lectures #18 & 19: plan - university of maryland...
TRANSCRIPT
Lectures #18 & 19: Plan • Stellar Evolution:
– Low-mass stars – High-mass stars
Stellar Evolution
• Most important factor determining a star’s fate is mass
• Stellar evolution is governed by competition between inward gravity 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
Post-Main Sequence Evolution: M < 8 Msun 2. H shell burning + inert He core → red giant
phase (R ~ 30 Rsun)
Evolutionary tracks of giant stars
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
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)
WD
The Hourglass Nebula
White Dwarfs • M ~ Msun • R ~ REarth • Magnetic field ~ 108 x that of Earth • Chandrasekhar mass limit = 1.4 Msun
→ Density ~ 106 kg / liter = 106 g/cm3 !
WD B
A
Sirius
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 gravity
→ Pressure increases towards center of star
star
star
star
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 à He @ T > 107 K) and helium burning (3 He à C @ T > 108 K) become possible: • C + He à O (oxygen) • O + He à Ne (neon) • Ne + He à 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 gravity > 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
Supernova Remnant
The Crab Nebula (1054)
Supernova Remnant
Evolution of M > 8 Msun stars
The Lifespan of a Massive Star
Low vs High-Mass Stellar Evolution
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 ~ few 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
"
Photon Sphere (photon in orbit)
Event Horizon (photon unable to escape)
Rs
1.5 Rs
Photon Sphere
Event Horizon
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 binary system
How do we find black holes? 3. Gravitational waves!!!
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 à 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
Black Hole
• Results from LIGO:
Black holes merging and gravity waves
Caltech-Cornell numerical GR group, http://www.black-holes.org
https://www.youtube.com/watch?v=YsZFRkzLGew
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…
Midterm #2 • Tuesday 11:00 – 12:15 am (PHYS 1412 – here!) • Be there early (~10:45 am) if you can • Material: Everything not covered by midterm #1,
starting with “Terrestrial Planets” • 20 multiple choice questions + 4 problems • No need to remember any formula • Bring your student ID card • Write on the scan sheet: your section #, student ID # • Use #2 pencil only • No books, calculators, phones, computers, hats, …