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Neutron Stars and Black Holes

Neutron stars and black holes evolve from old, dense stars.

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Neutron Stars and Black Holes

How dense can you get?

A neutron star has about 500,000 times Earth's mass. It is the size of Brooklyn, New York. These objects have an immense amount of gravity, but not as much as a black hole!

Neutron Stars

After a supernova explosion, the star's core is left over. This material is extremely dense. What happens next depends on the core's mass. The core might be less than about four times the mass of the sun. In this case, the star will become a neutron star. A neutron star (Figure below) is made almost entirely of neutrons. A neutron star has more mass than the sun; yet, it is only a few kilometers in diameter.

An artist's rendition of an ultra-dense neutron star

An artist's depiction of a neutron star.

A pulsar is a rotating neutron star that emits radiation in pulses. A pulsar can only be seen when the beam is pointing toward Earth. Pictured below is a nebular that looks like a cosmic hand (Figure below). There is a bright swirl of gas in the wrist of the hand. A very tiny but bright neutron star is in the center of that swirl.

A pulsar is a rotating neutron star that emits intense radiation in pulses

A neutron star at the center of a nebula.

A pulsar sits in the center of this nebula.

Black Holes

The core remaining after a supernova could be more than about five times the mass of the sun. In this case, the core collapses to become a black hole. Black holes are unimaginably dense. Not even light can escape their gravity (Figure below)! This is why they are black. We can't see black holes.

How can we know something exists if radiation can't escape it? A black hole affects the objects around it. It affects them with its gravity. Some radiation may leak out around the edges of a black hole. A black hole isn't a hole at all. It is the tremendously dense core of a supermassive star.

The light of these galaxies is bent by the presence of a black hole

The light of these galaxies is being bent by a black hole.


  • After a supernova explosion, the star's core is left.
  • If the core is less dense, it becomes a neutron star. A neutron star is made almost all of neutrons.
  • If the core is more dense, it becomes a black hole. No light can escape a black hole.


  1. What are the characteristics of a neutron star?
  2. What are the characteristics of a black hole?
  3. How do scientists know that black holes exist?
  4. Describe how a star forms a neutron star or a black hole and why it would form one or the other.

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Use the resources below to answer the questions that follow.

  1. What is the most distinctive feature of neutron stars?
  2. How large and how massive is a neutron star?
  3. What might cause neutron stars to release gamma rays?
  4. What if a neutron star exploded close to us in our galaxy?
  1. What do black holes do in space?
  2. What do black holes look like?

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black hole Super dense core left after a supergiant explodes as a supernova.
neutron star Remnant of a massive star after it explodes as a supernova.

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