1
00:00:00,580 --> 00:00:04,063
In the last video, we saw that if we started with a massive star,

2
00:00:04,063 --> 00:00:08,382
about nine to twenty times the mass of the Sun,

3
00:00:08,382 --> 00:00:14,136
and when it finally matures, the remnant of the star is roughly...

4
00:00:14,136 --> 00:00:15,759
or that remnant core of the star

5
00:00:15,759 --> 00:00:19,751
is roughly one and a half to three times the solar mass

6
00:00:19,751 --> 00:00:23,620
or the mass of the Sun, then this remnant right here,

7
00:00:23,620 --> 00:00:25,944
and let me just be clear: this is nine to twenty times

8
00:00:25,944 --> 00:00:29,748
is the mass of that star when it's in its main sequence.

9
00:00:29,748 --> 00:00:34,183
This one and a half to three times is the mass once it has shed off

10
00:00:34,183 --> 00:00:37,585
a lot of the, I guess outer material of the star,

11
00:00:37,585 --> 00:00:40,200
and this is really the mass of the remnant of the star.

12
00:00:40,200 --> 00:00:42,357
Kind of the core of the star.

13
00:00:42,357 --> 00:00:43,964
But if that remnant, once its stop fusing

14
00:00:43,964 --> 00:00:46,250
once its stops having outward pressure,

15
00:00:46,250 --> 00:00:50,200
once it has enough density, this we saw in the last video,

16
00:00:50,200 --> 00:00:54,441
will cause a supernova, it will cause a shockwave to move out

17
00:00:54,441 --> 00:00:55,874
through the rest of the material

18
00:00:55,874 --> 00:00:57,667
and essentially cause it blow up,

19
00:00:57,667 --> 00:01:01,885
and this will condense into a neutron star.

20
00:01:02,639 --> 00:01:05,011
Into a neutron star...

21
00:01:05,011 --> 00:01:07,200
Now in this video, what I want to talk about is

22
00:01:07,200 --> 00:01:09,410
what if we're starting with a star

23
00:01:09,410 --> 00:01:12,200
that has a mass more than--and this is give or take, we don't

24
00:01:12,200 --> 00:01:14,200
know the actual firm boundaries here,

25
00:01:14,200 --> 00:01:17,564
but what if we have a star, what if we have a star

26
00:01:17,564 --> 00:01:21,061
that is more than twenty times the mass of the Sun?

27
00:01:21,061 --> 00:01:22,867
And this is kind of the original mass,

28
00:01:22,867 --> 00:01:25,055
before the star burns itself out.

29
00:01:25,055 --> 00:01:29,077
Or when that star has kind of reached this old age,

30
00:01:29,077 --> 00:01:30,792
once it has that iron core,

31
00:01:30,792 --> 00:01:36,133
it has more than... So I could say the remnant..., the remnant...

32
00:01:36,133 --> 00:01:40,200
the dense remnant has more than three to four times

33
00:01:40,200 --> 00:01:42,470
the mass of the Sun.

34
00:01:42,470 --> 00:01:43,410
And remember,

35
00:01:43,410 --> 00:01:46,252
its gonna have three to four times the mass of the Sun,

36
00:01:46,252 --> 00:01:48,641
but it's going to be far denser. It's just gonna be a core.

37
00:01:48,641 --> 00:01:51,879
It's gonna be an iron/nickel core that's no longer fusing.

38
00:01:51,879 --> 00:01:53,564
So what happens to these stars?

39
00:01:53,564 --> 00:01:55,895
It turns out that these are so massive

40
00:01:55,895 --> 00:01:59,585
that even the neutron degeneracy pressure

41
00:01:59,585 --> 00:02:03,102
will not be enough to keep the mass from imploding.

42
00:02:03,102 --> 00:02:06,579
In these stars, all of the mass in these stars

43
00:02:06,579 --> 00:02:09,067
will just keep imploding, so the neutron...

44
00:02:09,067 --> 00:02:12,733
So we imagine in the first sun-like stars

45
00:02:12,733 --> 00:02:15,405
things would collapse into white dwarfs.

46
00:02:15,405 --> 00:02:17,590
Maybe i should draw that in white.

47
00:02:18,405 --> 00:02:21,477
So they would collapse into white dwarfs.

