The first confirmed black hole, Cygnus X-1, was regarded as proof that black holes exist because it a. is a compact object in a binary system b. emits X-rays from a hot accretion disk c. is accreting matter from a main-sequence companion d. has a period of 5.6 days e. has a mass of ten solar masses

Gravity waves produced by Black Hole mergers can be detected by

Assume that the mass of the core of a star that just went Supernova type II is $2.5 \mathrm{M}_{\odot}$ before and after the collapse, while the Radius changes from $10^3 \mathrm{~km}$, before the collapse, to 12 km , after the collapse respectively.(a) What is the change in potential energy of the core between the two stages before and after the collapse?(b) Knowing that the luminosity of the Sun is $4 \times 10^{26} \mathrm{~W}$ how many years would it take the Sun to release the same amount of energy?

Calculate the Schwarzschild radius for Milky Way.