Transcript Slide 1 - Physics @ IUPUI

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
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?
How it works:
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Eddington Valve
Star gets smaller and hotter.
Helium fully ionizes.
The Helium then absorbs most of the light.
So, energy out does not = energy
produced
Result
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Outer part of star heats up
That makes it expand
That makes it cool
That makes it go back to being only singly
ionized
• It can now radiate the energy
However
• The star has now over expanded
(overshot the runway)
• It can no longer hold up the outer part of
the sun
• So it falls back, and will overshoot the
other way going too far in
• The process then continues
What stars become RR Lyrae?
• Stars that will not supernova at the ends of
their life become RR Lyrae stars
• This is stars like our sun and up to a mass
of 4 times our sun.
• However they have lost a good fraction of
their masses by this point
• RR Lyrae stars are the stars in the middle
of the horizontal branch
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?
Anatomy of a Cepheid pulsation
• When smallest, hottest, and bluest, the
star is DIMMEST
• When largest, coolest, and reddest, the
star is BRIGHTEST
• This is because the change in size is
much greater than the change in temp
What will become a Cepheid
• For metal rich stars it is massive stars (> 4
solar masses)
• For metal poor they seem to become Type
II Cepheids and not RR Lyrae.
Unresolved Issue
• Clearly composition of the star has an
effect on the equation for period vs
absolute brightness
• However this total affect has not been
completely narrowed down
• This does lead to sizable errors in
measuring distances (10%)
• This error connects to the errors in
measuring the age of the universe
Conclusion
• Variable stars are quite simple to explain
in general and can be used for very
important distance calculations.
• All stars