Lecture 20: Black Holes Astronomy Spring 2014

Key Ideas: Black Holes are totally collapsed objects Gravity so strong not even light can escape Predicted by General Relativity Schwarzschild Radius & Event Horizon Find them by their Gravity X-ray Binary Stars Motions of Stars in the Galactic Center Black Holes do not make up the vast majority of dark matter

Gravity’s Final Victory A star more massive than 18M sun would leave behind a core with M  2 – 3 M sun : General Relavity predicts that nothing can stop gravitational collapse. Core would collapse into a singularity, an object with Zero radius Infinite density Well, not really, we lose track at meters

Black Holes The ultimate extreme object: Gravity so strong not even light escapes. Infalling matter gets shredded by powerful tides & crushed to infinite density. V esc exceeds the speed of light Black: It neither emits nor reflects light. Hole: Nothing entering can ever escape.

Schwarzschild Radius Light cannot escape from a Black Hole if it comes from a radius less than the Schwarzschild Radius: R S depends only on the mass of the core M = Mass of the Black Hole For M = 1 M sun, R S  3 km

The Event Horizon R S defines the Event Horizon: Events that happen inside R S are invisible to the outside Universe. Things that get inside R S can never leave the black hole. The “Point of No Return” for a Black Hole.

Info on Stephen Hawking and Event Horizon here would be great

Black Holes & Singularities If you can squeeze an object so it fits inside its Schwarzschild radius, then it has an event horizon and light cannot escape But GR predicts that a body inside of R Sch will collapse to a singularity, a point of infinite density

Collapse to a Point From Special Relativity E=mc 2 From General Relativity Matter tells spacetime how to curve Energy will also tell spacetime how to curve To resist gravity, we need to have a source of energy (thermal pressure, for example) But this is self-defeating. Increasing the energy = increasing the curvature Prediction of GR: You cannot win in certain cases

Gravity around Black Holes Far away from a black hole: Gravity is the same as for a star of the same mass. If the Sun became a black hole, the planets would all orbit the same as before. Close to a black hole: R < 3 R S, no stable orbits – matter spirals in. At R = 1.5 R S, photons orbit in a circle!

Black Holes Do Not Suck Black Holes wandering through the Galaxy do not pose a special hazard. They do not exert a stronger gravitational pull (create a larger curvature of spacetime) far away (~ stellar radius to a few AU) than “ordinary” matter There are large distance between stars, so the chances that the Sun will chance to get close enough to a black hole to care are remote So even though we can’t “see” them coming, I am not worried

Two observers: Jack & Jill Jack, in a spacesuit, is falling into a black hole, carring a blue laser beacon. Jill is orbiting the black hole in a starship at a safe distance in a stable circular orbit. Journey to a Black Hole: A Thought Experiment Jack Jill

From Jack’s point of view: Sees the ship getting further away. Flashes his blue laser once a second by his watch From Jill’s point of view: Each flash takes longer to arrive, and is Redder and fainter than the one before it. He Said, She Said Jack Jill Laser Flash

Near the Event Horizon... Jack Sees: His blue laser flash every second by his watch The outside world looks distorted Jill Sees: Jack’s laser flashes come ~1 hour apart Flashes are redshifted to radio wavelengths Flashes are getting fainter with each flash

Jill’s View from far awayJack’s view from 10R S

Just Outside the Event Horizon

Down the hole... Jill Sees: Sees one last laser flash after a long delay Flash is faint and at long radio wavelengths She never sees another flash from Jack… Jack Sees.. Not so much: Gets shredded by strong tides near the singularity and crushed to infinite density.

Jill’s Conclusions: The powerful gravity of a black hole warps space and time around it: 1.Time appears to stand still at the event horizon as seen by a distant observer. 2.Time flows as it always does as seen by an infalling astronaut. 3.Light emerging from near the black hole is Gravitationally Redshifted to longer (redder) wavelengths

Jack’s Conclusion: AAAAAAHHH HHHHH!...

Do Black Holes Exist? To make a black hole, you need to have a case where nothing can stop the gravitational collapse. Most of the time, something counteracts gravity Thermal pressure Electromagnetic pressure Degeneracy pressure Collapse of the iron-core of a massive star!

The Iron Catastrophe

Collapse of the Iron Core In the last seconds of the life of a massive star (M > 18 solar masses?), all Si has fused to iron in the core No new source of thermal energy to exert outward pressure Core collapses under gravity. Degeneracy pressure not enough to stop collapse Outer layers ejected, at least sometimes Forms a Type II SN

Seeing what cannot be seen… Q: If black hole are black, how can we see them? A: By the effects of their gravity on their surroundings: A star orbiting around an unseen massive object. X-rays emitted by gas superheated as it falls into the black hole.

X-Ray Binaries Bright, variable X-ray sources identified by X- ray observatory satellites: Spectroscopic binary with only one set of spectral lines  the companion is invisible. Gas from the visible star is dumped on the companion, heats up, and emits X-rays. Estimate the mass of the unseen companion from the orbit. Black hole candidates will have M  3 M sun

Artist’s Conception of an X-Ray Binary

Black Hole Candidates X-ray binaries with unseen companions of mass > 3 M sun, too big for a Neutron Star. Currently 20 confirmed black hole candidates: First was Cygnus X-1: 7 – 13 M sun Largest is GRS : 10 – 18 M sun Most are in the range of 4 – 10 M sun Estimated to be ~1 billion stellar-mass black holes in our Galaxy alone.

Supermassive Black Holes The centers of many galaxies have supermassive black holes (M~ solar masses) The Milky Way has rather a more pathetic black hole ~10 6 solar masses How black holes get to be so big is an important area of current research Merger of many smaller black holes Collapse of supermassive stars?

Stars near the Galactic Center (8000 parsecs away) orbit a massive, compact, dark object. Mass = 2 million times the Sun’s mass