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# Neutron Stars and Black Holes

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

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14.8 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 is shown in Figure below. This type of star 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 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. The Figure below is of a nebula that looks like a cosmic hand. 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 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 5 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 (See 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 being bent by a black hole.

### Vocabulary

• black hole: The super dense core left after a supergiant explodes as a supernova.
• neutron star: The remnant of a massive star after it explodes as a supernova.

### Summary

• After a supernova explosion the stars 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.

### Practice

Use this resource to answer the questions that follow.

Neutron Stars - Death Star at http://www.youtube.com/watch?v=h3Nodwb0vTU (3:41)

1. Describe the structure of neutron stars.
2. How large is a neutron star?
3. What causes neutron stars to release gamma rays?

1. Who is an expert on black holes?
2. What do black holes look like?
3. What are cosmic strings?
4. What would be created if two cosmic strings create a circle?

### Review

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?

### Notes/Highlights Having trouble? Report an issue.

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