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TRANSCRIPT
Astronomy 114
Lecture 21: Variable Stars, Supernovae
Martin D. Weinberg
UMass/Astronomy Department
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—1/16
Announcements
PS#5 posted, due next Wednesday: 4 Apr
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—2/16
Announcements
PS#5 posted, due next Wednesday: 4 Apr
Today:
Stellar pulsation
Supernovae
Standard Candles/Cosmic distance scale
Stellar Evolution, Chaps. 21 & 22
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—2/16
Special/interesting phases of stellar evolu-
tion
1. White Dwarf stars
2. Pulsating stars
3. Supernovae
4. Neutron stars
5. Black holes
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—3/16
What Causes Stellar Pulsation? (1/2)
By 1900, thousands of variable, pulsating stars, had beendiscovered by the source of the variability was not under-stood.
Pulsation period: days to years
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—4/16
What Causes Stellar Pulsation? (1/2)
By 1900, thousands of variable, pulsating stars, had beendiscovered by the source of the variability was not under-stood.
Pulsation period: days to years
Physical explanation (Part 1):
Consider a simple spring
Hang a mass from the spring and give it a kick
Oscillates with a characteristic frequency
If driving frequency matches characteristicfrequency: high amplitude oscillations
[movie]
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—4/16
What Causes Stellar Pulsation? (2/2)
A star has a springiness through gravity and pressure
Compress a star and it will spring back
Pull it outward and it will fall back in
Characteristic timescale: days to years (dependingon the size of the star).
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—5/16
What Causes Stellar Pulsation? (2/2)
A star has a springiness through gravity and pressure
Compress a star and it will spring back
Pull it outward and it will fall back in
Characteristic timescale: days to years (dependingon the size of the star).
Problems:
1. Not all stars are seen to pulsate
2. What is the driving mechanism?
3. The star requires an "engine" to continually supplypushes and pulls
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—5/16
The Opacity Valve (1/2)
The opacity of a gas is a measure of its ability to absorblight ("radiation"). General rule:
Hotter the gas, the lower the opacity
Atoms, particles absorb photon less readily
Cooler the gas, the higher the opacity
Atoms, particles absorb photon more readily
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—6/16
The Opacity Valve (1/2)
The opacity of a gas is a measure of its ability to absorblight ("radiation"). General rule:
Hotter the gas, the lower the opacity
Atoms, particles absorb photon less readily
Cooler the gas, the higher the opacity
Atoms, particles absorb photon more readily
Exception: as H and He are being ionized by increasingtemperature, the opacity gets large!
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—6/16
The Opacity Valve (2/2)
1. Gas is compressed, He or H ionization increases,opacity goes up, incresed pressure pushes staroutward[Closed valve]
2. Expanding layer cools, He or H ions recombine withelectrons, opacity drops, radiation escapes[Open valve]
3. Gas pressure now too low to support the envelope,star contracts, begins to heat . . .
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—7/16
The Opacity Valve (2/2)
1. Gas is compressed, He or H ionization increases,opacity goes up, incresed pressure pushes staroutward[Closed valve]
2. Expanding layer cools, He or H ions recombine withelectrons, opacity drops, radiation escapes[Open valve]
3. Gas pressure now too low to support the envelope,star contracts, begins to heat . . .
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—7/16
The Opacity Valve (2/2)
1. Gas is compressed, He or H ionization increases,opacity goes up, incresed pressure pushes staroutward[Closed valve]
2. Expanding layer cools, He or H ions recombine withelectrons, opacity drops, radiation escapes[Open valve]
3. Gas pressure now too low to support the envelope,star contracts, begins to heat . . .
Pulsation is driven by energy stolen from star’s luminosity
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—7/16
The Instability Strip
Cepheid variables ⇒
horizontal branch stars
RR Lyrae variables ⇒
giant branch stars
Long-period variables ⇒
asymptotic giant branchstars
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—8/16
Why fuss over variable stars?
Uniquely identified by their variability
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—9/16
Why fuss over variable stars?
Uniquely identified by their variability
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—9/16
Why fuss over variable stars?
Uniquely identified by their variability
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—9/16
Why fuss over variable stars?
Uniquely identified by their variability
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—9/16
Why fuss over variable stars?
Uniquely identified by their variability
Places them on the HR diagram
All intrinsically bright
Can be used as distance indicators
Essential steps in determining the cosmologicaldistance scale!
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—9/16
Why fuss over variable stars?
Uniquely identified by their variability
Places them on the HR diagram
All intrinsically bright
Can be used as distance indicators
Essential steps in determining the cosmologicaldistance scale!
Planetary Nebulae can also be used as distanceindicators
More than 20,000 in our Galaxy!
Also useful distance indicators
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—9/16
Supernovae (1/2)
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—10/16
Supernovae (1/2)
1. After iron core is formed and core fusion ceases,core collapses
2. Core reaches 5 × 109K: thermal photons are γ rays⇒ disassociate iron nuclei
3. Electrons combine with protons to form neutrons:e− + p → n + ν
4. Neutrinos escape the core, removing more energy
5. Core continues to collapse, reaches nuclear density
6. Can not easily compress core any further
7. Outer regions are still collapsing on to the core
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—10/16
Supernovae (2/2)
8. Bounces back at the nuclear density core
9. Wave propagates back through the star
10. Close to the surface, exceeds the speed of sound ⇒
shock wave
11. Nuclear reactions take place during shock wave,making elements more massive than iron
12. Total energy of the event: 1046 Joules
[movie]
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—11/16
What’s left?
Supernova remnants: Shells of expanding gas thatwe see as nebulae. Examples:
Crab nebula M1 (from 1054 AD, still expanding)
Veil Nebula (9000 BC)
SN1987A in the LMC
Stellar remnants: The cores of massive star SNbecome compact objects
Depending on the mass, they can becomeneutron stars or black holes
Many neutron stars seen as pulsars in supernovaremnants
Exotic elements dumped into interstellar medium
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—12/16
Crab Nebula
We expect to see oneevery 30–100 yearsper galaxy
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Veil Nebula
A114: Lecture 21—30 Mar 2007 Read: Ch. 22,23 Astronomy 114—14/16
Supernova 1987a (1/2)
Supernova in nearby companion Galaxy: Large Magel-lanic Cloud. Approx. 50 kpc from Sun
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Supernova 1987a (2/2)
Current appearance
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