February 28, 2020 0

Neutron Stars: Crash Course Astronomy #32

When an 8 – 20 solar mass star ends its
life, it does so with a bang: a supernova. And when it’s all over, there’s a couple
of octillion tons of superheated plasma expanding away from the explosion site at a fraction
of the speed of light, a whole mess of energy released in the form of light and neutrinos,
and a bizarre little ball of quantum nastiness in the center composed almost entirely of
neutrons. The properties of this neutron star are almost
as bizarre as things get in the Universe. And if it all seems rather alien to you, well,
that’s OK. For a little while, astronomers wondered if aliens really were behind what
they were seeing. Now I’m not sayin’ aliens. When we last left the core of a high mass
star, it was in a bad way: Milliseconds ago it was fusing silicon into iron, but now it’s collapsing
under its own immensely powerful gravity. The collapse takes a fraction of a second,
but a lot happens in that fraction of a second. In lower mass stars, the core supports itself
via electron degeneracy pressure, the result of a rule in quantum mechanics that says electrons
vehemently resist being squeezed together. But even electron degeneracy fails to stop
the collapse if the core has a mass more than about 1.4 times the mass of the Sun. That’s just
too much of a load to bear, and the collapse continues. Under these huge pressures, a funny thing
happens: Protons, electrons, and other subatomic particles get smashed together, and they merge
to form neutrons. And this happens to almost ALL of them. When the core collapses down
to about 20 km in diameter, it’s essentially a ball of neutrons with some protons and electrons
here and there that survived, and a crust of normal but highly compressed matter on
top. When this happens yet another effect comes
into play: neutron degeneracy. Like electrons, neutrons resist being squeezed too tightly
together, but this time the strength of the pressure is far, far stronger than for electrons.
If the core is less than about 2.8 times the Sun’s mass, the collapse runs into a wall.
It stops. This generates a huge shock wave, which, along
with a flood of energetic subatomic particles called neutrinos, blasts outwards, blowing
up the star. What’s left of the core after the metaphorical
smoke clears is a neutron star, one of the most bizarre objects in the Universe. Such a star would be extremely weird. Or really,
just EXTREME. Its mass would be more than that of the entire Sun, all packed into a
sphere maybe 20 km across. Now let’s just stop there for a sec and
let that sink in. The Sun has a mass 300,000 times the Earth. Imagine packing that all
into a ball THE SIZE OF A SMALL CITY. Too mind boggling? OK, think of it this way:
You are mostly empty space. Every atom in your body has a nucleus made of tightly packed
neutrons and protons, and electrons whizzing around outside them. If you could magnify
an atom to be 100 meters across, the nucleus would be roughly the size of a marble. Imagine
all that empty space between the nucleus and the electrons. That’s a normal atom. But in a neutron star, ALL OF THAT SPACE would
be filled with neutrons. All of it. Every nook and cranny inside the neutron star has
matter in it, all the way down to the scale of an atomic nucleus. This is what gives a
neutron star its mind-crushing properties. I’m now going to barrage you with very large numbers.
So, take a deep breath, and you might wanna sit down. Neutron stars are ridiculously dense. A single
cubic centimeter of neutronium, as neutron star stuff is usually called, has a mass of
about 400 MILLION tons. Want some perspective on that number? Well, very roughly, that’s
the total mass of every single car and truck in the United States. Imagine a couple of
hundred million vehicles, crushed down until they could all fit inside this six-sided die.
That’s neutronium. It’s so dense that, as far as it’s concerned,
normal matter is a slightly polluted vacuum. If you set it on the ground it would fall
right through the Earth. Now, anything that dense has a HUGE gravitational
pull. If you were on the surface of a neutron star… well, you’d be very dead, obviously
— like immediately, flattened down to a thickness of just a few atoms. And that’s because
a typical neutron star has a surface gravity 100 BILLION times stronger than Earth’s.
I have a mass of about 77 kilos, and here on Earth I weigh about 170 pounds. On a neutron
star, I’d weigh 17 trillion pounds. That’s 23,000 times the weight of the Empire State
Building. But wait! There’s more! In our introduction to the solar system I
mentioned that when you take a spinning object and shrink it the spin will increase—the
usual example is an ice skater drawing in his arms, increasing his rotation until he’s
a blur. The same is true for the star’s core when
it collapses into a neutron star. It may have had a very slow spin before the supernova,
maybe even taking weeks to spin once. But when it shrinks down to just 20 km across
and becomes a neutron star, that rotation will increase by a HUGE factor. A freshly minted
neutron star might spin several times per SECOND. The magnetic field increases as well. A star
like the Sun has an overall magnetic strength not too different from the Earth’s. But when
that core collapses, the strength of the field skyrockets, and a neutron star can easily
have a magnetic field several trillion times stronger than the Sun’s. That’s strong
enough to erase your credit card from a hundred thousand kilometers away. See? Ridiculous. All of these properties are brain-melting. But are
they real? Could an object like this really exist? Oh my, yes. The first neutron star was detected
in 1965, though not recognized for what it was at the time. A couple of years later another
one was found, and this time was correctly identified as a neutron star. But then things got…weird. In 1967, Jocelyn Bell was a graduate student
helping build a radio telescope. There was a persistent noise in their data they couldn’t
seem to fix. Bell studied it night after night, finally figuring out that the pattern wasn’t
a problem with their data, it was from an actual astronomical object. She had discovered
the first known pulsar. What’s a pulsar, you ask? Pulsars are neutron stars. In a nutshell,
their rapid rotation coupled with their incredibly strong magnetic fields launch twin beams of
