Deep space objects > Pulsars and neutron stars
Pulsars are spherical compact objects, the dimensions of which do not go beyond the boundaries of a large city. Surprisingly, with such a volume, they surpass solar in terms of massiveness. They are used to study extreme states of matter, detect planets outside our system, and measure cosmic distances. They also helped find gravitational waves that indicate energetic events like collisions of supermassive black holes. First discovered in 1967.
What is a pulsar?
If you look out for a pulsar in the sky, it looks like an ordinary twinkling star following a certain rhythm. In fact, their light does not flicker or pulsate, and they do not act as stars.
The pulsar emits two persistent narrow beams of light in opposite directions. The flickering effect is created due to the fact that they rotate (beacon principle). At this moment, the beam hits the Earth, and then turns again. Why is this happening? The point is that the light beam of a pulsar is usually not aligned with its axis of rotation.
If the blinking is created by rotation, then the speed of the pulses reflects that with which the pulsar is rotating. A total of 2,000 pulsars were found, most of which make one revolution per second. But there are about 200 objects that manage to make a hundred revolutions in the same time. The fastest are called milliseconds, because their number of revolutions per second is equal to 700.
Pulsars cannot be considered stars, at least not “living”. Rather, they are neutron stars that form after a massive star runs out of fuel and collapses. As a result, a strong explosion is created – a supernova, and the remaining dense material is transformed into a neutron star.
The diameter of pulsars in the Universe reaches 20-24 km, and the mass is twice that of the Sun. For you to understand, a piece of such an object the size of a sugar cube will weigh 1 billion tons. That is, you have something weighing the size of Everest in your hand! True, there is an even denser object – a black hole. The most massive reaches 2.04 solar masses.
Pulsars have a strong magnetic field that is 100 million to 1 quadrillion times stronger than Earth’s. For a neutron star to start emitting light like a pulsar, it must have the correct ratio of magnetic field strength to rotation frequency. It so happens that a beam of radio waves may not pass through the field of view of a ground-based telescope and remain invisible.
Why do pulsars rotate?
The slowness for a pulsar is one rotation per second. The fastest ones accelerate to hundreds of revolutions per second and are called millisecond. The rotation process occurs because the stars from which they were formed also rotated. But to get to that speed, you need an additional source.
Researchers believe that millisecond pulsars were formed by stealing energy from a neighbor. You can notice the presence of a foreign substance that increases the speed of rotation. And this is not good for an injured companion, which may one day be completely absorbed by the pulsar. Such systems are called black widows (after the dangerous species of spider).
Pulsars are capable of emitting light at several wavelengths (from radio to gamma rays). But how do they do it? Scientists cannot yet find an exact answer. It is believed that a separate mechanism is responsible for each wavelength. Lighthouse beams are composed of radio waves. They are bright and narrow and resemble coherent light, where the particles form a focused beam.
The faster the rotation, the weaker the magnetic field. But the speed of rotation is sufficient for them to emit the same bright rays as slow ones.
During rotation, the magnetic field creates an electric field, which is able to bring charged particles into a mobile state (electric current). The area above the surface where the magnetic field dominates is called the magnetosphere. Here charged particles are accelerated to incredibly high speeds due to the strong electric field. With each acceleration, they emit light. It is displayed in the optical and X-ray range.
What about gamma rays? Research suggests that their source should be sought elsewhere near the pulsar. And they will resemble a fan.
Search for pulsars
Radio telescopes remain the main method for searching for pulsars in space. They are small and faint compared to other objects, so you have to scan the entire sky and gradually these objects get into the lens. Most were found with the help of the Parks Observatory in Australia. A lot of new data will be available from the Quadrant Kilometer Antenna Array (SKA) starting in 2018.
In 2008, the GLAST telescope was launched, which found 2050 gamma-emitting pulsars, of which 93 were millisecond. This telescope is incredibly useful as it scans the entire sky, while others only highlight small areas along the plane of the Milky Way.
