If you can't see neutron stars, how do you know they are there?
Specially designed telescopes can sense the X-rays that neutron stars produce. However there is another way that neutron stars can be identified.
Stars have magnetic fields associated with them. (Some planets do too.) When a supergiant's core is compressed into a neutron star the magnetic field is compressed with it and that causes the neutron star to have an amazingly strong magnetic field. We say it has a high "magnetic field density". These tight magnetic fields cause electrons and protons in the area, orbiting the neutron star, to emit light! This light is emitted due to the way the magnetic field interacts with the charged particles (protons and electrons) and is similar to the way a radio transmitter sends out a signal. It isn't important to understand exactly how the light is made but it's worth knowing that most of the light is emitted along the magnetic axis of the star because that is where the magnetic field lines are squeezed the tightest and are the strongest. If the field lines are incredibly strong the light emitted will be at the high end of the light spectrum - ultraviolet or even X-rays. Sometimes the emitted light is in the visible range. Most neutron stars, however, have magnetic fields which cause their neighboring atoms to emit light in the lowest part of the spectrums - radio waves. These radio waves travel through space and are easily detected by radioastronomers using radiotelescopes here on Earth.
This gives rise to an important property of most neutron stars
- but to understand it you have to think about the axis of the
neutron star's magnetic field and its axis of rotation. They are
NOT the same axis. (Usually.)
You are familiar with the Earth's magnetic field axis and its axis of rotation, so let's start with something you know. You know (from your January lessons) that the Earth's axis of rotation points towards Polaris. If you look at a globe or map of the Earth, you will see there is a clearly defined "North Pole" at the top. (The latitude of the North Pole is 90oN.) This is called the "geographic North Pole". The axis of rotation is the axis about which the planet (or star) rotates. You know (from your lesson on the auroras) that the Earth's magnetic North Pole is NOT at the geographic North Pole. Instead, the Earth's magnetic North Pole is in Canada! | ![]() |
Imagine a line drawn through the Earth's magnetic North Pole and out the
other side (at the Earth's magnetic South Pole, somewhere in Antarctica,
but NOT at the geographic South Pole).
By the way, this has nothing to do with the Earth's "tilt" (of about 23.5o) which is the angle of the Earth's rotational axis with respect to the Earth's orbital plane. (We will return to this tilt of the Earth's axis in a few months when I teach you about the seasons.) Don't allow the tilt of the Earth to confuse you. Here, in this lesson, we are talking about the angle of the magnetic field with respect to the rotation axis. Indeed, to keep things simple I have drawn the Earth with its rotational axis straight up (ignoring its tilt).
Anyway, let's stay on track by thinking about the Earth's magnetic line - running from magnetic north (in Canada), through the Earth and coming out of the Earth at the magnetic south (in Antarctica).
![]() |
Extend that line into space
on either side of the planet and you have the Earth's "magnetic
axis". It will be at an angle from the rotational axis.
That means as the Earth rotates, its magnetic axis will swing along with it. While the geographical axis constantly points to Polaris, the magnetic axis makes a nice circle though the sky. If you were far off in space and the Earth's magnetic North Pole was pointing at you right now, it would swing away from you as the Earth rotated. It would not return (to point at you) for 24 hours (one Earth rotation). | ![]() |
What's this gotta do with neutron stars!?
Most neutron stars also have magnetic fields that are not aligned with their rotational axis so they produce a "beam" of emitted light, mostly radio waves, that circles through its "sky".
![]() |
Most of the neutron star's emitted
radio wave is sent along the magnetic axis because the magnetic
field strength is at is greatest along the magnetic field axis.
As the neutron star rotates, its beam of radio waves will swing away from us and not return - not point again in our direction - until the neutron star has completed a rotation. Therefore a neutron star's radio beam will wink on and off at a regular interval. The length of that interval depends upon the time it takes the neutron star to rotate. | ![]() |
The effect is very much like the pulses of light that a lighthouse produces as it rotates its beam.
