Sky and Telescope - July 2017 - 19

G REGG DINDER M A N / S&T, SOURCE: FERYA L ÖZEL; SPAG HE T TI: M. E. CA PL A N & C. J. HOROWIT Z / A R XIV:16 0 6.036 46 (2016)

AROUND THE BEND
Because they're so
compact, neutron stars
distort the spacetime
around them (possible
paths in turquoise).
Outgoing starlight is
redirected accordingly. A telescope
would normally only
detect light from the
hemisphere of a star
that's facing it. But
the more compact the
star, the farther around
the star the telescope
"sees." Shown are the
paths (light blue) of
light emitted by two
neutron stars of the
same mass (1.6 Suns)
but different sizes: The
larger star has a radius
of 14 km, the smaller
one of 8 km.

Larger neutron star

To Earth

as seen from Earth

Smaller neutron star

To Earth

times higher than the densest natural elements on Earth.
Under those conditions, neutrons not only start vehemently repelling one another but also interacting in new
ways. This is because neutrons are examples of particles called
fermions, which require increasingly higher energy to be conﬁned closer and closer together. To counter this rising energy,
neutrons may ﬁnd it "energetically favorable" - basically,
less hassle - to dissolve into their even smaller constituents,
called quarks, creating a quark soup. Alternatively, they may
form different combinations of quarks than what normally
make up a neutron or proton. Such hyper-nuclei, called hyperons, can be created in laboratories but survive only for a short
time. In neutron stars, they might be stable.
Yet another possibility is that fermions pair up with one
another to form a type of particle called a boson. These particles, which behave differently from fermions, can transition
into an unusual superﬂuid state of matter (the same state
that is observed in low-temperature helium ﬂuids, superconductors, and some other metals, called a Bose-Einstein condensate). This state will have strange properties, such as ﬂowing
without friction. If this transition indeed occurs inside the
core - and we have good reason to think that it does - it will
relieve some of the pressure built up by the high densities that
matter experiences there.
But which of these possibilities actually take place in a
neutron star's interior? One of the things that complicate this
puzzle is that, while we've observed hyperons and many types
of bosons as standalone particles in laboratories, a quark has
never been observed by itself, in what is called an "unconﬁned" state. Therefore, predicting and testing the behavior of
quark matter becomes very difﬁcult.

as seen from Earth

NUCLEAR PASTA
At the bottom of a
neutron star's inner
crust, the density is
so high that nuclei
may transform into
extended tubes (called
"spaghetti"), sheets
("lasagna"), and other
strange phases of
matter. These are
called nuclear pasta.
The pasta layer would
have both solid and
liquid properties, akin
to liquid crystals.

But if one could see into the cores of neutron stars and
prove that they contain quark matter, it would constitute a
major advance in our understanding of these smallest constituents of matter.

Probing the Invisible
How would astronomers go about probing the deep interiors
of not only the densest but also the smallest stellar objects
in the universe? At roughly 20 km (12 miles) across, neutron
stars are smaller than some of the solar system's asteroids.
Yet they pack into that tiny volume up to two times the mass
of the Sun. And unlike asteroids, they are hundreds to many
thousands of light-years away.
It turns out that measuring the exact sizes of neutron
stars, which can be done from a distance, provides the best
possible tool for getting a complete picture of their interiors. Our calculations tell us that, if only neutrons remain
in the interior, the pressure building up from the repulsive
interactions will support a star of a particular size. If any
constituents other than neutrons form, their interactions
would cause a different amount of repulsion, creating a star
of a different size. Thus, astronomers can discover the possibilities in the realm of physics simply by measuring exact
diameters of these stars.
Astronomers are used to measuring the sizes of far-away
objects by collecting and analyzing the light they emit.
Indeed, nearly all our knowledge about the sizes of normal
stars comes from measuring both the total light emitted by
the star, referred to as its luminosity, as well as the breakdown of that emission into different wavelengths of light,
known as its spectrum. The spectrum of the star allows us
s k y a n d t e l e s c o p e .c o m

* J U LY 2 017

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Sky and Telescope - July 2017

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