Goals Explain how neutron stars form Explain what pulsars are

Chapter 13: Neutron Stars and Black Holes


Goals

• Explain how neutron stars form • Explain what pulsars are

• Explain what gamma ray bursts are

• Explain how Einstein’s theories apply to these objects

• Explain how black holes form


When a massive star begins its core collapse, the electrons get compressed into the protons to form neutrons.

It is the formation of the neutrons which caused the core to bounce back and eject the outer most atmosphere of the star.

All that is left is a hot compressed sphere of neutrons. This is a neutron star. They are about the size of a major city but more massive than our Sun

Chapter 13: Neutron Stars and Black HolesWhen all that mass shrinks down to the size of a city, the conservation of angular momentum means that it will rotate faster. Some neutron stars rotate 1,000 times per second!!

Also, as the object shrinks the strength of the magnetic field increases to one trillion times that of Earth’s.

The surface of the neutron star is so hot that mater get accelerated to high speeds.

If the magnetic axis is tilted with respect to the rotation axis, mater streams out along the poles.


If you happen to be in the line of sight of the beam of light, you will see a pulse every time it points at you.

Since it is rotating up to 1,000 times per second, you will see 1,000 pulses per second.

We call these objects Pulsars.


Because of their high mass, Pulsars are not likely to speed or slow down much in our lifetimes. Because of this they make very good clocks. The Naval Observatory uses Pulsars to check their atomic clocks.

Because their pulses are so precise, radio astronomers can easily detect planets around pulsars because of their Doppler shifts.

The first “Planets” were discovered in this way.

Chapter 13: Neutron Stars and Black HolesGamma Ray Bursts

Discovered by military satellites looking for nuclear test ban violations in the 1960s.


Their random distribution means they are not part of the Milky Way.

By observing these first in the gamma ray and then x ray and visible wavelengths, we are able to determine host identity and therefore distances some near the limit of the observable universe.


Two theories for Gamma ray bursts.

Chapter 13: Neutron Stars and Black HolesIf a neutron star exceeds about 3 solar masses, then even the neutron pressure is not enough to hold off gravity for long.

However, we have no more “tricks” up our sleeve. We know of no form of matter after neutrons which can hold off gravity.

The resulting object should continue to contract until the gravitational force is so strong that light itself cannot escape.

We call such an object a “Black Hole”

Black Hole


The “event horizon” is the actual boundary where light cannot escape. It is sometimes called the Schwarzschild radius after the scientist who proposed it. It depends on the mass of an object.

Earth is 1 cm, Jupiter is 3 m, Sun is 3 km, 3 solar mass star is 9 km.

A 3 solar mass neutron star lies within its own event horizon and thus becomes a black hole. This is the ultimate fate of any star 20-30 the mass of the Sun.


The gravitational field around an object is like pulling a rubber sheet.

If you pull too hard that part of it gets pinched off form the rest of the universe.


Except for being invisible, black hole are no different than any other object.

They are no cosmic vacuum sweepers.

If our Sun was a black hole, all the planets would continue to revolve around it. We just would get no light.

But they are cosmic heaters because material gets compressed so much when it gets near them.

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Goals Explain how neutron stars form Explain what pulsars are