What Is A Neutron Star?
A neutron star is the collapsed core of a giant star which before collapse had a total mass of between 10 and 29 solar masses. It is a celestial body of very high density, composed primarily of neutrons (subatomic particles with no net electric charge, with a slightly larger mass than protons).
They are the ancient remnants of stars that have reached the end of their journey through space and time. Despite their small diameters (about 20 kilometres), neutron stars have nearly 1.5 times the mass of our Sun. These small but mighty objects are incredibly dense. In other words, if you were to weigh it, a teaspoon of neutron star material would weigh around a billion tonnes!
How are Neutron Stars formed?
To understand how neutron stars are formed, you first need to know the lifecycle of a star. A star is formed when a large amount of gas (mostly hydrogen) starts to collapse in on itself due to gravitational attraction. During contraction, the atoms of the gas collide with each other with more frequency and speed. As a result, the gas heats up. Eventually, the gas becomes so hot, that the hydrogen atoms no longer bounce off each other but instead coalesce to form helium. The heat released from this reaction is what makes a star shine.
Stars remain stable for a long time maintaining the balance between nuclear reactions and gravitational attraction. When a star runs out of fuel, it begins to cool and starts to contract. The death of a star results in a luminous stellar explosion called ‘Supernova’.
When stars 4 to 8 times massive than our Sun explode in a violent supernova, their outer layers blow off in a spectacular display leaving behind an extremely dense and small core that continues to collapse. Gravity presses the material in on itself that protons and electrons combine to make neutrons yielding the name “neutron star”.
Types Of Neutron Stars
Neutron Stars are some of the most astonishing objects present in our universe after Black Holes. There are some neutron star types that are known to us.
- Radio pulsars
- Recycled pulsars
- Magnetar
- Soft gamma-ray repeater (SGR)
- Anomalous X-ray pulsar (AXP)
- Low-mass X-ray binaries (LMXB)
- Intermediate-mass X-ray binaries (IMXB)
- High-mass X-ray binaries (HMXB)
- Accretion-powered pulsar or X-ray PulsRadio Pulsars
Radio pulsars were discovered around 50 years ago, and still, astronomers and scientists are not entirely confident of the nature of this celestial body.
Radio pulsars are generally accepted to be highly-magnetized, rapidly rotating neutron stars with a light-house beam of radiation that produces the pulsed emission. These are also known as white dwarfs as they emit a beam of electromagnetic radiation. The discharge can only be seen when the shaft of the emission is pointed towards the Earth.
One way you can think of a pulsar (from pulse and -ar as in quasar) is like a lighthouse. The beam coming from a lighthouse spreads across the sky. Even though it is there shining always, you can only see it when it is pointing in your direction.
Pulsars are also believed to be a source of ultra-high-energy cosmic rays.
Recycled pulsars
Pulsars that have undergone a binary history are called as Recycled Pulsars. Many astronomers call recycled pulsars as millisecond pulsars. Both these terms are mostly used interchangeably among the astronomy community. The theory behind the origin of millisecond pulsars is that they are old and rapidly rotating neutron stars which have been recycled through the accretion of matter. The first-millisecond pulsar was the celebrated 1.55 ms pulsar PSR B1937+21, which, for over 20 years, was the fastest pulsar known. Many of them are found in globular clusters.
Magnetar
A magnetar is a type of a neutron star with a powerful magnetic field. The field decay powers the high-energy emission, particularly X-ray and gamma rays. The magnetic field of a magnetar can go up to 1 quadrillion gauss which is 1,000 trillion times the magnetic field of Earth.
A theory regarding Magnetars was proposed in 1992 by Robert Duncan and Christopher Thompson, but the first burst of gamma rays from it was detected on March 5, 1979.
These objects are around 20 kilometres in diameter with a mass of 2–3 times that of the Sun.
The differentiating feature between a magnetar and other neutron stars is that a magnetar has a very high magnetic field. Also, other neutron stars have a rotating speed of about once in every 1 to 10 seconds, but a magnetar rotates once in less than 1 second.
These are known to be the most magnetic stars in the sky.
Soft gamma-ray repeater (SGR)
Soft-gamma ray repeater is a type of a neutron star which emits large bursts of X-rays and gamma-rays at irregular intervals. It is also believed that they are a type of magnetar.
On March 5, 1979, an unusual gamma-ray burst was detected and earlier it was known to be originated from a supernova remnant. The photons present in gamma-rays were less energetic, and it was not a normal one. Later it became clear that the bursts came from a single region attributing to the presence of a soft-gamma ray repeater.
Anomalous X-ray pulsar (AXP)
Anomalous X-ray pulsars are young and highly magnetized neutron stars. These are now widely believed to be magnetars. They are characterized by slow rotation periods of ~2–12 seconds. The existence of soft-gamma ray repeaters motivated the discovery of anomalous X-ray pulsars. As of 2017, there are 12 confirmed and two candidate AXPs.
Low-mass X-ray binaries (LMXB)
These are called so because they are a class of binary stars that are luminous in X-rays. Transfer of mass takes place from one component called, donor to another component, accretor, which is a neutron star or a black hole. The companion star in a low-mass X-ray binary is a relatively dim late-type star. These systems tend to be optically faint, with less than 1% of the radiation visual wavelengths spectrum. Majority of emissions being X-rays, these are known to be one of the brightest X-ray sources in the universe.
These are generally found in and around the disk of Milky Way. Some observations suggest that there may be anywhere between 1,200 and 2,400 black hole LMXBs in our galaxy.
Intermediate-mass X-ray binaries (IMXB)
An intermediate-mass X-ray binary (IMXB) is a binary star system having one of the components as a neutron star or a black hole. The second component is a star of intermediate-mass. Low-mass X-ray binary systems are the source of intermediate-mass X-ray binary.
High-mass X-ray binaries (HMXB)
If the companion star in an X-ray binary system has a mass greater than 10 solar masses, then it is known as a high-mass X-ray binary.
A high-mass X-ray is a binary star system having intense X-rays. The normal stellar component is a massive star: usually an O or B star, or a blue supergiant. In the high-mass X-ray binary system, the enormous star dominates the light emission while the compact object is the dominant source of X-rays.
Once an HMXB reaches the end of its life, it can become a single red giant with a neutron core or a single neutron star. It is only accurate if the periodicity of the binary was less than 1 year. With a periodicity of more than a year, the HMXB can become a double neutron star binary if uninterrupted by a supernova.
It is interesting to note that the first discovered black hole Cygnus X-1 is an example of a high-mass X-ray binary system.
Accretion-powered pulsar
Accretion-powered pulsars or X-ray pulsars are a class of astronomical objects exhibiting the periodically varying intensity of X-ray emissions. The period of X-ray emissions varies from a fraction of second to as much as several minutes.
An accretion-powered pulsar is a type of binary system consisting of a highly magnetized neutron star. The magnetic field strength at the surface of the neutron star is typically about 10 raise to 8 Tesla, over a trillion times stronger than the strength of the magnetic field measured at the surface of the Earth.
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