BLACK HOLES

WHAT IS A BLACK HOLE? A few definitions from the dictionary: Region of space resulting from the collapse of a star Region of space with strong gravitational field Dungeon or dark cell in a prison Project that requires large effort with no return Concentration of mass with a gravitational field so strong that not even light can escape its grip

Schwarzschild discovers that General Relativity predicts the presence of “singularities”, regions where gravity is so strong that not even light can escape… Einstein refutes… “too bizarre to be true”…. … but research continues…. BLACK HOLES are predicted….

Escape velocity Earth : 11.2 km/s Mars: 5 km/s Asteroid Ida (radius 50 km) : 10 m/s –You can jump maybe 4 m/s Sun : 620 km/s Black event horizon : c inside event horizon: >c outside event horizon: <c

Properties of Black Holes… e.g. Time dilation, gravitational redshift, “spaghettification” due to tidal forces red-shifted red-shifted into oblivion From inside this region no information can escape

It was thought to be a ridiculous idea, at first. But, yes. Oh, yes.

Theoretically: Black holes believed to be associated with the “death” of massive stars

The Sun will not turn into a black hole. It’s not massive enough for its force of gravity to overcome the natural pushback of atomic structure. But if it did, its event horizon would have a radius of 3 km (compared to the km it is now) The EH of the Earth as a BH would have a 9mm radius! The actual mass of the sun would be at the center of this EH sphere, compressed to a point.

What do we see when we look at a black hole ? Not much. We kinda hafta wait for it to interact with something.

Observationally: several pieces of evidence (astrophysicists like to classify by the mass of our Sun) ) Small-mass BHs in X-ray binaries a few Supermassive BHs in galaxy centers and Quasars

X-RAY BINARIES A Star “feeding” a Black Hole One star has reached the end of its life, and turned into a black hole. It’s close enough so it can pull material off its companion. As this material spirals in, it speeds up, heats up and gives off ultra violet, x-rays, and even gamma rays. We think this is what makes Cygnus X-1 the brightest x- ray source in the sky.

QUASARS L~10 46 erg/s ~ L sun !! Quasars (quasi-stellar objects) are superenergetic distant objects. We now think they are very distant galaxies with Active Galactic Nuclei (AGN). Material is falling into a supermassive black hole at the center, and jets form at the axes of spin that radiate away some escaping energy.

Do we ever “witness” the birth of a NEW Black Hole? We probably do when we “see” a…. GAMMA-RAY BURST GRBs radiate more energy than the rest of the universe Combined, for a few moments.

These are the speeds of the stars closest to the center of the Milky Way. They’re orbiting something. Something massive. Something Invisible. A supermassive black hole, methinks.

How much does light bend near a star? A bit, near the Sun A lot more, near the event horizon of a BH. In a the EH ! So, wow, you could see the back of your head at the EH. If you could stand there. Which you can’t. If you think about it, this is a bit like a wormhole. It’s going to do some awful things to time.

These stars are actually in different places. They could even be behind us !

A Black Hole is a hole from all directions Not just from the front, like holes you’ve seen before

And there’s tidal forces. Long before you get to the EH, the difference on the force of gravity between your head and your feet will be so extreme, you get stretched. Even your atoms get pulled apart.