1. Dead & Variable Stars

2. Type I vs Type II Supernovae
The time dependence of the intensity of the light from a supernovae is called a light curve. Light curves with two characteristic shapes are observed:
Type I Supernova (carbon-detonation): Slight delay in the decay of the brightness. Spectrum shows little hydrogen or helium. the progenitor star is a carbon core of an older star
Type II Supernova (core-collapse): Spectrum shows more hydrogen and helium (outer layers). Situation as discussed above

3. Type I – “Carbon Detonation”
Implosion of a white dwarf after it accretes a certain amount of matter, reaching about 1.4 solar masses
Very predictable; used as a standard candle
Estimate distances to galaxies where they occur
Chandrasekhar limit

4. Type II – “Core Collapse”
Implosion of a massive star
Expect one in our galaxy about every hundred years
Six in the last thousand years; none since 1604

5. Supernova Remnants Crab Nebula (M1) from Supernova in 1054 AD
recorded by the Chinese and Native Americans, among others
Was visible in daytime (it’s relatively close)
Many supernovae in other galaxies have been observed in the 20th century, most notably Supernova 1987 A, in the Large Magellanic Cloud.
From Vela Supernova
Crab Nebula

6. SN1987A
Lightcurve
Occurred in LMC, a small nearby galaxy (knew distance immediately)
Progenitor was known previously (a blue supergiant)
Neutrino pulse observed ~20 hours before light
Bright ring in pic probably the planetary nebula previously blown off, heated by radiation from the blast

7. Formation of the Elements
Light elements (hydrogen, helium) formed in Big Bang
Heavier elements formed by nuclear fusion in stars and thrown into space by supernovae
Condense into new stars and planets
Elements heavier than iron form during supernovae explosions
Evidence:
Theory predicts the observed elemental abundance in the universe very well
Spectra of supernovae show the presence of unstable isotopes like Nickel-56
Older globular clusters are deficient in heavy elements
GCs contain “first generation” stars – later generations have higher proportion of heavy elements

8. What’s Left? Type I supernova Type II supernova Nothing left behind
While the parent star is destroyed, a tiny ultra-compressed remnant may remain – a neutron star
This happens if the mass of the parent star was above the Chandrasekhar limit

9. More Massive Stars end up as Neutron Stars
The core cools and shrinks
Nuclei and electrons are crushed together
Protons combine with electrons to form neutrons
Ultimately the collapse is halted by neutron pressure, the core is composed of neutrons
Size ~ few km
Density ~ 1018 kg/m3; 1 cubic cm has a mass of 100 million kg!
Manhattan

10. How do we know Neutron Stars exist? Pulsars
First discovered by Jocelyn Bell (1967)
Little Green Men?!? Nope…
Rapid pulses of radiation
Periods: fraction of a second to several seconds
Small, rapidly rotating objects
Can’t be white dwarfs; must be neutron stars
Hewish (Bell’s advisor): explanation, 1974 Nobel Prize
Can’t be white dwarfs because they would fly apart – too big!

11. The “Lighthouse Effect”
Pulsars rotate very rapidly
Extremely strong magnetic fields guide the radiation
Results in “beams” of radiation, like in lighthouse
Spinning so fast b/c of angular momentum, like an ice skater
We can see them if the “beam” sweeps across the earth
Width of beam may only be a few degrees
Other wavelengths also produced, but radio typically the strongest

12. Super-Massive Stars end up as Black Holes
If the mass of the star is sufficiently large (M > 25 MSun), even the neutron pressure cannot halt the collapse – in fact, no known force can stop it!
The star collapses to a very small size, with ultra-high density
Nearby gravity becomes so strong that nothing – not even light – can escape!
The edge of the region from which nothing can escape is called the event horizon
Radius of the event horizon called the Schwarzschild Radius

13. How might we “see” them? Radiation of infalling matter
Gravitational effects, if evidence that an unseen partner is both very massive and very small

14. Evidence for the existence of Black Holes
Fast Rotation of the Galactic Center only explainable by Black Hole
Other possible Black Hole Candidates:
Cygnus X-1 (X-ray source), LMC X-3
Observational evidence very strong

15. Novae – “New Stars”
Actually an old star – a white dwarf – that suddenly flares up
Accreted hydrogen begins fusing
Usually lasts for a few months
May repeat (“recurrent novae”)

16. Variable Stars
Eclipsing binaries (stars do not change physically, only their relative position changes)
Nova (two stars “collaborating” to produce “star eruption”)
Cepheids (stars do change physically)
RR Lyrae Stars (stars do change physically)
Mira Stars (stars do change physically)

17. Eclipsing Binaries (Rare!)
The orbital plane of the pair almost edge-on to our line of sight
We observe periodic changes in the starlight as one member of the binary passes in front of the other

18. Cepheids Named after δ Cephei Period-Luminosity Relations
Two types of Cepheids:
Type I: higher luminosity, metal-rich, Pop. 1
Type II: lower lum., metal-poor, Population 2
Used as “standard candles”
“yard-sticks” for distance measurement
Cepheids in Andromeda Galaxies established the “extragalacticity” of this “nebula”

19. Describe how variable stars, e. g
Describe how variable stars, e.g. Cepheids, can be used as cosmic yardsticks
k: Cepheid stars oscillate between two states: In one of the states, the star is compact and large temperature and pressure gradients build up in the star. These large pressures cause the star to expand. When the star is in its expanded state, there is a much weaker pressure gradient in the star. Without the pressure gradient to support the star against gravity, the star contracts and the star returns to its compressed state. [Does not answer the question! Answers the question: What happens when a Cepheid changes its brightness?]
A: We can infer their brightness therefore enables us to measure distance [Short but good, although better use luminosity ISO brightness]

20. Cepheids
Henrietta Leavitt (1908) discovers the period-luminosity relationship for Cepheid variables
Period thus tells us luminosity, which then tells us the distance
Since Cepheids are
brighter than RR Lyrae,
they can be used to
measure out to further
distances
Extends cosmic distance ladder out to as far as we can see Cepheids (5 Mpc or more, that is, out to neighboring galaxies)

21. Properties of Cepheids
Period of pulsation: a few days
Luminosity: suns
Radius: solar radii

22. Properties of RR Lyrae Stars
Period of pulsation: less than a day
Luminosity: 100 suns
Radius: 5 solar radii

23. Distance Measurements with variable stars
Extends the cosmic distance ladder out as far as we can see Cepheids – about 50 million ly
In 1920 Hubble used this technique to measure the distance to Andromeda (about 2 million ly)
Works best for periodic variables
15 Mpc takes us to neighboring galaxies

24. Cepheids and RR Lyrae: Yard-Sticks
Normal stars undergoing a phase of instability
Cepheids are more massive and brighter than RR Lyrae
Note: all RR Lyrae have the same luminosity
Apparent brightness thus tells us the distance to them!
Recall: B  L/d2