Black Holes, or the Monster at the Center of the Galaxy
Learning Objectives! How do black holes with masses a few times that of our Sun form? How can we observe such black holes?! 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?
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
Cygnus X-1! A binary system with an unseen 15 solar mass companion! Spectrum of X-ray emission consistent with material rapidly orbiting 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
The Monster at the Center of the Galaxy 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
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 they must 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 them towards each other! We have a name for a force that makes 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 constant speed, 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 is curved by placing a mass in it! Because gravity is really 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 of light by massive objects is called gravitational lensing
GR Proofs: Gravitational Redshift! Light loses energy as it climbs out of a gravitational field so its wavelength increases (a redshift)! Remember, as energy decreases wavelength increases! As with light bending, the effect is small but measurable! First directly measured by Pound and Rebka in 1959
GR Proofs: Time Dilation! Light moves slightly more slowly in large gravitational fields! As we measure time by receiving light from events, time moves more slowly near a massive object! Directly measured by Hafele and Keating in 1971
! When lots of mass is packed into a small region of spacetime, the spacetime curves so much that it closes in on itself Black Holes! Photons flying outward from such a massive object arc back inward White dwarf! If light cannot escape the gravity of an object, then that object is a Black Hole
Black Holes! Photons just escaping a BH lose energy climbing out. So objects closer to BHs appear redder. This is the gravitational redshift White dwarf! 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.
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 Holes have 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
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 and head move away and are 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