lost in some recent reports
announcing we should be spotting the next peak by September.
T CrB is at a declination of 25 north, which means its
visible for most of the year from Europe and North America. The exception is
around September-November, when the Sun is annoyingly close to it in the sky,
so we’d really rather it didn’t time its explosion then. Southern hemisphere
observers have a considerably narrower viewing window.
When the explosion comes, NASA predicts T CrB should be
visible to the naked eye for a few days, followed by a week or so where
binoculars should still be sufficient. To find it, look between the very bright
stars Vega and Arcturus, a little closer to the latter, and about seven degrees
from Alphecca, which should be of similar or slightly greater brightness.