energy away from the star, like the beams from a lighthouse. The beams sweeps around
as the star rotates, and from Earth we see this as a pulse, a blip, of increased brightness.
This pulse can be detected in visible light, radio waves, and even X-rays! The spin of a neutron star is amazingly stable,
making these pulses act like a very accurately timed cosmic clock. In 1967, no one could
believe a natural object could do this, and this object was half-jokingly given the name
LGM-1. Little Green Men 1. Now we know of over a thousand pulsars in
just our galaxy alone, and we know they are the leftover cores of massive stars that exploded.
Some spin with periods many seconds or even minutes long. Some are in binary systems;
another normal star orbits them. If they’re close enough together the neutron star can
rip material off the other star and feed on it. This increases the pulsar’s spin, and
we know of a few that have incredibly rapid rotation rates; some spin hundreds of times
per second! These are called millisecond pulsars, and if they spun much faster the centrifugal force
would rip them apart despite their tremendous gravity! Even after a thousand years, a pulsar can
still be a force to reckon with. There’s a pulsar in the center of the Crab Nebula,
the remains of a star that exploded to create that supernova remnant. A substantial fraction
of the light emitted from the nebula is powered by the pulsar itself; its fierce output energizes
the nebula, causing it to glow brightly even after a millennium. I’m telling ya, thinking about neutrons
stars makes the hair on the back of my neck stand up. And I haven’t even mentioned magnetars yet. Neutron stars are more than just weird little
balls of neutrons. They have a crust, probably a few centimeters thick, made of highly compressed
but more or less normal matter, squeezed into a kind of highly stiffened crystal state.
The magnetic field of the star penetrates this crystalline crust and stretches out for
quite a distance. In some neutron stars the magnetic field is
even stronger than usual, and can be — get this — a QUADRILLION times stronger than
the Sun’s. These über-powerful neutron stars are given the name magnetars. They’re
relatively rare; maybe 10% of all neutron stars are born as magnetars. And they have
short lifetimes; the magnetic field is so strong it acts like the brakes on a car, slowing
the neutron star’s spin. That spin helps power the magnetic field, so the field weakens
as the star slows. But while they’re around, magnetars are the
most magnetic objects in the Universe. And with great power comes great responsibility…
if your responsibility is to be one of the scariest objects in the Universe.
Why? In a neutron star, the crust and magnetic
field are locked together, so a change in one affects the other. The crust of the star
is under incredible strain due to the intense gravity and rapid rotation. If the structure
slips, it can snap, creating a star quake — like an earthquake, but just a WEE bit
stronger. In an earthquake, huge amounts of energy are released when the Earth’s crust
shifts and snaps, enough to destroy buildings and quite literally move mountains. But in a neutron star this effect is multiplied.
Hugely. Remember, the crust is phenomenally dense, and the gravity is enormous. If the
crust strains and snaps, dropping just a single centimeter, the resulting release of energy
is vast beyond imagining. This energy is released as a tremendous explosion
in the crust, shaking it. This also shakes the magnetic field, which reacts…poorly. When the Sun’s magnetic field throws a tantrum,
we get a solar flare, which can be as powerful as billions of nuclear bombs. A magnetar flare
dwarfs that into insignificance. It can be trillions of times stronger than a solar flare
— in a fraction of a second, a magnetar can release as much energy as the Sun gives
of in a quarter of a million years. In 2004, astronomers were stunned when a huge
blast of X-rays slammed into orbiting satellites. One of these satellites, named Swift, actually
had its detectors saturated with X-rays, even though it wasn’t even pointing at the source
at the time! The X-rays literally came right through the side of the satellite with such
intensity that Swift — which was designed to detect powerful X-ray sources — was momentarily
blinded by them. The source of this X-ray burst was quickly
determined to be a magnetar called SGR-1806-20, and the effects were incredible: It actually
compressed the Earth’s magnetic field, and partially ionized the Earth’s upper atmosphere. Oh, did I mention that this magnetar is 50,000
light years away? That’s halfway across the galaxy. That’s INCREDIBLE. At a distance
of 500 quadrillion km its effects were felt more strongly than a powerful flare from the
Sun! The good news is that there are very few magnetars
like this in the galaxy, probably fewer than a dozen. Also, catastrophic explosions like
the one in 2004 are rare; if one had had happened any other time in the past 40 or so years
we would’ve detected it. And frankly, it’s really cool that we had astronomical satellites
orbiting the Earth which could study it! We’ve come a long way in understanding neutron
stars since they were first discovered, but there’s much about them we don’t understand.
Every time we find out more we find out they’re even weirder than we first thought. And yet, for all that, they’re not the weirdest
things in the sky. Not by a long shot. That place is held pretty securely by the other
type of object created in a supernova: a black hole.
Stay tuned. Today you learned that when a star between
8 and 20 times the Sun’s mass explodes, the core collapses to form a neutron star.
Neutrons stars are incredibly dense, spin rapidly, and have very strong magnetic fields.
Some of them we see as pulsars, flashing in brightness as they spin. Neutrons stars with
the strongest magnetic fields are called magnetars, and are capable of colossal bursts of energy
that can be detected over vast distances. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch even more awesome
videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,
and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited
by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought Café.

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