What is Event Horizon?

Its the condition when escape velocity of a body reaches speed of light.

At event horizon even light cant escape the body termed as black hole.

Schwarzschild Radius:

It is the radius of a body at which it can behave as a black hole.

Schwarzchild radius of a body of mass M is given by R = 2GM/c²

where c is  speed of light.

1) Schwarzschild radius of Sun:

Mass of Sun= 2 x 10^30 Kg

G= 6.6 x 10^-11 units.

c= 3 x 10^8m/sec

Putting the values in above formula, R=3Km.

This means if we compress sun to the radius of 3Km it will act as a black hole.

2)  Schwarzschild radius of Sagittarius A:

Mass M= 2.6 million solar mass.(2.6 million times mass of sun)

             = 5.2 x 10^ 36 Kg.

G= 6.6 x 10^-11 units.

c= 3 x 10^8m/sec

Putting the values in above formula, R=7.6 x 10^9m.