PX158 - I7d - blackholes

above the chandrashekhar limit for neutron stars, there is no escaping collapse into a blackhole
they are very dense objects
not even light can escape their gravity
best described by general relativity
but the concept can be thought of in newtonian gravity
there could be objects with escape velocities larger than that of light:

\begin{aligned} \frac{1}{2} m v_{e s c}^{2} & = \frac{G M m}{R} \\ v_{e s c}^{2} & = \frac{2 G M}{R} \end{aligned}

$M =$ mass of planet/star, $R =$ its radius

for light:

R = \frac{2 G M}{c^{2}} = 3 (\frac{M}{M_{⊙}}) k m

- $R$ is the schwarzschild radius
- for the sun, $R_{s c h w .} = 6 k m$ ; for the earth, $R_{s c h w .} = 6 c m$

blackholes do not radiate (in the conventional sense)
nothing ever falls directly onto it due to too much angular momentum
a disc of matter is formed in which gas heats up through friction
the falling of matter is called accretion
blackholes that accrete are called 'quasars'
accretion discs are very luminous
accretion discs are stable to $\sim \frac{6 G M}{c^{2}} = 3 R_{s c h w .}$
energy released by matter falling into a blackhole:

E_{g r a v .} = \frac{G M m}{R} = \frac{1}{6} m c^{2}

efficiency $\approx 17 %$ (cf: $H$ -fusion $\sim 0.7 %$ )