Sky and Telescope - July 2017 - 18
Strange Stellar Physics
Outer crust: ions, electrons
Inner crust: ion lattice soaked
in superfluid neutrons
Outer core: superfluid neutrons,
Inner core: unknown
~2×1015 g cm −3
~2× nuclear density
2×1014 g cm −3
4×1011 g cm −3
diagram not to scale
WHAT LIES WITHIN Here is one approximation of what a neutron star's
interior might look like. The "neutron drip" boundary is the density at
which no more neutrons can be added to nuclei; the few nuclei that exist at the neutron drip density have 20 to 40 times more neutrons than
protons and sit in a vast sea of free neutrons. Inside that boundary the
density skyrockets, surpassing the maximum packing ability of neutrons
and potentially creating exotic matter phases. The predicted density in the
core is 100 trillion times greater than lead at room temperature.
differently than the cold matter inside a neutron star. (It's
"cold" not because the temperature is low, but because the
thermal energy is so small compared to the energies of other
internal interactions that it's unimportant.) Second, the weak
interaction acts over a relatively long time scale, much longer
than the time that particles have to interact with one another
when crushed against each other in a collider. Imagine that
a plane were ﬂying nearby, in the opposite direction to yours.
You might have time to catch a glimpse of a passenger on
that ﬂight, but you certainly wouldn't have time for a hearty
handshake - even if it were physically possible.
Still, if the creation of neutrons during the implosion were
all that could happen to the core's atomic nuclei, astronomers
would by now consider the question of what lies inside a neutron star a solved problem. But transforming run-of-the-mill
atoms into a super-dense soup of (almost) entirely neutrons
turns out to be only the beginning of the particles' journey.
The Fate of Collapsing Matter
As neutrons become squeezed further together in neutron
star cores, they reach densities that are difﬁcult to fathom.
While the star's crust may look more or less like normal matter, the core can reach densities that are a hundred trillion(!)
into quark soup
Normal nuclei and atomic structure
Quarks regroup to
Leaves about 1 proton
for every 10 neutrons
in stellar core
Density increases by 100 trillion times
Particles form a
FATE OF COLLAPSING MATTER When a star collapses to form a neutron star, the atoms in the core (left) are crunched together, with the majority of the electrons and protons combining to form neutrons. Protons and neutrons are each made of three subatomic particles called quarks
(center). But under such incredible densities, these nuclei might transform further (right): The protons and neutrons could dissolve into a quark
soup; the quarks could change and regroup to form particles called hyperons, which contain at least one strange quark; the nuclei could unite in a
single quantum state, called a Bose-Einstein condensate; or something else that we haven't imagined could be created.
J U L Y 2 0 1 7 * SK Y & TELESCOPE
IN TERIOR: B. LINK (M ON TA N A STATE UNIV.) / N ASA; COLL A PSING M AT TER: CO MPOSITION: G REGG DINDER M A N / S&T, BOSE-EINSTEIN CONDENSATE: OIST
H, He . . .