The Death of High mass Stars

1. The Death of High mass Stars

2. What Determines A Star’s Fate? • Stellar remnants larger than ~ 5M face yet another fate -- a black hole. • At the end of a stars life, the star will start to contract (see previous notes) • If a star is not large enough, it will either become a white dwarf or neutron star .

3. What is a Black Hole? • A black hole is an object whose escape velocity near its "surface" exceeds the speed of light. The "surface" is called theevent horizonas we are unable to observe anything inside this distance.

4. This picture shows the light trajectories as a star gets closer and closer to the black hole. (a) only a small portion of the light is directed close enough to the black hole to be captured. (b) the star is close enough that the amount of light captured is increased because of the strong gravitational field of the black hole that bends the paths of the photons inward toward it.

5. (c) the star is much closer and this bending is so strong that fully half the light is captured. The amount of light captured goes up further as the star approaches closer, (d) nearly all of it does - just a little that happens to be emitted directly outward can get away. When the star gets still closer, no light gets out - it is then inside the event horizon. Nothing that happens inside the event horizon can be detected from outside, since no light (or anything else) can get out to show that something took place!!

6. How Big is a Black Hole? • We can estimate the size of the event horizon from the escape velocity • M = the mass of the central object; R = distance of orbiting object • This R is called the Schwarzschild radius; it defines the position of the event horizon. • For an object with M = 6 M, R = 18 kilometers! • So black hole with a stellar type mass will have a "radius" of only a few tens of km, even smaller than a neutron star.

7. I’m Falling into a Black Hole!! • Einstein's theory states that if we were falling into the black hole, we would find no slowing down in time and would just plunge through the event horizon and be torn apart. • Thus, the appearance of the collapse of the star depends completely on how and where we observe it (Relativity) • What happens if you are a some larger distance from a black hole? • You will probably be surprised to feel such a strong gravitational force from what may appear to be empty space! In fact, at first it may not be obvious where the black hole is

8. How do we know Einstein is Right? • People looked for the "deflection" of starlight as it passed close to a massive object like the sun. The effect is seen exactly as Einstein's theory predicts it.

9. How do we look for stellar remnant black holes? • Although black holes are really black, when matter falls into them it can heat up so much it glows in x-rays

10. One of the best examples is the apparently ordinary hot star HDE 22685. Not only is it a bright X-ray source, but it is a spectroscopic binary with an unseen companion with M > 3M, which is therefore likely to be a black hole. The system is shown to the right.

11. How to find Black Holes • Another way to find black holes is to look for spectroscopic binaries where one star is invisible and where the velocities imply that a very massive object must be present (essentially the same as above but without the requirement that an "accretion disk" of hot gas be present).

12. Works Cited • http://ircamera.as.arizona.edu/NatSci102/lectures/blackhole.htm