Life After Cosmic Death: Neutron Stars, Pulsars, and Magnetars
Many celestial objects fascinate astronomers: among them are black holes, quasars, galaxy superclusters, pulsars, magnetars, and neutron stars. Today, we're going to be talking about the latter three. Let's explore how they are formed, how they're detected, how they influence the universe around them, and what discoveries astronomers have made regarding them. To understand what pulsars and magnetars are, we must first delve into the mechanics of neutron stars.
What are neutron stars?
There are many different types of stars scattered throughout the universe. They range from extremely small and dim red-dwarfs to massive and luminous supergiants. It may seem counterintuitive, but typically the smaller the star, the more stable it is and the longer its lifespan. It burns through its hydrogen content at a much slower rate than its supergiant counterparts. Therefore the more massive the stars, the quicker they typically extinguish their gaseous fuel. Once they reach the end of their lifespans, massive stars – typically those weighing between four and eight solar masses – implode on themselves and shed their outer layers of gas, becoming a phenomenon known as a supernova.
Some stars are so massive – usually weighing about 20 to 100 times more than the Sun – the collapse continues until the core becomes an infinitely dense point, forming stellar-mass black holes. Those that are smaller leave behind what is known as a neutron star. Believe it or not, these are among the most compact objects in the universe, and they're extremely small, measuring just about 12.5 miles (20 km) across. They're also so dense that they weigh as much or more than the Sun. According to NASA, "One sugar cube of neutron star material would weigh about 1 trillion kilograms (or 1 billion tons) on Earth – about as much as a mountain."
So how does this once-massive star become such a dense object? Well, the process is kind of complicated, but extremely interesting. The matter that makes up stars contains both electrons and protons, but once the star runs out of fuel and begins to collapse, most of the atoms are compacted together tightly and converted into neutrons. All of the elements that once made up the star's core become neutrons. Throughout the process, the neutrons release immense amounts of subatomic particles called neutrinos. In fact, it's estimated that one supernova radiates 10 times more neutrinos than there are particles, protons, neutrons, and electrons in the sun.
Above 1.44 solar masses, the neutrons are able to stop the star's core from completely collapsing in on itself to form a black hole thanks to a phenomenon known as neutron degeneracy pressure. As the star contracts, the neutrons are forced into higher and higher energy levels. This creates an effective pressure that prevents further gravitational collapse, forming a neutron star.
According to NASA, if we were to shrink a Boeing 747 into a neutron star, it would be the size of a mere grain of sand, whereas the Earth would be the size of a basketball.
Often, neutron stars are difficult to detect because they are incredibly small, and they don't emit much radiation. However, sometimes they do emit pulsating X-rays or electromagnetic radiation that is detectable, and sometimes their rotational axis and magnetic dipole axes are misaligned, which brings us to...
What are pulsars?
All pulsars are neutron stars, but not all neutron stars are pulsars. Some neutron stars are positioned in such a way, relative to Earth, that we can see their rotational axis. They spew large quantities of radio waves and other forms of electromagnetic radiation at regular intervals, making them something called pulsars. Pulsars spin at extreme speeds – sometimes as fast as 70,000 km per second – even faster if the object happens to be located in a binary system. From our perspective, they just look like flickering stars, but they "blink" in a predictable, rhythmic way. In reality, they don't actually blink at all, it just appears that way because of how quickly they spin.
What sets pulsars apart is the fact that they have very strong magnetic fields, which funnel jets of particles out along both the magnetic poles. These accelerated particles produce powerful beams of light. Like a lighthouse, the light comes in and out of view, giving off the impression that this tiny object, which again, is merely the size of Manhattan, is flickering on and off like a candle in the wind.
Another thing that factors into the manner in which pulsars come in and out of view is that they often have a misalignment between their magnetic field and their spin axis. So, the beams of particles and light are swept around as the star rotates. When the beam sweeps over the Earth, the pulsars appear to turn on and off.
Pulsars and neutron stars have extremely strong magnetic fields. So strong, in fact, their magnetic fields can range from 100 million to more than a million billion times stronger than the magnetic field we experience on Earth's surface. You would die if you were to stand on (or near) the surface of a neutron star or pulsar because the gravity is so extreme. Moreover, it's estimated that if you were to stand at the equator of a pulsar, the rotational velocity may be more than 1/10th the speed of light. Basically, you don't want to find out what would happen if you stood on their surface. NASA notes that the gravitational pull is so powerful, "a marshmallow impacting the star's surface would hit with the force of a thousand hydrogen bombs."
Approximately one out of ten neutron stars become terrifying celestial objects known as a magnetar.
What is a magnetar?
While extremely rare, magnetars are among the strongest, most magnetic objects in the universe. Per NASA, "magnetars have magnetic fields a thousand times stronger than ordinary neutron stars that measure a million billion Gauss, or about a hundred-trillion refrigerator magnets. For comparison, the Sun's magnetic field is only about 5 Gauss."
As we mentioned before, neutron stars and pulsars can become stronger when they belong to binary star systems. One theory for how magnetars form is through the interactions of two very massive stars orbiting one another in a compact binary system. Alternatively, they may form when a neutron star siphons gaseous materials from a living star. Magnetars are like vampire neutron stars in that way. Some steal material from their companions to become stronger. Others function differently, but the most basic could strip all credit cards on the planet from less than half an Earth-Moon distance away.
Oddly enough, some magnetars get their magnetic energy from a phenomenon similar to earthquakes, called star-quakes. They form when the magnetar's crust is heated up by gravitational stresses and the object's extreme magnetic field to the point that it emits an extreme amount of energy, called a gamma-ray burst.
One of the most powerful gamma-ray bursts of all time came from halfway across the galaxy in 2004. Known as SGR 1806-20, it ionized the Earth's upper atmosphere and generated more energy in 1/5th a second than our Sun does in over 250,000 years. If it were closer, it could have caused a mass extinction event on our planet. In other words, it was a more powerful blast than a supernova.
Each of these objects is unique and amazing in its own way. While what we know is already fascinating, there's still much about them to be learned and understood.