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How Big is a Neutron Star? Smaller Than You Think

Posted by Guy Pirro 03/20/2020 12:15AM

How Big is a Neutron Star? Smaller Than You Think

Neutron stars are compact, extremely dense remnants of supernova explosions. They are about the size of a typical city with up to twice the mass of our Sun. Neutron stars are so dense and compact, that you can think of the entire star as a single atomic nucleus. How the neutron-rich, extremely dense matter behaves is unknown and it is impossible to create such conditions in any laboratory on Earth. Physicists have proposed various models, but it is unknown which, if any, of these models correctly describe neutron star matter in nature. An international research team led by members of the Max Planck Institute for Gravitational Physics in Germany has obtained new measurements of how big neutron stars are. Their results show that a typical neutron star has a radius close to 11 kilometers. They also find that neutron stars merging with black holes are in most cases likely to be swallowed whole, unless the black hole is small and/or rapidly rotating. This means that while such mergers might be observable as gravitational-wave sources, they would be invisible in the electromagnetic spectrum.


  • Basser53 [John Neumann]
  • 03/22/2020 02:10PM
Some confusion here. You quote the radius of a typical neutron star as 11 kilometers. You also say a typical neutron star is 11 kilometers across (diameter). These are two very different measurements.


Yes, Thank you -- You are absolutely correct -- Good catch.

It should read "22 kilometers across (diameter)."

It's gratifying to see that folks are actually reading these news items rather than just looking at the pretty pictures and videos.

Thanks for pointing this out.


So how much would a teaspoon, or perhaps a cc, of the matter weigh, here on earth ?

So how much would a teaspoon, or perhaps a cc, of the matter weigh, here on earth ?

So how much would a teaspoon, or perhaps a cc, of the matter weigh, here on earth ?

OK, (and please correct me if I am wrong, but) to the best of my (efforts of) calculation... given and using the stated figures in the article...
One cc of the neutron star matter would “weigh” here on earth approx 713.54 billion kg, or 1.573 trillion lbs. And going a step further for comparison, and illustrative purposes, (not to mention the sheer fun of it :) given the estimated weight of the earth at 5.974 x 10^21 kg or 1.317 x 10^25 lbs. (not sure if that includes people animals trees etc, or not), it would take only a cube of such matter 203 meters (666 ft) on a side, to EQUAL the mass of the earth. And that’s not putting it lightly.

OK Michael, for the fun of it, I worked out the answer from information found in the article and came out with an answer very close to yours.

As a side note, since an atomic nucleus is about 300 billion Kg (Kilograms) per cc (cubic centimeter) we should expect the answer for a neutron star to be somewhere in that ballpark.

You’re answer is 713.54 billion Kg per cc. My answer is 499.8 billion Kg per cc. Not bad since we took different approaches. Either answer is acceptable since we are working with imprecise numbers.

Here’s my approach. I used two pieces of info from the article:

1) Radius of Neutron Star = 11 Km (Kilometers)
2) Mass of Neutron Star = 1.4 times the mass of the Sun

Then I used these formulas:

Volume of Sphere = (4/3) x (Pi) x (Radius)^3

Density = Mass/Volume


Volume of Neutron Star

Volume of Sphere = (4/3) x (Pi) x (Radius)^3

V= (4/3) x (3.14) x (11 Km)^3

V= 5.572 x 10^18 cubic centimeters


Mass of Neutron Star

Mass = (1.4) x Mass of Sun

Mass = (1.4) x (1.989 x 10^30 Kilograms)

Mass = 2.785 x 10^30 Kg


Density of Neutron Star

Density = Mass/Volume

Density = (2.785 x 10^30 Kg) / (5.572 x 10^18 cc)

Density = 4.998 x 10^11 Kilograms per cubic centimeter

(or 499.8 billion Kg per cc)


Hope this helps,

Guy Pirro