48
00:02:21,477 --> 00:02:23,282
No, thats not white either.

49
00:02:23,529 --> 00:02:26,800
There you go. They would collapse into white dwarfs eventually.

50
00:02:26,816 --> 00:02:28,425
So this is a white dwarf.

51
00:02:29,933 --> 00:02:31,729
White dwarf.

52
00:02:31,729 --> 00:02:33,692
And here the pressure that is keeping this

53
00:02:33,692 --> 00:02:36,641
from collapsing further is electron degeneracy pressure.

54
00:02:36,641 --> 00:02:38,308
The atoms are squeezed so much

55
00:02:38,308 --> 00:02:40,641
that the electrons are essentially keeping them

56
00:02:40,641 --> 00:02:42,032
from squeezing any more.

57
00:02:42,032 --> 00:02:43,477
But if the pressure gets large enough then

58
00:02:43,477 --> 00:02:46,329
you have the neutron star. So you have even

59
00:02:46,329 --> 00:02:48,425
more mass in even a smaller...

60
00:02:48,425 --> 00:02:49,880
And I'm not drawing this to scale.

61
00:02:49,880 --> 00:02:51,205
Neutron stars are tiny.

62
00:02:51,205 --> 00:02:54,243
White dwarf stars are on the scale

63
00:02:54,243 --> 00:02:57,067
of an earth like planet. Neutron stars we learned in

64
00:02:57,067 --> 00:02:58,918
the last video are on the scale of a city!

65
00:02:58,918 --> 00:03:00,976
So they are super dense, super tiny and this has

66
00:03:00,976 --> 00:03:02,973
more mass then this over here.

67
00:03:02,973 --> 00:03:04,847
In fact, maybe I'll just draw this as a dot,

68
00:03:04,847 --> 00:03:06,786
just so you have a sense how dense it is.

69
00:03:06,786 --> 00:03:09,890
It's really just like one big atomic nucleus, or...

70
00:03:10,521 --> 00:03:11,381
Well, it's still small.

71
00:03:11,381 --> 00:03:12,158
It's like the size of a city.

72
00:03:12,173 --> 00:03:14,036
It's like a nucleus the size of a city.

73
00:03:14,036 --> 00:03:17,314
But this right here is a neutron star.

74
00:03:18,345 --> 00:03:19,333
Neutron star.

75
00:03:19,333 --> 00:03:21,317
And what's unintuitive about what I'm drawing is

76
00:03:21,317 --> 00:03:23,685
each of these smaller things have more mass...

77
00:03:23,685 --> 00:03:26,518
This overcame the electron degeneracy pressure

78
00:03:26,518 --> 00:03:27,836
to collapse even further.

79
00:03:27,836 --> 00:03:29,518
But if the mass is large enough,

80
00:03:29,518 --> 00:03:31,492
and this is what we're talking about in this video,

81
00:03:31,492 --> 00:03:35,598
even the neutron degeneracy pressure will not be able

82
00:03:35,598 --> 00:03:37,333
to keep that mass from collapsing.

83
00:03:37,333 --> 00:03:40,915
And there's even theoretical quark stars,

84
00:03:40,915 --> 00:03:42,679
where the quark degeneracy pressure...

85
00:03:42,679 --> 00:03:47,600
But even beyond that, all collapses into a single point,

86
00:03:47,600 --> 00:03:50,684
and I'm simplifying here, but it collapses

87
00:03:50,684 --> 00:03:54,338
into a single point of infinite density.

88
00:03:54,338 --> 00:03:59,003
Infinite mass density.

89
00:03:59,988 --> 00:04:03,667
And this is really the mass of a black hole.

90
00:04:03,667 --> 00:04:05,188
And I'm calling it the mass of a black hole,

91
00:04:05,188 --> 00:04:07,413
because there's different ways how you could view

92
00:04:07,413 --> 00:04:09,559
where a black hole starts and ends

93
00:04:09,559 --> 00:04:11,646
or what exactly is the black hole.

94
00:04:11,646 --> 00:04:14,195
So this is all the mass,

95
00:04:14,195 --> 00:04:19,971
all the mass of the black hole.

96
00:04:19,971 --> 00:04:23,498
Or we could say, of the original star.

97
00:04:23,498 --> 00:04:25,456
So when we're talking about that remnant

98
00:04:25,456 --> 00:04:27,477
being three or four solar masses,

99
00:04:27,477 --> 00:04:30,425
all of that mass is now being contained...