Finding different wavelengths can be problematic. The fact is that radio waves are incredibly powerful, but they may simply not hit the telescope lens. But gamma rays spread over more of the sky, but are inferior in brightness.
Scientists now know about the existence of 2,300 pulsars found by radio waves and 160 by gamma rays. There are also 240 millisecond pulsars, of which 60 produce gamma rays.
Pulsars are not only amazing space objects, but also useful tools. The light emitted can tell a lot about internal processes. That is, researchers are able to understand the physics of neutron stars. The pressure in these objects is so high that the behavior of matter differs from the usual one. The strange stuffing of neutron stars is called “nuclear paste.”
Pulsars are very useful because of the accuracy of the pulses. Scientists know specific objects and perceive them as a cosmic clock. This is how conjectures about the existence of other planets began to appear. In fact, the first exoplanet found orbited a pulsar.
Do not forget that pulsars continue to move during “blinking”, which means that they can be used to measure cosmic distances. They were also involved in testing Einstein’s theory of relativity, like moments with gravity. But the regularity of the pulsation can be disrupted by gravitational waves. This was noticed in February 2016.
All pulsars gradually slow down. The radiation is powered by a magnetic field created by rotation. As a result, it also loses its power and stops sending beams. Scientists have drawn a special line where gamma rays can still be detected in front of radio waves. As soon as the pulsar sinks lower, it is decommissioned to the pulsar cemetery.
If a pulsar was formed from supernova remnants, then it has a huge energy reserve and a fast rotation speed. Examples include the young object PSR B0531 + 21. In such a phase, it can stay for several hundred thousand years, after which it will begin to lose speed. Middle-aged pulsars make up the majority of the population and only produce radio waves.
However, a pulsar can extend its life if there is a satellite nearby. Then he will pull out his material and increase the speed of rotation. Such changes can occur at any time, so the pulsar is able to revive. Such a contact is called a low-mass X-ray binary system. The oldest pulsars are millisecond pulsars. Some are billions of years old.
Neutron stars are rather mysterious objects that exceed the solar mass by 1.4 times. They are born after the explosion of larger stars. Let’s get to know these formations better.
When a star explodes, 4-8 times more massive than the Sun, a core with a high density remains, which continues to collapse. Gravity pushes on the material so hard that it forces protons and electrons to merge to appear as neutrons. This is how a high-density neutron star is born.
These massive objects are capable of reaching a diameter of only 20 km. To make you aware of density, just one spoonful of neutron star material will weigh a billion tons. The gravity on such an object is 2 billion times stronger than Earth’s, and the power is enough for gravitational lensing, allowing scientists to see the back of the star.
The thrust from the explosion leaves a momentum that causes the neutron star to rotate, reaching several revolutions per second. Although they can accelerate up to 43,000 times per minute.
When a neutron star is part of the binary system where the supernova exploded, the picture is even more dramatic. If the second star was inferior in massiveness to the Sun, then it pulls the companion’s mass into the “Roche petal”. It is a spherical cloud of material that revolves around a neutron star. If the satellite was 10 times more than the solar mass, then the mass transfer is also tuned, but not so stable. The material flows along the magnetic poles, heats up and X-ray pulsations are created.
By 2010, 1,800 pulsars were found using radio detection and 70 through gamma rays. In some specimens, planets were even noticed.
Types of neutron stars
For some representatives of neutron stars, jets of material flow almost at the speed of light. When they fly past us, they flash like the light of a lighthouse. Because of this, they were called pulsars.
When X-ray pulsars take material from their more massive neighbors, it comes into contact with a magnetic field and creates powerful beams that can be seen in the radio, X-ray, gamma, and optical spectrum. Since the source is located in a companion, they are called accreting pulsars.
Rotating pulsars in the sky obey the rotation of stars because high-energy electrons interact with the pulsar’s magnetic field above the poles. As the matter inside the pulsar’s magnetosphere accelerates, this causes it to emit gamma rays. The release of energy slows down the rotation.
Magnetar magnetic fields are 1000 times stronger than those of neutron stars. Because of this, the star is forced to rotate much longer.