Neutron stars are one of the most extreme and exotic objects in the known universe.
What are neutron stars?
Neutron stars are the remains of the cores of massive stars that have reached the end of their lives.
They are one of the two possible evolutionary endpoints of the most massive stars, the other being black holes.
The densest stellar objects, apart from perhaps, whatever exists at the heart of a black hole, neutron
stars are some of the universe's most extreme objects.
NASA estimates that there are as many as a billion neutron stars in our Milky Way galaxy. The majority of
neutron stars discovered thus far are young neutron stars that sweep energetic radiation over the Earth as
they rotate at incredible speeds. Older neutron stars that have had billions of years to slow their spin
and cool are less conspicuous but are no less fascinating.
How do neutron stars form?
he life of a star, no matter its size, is a balancing act between the inward "push" of gravity and the outward
push provided by photons generated as they conduct nuclear fusion, the forging of heavy atomic nuclei from
light nuclei, at their cores. When stars run out of hydrogen to fuse into helium, they reach the end of
their main sequence of nuclear-fuel-burning lives. The outward energy ceases, and gravity wins out, causing
the core of the star to collapse in on itself. As this happens, nuclear fusion in the outer shell of the
star continues and this causes these outer layers to "puff out." These shed outer layers cool around the
still collapsing core, which, if it is massive enough, will begin a new round of nuclear fusion, forging
helium into heavier elements like carbon.
Even stars with masses between 10 and 20 times that of the sun, reach a limit to the heavy elements they can
forge (Green, Jones, 2015, pg 251), usually ending up with a core of almost pure iron. Even this heavy
element isn't dense enough to prevent massive cores from further collapse. As this occurs the gravitational
pressure is so intense that the negatively-charged electrons and positively-charged protons that comprise
the iron nuclei in this stellar core are crushed together, creating a sea of uncharged, or neutral neutrons.
Some massive stellar cores are at this point saved from further collapse by a quantum phenomenon called
"neutron degeneracy pressure," which occurs when such a density is reached that neutrons can no longer be
packed any closer together, leaving them as neutron stars.
WHAT HAPPENS WHEN TWO NEUTRON STARS COLLIDE?
Neutron stars can exist in isolation, only detectable by their surface temperature, or they can dwell in
partnerships with "ordinary" stars, often siphoning off their material, or in some cases, they can exist in
binary systems with another neutron star.
In these circumstances, according to Einstein's theory of general relativity, as these binary neutron stars
orbit each other they create ripples in space-time called gravitational waves. Just like material falling
to the surface of a neutron star grants it angular momentum, as gravitational waves ripple away from binary
neutron stars they carry angular momentum out of the system.
Loss of angular momentum causes the neutron stars to draw close together, and as this happens they radiate
gravitational waves even more strongly, increasing the rate at which angular momentum is lost.
Eventually, this causes the neutron stars to collide and merge to create an even larger neutron star. The
violent event, an explosion known as a kilonova, which after as long as a billion years of prelude with the
stellar remnants dancing around each other, lasts just a few milliseconds. Kilonovas release energy
brequivalent to millions of times that of the sun, sending out an intense burst of space-distorting
gravitational waves and a short but powerful burst of gamma rays, and be responsible for forging heavy
elements like gold, silver, and platinum.
