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Goal: To understand special
stars.
Objectives:
1) To learn about Black holes
2) To learn about Neutron Stars
3) To understand Stars that erupt.
4) To understand Variable stars
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!
• They also spin and have magnetic fields.
• Pictured is the Crab Nebula – which
supernovaed in 1054.
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.
Energy has to come from
somewhere.
• Where does the energy the pulsars emit
come from?
• A) heat
• B) nuclear fusion
• C) gravity
• D) Spin
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?
White dwarfs in binaries
• White dwarfs also can flare up in a binary
system.
• On a white dwarf though, the matter falling in
stays as Hydrogen.
• Eventually the temperatures on the surface go
up and the density of Hydrogen gets high
enough to fuse into Helium.
• This creates a quick burning which is known as
a nova (not to be confused with supernova).
Black Holes in
binaries
• Just like with Neutron stars and white dwarfs,
black holes will create an accretion disk.
• However, you can see nothing from the actual
accretion, so all you get to see is the accretion
disk.
• On the plus side, the accretion disk goes down
to a few km in size at which point the gas has
been heated quite a bit (infalling gas is slowed
by frictional heating and interactions with the
magnetic field).
• The innermost parts will emit X-rays!
Astro-mercial
• But wait there’s more!
• JETS!
• Materials racing
outward at close to
the speed of light
and going for up to
millions of light
Years!
• (NGC 5532)
Variable stars
• Usually stars are held in equilibrium.
• If they expand then they cool and that ends the
expansion.
• Their cores are stable.
• The star is stable.
• However, there is a region on the HR diagram
where this is not the case – the instability strip.
• In this region stars will pulsate – that is they will
expand and contract.
• This causes the star to get brighter and dimer.
Variable types
• RR Lyrae – lower mass stars after they undergo
their Helium flash (the sun will do this someday).
• RR Lyrae are Horizontal Branch stars.
• Metal rich and Metal poor Cepheid variables
(Type I and II).
• These are the higher
mass stars which pass
back and forth through
the instability strip.
Observing RR Lyrae
• RR Lyrae have periods of about 0.3 to 0.5
days.
• A) Why do you think those periods are so
short?
• B) Why is this length of period a really bad
thing when it comes to observing the star
(hint, when can a good telescope look at
stars?)?
Absolute Magnitude
• RR Lyrae have an average absolute
magnitude of 0.75.
• Why is that an advantage?
• What is the disadvantage if you are
looking at other galaxies?
Why variable stars are important
• Variable stars have a relationship between their
period of pulsation and their absolute brightness.
• The longer the period, the bigger the star is, and
the brighter it is (sort of like a bigger bell has a
larger period of vibration).
• This allows us to measure distances (especially
since these are very bright stars which can be
seen a LONG distance away)!
• In fact, the distance to Andromeda was first
attempted to be estimated using Cepheid
variables.
Distance to Andromeda
• Edwin Hubble tried to estimate the
distance to Andromeda using Type II
Cepheids (metal poor).
• Type II Cepheids are in the globular
clusters.
• However, he made a slight mistake.
Type I Cepheids (metal rich):
Mv = -2.81 log(Period in days) -1.43
• Type I Cepheids (metal rich)
• ones in the disk of our galaxy
• have a pretty exact relationship between
variability period and average absolute
magnitude.
• The brightness of Type Is is 4 times
greater than Type IIs
Distance misestimated
• So, Hubble underestimated the distance to
Andromeda by a factor of 50.
Even today
• We still know the distance to the
Andromeda galaxy using the Type I (metal
rich) Cepheids.
Profile of a Cepheid Variable
• Cepheids expand and contract.
• As they do they change color (and
temperature).
• As they expand they cool and turn redder.
• As they condense they get hotter and turn
bluer.
• When do you think they are brightest?
Conclusion
• There are some very special stars out
there.
• Many are in binary systems and do very
weird things (and I have not covered the
extremely rare ones such as Helium or
Carbon stars).
• Variable stars are quite simple to explain
in general and can be used for very
important distance calculations.