# Caty Pilachowski IU Astronomy Mini-University 2014.

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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…. About 1 centimeter About 3 kilometers 100 billion times smaller than a proton

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

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! from xkcd (www.xkcd.com)

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

A B C A.Nothing – it would continue to orbit the Earth B.The Moon would spiral into the Earth C.The Moon would fly off into space

And the answer is…. The Moon would continue to orbit the Earth just as before… A!A!A!A!

a

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 Gravity Distance x Distance Mass ~

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, 18 th century British scientist, Fellow of the Royal Society

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 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 star

About 20 candidates known in the Milky Way About 20 candidates known in the Milky Way Masses 4-12 times the mass of the Sun Masses 4-12 times the mass of the Sun A few thousand light years away A few thousand light years away The galaxy contains many more yet to be discovered The galaxy contains many more yet to be discovered

1974 – A strong radio source was discovered in a radio- wavelength survey of the Galactic Center region

Sgr A* in X-rays 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*

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

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! 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 super- massive black holes at their centers

How do Monster black holes form? Does the galaxy make the black hole? -or- Does the black hole make the galaxy?

Black Hole First 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

Galaxies First: I 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

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 Galaxies First: II

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

Black holes in the center of galaxies will eventually merge together Merging Black Holes

Black Hole Eats a Star A star in the galaxy RX J1242-11 came too close to its black hole The star was tidally shredded 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 (yellow ) 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 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!

Courtesy A. Feild / STScI / NASA