All stars rotate, even supergiants. When the core of a supergiant
collapses and produces a supernova Type II, it causes a great
deal of the star's core mass to contract. As the core contracts
its spin speeds up. This is an easy bit if physics to understand
(for a change ). If you watch an ice skater perform you may have
noticed that she will pull her arms in to make herself spin faster.
She'll move her arms outward to slow her spin. This change in
"spin speed" is caused by the conservation of her angular
momentum. Angular momentum is a spinning version of momentum
- when something spins it stays spinning (ignoring friction) and
if the radius of spin is changed its rate of spin changes to compensate.
A spinning skater can increase the rate of her spin by decreasing
the radius of her mass. She does this by drawing her arms in towards
the center of her body (the center of her spin). That tiny redistribution
of her mass, bringing the mass of her arms inward, can increase the rate
of her spin several fold.
Now imagine that same physics applied to a collapsing supergiant's
core as it becomes a neutron star. The core is already pretty
massive, like a white dwarf, and it may have a rotation of (say)
once per week, an average rotation rate for a star. As the core
collapses it moves most of its mass closer towards the center
of its spin and it packs it in VERY tight. Remember, the core
of a supergiant is about the size of the Earth but it gets squeezed
into a diameter of only 10 kilometers when it becomes a neutron
star. That's a HUGE redistribution of mass and it causes a HUGE
increase in the rate of spin.
The slowest neutron stars take only
a few seconds to compete one rotation and most neutron stars spin
much faster!
A rapidly rotating neutron star whose magnetic pole is off-center to its axis of spin will appear to send out pulses of light (possibly X-rays, visible light, etc. but most likely radio waves) in regular, rapid bursts. This is called a pulsar. Each time the neutron star's magnetic axis points our way it will shine its beam of light at us and then turn it off as it rotates its magnetic field away. Each pulse is caused by a complete rotation.
Astronomers had speculated about the existence of neutron stars since the 1930s but it wasn't until the dawn of radioastronomy that their existence was actually proven. Neutron stars were discovered by finding pulsars.
In 1968 some radioastronomers detected several radio signals
from outer space. The signals were from specific areas of
the sky, powerful and rapid. They were amazed at how
regular the radio pulses were. They didn't know about the physics
of radio waves from rotating neutron stars and they couldn't imagine
any natural phenomena that would create such "clock like"
regularity. They went so far as to speculate that they were receiving
signals from an intelligent, extraterrestrial species! In their
data books, alongside the tracings of these radio signals, they wrote
the initials "LGM" to stand for "Little Green Men"!
But before calling the president or the papers they decided to
call some physicists to get their opinions. The subject of neutron
stars came up. They were told that that the magnetic fields of neutrons stars could produce
radio waves which would flash "on and off" as they rotated.
It soon became clear that they had not detected aliens
, but neutron
stars
.
Over 300 pulsars have now been identified and there may be over a million pulsars in our galaxy.
Pulsars are considered the best "time keepers" in the
universe. The very large mass of a neutron star means that they
have a great deal of angular momentum so it takes a lot of energy
to speed up or slow down a neutron star. That makes the timing
of the radio wave pulses very accurate - as accurate as an atomic
clock, the most accurate time-keeping device on Earth. Some scientists
have speculated that a space-faring civilization (aliens or perhaps
our descendants) could use distant pulsars to help with navigation. By measuring the position of several
pulsars, and knowing which pulse came from which pulsar
(based upon their pulse rate which will not change), you could
figure out your position in the galaxy. This is similar to the
way sailors can use lighthouse pulses to plot their position in
a harbor. The idea of using pulsars to determine your position
in space is a good one. Indeed, NASA scientists liked the idea
so much that they placed a "pulsar map" on the deep-space
probes called Pioneer. If an alien civilization ever stumbles
onto a Pioneer craft, they will be able to understand the map's
use of pulsars to show where the space probe came from. (In order
to send the invasion force to the correct star system! )
The precise timing of pulses from pulsars allow astronomers to make very precise observations. This data has been used to infer the existence of Earth-sized objects orbiting several pulsars and to confirm theories about gravitational radiation (involving concepts beyond the scope of this course).
