Download Slide 1 - Physics @ IUPUI

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.