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Black Holes
Gravity is not a force – it is the curvature of space-time
- Objects try and move in a straight line. When space is
curved, they appear to move in a curved path
Escape Velocity
Earth: 11 km/s
Sun: 620 km/s
White Dwarf: 6,500 km/s
Neutron Star: 165,000 km/s
• 1783: Rev. John Mitchell noticed a strange consequence of Newton’s laws of motion
and gravity: an object with the same density as the Sun but 500 times its radius would
have an escape velocity greater than the speed of light
Speed of light = 300,000 km/s (180,000 miles/s)
• Escape velocity of a neutron star is already close! How much further would it have to
collapse before the escape velocity exceeded the speed of light? Down to 3 km
If an object has an escape velocity greater then the speed of light,
then nothing, not even light, can escape its surface. From the outside,
We would never see anything come from that object. It is a Black Hole.
Schwarzchild Radius
• How small an object must become before its escape velocity exceeds the speed
of light.
Sun: 3 km
Earth: 0.9 cm (1/3 of an inch)
A human: 10-23m (10,000,000 times smaller than a proton)
Black Holes
• At the end of the lives of stars greater than 25-30 MSUN,
neutrons in the collapsing core eventually cannot provide
enough pressure to support the proto-neutron star against
its gravity, and it collapses further. What happens next?
• Answer: it collapses inwards until all of its matter is within
The Schwarzchild radius (3km). It is then a black hole.
• We call Black Holes that form this way stellar mass black holes.
They have masses 3 – 10 MSUN
Black Holes
• Any event that takes place inside the Schwarzchild radius of a black hole can
never be seen by the outside universe. The Schwarzchild radius is effectively the
boundary of the black hole (its surface), and is called the event horizon.
• The only things we can tell about the matter inside the black hole are
• Its mass
• Its spin
• Its electric charge
• Far from the black hole, gravity appears to be normal
• If the Sun were to suddenly collapse into a black hole, the orbits of the
planets would be unaffected (we wouldn’t be ‘sucked in’)
• Very close to the black hole, gravity is extremely strong. If you get closer than 3
times the Schwarzchild radius (9 km for a solar-mass black hole), you would be
unable to escape.
Singularity
Journey into a Black Hole
How do we see Black Holes?
We don’t directly, but we see their effects on nearby gas and stars
• Stars orbiting black holes appear to ‘wobble’ when we observe
Them
• We see X-rays from discs of gas falling into a black hole
How do we see Black Holes?
We don’t directly, but we see their effects on nearby gas and stars
• Stars orbiting black holes appear to ‘wobble’ when we observe
Them
• We see X-rays from discs of gas falling into a black hole