# Gravity and neutron stars

• B
• Simon Peach
In summary, a neutron star is a star that's been compressed from say a sun sized star to the size of a city, every thing OK upto now. Then it goes on to say that it has, the neutron star, enormous gravity, this is were I started to think; why is the gravity stronger than the gravity of the original star? The neutron star holds the same amount of matter as the original star so shouldn't the gravity be the same albeit the matter is more compacted. Does the compaction have an effect on the gravity? If so why? Same for a Black hole, in this sense just a more compact neutron star.f

#### Simon Peach

I've just watched a vid about jets of matter and neutron stars. It was stated in it that a neutron star is a star that's been compressed from say a sun sized star to the size of a city, every thing OK upto now. Then it goes on to say that it has, the neutron star, enormous gravity, this is were I started to think; why is the gravity stronger than the gravity of the original star? The neutron star holds the same amount of matter as the original star so shouldn't the gravity be the same albeit the matter is more compacted. Does the compaction have an effect on the gravity? If so why? Same for a Black hole, in this sense just a more compact neutron star.

Simple answer: The total gravity of a neutron star doesn't increase. At a given distance from neutron star you would feel the same gravity as you would from a main sequence star of the same mass. However, being more compact, you can get much closer to the center of a neutron without reaching its surface than you can the main sequence star, and gravitational force does depend on how far you are from the center. With a sun sized star the surface is some 695,000 km from the center and the g-force felt there would be 28 times stronger than at the surface of the Earth. A sun massed neutron star would be be ~9 km in radius and the g-force at its sufrace would be ~1.76e11 times stronger than on the surface of the Earth.

Buzz Bloom and unusually_wrong
So I was right in my thoughts that for a given mass regardless of it's compaction the gravity is the same. This brings up another query: Why is it only black holes colliding that is produce gravity waves that are detected? Is it because the said B/H have such a high density?

Why is it only black holes colliding that is produce gravity waves that are detected? Is it because the said B/H have such a high density?

Yes. The gravity very far away from a black hole is identical to a star of equal mass. But when you compress all that mass into a very small volume you can get very close to all of that mass. The Sun, for example, is more than a million kilometers across, and if you were just above the photosphere you would still be more than a million kilometers from all of the mass on the other side of the Sun. Hence the gravity from that bit of mass isn't nearly as strong as the gravity from the points in the Sun near you. But when you compress everything into a small sphere perhaps a few dozen kilometers across, you're suddenly very close to all of that mass and the gravity is many orders of magnitude stronger.

YoungPhysicist
So I was right in my thoughts that for a given mass regardless of it's compaction the gravity is the same. This brings up another query: Why is it only black holes colliding that is produce gravity waves that are detected? Is it because the said B/H have such a high density?
Theyv'e also detected gravitational waves produced by merging neutron stars.
As two objects orbit each other, they emit gravitational waves. This causes the system to lose energy and they spiral in towards each other. In most cases this is slow process. It speeds up and the energy of the gravitational waves increase as they get closer. The closer they get, the faster they orbit, the faster they orbit, the greater the energy lost to gravitational waves. Black holes and Neutron stars are so compact that they can get very close to each other before the actual collision and thus can orbit very fast, producing quite a bit of gravitational radiation.

Thank you all for explaining