Exam on Wednesday
• Short answer and essay questions • Mostly qualitative • Don’t need to memorize formula, but do
need to know which way formula go
• Review session today, 4:30, here Tuesday, 5:00, 4421 Sterling Hall
Evidence for Black Holes
• Stellar Mass Black Holes – X-ray binaries – Accretion Disks – Orbits
• Supermassive Black Holes – Spectra – Masers – Jets
Type Ia
• Start with a White Dwarf • Somehow get above Chandrasekhar limit
– Accretion from companion? – Merger?
• Start a little nuclear burning in the core
Type Ia Supernova
• Burn most of the star up to Fe/Ni
• No compact remnant left, no pulsar
Tycho’s SN (1572) remnant
HOW TO DETECT A BLACK HOLE • Effects on matter/light outside the horizon
– gravitational attraction of other bodies – “dark star” with mass
• distinguish from normal star, white dwarf, neutron star
• Accretion (swallowing) of gas – gas heated by compression/turbulence in strong gravity
field X-rays – but need a source of gas
• accretion from interstellar matter insignificant • mass transfer in binaries to the rescue
SunM3>
HOW TO DETECT BLACK HOLES
1. Mass of “compact “ companion in close binary system (stellar remnants only)
X-ray binary (artist’s impression)
HOW TO DETECT BLACK HOLES
3. Random motions of stars in galaxy’s nucleus (supermassive holes)
Globular cluster M3 (similar appearance to a galactic nucleus)
Too much “swirl” (angular momentum) … …makes it more
like a whirlpool
Gas almost never falls directly into a black hole
ACCRETION DISK • Like a flattened whirlpool
• Gas must give up angular momentum to go down the drain
VISCOSITY (~FRICTION)
ACCRETION DISKS ALLOW US to PROBE the HORIZON
GRAVITY
MOTION
HEAT
RADIATION (X-rays, UV…)
Energy flows from one form to another...
matter swirling inward
friction
ENERGY FLOW IN ACCRETION DISK
GRAVITATIONAL POTENTIAL ENERGY
KINETIC ENERGY
HEAT
RADIATION
Energy flows from one form to another...
falling matter
compression/turbulence
particle collisions, etc.
CAN WE IMAGE BLACK HOLES?
• HUBBLE: read newspaper @ 1 mile – Optical/UV telescope in space – Falls short by 100,000
• VLBA: read newspaper in Chicago – Transcontinental radio telescope – Falls short by 1,000
• Event Horizon Telescope: read paper in LA – High frequency VLBA (being built) – Can do if for 2 Black Holes
• MAXIM: Read newspaper on moon – X-ray interferometer in space (2040?)
…NOT YET, BUT SOMEDAY…?
MAXIM = Micro- arcsecond X-ray Imaging Mission
DETECTOR SPACECRAFT
CONVERGER SPACECRAFT
200 M
COLLECTOR SPACECRAFT (32 PLACES EVENLY SPACED)
10 KM
5000 KM
CONSTELLATION BORESIGHT
Hub Spacecraft
SPACECRAFT DELAY LINE
Why is it easier to image a supermassive black hole?
• Black hole radius scales as M
• Even though they are far away, still appear large
2 biggest apparent Black Holes
• Sag A* - Milky way center – 26,000 light years away – 4.3 million Msun
• M87 – 54 million light years away – 6.6 billion Msun
• Which looks bigger?
EVOLUTION OF CLOSE BINARIES
• “Algol Paradox” and its resolution • Roche lobe = “sphere” of influence
– actually teardrop shaped
• Matter flows across Lagrange point • Too much angular momentum
ACCRETION DISK
ALGOLS CAN EVOLVE INTO X-RAY BINARIES
• Crucial that mass ratio flips – otherwise stars can fly apart
• Compact star either NS or BH – depends on mass of precursor
• Two modes of mass transfer – stellar wind: star smaller than Roche
lobe – “Roche lobe overflow”: star swells to
fill Roche lobe
NEUTRON STAR VS. BLACK HOLE: …how to tell
• BH if: – mass > 3 Msun (reliable) – distinctive spectrum (unreliable ????)
• NS if: – pulsing (X-ray pulsar) – evidence of nuclear explosions on surface
(X-ray burster)
BINARY MASS FUNCTION depends on...
• Orbit period: easy • Doppler shift of
normal star: easy • Mass of normal star:
hard • Orbit inclination: hard
0.1 10 1 100
Log Mass (solar units)