Sky and Telescope - July 2017 - 16

STRANGE STELLAR PHYSICS

by Feryal Özel

Neutron Stars
The Inside Story of

It

is a rare opportunity when we can use astronomical observations to push the frontiers of physics in a
way that is not possible in any laboratory on Earth.
Yet neutron stars, the dead, dense remnants of massive stars,
provide us with just that opportunity.
Crushed under the inward pull of gravity once a massive
star's fuel is exhausted, the stellar core that becomes a neutron
star reaches matter densities that are not naturally encountered anywhere else in the universe. (Black holes don't count:
Although they may have inﬁnite energy densities, they hide
behind horizons and are inaccessible.) In fact, the matter that
makes up neutron stars has fundamentally changed character,
taking us into regimes that are still poorly understood in physics. As a result, a lot of recent efforts have focused on probing
these stars' interiors with astronomical observations.
To be sure, there are some aspects of that unusual matter
that we can conﬁdently predict, based on theoretical calculations and laboratory experiments. For example, during the implosion of the star, the electrons that normally
surround the nucleus of an atom in the core get pushed into
the atomic nuclei. There, they combine with the protons
through weak interactions, one of the four types of interactions that take place between particles in the universe. (The
other three are strong, gravitational, and electromagnetic.)
That combination produces a neutron - thus giving this new
star its name - as well as nearly massless, ghostly particles
called neutrinos that rapidly escape from the star, carrying
with them a large amount of energy. The ﬁrst observational
conﬁrmation of this process happened with Supernova
1987A, observed in 1987 in the Large Magellanic Cloud (S&T:
Feb. 2017, p. 36), when two neutrino observatories detected
a burst of particles around the same time as the supernova's
visible light appeared in the sky.
Scientists can't create such neutron-rich matter. Simply
smashing nuclei together and squeezing them to high densities in heavy ion colliders, such as the RHIC experiment in
Brookhaven National Laboratory, doesn't work. First, collisions in accelerators create very hot matter, which behaves

16

J U L Y 2 0 1 7 * SK Y & TELESCOPE

Astronomers
are using X-rays,
gravity & clever
calculations to
discover what lies
within these
extraordinary
objects.

Sky and Telescope - July 2017

Table of Contents for the Digital Edition of Sky and Telescope - July 2017

Contents
Sky and Telescope - July 2017 - Cover1
Sky and Telescope - July 2017 - Cover2
Sky and Telescope - July 2017 - 1
Sky and Telescope - July 2017 - Contents
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Sky and Telescope - July 2017 - Cover3
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