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Monster Black Holes. Caty Pilachowski IU Astronomy Mini-University 2014. What is a black h ole , anyway?. A black hole is…. …an object whose gravity is so intense that light cannot escape. How Big Are Black Holes?. A black hole’s size depends on its mass….

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caty pilachowski iu astronomy mini university 2014

Monster Black Holes

Caty Pilachowski

IU Astronomy

Mini-University 2014

a black hole is
A black hole is….

…an object whose gravity is so intense that light cannot escape

how big are black holes
How Big Are Black Holes?

A black hole’s size depends on its mass….

100 billion times smaller than a proton

About 1 centimeter

About 3 kilometers

strong gravity
Strong Gravity!
  • Strength of gravity changes rapidly with distance
  • The closer you are, the stronger the gravity
  • Spaghettification!
  • Small black holes have strong “tides”
black hole safety
Black Hole Safety!

At larger distances, a black hole’s gravity is exactly the same as if it were “normal” matter

  • Safe limit: hundreds or thousands of times the black hole’s diameter
black holes are not intuitive
Black holes are not intuitive!

from xkcd (

earth and moon
Earth and Moon?

What would happen to the Moon if the Earth were suddenly to collapse into a Black Hole?



  • Nothing – it would continue to orbit the Earth
  • The Moon would spiral into the Earth
  • The Moon would fly off into space



and the answer is
And the answer is….

The Moon would continue to orbit the Earth just as before…



Black Hole



escape velocity
Escape Velocity


Isaac Newton, 1728

A Treatise of the System of the World

  • A cannonball fired with enough velocity will orbit the Earth
  • With a high enough velocity, it will escape Earth entirely

Distance x Distance



dark stars
Dark Stars
  • What happens if a star’s gravity is so strong that its escape velocity is faster than the speed of light?
  • From the speed of light, Michell calculated that light could not escape from a star 500 x the Sun’s radius and the same density (125,000,000 solar masses!)
  • The star would be dark!
  • John Michell, 18th century British scientist, Fellow of the Royal Society
black holes
Black Holes!

In the 1930s – Chandrasekhar, Oppenheimer, Snyder predict that massive stars can collapse into something denser – a black hole

  • J.A. Wheeler popularized the term “black hole”
cygnus x 1 the first black hole
Cygnus X-1: The FIRST Black Hole
  • Discovered in 1972
  • A blue supergiant star orbits an invisible companion
  • Bright in x-rays
  • HDE 226868, near Eta Cygni in Cygnus
evidence for black holes
Evidence for Black Holes
  • Effect of gravity on nearby objects – Mass!
  • Accretion disks
  • Accretion disks emit x-rays as matter falls in
  • But it’s hard to tell the difference between a black hole and a neutron star
stellar mass bhs
Stellar Mass BHs
  • About 20 candidates known in the Milky Way
  • Masses 4-12 times the mass of the Sun
  • A few thousand light years away
  • The galaxy contains many more yet to be discovered
sagittarius a
Sagittarius A*
  • 1974 – A strong radio source was discovered in a radio- wavelength survey of the Galactic Centerregion
sgr a in x rays
Sgr A* in X-rays
  • Strong X-ray emission from Sgr A*
  • X-ray flares!
stars orbit sgr a
Stars Orbit Sgr A*

Even though Sgr A* is 26,000 light years away, we can see stars orbiting around it

stars orbit sgr a1
Stars Orbit Sgr A*

The orbits of stars around Sgr A* tell us the mass of the central object

  • The mass of Sgr A* is 4,000,000 times the mass of the Sun
other galaxies have black holes too
Other galaxies have black holes, too!

The giant elliptical galaxy M87 contains a massive black hole

3.5 BILLION solar masses!

all large galaxies contain central black holes
All large galaxies contain central black holes

Hubble has examined many large galaxies and found super-massive black holes at their centers

how do monster black h oles f orm
How do Monster black holes form?

Does the galaxy make the black hole?


Does the black hole make the galaxy?

black holes grow with galaxies
Black Holes Grow with Galaxies
  • Black holes accrete gas
  • Black holes eat stars
  • When galaxies merge, their central black holes also merge
  • NGC 5033 hosts TWO super-massive black holes!
  • The black holes will eventually merge into one
merging black holes
Merging Black Holes

Black holes in the center of galaxies will eventually merge together

black hole eats a star
Black Hole Eats a Star

Artist’s Conception

  • A star in the galaxy RX J1242-11 came too close to its black hole
  • The star was tidally shredded
  • Strong x-ray flare

BH Mass ~ 100 million suns

the doomed cloud
The Doomed Cloud
  • Small gas cloud discovered in 2011
  • Moving almost directly toward Sgr A* black hole (yellow )
  • Velocity ~ several million km/hour!
  • About 3 Earth-masses of gas (but is a star inside?)
the cloud in 2013
The Cloud in 2013
  • Closest approach to Sgr A* in early 2014, about 2200 x the black hole’s radius
  • As the cloud approaches Sgr A*, gravity “spagettifies” the cloud
2014 observations
2014 Observations

The leading edge of the cloud has whipped around Sgr A* at 10,000,000 km/hour!

Computer Simulation

  • We’re still not sure what it is…
    • An isolated cloud
    • A “wind bubble” around a star
the cloud s future
The Cloud’s Future…
  • Current observations suggest the cloud contains a star…

Computer Simulation

black holes affect galaxy evolution
Black Holes Affect Galaxy Evolution

High energy jets from feeding black holes may regulate star formation, gas accretion, and

galaxy growth!

Only a fraction of accreted material ends up in the black hole – most is shot back out in a jet!


Black holes are a natural phenomenon governed by the laws of science

  • Black holes come in sizes from a few solar masses to billions of solar masses
  • The Milky Way, like all large galaxies, contains a monster black hole at its center
  • Black holes grow by feeding on stars and gas
  • Monster black holes regulate their galaxy’s growth