Introduction
First off, Neutron Stars are not to be considered as stars. Neutron stars are basically cores of dead stars (stars within the range of solar masses 1 to 3) that run out of fuel and collapse. However, we’ll be talking more about that later. Neutron stars have 2 billion times more gravity than the Earth.
Formation of Neutron Stars
So first, to know how Neutron stars form, We need to know the lifecycle of High mass stars.
What are High mass stars?
Stars are classified into different types based on their mass. The mass of stars is calculated in ‘Solar Mass’. 1 solar mass is the mass of our Sun. So 2 solar masses would be the approximate mass of 2 Suns and so on. High mass stars are stars with 8 solar masses or more. Some examples of High mass stars are Betelgeuse, Rigel, etc.
All stars are formed from Nebulae. A nebula is a very big gas cloud floating around in space. These nebulae are often thousands or even tens of thousands of light-years in diameter with very low densities. That means, there is a very small amount of matter present in a specific volume. As you may already know, stars are mostly made up of hydrogen and helium gases, nebulae are also made up of hydrogen and a bit of helium gas as well. So, you might be wondering how stars form from a hydrogen and helium gas cloud? The gas particles in the nebula have their own gravitational field. Hence nearby gas particles get attracted towards each other and then a small high density (when compared to the overall density of the nebula itself) lump of gases form in that particular region of the nebula. As time passes, this lump of gas particles grow larger in size and density. After about 10 million years, the density of the lump of gas particles increases and becomes high enough to start nuclear fusion. At this stage, the lump of gases is called a protostar. After another few million years, the protostar slowly becomes stable and is later called a complete star.
There are a few forces that act on a star, they are radiation pressure and gravitational pressure. These two forces have to be balanced for a star to exist in a stable form otherwise the star can expand too much or collapse on itself. Higher mass stars burn their fuel quicker, hence they have a shorter lifespan. Since they burn their fuel very fast, about midway through their lifespan, the radiation pressure of the high mass star overcomes its gravitational pressure. When this happens, the stars begin to expand rapidly. And then, a red or blue supergiant (depending on the colour of the star) is formed. But as the star expands, the pressure in the core of the star decreases and the nuclear fusion in its core decreases. Then, the gravitational pressure overcomes the radiation pressure and the star collapses in a supernova explosion.
All the matter present in the star except for the core gets blasted into space and the core of the star becomes a neutron star or black hole (depending on the mass of the core of the star).
Composition of a Neutron Star
As the name implies, neutron stars are composed of neutrons as mentioned earlier. However, the name can sometimes be misleading, because neutron stars aren’t made up entirely of neutrons.
As you can see, the outermost layer contains atomic nuclei and free electrons. The next layer contains heavier atomic nuclei, free neutrons and electrons. Next, we have the core, now this is where things get a little more complicated. In this layer, there is a quantum liquid with protons, neutrons and electrons all existing in a soup.
So what is a quantum liquid?
A quantum liquid is not actually a liquid, but it’s a fraction of protons and electrons that behave similarly to a “fluid”. One example in superconductivity is when quasi-particles (quasi-particle describes a physical concept in which the elementary excitations in solids are treated like spin waves, as particles) are made up of pairs of electrons and a phonon (a phonon is a collective excitation in a periodic, elastically arranged in an atom or molecules in condensed matter) acting as a boson.
The components of the inner core of the neutron star however are strange. The inner core literally contains unknown ultra-dense matter as shown in Fig-4. Then some neutrons and protons remain as particles but they break down into constituent quarks and even hyperons. It’s really tough to observe this deep inside a neutron star, and therefore scientists find difficulties in studying this unknown matter. However, NASA is planning to use X-ray spectroscopy to gain more knowledge about this, but as of now, it still remains as a mystery.
Mass, Density, Degeneracy pressure and Gravity
Just one teaspoon of neutron star matter will be heavier than Mount Everest!
Neutron stars are extremely heavy, in fact, just one teaspoon of matter from a neutron star will have a mass of approximately 5.5×1012 kg (55 followed by 12 zeroes), or about a whopping 5 billion tons!
The density of a neutron star is tremendous, ~10¹⁷kg/m³.
