Question

A black hole with a mass of 10 9 M Sun brightens rapidly and increases its luminosity by a factor of two. Approximately how long would it take for an observer on Earth to see the black hole reach its maximum brightness?

a. 3 weeks b. 3 minutes c. 3 days d. 3 hours

Answer 1

Here, A black hole with a mass of 10 9 M Sun brightens rapidly and increases its luminosity by a factor of two

relativity predicts that as an object collapses to form a black hole, it will eventually reach a point of infinite density. What that really means is that the theory of relativity breaks down at this point, and no one knows what happens at the center of a black hole - we would need a viable theory of quantum gravity in order to understand this.

But here's something that you might find useful: when we talk about the "size" of a black hole, we usually talk about something called the Schwarzschild radius. The Schwarzschild radius is the "point of no return" - once you get closer to the black hole than it, you can never escape. Consequently, the escape speed at the Schwarzschild radius is equal to the speed of light, and the value of the Schwarzschild radius works out to be about (3x10⁵ cm) x (M / M_sun),

1. where M is the mass of the black hole and M_sun is the mass of the Sun.
2. M for a black hole in our galaxy is around 10 times the mass of the Sun,
3. but for supermassive black holes at the centers of galaxies it can be millions or even billions.)