# Homework Help: Astronomy home black holes

1. Jan 27, 2012

### amy21

1. Photons approaching a black hole will initially
a. all of the above are possible depending on distance from the event horizon
b. pass by with their direction changing (bending/curving)
c. fall into the black hole
d. go into orbit around the black hole
e. pass by without changing direction

am thinking the answer is between d/ a because
The photon sphere is a spherical boundary of zero thickness such that photons moving along tangents to the sphere will be trapped in a circular orbit. The orbits are dynamically unstable. hence any small particle of infalling matter will grow over time, either setting it on an outward trajectory escaping the black hole or on an inward spiral eventually crossing the event horizon.While light can still escape from inside the photon sphere, any light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. Hence any light reaching an outside observer from inside the photon sphere must have been emitted by objects inside the photon sphere but still outside of the event horizon.

2. Jan 27, 2012

### phinds

a. all of the above are possible depending on distance from the event horizon

since there's nothing above "a", this one's going to be hard to evaluate.

As for the rest, do you think maybe there might be some difference depending on whether the photons are aimed directly at the BH or way off to one side?

3. Jan 27, 2012

### amy21

that is what confused me because they all can happen just depends on distance from the event horizon and since there was no option above a i was confused .
but thanks!
a quick question if there are Two stars that have the same temperature, but one has twice the radius of the other. The luminosity of the larger star compared
to the smaller star will be how great? .. am thinking it would be 2twice as great right ?