What is a neutron star?

Neutron Star Definition

Neutron star definition.

A neutron star is an incredibly compact star, which is only 10-30 km. in diameter, but weighs 1.4 – 2.3 times as much as the Sun. A neutron star can be formed after a heavy star dies in a supernova explosion. When a heavy star has burned all its hydrogen into helium, using fusion, the temperature rises in its core until it is warm enough for burning helium. When the helium is burned, the temperature rises further and the process continues until nickel 62 is formed. Nickel has the highest binding energy per nucleus at the core. Thus, the star can not form more energy by merging or fissioning nickel, and therefore there is no energy to withstand gravity pressure. This causes the star to collapse under its own weight in a very short time.

Therefore, the electrons (which are negative) are pushed into the positive atomic nuclei. This means that the star consists mainly of neutral core particles – neutrons. The substance surrounding the nucleus encounters an impermeable wall, and therefore an outward pressure wave causes the material to flow into space, in a giant explosion called a supernova. Once all the gas is blown away, only the neutron star is left and only a teaspoon of the interior would weigh 100 million tons here on earth. The finished neutron star can not be pressed more together.

When a star becomes a size that is too big to support itself, it breaks down because of its own enormous gravity. When this happens, the star is broken in what is called a supernova. Sometimes, out of the ashes of such a devastating event comes another type of star, a so-called neutron star, consisting mainly of neutrons, which are electrons and photons together. Neutron stars are extremely hot and have a high pressure from gravity.

Neutron stars are supposed to be the remainings of a normal star which concluded its life as a supernova. In a supernova explosion, a star first blows its outer layer, creating a very compact star called  white dwarf. At one point, the white dwarf passes a critical border for its size and collapses. At the collapse, the substance is squeezed so much that its electrons are absorbed in the protons of the atomic nuclei, and neutrons are formed. A neutron star can therefore also be perceived as one giant nuclear nucleus consisting exclusively of neutrons. Neutron stars are so small that it is almost impossible to ebserve them. But they are often detected by the radiation they emit. The emission of radiation is related to the star’s rotation, which can be at several hundred rpm.

History of Neutron Stars

Neutron stars have been known since the beginning of the galaxy, but they were discovered and began to be investigated in 1933 when Walter Baade and Fritz Zwicky emerged with the idea of ​​a neutron star existence. It was a year after the neutron particle was discovered. The first neutron star was found in Crab Street in 1965 by Antony Hewish. The study of neutron stars continued and more was discovered through the 1970s and lasted till today. Studying these stars also helped the discovery of pulsaries.


Neutron stars are relatively small, but they have a large mass. Typical neutron stars have a radius at a location between 10 and 20 km, which corresponds to a diameter of about 12 miles. Our sun, in contrast, is about 50,000 times as high as the average neutron star. Although the neutron star is smaller, its particles are very dense and that gives it a lot that is 1.5 times larger than the Sun. The crust of a neutron star is very thin compared to the rest of the star, only one mile thick, consisting mainly of neutrons, with a touch of other particles.

A star’s destiny depends on its mass. The heavier a star is, the shorter the time it lives (it is said that a star “lives” as long as nuclear core fusion processes occur in its interior parts).

For example, the sun is a typical average star with a mass of about 2 * 10 30  kg (M sol ) and it is expected to have a life span of about 10 billion. year. As the solar system already has an age of just 4.5 billion years, the Sun is now about halfway through its cycle. There are stars with a mass in the range of barely 0.1 M solar and up to over 100 M of sun .

In the vast majority of a star’s life, energy is released by the fusion of hydrogen. It is precisely the radiation pressure of this released energy that prevents the star from collapsing under the influence of its own weight. In the final phase of its lifetime (the last 10%), the star breathes and becomes a giant star, while the inner parts of the star are pulling together.

The star’s radius typically becomes several hundred times larger in this final stage. In the case of the Sun, it will grow from its current radius of approx. 700 000 km and all the way to Earth’s lane (150 million km). In this final phase of its life cycle, the star can survive through the fusion of a sequence of heavier elements (helium → carbon → oxygen → neon → magnesium → sulfur → silicon → iron). How far a star’s inner parts could be reached in this process depends on the mass of the star.

When energy can no longer be released by merger, the outbound radiation pressure stops and the star collapses and becomes a so-called compact object. Stars that have a mass less than 8 M sun end their days like a white dwarf. The outer parts of the original star are thrown into a beautiful planetary fog (which has nothing to do with the formation of planets, as you thought in ancient times). If the original star has a mass of more than 8M of sun , it will explode into a supernova explosion and leave a neutron star (In the most extreme case where the star initially weighs more than 25M of sun it will collapse into a black hole).

A typical neutron star

A typical neutron star has a radius of only 10 km and a mass of 1.5 M solar . A cubic centimeter material from a neutron star has a mass of hundreds of millions of tons. In other words, it is extremely compact and can be considered as one major nuclear nucleus. As the name suggests, a neutron star consists mainly of neutrons, but also a smaller proportion of protons.

When the neutron stars are so small, they are very hard to spot with an optical telescope. Fortunately, however, some neutron stars have a very powerful magnetic field and a fast rotation that emit radio waves in a cone-shaped beam – exactly like a lighthouse that glows over the dark ocean at night. These neutron stars are called radio pulses and can be observed with large radio telescopes.

Another type of neutron stars we can detect here from Earth is neutron stars in so-called binary systems – ie. neutron stars circle around another star. If the two stars are close enough to each other, the neutron star can, in other words, eat the gas from the other star, thereby emitting a powerful X-ray. These neutron stars can thus be seen with X-ray detectors aboard satellites in circulation around the Earth.

The first neutron star was discovered in 1967 and today we know more than 2000 neutron stars in Mælkevejen. The hitherto closest known of these are at a distance of approx. 280 light years

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