Goal: To understand special stars.

Objectives:

1) To learn about Neutron Stars

2) To learn about Pulsars

3) To understand Stars that erupt.

Special stars – neutron stars

• Neutron stars are stars that are about 1.4 times the mass of our sun and made entirely of neutrons.

• These stars are only a few km in size.

• They are essentially a giant atom!

• Densities are HUGE (100 trillion times that of water)!

• They also spin and have magnetic fields.

Crab

• Pictured is the Crab Nebula – which supernovaed in 1054.

Special stars – Pulsars

Special stars – Pulsars• Pulsars are neutron stars.• They spin very quickly (once per second to a

thousand times per second).• The stars have strong magnetic fields, and

only beam light from their pole (sort of like a lighthouse floodlight).

• The pulses normally come in the radio.• However, they also emit a lot of X-rays.• The Crab for example spins 30 times per

second.

Other types of Pulsars

• Hot spot – some emit x-rays from a hot spot that rotates around.

• Accretion – this one is a combination of the other two.

• Material tends to accrete more along the magnetic poles creating a magnetic hot spot.

From NASA

Again NASA

Energy has to come from somewhere.

• Where does the energy the pulsars emit come from?

• A) heat

• B) nuclear fusion

• C) gravity

• D) Spin

Magnetars

• Some neutron stars have a magnetic field 1000 times stronger than the others

• Not sure how they form but may have to do with formation as they collapse

• However they seem to have slower rotations (seconds)

• Only seem to last 10,000 years or so• Hard to observe flare up only very

randomly

Neutron Stars in binary systems• Remember that most stars are in binary

systems!• At the end of the life of the biggest star,

sometimes the other stars get away because the dying star looses a lot of mass.

• Sometimes they stay together.• Then, when the smaller star evolves…

binary systems – Roche Lobes• As a star expands it has a looser and looser

hold on its own materials (gravity decreases by the radius squared).

• At some point a companion star will have more influence over the outermost parts of the star than the star itself does!

• This is called the Roche Lobe.• If a red giant expands past its Roche Lobe, the

companion star will accrete materials from it.

But, what happens when you accrete matter onto a few km ball of

mostly neutrons?• Well, at first the Hydrogen falls way down onto

the surface.• This produces energy that helps to power the

constant emission of X-rays by the neutron star.• Then, the H is fused into He and crushed onto

the surface of the neutron star.• Soon you build up a layer of He (sort of like a

layer of snow).

He bomb

• When the He layer is about 1 m thick, the Helium ignites!

• As we saw with the Helium flash for a star, this is a tricky time.

• The burning He heats the surface of the star – which speeds up the production of Helium!

• The result is a spectacular explosion (although not as spectacular as a supernova)

• This produces an X-ray burster!

One other side effect

• Another side effect of accreting matter is a change to the spin.

• Will the spin get faster or slower?

If all that was not strange enough

• One seems to have a planets and maybe an asteroid belt

• Formed from the debris field of its supernova

Two binary neutron stars

• Can eventually (do to gravity waves) merge together and supernova forming a black hole.

Finally

• There is one more thing to change spin

• Sometimes there is a break in the crust that can make the star shrink down a little bit or change its state

• This makes it spin faster usually.

Conclusion

• Neutron stars are very strange and interesting stars.

• Some are in binary systems and do very weird things

• Their very strong magnetic fields seem to be many of the causes of their strangeness.