Next, we have the gravitational field of a Neutron star. As we have mentioned earlier, the mass of a neutron star is immense. So Unsurprisingly, the gravity of the neutron star would also be spectacular. It clocks in at about 2×1012 m/s². In comparison, the Earth’s gravitational field is only a mere 9.81m/s².
In other words, the gravity here is 2 billion times more powerful. Not only this, but the gravity of a neutron star is so powerful that it has the ability to bend radiation through gravitational lensing, this allows astronomers to observe the backside of a neutron star.
Finally, we have the degeneracy pressure of a neutron star. First off, let’s take a look at what neutron degeneracy is. Neutron degeneracy is a stellar application of the Pauli Exclusion Principle (which states that no two fermions can be in the same quantum state). No two neutrons can occupy the same exact states. This is true as long as the pressure from a star is below the Chandrasekhar limit(solar masses 1.44). If the star is above the limit, protons and electrons collide with each other leaving the core and forming a neutron star. As mentioned earlier, this is because it creates an effective pressure that prevents more gravitational collapse.
How to Detect Neutron Stars
Neutron stars emit a lot of radiation in the form of electromagnetic waves (light) with a wavelength of mostly X-rays and also some Ultra-Violet wavelength radiation. We have already detected roughly 3000 neutron stars. But, we can’t see X-rays or UV rays with our naked eyes. So we will need an equipment that can detect these wavelengths of radiation. So, we use the X-ray telescopes to detect them. Using these telescopes, we can’t directly look through the telescope to see these wavelengths of radiation because our eyes can’t see it. So, these X-ray telescopes are just normal telescopes with better and larger mirrors to capture as much radiation as possible. In these X-ray telescopes, there are X-ray detecting sensors and cameras placed in the eyepiece(the part of a telescope that is closest to the eye when looked through the telescope) instead of our eyes.
X-rays have a very short wavelength. Electromagnetic radiation with short wavelengths like X-rays and UV rays gets scattered in the Earth’s atmosphere. Because of this, we get a blurred image. So, scientists have spent a lot of time and have done a lot of research to find a solution. Then, they got a brilliant idea! Here, the Earth’s atmosphere was the main problem. Then, they realised that the best solution is to just remove the Earth’s atmosphere from the problem. So, they had to send these telescopes to space because in space, the Earth’s atmosphere cannot interfere. That is how space telescopes were first invented. Almost all X-ray telescopes are Space telescopes. A good example for this is the Chandra X-ray Observatory.
In the above image, we can only see the strong X-ray source as a small cloud/disc. We still don’t have the technology to actually see the Neutron star. What we see here is the X-ray radiation emitted by the accretion disc of the neutron star and not the neutron star itself. But there is still a problem with trying to detect strong X-ray sources and that problem is also the reason why we have only detected roughly 3000 neutron stars till date. Well then, what exactly is the problem? Well, the problem is that neutron stars/ their accretion discs don’t always emit radiation. We have only so far detected neutron stars which are mostly younger than about 1 million years. So, you might be thinking, ‘A million years old? That’s a long time!’ A million years is a long time but not on an astronomical scale. For example, our Sun has a lifespan of about 10 billion years. We have only discovered very young neutron stars because the amount of radiation emitted by the neutron star itself slowly decreases. Because of this, the radiation emitted by neutron stars older than a million years is too weak for us to detect with the X-ray telescopes and the technology we have currently. But we have still detected some neutron stars which are older than a million years. Because when a lot of matter comes near a neutron star, the neutron star will absorb the matter and then create an accretion disk around the neutron star.
This accretion disc contains the matter which is getting absorbed by it. So, the matter orbits the neutron star as it is in the accretion disc. The accretion disc gets so hot due to friction and it starts emitting a very high amount of X-ray radiation(higher than the amount of radiation emitted by the neutron star itself) and then due to the very strong magnetic field of a neutron star, the radiation and some of the matter gets funneled out of the two poles of the neutron star to form the so-called light jets. These light jets are very very bright in the X-ray wavelength and they can be very easily detected with X-ray telescopes. But a neutron star absorbing nearby matter is very rare because there is usually not a lot of matter near a neutron star. There is another way of detecting neutron stars. That is to use their strong gravitational field to our advantage. If it’s a very massive neutron star, then we can detect their gravitation fields interfering with the movements of other low mass stars. Sometimes we can detect some stars completely orbiting around the neutron star. But sometimes it can also be a Black Hole as well. Then, using some calculations, we can find out where the source of the strong gravitational field is. This is how we can detect Neutron stars.
