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ASTRO 101: Black Holes Module 10
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Notes for ASTRO 101 based on the official Coursera videos for the class. Covers Module 10.
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Module 10Refraction, Gravitational Lensing, and Black Hole Physics
Refraction and Gravitational Lensing
● Refraction: Light bends when entering a material (e.g.,glass), forming lenses that
magnify, diffuse, or distort light.
● Gravitational lensing: Objects with a gravitational field bend light as it travels
through curved spacetime.
○ Example: The Cheshire Cat galaxy group, where light from distant galaxies is
bent by the mass of closer galaxies, creating arcs and distorted images.
Neutron Stars and Gravitational Lensing
● Neutron stars: Dense stars with strong gravitationalfields that bend light emitted
from their surfaces.
○ Light paths curve, allowing parts of the star typically hidden to become
visible.
○ Observing this effect helps distinguish neutron stars from black holes.
● Example analogy: If a head were as massive as a neutron star, light from the back
would curve around, making the entire head visible.
Neutron Star Hotspots
● Hotspots on neutron stars emit light influenced by the star’s gravity.
● Without gravitational effects:
○ The hotspot is visible only during part of the star’s rotation.
○ Brightness varies significantly over time.
● With gravitational lensing:
○ The hotspot remains visible throughout the rotation.
○ Light is distorted, reducing brightness variation.
Doppler Effects on Rotating Stars
, ● Neutron stars rotate rapidly (e.g., equatorial speed = 30% of light speed).
● Doppler effect:
○ Light from the approaching side is blueshifted (appears brighter).
○ Light from the receding side is redshifted (appears dimmer).
● Doppler boosting: Amplifies brightness differences between blueshifted and
redshifted light.
NICER X-ray Telescope
● Measures gravitational lensing and Doppler-boosted light from neutron star
hotspots.
● Observes X-rays from neutron stars and black holes.
Accretion Disks Around Black Holes
● Structure: Gas orbiting a black hole, often depictedas flat.
● Gravitational lensing distorts the accretion disk, making the back visible as an arch
above and below the black hole.
● Example model: Vinyl record analogy:
○ Front disk is directly visible.
○ Light from the back bends around the black hole, creating a distorted view.
Accretion Disks in Media
● Interstellar’s depiction:
○ Realistic gravitational lensing effects.
○ Excluded jets and Doppler boosting for simplicity.
○ Accretion disk modeled as cooler and safer for astronauts.
Photon Paths Near Black Holes
● Photon sphere: Light orbits black holes in sphericalshells.
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