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ASTRO 101: Black Holes Module 5
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Notes for ASTRO 101 based on the official Coursera videos for the class. Covers Module 5.
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Module 5Black Hole Jets:
● Black holes, like Cygnus X-1, appear dark but can create bright regions around
them due to material they interact with.
● The light seen near black holes comes from matter in their accretion disks or jets,
not from the black hole itself.
● Astrophysical jets, cone-like structures formed near black holes, emit energy
equivalent to thousands of suns and extend far into space.
● Jets are powered by material falling into black holes and interacting with magnetic
fields, creating light through synchrotron radiation.
● Relativistic jets, traveling up to 10% the speed of light, align with a black hole's spin
axis but may slightly wobble, causing a "lighthouse effect."
● Jets from Cygnus X-1 stretch up to 600,000 times the Earth-Sun distance when
observed in radio wavelengths.
● The jet of the galaxy M87 spans 5,000 light years, displaying bends from wobble
and interaction with surrounding space.
● Doppler shifts affect the jet's appearance: material moving toward us appears
brighter and bluer, while material moving away looks redder and dimmer.
● Pictor A's black hole produces jets 800,000 light years long, over eight times the
size of the Milky Way, making some black hole jets the largest structures in the
universe.
● Radio lobes, or red clouds of radiation, are another feature of the environment
around black holes, often linked to their jets.
● Black holes are known for pulling matter in, but they also eject high-speed jets of
material.
● The jets are formed from material that narrowly avoids falling into the black hole
and is expelled at high speeds.
● Magnetism is a key factor in keeping these jets long and straight as they shoot out
into space.
● Black holes may play a significant role in the origin of magnetism in the universe.
● Magnetic fields are blasted into space by black holes, potentially spreading
magnetism across the cosmos like a high-speed water jet.
, Black Hole Companions:
● Black holes can have low-mass or high-mass companion stars, and the type of
companion determines how the black hole is fed.
● High-mass starstransfer mass through strong stellarwinds, whilelow-mass stars
require close proximity to transfer mass.
● Cygnus X-1, a high-mass x-ray binary, has a blue supergiant companion star, HDE
226868, orbiting at just 0.2 astronomical units from the black hole.
● Systems like Cygnus X-1 emit strong x-rays due to active feeding, and such systems
are calledx-ray binaries.
● X-ray binariescontain a compact object (black holeor neutron star) and a
companion star, with classification based on the companion star's mass, not the
compact object's type.
● Companion stars inlow-mass x-ray binaries (LMXBs)are less massive than the
Sun, while those inhigh-mass x-ray binaries (HMXBs)are at least ten times the
Sun's mass.
● There is also an intermediate category, but these systems behave similarly to
LMXBs.
● The classification is based on companion star mass because the system's
properties, such as orbital period and mass transfer, depend more on the
companion star than the compact object.
● Low-mass stars orbit closer to black holes, resulting in shorter orbital periods,
sometimes less than an hour.
● High-mass stars orbit farther from the black hole, with orbital periods of several
days.
● The feeding rate and orbital dynamics differ significantly between LMXBs and
HMXBs, influenced by the companion star's size and mass.
Accretion:
● Black holes in binary systems can feed on companion stars through two primary
mechanisms:Roche lobe overflowandwind-fed accretion.
Roche Lobe Overflow:
● TheRoche lobeis a teardrop-shaped region aroundeach star in a binary system
where the star's gravity dominates.
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