54 | New Scientist | 21/28 December 2019
The cosmos is home to some truly rogue stars.
Stuart Clark introduces the most outlandish
W
ITH hundreds of billions of stars in
our galaxy alone, you would expect
a few oddballs – and you would
be right. Stars do follow a more or less set life
path, determined by their mass. But we are
increasingly finding that the details of those
lives can diverge more than we ever imagined.
In some cases, we are discovering stellar
characteristics and habits so outlandish that
they challenge our understanding of physics.
From a cannibalistic star to one that makes
impossible elements and another that refuses
to die, here is our introduction to some of the
strangest stars in the universe.The one that got away
Our galaxy is leaking stars. That is the only
conclusion astronomers have been able to
draw from the discovery of a few dozen stars
travelling so fast that not even the gravity
of the Milky Way can hold on to them. The
record holder is S5-HVS1, which clocks in at
1700 kilometres per second – so fast that it has
already broken out into the lonely reaches of
intergalactic space. But how has an enormous
ball of gas accelerated to such a speed?
When Warren Brown at the Harvard-
Smithsonian Center for Astrophysics identified
the first hypervelocity star in 2005, it appeared
to have come from the centre of the galaxy.
That pointed the finger at the supermassive
black hole there. Brown’s calculations showed
that if a pair of stars passed close enough, the
black hole would snatch hold of one of them
and shoot the other off into space.
The plot thickened last year, when Monica
Valluri at the University of Michigan led a
team to investigate LAMOST-HVS1, the closest
hypervelocity star to the sun. This one seems
to have been ejected from a star cluster, whichis odd, because there are no black holes in star
clusters – or so we thought. But it would make
sense if these clusters contained a suspected
class of black holes, known as intermediate
mass black holes, that have so far evaded
definitive detection. “It was very surprising,”
says Valluri. “If we find more hypervelocity
stars coming from star clusters, then we really
need to investigate what’s sitting at the centre
of those clusters,” she says.
Perhaps the most intriguing thing about
hypervelocity stars, however, is what they
could reveal about dark matter. This is the
hypothetical substance thought to hold
galaxies together. By studying the trajectories
of the hypervelocity stars ejected from
the galactic centre, astronomers hope to
deduce the shape of the dark matter cloud
surrounding the Milky Way. That could give
us a better idea about how dark matter
interacts with itself, providing a vital new
clue to its identity.The star that makes
“impossible” elementsWhat it lacks in vowels, Przybylski’s star
(pronounced shi-BILL-skee) makes up for
in intrigue, being the only star suspected to
point to a radical extension of atomic physics.
The mystery began in 1961, when
astronomer Antoni Przybylski discovered
something peculiar about the spectrum of
light from star HD 101065. Spectra are
nature’s barcodes. They feature dark lines
superimposed on a spectrum of colours,
with the positions of the lines relative to the
colours telling us about the star’s chemical
constituents. Ordinarily, the patterns are
easy enough to decipher, but in the case RE
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