Types of Neutron stars
Neutron Stars are mainly of 3 types. They are: Magnetars, Pulsars, Magnetars+Pulsars. Neutron stars are categorized into these 3 based on their characteristics and properties. But one thing to be remembered is that they are all normal neutron stars, but at different stages.
Image Credit: NASA/JPL-Caltech
a) Magnetars
A magnetar is a neutron star with a very strong magnetic field. A magnetar’s magnetic field is thousand times stronger than that of the Earth. The magnetic field of an object is calculated using a unit called Tesla(T). The strength of the magnetic field of a magnetar is about 10,000,000,000 Tesla while the Earth’s magnetic field is less than 0.000030 Tesla. Magnetars are very hard to find and astronomers have found only about 30 magnetars till date.
b) Pulsar
We have discovered about 3000 neutron stars till date and most of them are pulsars. These pulsars emit a lot of radiation in the form of X-rays and UV rays. Other types of neutrons stars also emit a lot of radiation in the forms of X-rays and UV but, pulsars emit more radiation than other types of neutron stars. We can only observe neutron stars using the X-Ray and UV- rays which it emits. So, we use X-ray telescopes to observe them. If you observe a strong X-ray source from any part of the sky, there is a 90% chance that it is a pulsar but it can also be a Quasar(a type of black hole). Because of the neutron star’s strong gravitational field, the neutron star can absorb any matter which comes very close to the neutron star. When that happens, an accretion disc is formed around the neutron star.
The accretion disc contains extremely hot matter which can reach temperatures of almost 1,000,000°Kelvin. The matter in the accretion disc is so hot that it emits high-energy X-ray radiation. Not only magnetars but all neutron stars have a very strong magnetic field. Because of this magnetic field, some of the matter and energy from the accretion disc gets funneled to the two poles of the neutron star and a lot of X-ray radiation gets emitted as a light jet from its two poles. These light jets from its poles are very very bright and observed as the second brightest things ever discovered in the universe. So here, the neutron star itself doesn’t really emit X-ray radiation. But why are they called pulsars you might ask? Well, the magnetic poles and their rotational axis are not aligned correctly. Neutron stars have a fast rotation speed. Some neutron stars have a rotational period of about 25 rotations per second. So, when viewed from Earth, it looks like the pulsar is blinking or the light from it is pulsing. So, when a neutron star starts pulsing, then it’s called a Pulsar.
c) Magnetar+Pulsar
Some neutron stars have both the properties of Magnetars and Pulsars. Hence, they are called Magnetars+Pulsars. Scientists and Astronomers have only discovered 6 neutron stars which have both the properties of Magnetars and Pulsars.
Lifespan and Death of Neutron Stars
Neutron stars are one of the longest living objects ever discovered in the universe. Some scientists have estimated the current age of the longest living neutron star in the universe to be around 10 billion years old. Also, we have to remember that our universe is only estimated to be 13.8 billion years old! We don’t exactly know how long a neutron star can live. Because we usually say that a star dies when the fuel in the star runs out or the nuclear fusion in the star stops. Because a neutron star is the dead core of a high mass star, there is no nuclear fusion or any energy/heat source with the neutron star. After about 10 billion years in the life of a neutron star, it will just cool down and the black body radiation from the neutron star will also stop. Then the neutron star becomes almost invisible as it does not emit any radiation. But we can sometimes detect them if they emit light jets from their accretion discs after absorbing a lot of matter. So, the life span of the neutron star or how it dies is unknown. There is a lot of research going on to find the answer! What scientists believe is that we can only find the lifespan and the death of a neutron star if we know the yet unknown ultra-dense matter present in the core of these neutron stars.
Read another article written by one of the authors: “The Evolution of Stars“
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