What is a Neutron Star? - Mass, Density & Weight

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Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Neutron stars are dense objects found throughout the universe. Composed of neutrons, they have more mass than the sun, but are only a few kilometers wide. This lesson will cover the properties of neutron stars.


A neutron star is composed almost entirely of neutrons, so let's first take a look at what a neutron is. Neutrons are found in the nuclei of every particle except hydrogen. A neutron is a fundamental particle of an atom, having approximately the same mass as a proton. It is located in the nucleus along with protons. A neutron has a mass equal to 1,838 electrons and a mass equal to 1.0014 protons.

Properties of a neutron

In order for a neutron star to form, high temperatures and pressures are needed. Such pressures and temperatures only occur in the aftermath of a supernova during the collapse of massive stars. The resulting neutron star is composed almost entirely of neutrons. Neutron stars are some of the densest known objects in the universe, with a diameter roughly equal to an average city, but a mass equal to nearly 500,000 Earths or 2 suns.

Structure of a neutron star
neutron star


In astronomy, large masses are usually measured in solar masses; one solar mass is equal to the mass of our sun (1.9891 x 10^30 kilograms). A typical neutron star has a mass between 1.3 and 2 solar masses. This means neutron star can have twice the mass as our sun. For example, a neutron star named 1002.3825, discovered by Cornell University in the 4U region of the universe, was found to have a mass of 1.58 +/- 0.06 solar masses or 3.13 x 10^30 kilograms.


While the mass of neutron stars is almost beyond comprehension, the diameter is more within our normal range of measurement. The average range in diameter for a neutron star is only 10 - 20 kilometers. Going back to our example above, the neutron star 1002.3825 has a diameter of D = 18.22 +/- 0.80 km.


The density of neutron stars is again, almost beyond comprehension. The density of a neutron star has a range of 8 x 10^13 g to 2 x 10^15 g per cubic centimeter. Although hard to believe, the density of a neutron star is about equal to the the density of the nucleus of an atom.

The intense density of a neutron star means that the gravity on the surface is 2 x 10^11 - 3 x 10^12 times stronger than the gravity here on Earth. If a rocket ship were to try to escape the gravitational field a neutron star, it would have to travel about 10,000 to 150,000 km per second, which is half the speed of light.


A neutron star is the result of the collapse of a star after a supernova explosion. The resulting neutron star is made up almost entirely of neutrons. Neutron stars are some of the most dense objects in the universe, having a mass up to two times our sun, but a diameter ranging only between 10 - 20 kilometers.

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