Monster Black Holes

Caty Pilachowski
IU Astronomy
Mini-University 2014
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….
100 billion times
smaller than a proton
About 1 centimeter
About 3 kilometers
• Strength of gravity changes rapidly
with distance
• The closer you are, the stronger the
Small black holes have
strong “tides”
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!
from xkcd (
Earth and
What would happen to the
Moon if the Earth were
suddenly to collapse into a
Black Hole?
A. Nothing – it would continue
to orbit the Earth
B. The Moon would spiral into
the Earth
C. The Moon would fly off into
And the
answer is….
The Moon would continue to
orbit the Earth just as
Escape Velocity
A cannonball fired with
enough velocity will
orbit the Earth
With a high enough
velocity, it will escape
Earth entirely
Gravity ~
Distance x Distance
Isaac Newton, 1728
A Treatise of the System of the World
John Michell, 18th century
British scientist, Fellow of
the Royal Society
• 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!
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:
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
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
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
1974 – A strong
radio source was
discovered in a
survey of the
Galactic Center
Sgr A* in
Strong X-ray
emission from
Sgr A*
X-ray flares!
Sgr A* is not visible in
visible or infrared light
But many stars surround Sgr A*
Even though
Sgr A* is
26,000 light
years away,
we can see
stars orbiting
around it
Stars Orbit
Sgr A*
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
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
Hubble has
examined many
large galaxies and
found supermassive black
holes at their
How do Monster black
holes form?
Does the galaxy
make the
black hole?
-orDoes the black
hole make
the galaxy?
Dense clump of
primordial gas
becomes a black hole
The black hole’s
gravity attracts more
gas into a disk
The gas forms stars
to make a galaxy
Black Hole
Galaxies First:
A single massive
star forms
The star collapses to
form a few hundred
solar mass black hole
The black hole eats
gas, stars, and other
black holes to grow
Galaxies First:
A cluster of massive stars
forms in a baby galaxy
The cluster collapses
under gravity to form
a massive black hole
The black
hole grows
Black Holes Grow with
Black holes accrete gas
Black holes eat stars
When galaxies merge, their central black holes
also merge
NGC 5033 hosts TWO supermassive black holes!
The black holes will
eventually merge into one
Merging Black Holes
Black holes in the center of galaxies will
eventually merge together
a Star
• A star in the galaxy
RX J1242-11 came
too close to its black
• The star was tidally
• Strong x-ray flare
Artist’s Conception
BH Mass ~ 100 million suns
Accreting Gas
The Doomed Cloud
Small gas cloud discovered in 2011
Moving almost directly toward Sgr A* black hole
Velocity ~ several million km/hour!
About 3 Earth-masses of gas (but is a star inside?)
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
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…
Current observations suggest the cloud
contains a star…
Computer Simulation
Black Holes Affect
Galaxy Evolution
Only a fraction of accreted material
ends up in the black hole – most is
shot back out in a jet!
High energy jets from
feeding black holes
may regulate star
formation, gas
accretion, and
galaxy growth!
Courtesy A. Feild / STScI / NASA

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