WEBVTT FILE

1
00:00:00.366 --> 00:00:04.166
Accretion. In short, accretion
means growth by accumulation.

2
00:00:04.500 --> 00:00:08.100
Land on Earth can form from accretion
of soil deposited by rivers or sea.

3
00:00:08.500 --> 00:00:11.900
Planetary bodies can also grow
from accretion as gravitational forces

4
00:00:11.900 --> 00:00:14.100
attract debris and other matter.

5
00:00:14.100 --> 00:00:16.366
But we want to talk about
accretion in astronomy.

6
00:00:16.366 --> 00:00:18.666
Accretion is one way
we can see black holes.

7
00:00:18.966 --> 00:00:21.066
Black holes have intense
gravitational mass,

8
00:00:21.066 --> 00:00:23.833
pulling in everything in the vicinity,
including light.

9
00:00:24.200 --> 00:00:27.533
Without that light,
we can't see black holes, but we can see

10
00:00:27.533 --> 00:00:30.633
the disk-shaped flow of gas and dust
that forms around some black holes.

11
00:00:30.900 --> 00:00:33.133
We call these accretion disks,
and they can form around

12
00:00:33.133 --> 00:00:36.033
black holes of any size,
as well as around some stars.

13
00:00:36.600 --> 00:00:40.700
Material in black hole accretion disks
is really hot, like millions of degrees.

14
00:00:41.066 --> 00:00:42.233
This is caused by the collision

15
00:00:42.233 --> 00:00:45.433
of particles in the accretion disk
as they are pulled into their black hole.

16
00:00:46.200 --> 00:00:49.200
These collisions can cause
accretion disks to release X-rays,

17
00:00:49.366 --> 00:00:51.400
which can allow us
to locate some black holes.