Of course, all that spinning can't go on forever so astronomers carefully observe pulsars to collect data on their "spin-down rate". These observations show that most pulsars slow down at a very slow, steady rate. However, a few pulsars have sudden, "hiccups" in their otherwise perfect pulsing. These hiccups have been interpreted as being caused by the unusual quantum physics going on just below the crust of a neutron star and have been nicknamed "neutron star quakes".
Great! Where can I find a pulsar? Or a supernova?
Well, you need a radiotelescope to detect most pulsars. On the other hand, a supernova is easy to see because it is so bright!
As recently as 1987 a supernova was seen in the sky of the Southern Hemisphere in a nearby galaxy called the Large Magellanic Cloud. When I say "nearby" I am talking in relative terms. In fact the Large Magellanic Cloud is so far away that it takes 169,000 years for its light to reach us. (It's 169,000 light-years away.) So the star that went supernova did so, not in 1987, but 169,000 years earlier. Astronomers named it "supernova 1987A" - boring but easy to remember. Just before it became visible, supernova 1987A showered us with a neutrino burst. You can't feel neutrinos, so you wouldn't have even known it! Within a few days supernova 1987A reached a maximum magnitude of 2.3 and it was easy to see for several weeks, if you lived far enough south.
The progenitor star, called Sanduleak, was a blue supergiant. Based upon calculations and what is know about the physics of stars, astronomers estimate that it was only 20 million years old and about 20 solar masses. (Didn't I tell you big stars age quickly?) It is thought that Sanduleak was at one time a red supergiant but it had undergone a "spasm", a minor explosion, that caused it to lose a bit of its outer shell. That would have exposed its deeper, hotter, inner shells and allowed more heat to come to the surface giving it a hotter surface temperature and thus a bluer color. When it finally exploded it wasn't as bright as it could have been had it been a proper red supergiant.
It's been over ten years now but the shell of ejected materials
is still blocking our view of the center of the explosion and
the debris is probably blocking any radio waves. So far, no pulsar
has been detected from 1987A, but give it a few more decades for
the dust to clear.
The rich materials left over from a supernova explosion produce a thick cloud referred to as a supernova remnant (SNR).
Is there a supernova remnant in the northern sky?
Yes, there are several.
Perhaps the most famous is the Crab Nebula in TAURUS. In the year 1054 Chinese astronomers recorded a "new star" in the constellations we call TAURUS. Actually it was a supernova that exploded 6,000 years earlier but it took that long for the light to reach Earth. They recorded this "new star" as becoming the brightest object in the night sky, not counting the Full Moon. Like all "new stars" it didn't last long and within a few weeks it was gone. Today this nebula is a very dim object (magnitude 10) so you need powerful binoculars to see it. You may not be able to find it but it's still nice to know that it lies just to the right of Alheka, the 3.0 magnitude B-type star that is at the tip of TAURUS' right horn. | ![]() |
In 1731 John Bevis used a low power telescope to discover the nebula left behind by this old supernova and about a hundred years later the Earl of Rosse used his powerful telescope (72-inch diameter) to see the nebula in more detail. He thought it looked like a crab so he named it the Crab Nebula.
Here's a photo of the Crab Nebula (photo credit to NOAO) taken through a powerful telescope.
There is so much energy associated with this explosion that it still radiates in nearly every part of the spectrum, from radio waves to X-rays. This photo is of the visible spectrum. The nebula is moving away from the center at a rate of a few thousand kilometers per second and will eventually drift far off into space and form a nebula rich in heavy elements. In 1969, the year after the first pulsars ("LGMs") were discovered, astronomers detected a very faint flashing object in the center of the Crab Nebula. This is one of the few pulsars that emits light in the visible parts of the spectrum. The Crab Nebula's pulsar sends us a flash of visible light (and radio waves too) over 30 times each second! | ![]() |
What happens if you have a super, supergiant? You know - very massive.
To find out, continue on to the next lesson and learn about black holes!