Black Holes, or the Monster at the Center of the Galaxy

! How do black holes with masses a few times that of our Sun form? How can we observe such black holes? How do fluctuations in light tell us maximum sizes?

! Where and how might you observe a supermassive black hole? How many times more massive than a solar mass black hole is a supermassive black hole?

! Why is a black hole black? Can gravity affect light? What is curved space time? What is a gravitational lens?

! What is a gravitational redshift? What is time dilation?! What is an Event Horizon and how does it depend on

black hole mass? About how big is the Event Horizon of a black hole that is a few times the mass of our Sun?

Learning Objectives

Stellar Evolution Recap

How do we see a black hole?! If a BH emits no light, how can we ever see it?! Just like for binary stars we can measure the mass

of invisible objects from the orbits of visible objects

!Plus, if a BH is in a binary system, it can steal matter from its companion star

!This matter falling toward a BH forms an orbiting accretion disk, depositing matter onto the Black Hole

!The ultra-hot, violent gas falling into the BH emits powerful X-rays

! A binary system with an unseen 15 M⊙

companion! Spectrum of X-ray

emission consistent with material rapidlyorbiting a black hole

! Rapid fluctuations suggest an object that is only a few km in diameter!Nothing can travel faster than the speed of light.

So time fluctuations tell you maximum sizes

Cygnus X-1

Orbits of stars near the

Galactic center prove there is an object ~ 4 million times the mass of the Sun at

the center of our Galaxy

This object is in a region

that is smaller than

a neutron star...only a supermassive

black hole can explain

this

The Monster at the Center of the Galaxy

General Relativity (GR)! Einstein’s General Theory

of Relativity tells us how objects in spacetime are accelerated by gravity

! Objects tell spacetime what shape it is

! Curved spacetime tells objects how to move

! Gravity is a geometric effect...it is the curving of spacetime around masses

General Relativity (GR)! Gravity is geometry? How does

that even make sense?! Consider two masses,

initially widely separated, falling directly to a point at the center of the Earth

! How would they fall?! They start widely separated

but by the center of the Earth theymust be “on top of each other” center

General Relativity (GR)! Now, think about what this would look like if you

didn’t know the masses were falling to a center

! The masses look like “some force” is attracting themtowards each other

! We have a name for a force thatmakes masses appear to move towards each other…gravity

! Curved geometry can make objects move towards each other. Einstein’s insight was that placing an object in spacetime curves spacetime

Curved Spacetime

! No matter = Flat Spacetime

! Things roll along at a constantspeed, nothing speeds up, slows down or is deviated by the spacetime

Curved Spacetime! Massive object

= Indentation in spacetime

! The underlying space an object is passing through iscurved by placing a mass in it

! Because gravity isreally curved space a mass can even accelerate a massless object (i.e. light, just like everything else, is influenced by gravity)

GR Proofs: Strong Lens! This is one quasar,

but there are 4 images of it

! The light travels in multiple directions, bent by a massive foreground galaxy (in the middle)

! This bending oflight by massive objects is called gravitational lensing

GR Proofs: Gravitational Waves

Black Holes! When lots of mass

is packed into a small region of spacetime, the spacetime curves so much that it closes in on itself

! Photons flying outward from such a massive object arc back inward

! If light cannot escape the gravity of an object, then that object is a Black Hole

White dwarf

! Photons just escaping a BH lose energy climbing out. So objects closer to BHs appear redder. This is the gravitational redshift

! Such photons take more time to climb out of a BH. We measure time by receiving photons of light from events. Thus, events that are occurring closer to BHs appear to take longer to happen. This is time dilation.

Black Holes

White dwarf

Inside a Black Hole! The mass in a Black Hole collapses to a single

point – called a singularity! A Black Hole is separated from the rest of the

Universe by a boundary, the Event Horizon! The Event Horizon is the

distance at which light would fall into a Black Hole rather than escape it

! More massive Black Holeshave larger Event Horizons

! No light (so no events)can be seen from within an Event Horizon

A 3 M⊙ black hole’s event horizon is about the same size as a small city

First image of a Black Hole

Credit: Event Horizon Telescope/ESO

10-8 degrees

Falling Into a Black Hole! Imagine that you free-fall,

feet-first, into a Black Hole of 3 M⊙ (3 solar masses) from a distance of 1 AU (1 Astronomical Unit)

!You would fall slowly at first, accelerating as you approach the Black Hole

!Your watch would show it took ~ 2 months for you to pass the Event Horizon

!But, you would die long before then...

What Your Friends See (safely back 1 AU from the BH)

!Lets say you have a blue light that is blinking once per second

!As you approached the Event Horizon, the light would blink more slowly and look more red!Time dilation, Gravitational redshift

!As you cross the Event Horizon time is fully dilated and light is fully redshifted

!The photons that leave you as you cross the Event Horizon are the final ones your friends see

!Your light would be very red (radio waves) and you’d appear frozen forever at the Event Horizon

Once You’re In, You’re In!Once you cross the Event Horizon, nothing can

prevent you from falling to the central singularity!You fall to the singularity in a finite amount of time

(you’re traveling near the speed of light, now) !You would have an immensely distorted view of

above and below, because of tidal effects: !Your waist falls faster than your head, your feet faster

than your waist. From your waist’s view, your feet & head move away and so would appear redshifted

!Below, everything is so distorted you can never, ever see the singularity...as you fall into it! Nobody knows what would happen next….

Next Time

Our Milky Way