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Lectures #18 & 19: Plan • Stellar Evolution:
– Low-mass stars – High-mass stars
Stellar Evolution
• The most important factor determining a star’s fate is the mass
• Stellar evolution is governed by competition between inward gravitational force and outward pressure (the need to maintain hydrostatic equilibrium)
Stellar Evolution: Early Stages • Collapse of proto-stellar cloud → Heating from contraction → Accretion disk → Jets
• Ignite hydrogen fusion (�burning�) into helium in the core → Enter main-sequence phase
Dense regions in molecular clouds
The birthplace of stars!
“Stellar Nursery” The Eagle Nebula
Protostars
Bipolar Flows
Post-Main Sequence Evolution: M < 8 Msun 1. H core depleted → core contracts → envelope
expands → star leaves main sequence
Lifetime on main sequence: ~ 1010 / M2 years (reminder)
Post-Main Sequence Evolution: M < 8 Msun 2. H shell burning + inert He core → red giant
phase (R ~ 30 Rsun)
Transition to Red Giants
Post-Main Sequence Evolution: M < 8 Msun 3. Desperate attempt to maintain hydrostatic equilibrium:
Ignite He in core (�helium flash�) → contracts back to yellow giant phase (R ~ 10 Rsun)
He burning: “triple alpha reaction”
H burning: “p – p reaction”
A (temporary) new lease on life
He core burning
Post-Main Sequence Evolution: M < 8 Msun 4. He core depleted → inert C core + H, He
burning shells → envelope expands → red supergiant phase (R ~ 300 Rsun ~ Mars’ orbit!)
5. Shed outer envelope → planetary nebula 6. Core remnant = white dwarf
(no more energy source – simply cooling)
white dwarf
The Hourglass Nebula
White Dwarfs • M ~ Msun • R ~ REarth • Magnetic field ~ 108 x that of Earth • M ≤ Chandrasekhar mass limit = 1.4 Msun
→ Density ~ 106 kg / liter = 106 g/cm3 !
B
A
Sirius
white dwarf
Evolution of M < 8 Msun stars
Evolution of M ~ Msun Stars
The Life-path of the Sun
What happens in a star of higher mass?
Reminder: Hydrostatic Equilibrium Outward pressure = Inward gravitational force
→ Pressure increases towards center of Sun
Reminder: Gas Pressure
Pressure = Constant × Temperature × Density
High-Mass Stars (M > 8 Msun) • So high-mass stars must have higher core
temperatures and densities than low-mass stars • Nuclear reactions other than hydrogen burning
(4 p ! He @ T > 107 K) and helium burning (3 He ! C @ T > 108 K) become possible: • C + He ! O (oxygen) • O + He ! Ne (neon) • Ne + He ! Mg (magnesium) • And then silicon, sulfur and even up to iron!
Post-Main Sequence Evolution: M > 8 Msun
Nuclear reactions in massive stars
Post-Main Sequence Evolution: M > 8 Msun
Successive stages of shell and core burning produce ever heavier elements until it reaches iron …
Post-Main Sequence Evolution: M > 8 Msun
- It actually costs energy to build elements heavier than iron by fusion.
- Disastrous consequences: inward gravitational force > outward pressure…
Post-Main Sequence Evolution: M > 8 Msun
→ Catastropic collapse! → Electron + proton → neutron (in core) → Core bounce → Kaboum! Supernova explosion! (accompanied by other nuclear reactions
that create atoms heavier than iron)
Before and After – a Supernova (SN 1987a in the Large Magellanic Cloud)
Supernova Remnant: SN 1987A
Mm-wave + visible + X-rays
Supernova Remnant
Supernova Remnant: Tycho Brahe (1572)
X-rays
Supernova Remnant: The Crab Nebula (1054)
Visible
Evolution of M > 8 Msun stars
The Life of a Massive Star
Low vs High-Mass Stellar Evolution
Stellar Remnants after Supernova
i. If final core mass < 3 Msun → Neutron star
ii. If final core mass > 3 Msun
→ Black hole iii. No remnant!
Neutron Star
• Giant ball of neutrons! • M ~ 1.4 – 3 Msun • R ~ 10 km
• Very strong magnetic field – 1012 x that of Earth
• Fast rotator
→ Density ~ 4 x 1014 g / cm3 ~ humanity / cm3 !
→ Pulsar
Pulsar: the Explanation
The Crab Nebula Pulsar
Pulsar
http://www.jb.man.ac.uk/~pulsar/Education/Sounds/
Black Hole
• Gravity’s Ultimate Triumph! • Vesc
2 = 2 G M / R
If Vesc = c (speed of light) RS = 2 G M / c2 = Schwarzschild radius = size of �event horizon� • If M = 1 Msun → RS ~ 3 km
Mass warps space!
Black Holes
Light Bending
May 29, 1919: Solar eclipse proved theory of general relativity (Einstein)
Sun
Light Bending
Light bending near a black hole
Light near a Black Hole Sitting back-to-back but seeing eye-to-eye
Photon Sphere (photon in orbit)
Event Horizon (photon unable to escape)
Rs
1.5 Rs
Photon Sphere
Event Horizon
Photons are orbiting the black hole at R = 1.5 Rs!
Strong Tides near Black Holes → �Spaghettification�!
How do we find black holes?
Here!
How do we find black holes?
1. Motion of a visible companion star in orbit around the black hole
2. Strong X-ray source due to mass accretion
Light curve from a black hole X-ray binary system
How do we find black holes? 3. Gravitational waves!!!
Black holes merging and gravity waves
https://www.nytimes.com/video/science/100000004200661/what-are-gravitational-waves-ligo-black-holes.html
Black Hole • LIGO: Laser Interferometer Gravitational-Wave Observatory - It is actually two observatories: one in Louisiana, another in
the state of Washington (~10 milli-second apart at v = c !)
4-km baselines
Black Hole • LIGO: Laser Interferometer Gravitational-Wave Observatory - Needs to measure changes in space-time of <1 part in 1022 !!!
Over 4-km baselines, accuracy needed is less than 4 km x 10-22 = 4 x 10-19 m ~ 0.0005 proton radius!
- Uses two laser beams at 90 degrees of each other to measure small displacements of test masses hung by pendulums
Black Hole
• Results from LIGO: - GW150914 event:
merger of a pair of black holes of 36 + 29 Msun ! 62 Msun - 3.0 Msunc2 is radiated in gravitational waves - First detection of: gravitational waves, black hole binary,
black hole with mass ~25 Msun and above
- Fast timeline: " 2015 September 14: LIGO detection " 2016 February 11: LIGO research announcement " 2017 October 3: Nobel Prize in Physics awarded
to Kip Thorne, Rainer Weiss, and Barry Barish
Black Hole
• In principle, there is no upper limit to a black hole’s mass
– MBH = 106 – 109 Msun in the centers of many galaxies ! – We will discuss them later…
Neutron Star Merger!
(Troja+17)
• Newest results from LIGO: - GW170817 event:
A pair of neutron stars of ~1.3 Msun each merged into a ~ 2-3 Msun neutron star or black hole - First detection of electromagnetic radiation from a
gravitational wave event - Usher in a new era of “multi-messenger astronomy”!
Activity #9 What is the escape velocity at the event horizon of a black hole?
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