100
00:04:30,425 --> 00:04:31,185
Well, not all of it.

101
00:04:31,185 --> 00:04:33,720
Some of it was released as energy during the supernova,

102
00:04:33,720 --> 00:04:35,441
and that was also true for the neutron star.

103
00:04:35,441 --> 00:04:38,631
But most of that mass is now being contained

104
00:04:38,631 --> 00:04:41,467
in this infinitely small point.

105
00:04:41,467 --> 00:04:46,513
And you'll hear physicists and mathematicians talk about singularities.

106
00:04:46,944 --> 00:04:48,292
Singularities.

107
00:04:48,292 --> 00:04:52,467
And singularities are really points, even in mathematics,

108
00:04:52,467 --> 00:04:54,010
where everything breaks down,

109
00:04:54,010 --> 00:04:55,933
where nothing starts to make sense anymore,

110
00:04:55,933 --> 00:05:00,498
where the mathematical equations don't give you a defined answer.

111
00:05:00,498 --> 00:05:01,677
And this is a singularity,

112
00:05:01,677 --> 00:05:06,318
because you have, you have a ton of mass in an infinitely small space.

113
00:05:06,318 --> 00:05:09,005
You essentially have an infinite density right here.

114
00:05:09,005 --> 00:05:10,790
And this is hard to visualize,

115
00:05:10,790 --> 00:05:14,133
but you have kind of an infinite curvature in space-time right here.

116
00:05:14,133 --> 00:05:15,653
And I can't visualize that.

117
00:05:15,653 --> 00:05:16,941
So maybe we'll think about that in more videos.

118
00:05:16,941 --> 00:05:19,788
But the reason why I said that it's...

119
00:05:19,788 --> 00:05:22,574
There's different ways to think about what a black hole is,

120
00:05:22,574 --> 00:05:23,967
about where it starts and ends.

121
00:05:23,967 --> 00:05:25,375
This is where the mass is.

122
00:05:25,375 --> 00:05:27,080
And if there was any other mass over here

123
00:05:27,080 --> 00:05:29,547
it would obviously become attracted to this mass

124
00:05:29,547 --> 00:05:32,204
and then become part of that singularity.

125
00:05:32,204 --> 00:05:35,800
It would add to that mass, that already huge mass

126
00:05:35,800 --> 00:05:38,641
that's in an infinitely small point in space.

127
00:05:38,641 --> 00:05:41,262
But the reason why the boundary is hard to define is

128
00:05:41,262 --> 00:05:45,067
because there is some point at which, there is some point in space

129
00:05:45,067 --> 00:05:47,324
around that singularity, at which,

130
00:05:47,324 --> 00:05:49,414
no matter what that thing is,

131
00:05:49,414 --> 00:05:51,829
no matter how much energy that thing has,

132
00:05:51,829 --> 00:05:56,400
it will not be able to escape the gravitational influence

133
00:05:56,400 --> 00:05:58,085
of the black hole.

134
00:05:58,085 --> 00:06:00,518
Of that ultra dense mass.

135
00:06:00,518 --> 00:06:03,729
So even if it was electromagnetic radiation,

136
00:06:03,729 --> 00:06:05,529
even if it was light,

137
00:06:05,529 --> 00:06:09,199
and even if it was light that shone away from the mass,

138
00:06:09,199 --> 00:06:12,169
it will eventually have to go back.

139
00:06:12,169 --> 00:06:15,533
It will not be able to escape the gravitational influence.

140
00:06:15,533 --> 00:06:20,759
And so the boundary where, if you're within that boundary,

141
00:06:20,759 --> 00:06:22,041
that's really a sphere.

142
00:06:22,041 --> 00:06:24,733
So that boundary around the singularity...

143
00:06:25,748 --> 00:06:28,644
And that boundary around the singularity where

144
00:06:28,644 --> 00:06:31,041
if you're within the boundary, no matter what you do,

145
00:06:31,041 --> 00:06:32,977
no matter if you're electromagnetic radiation,

146
00:06:32,977 --> 00:06:34,308
you're still going to...,

147
00:06:34,308 --> 00:06:36,537
you're never going to be able to escape the black hole.

148
00:06:36,537 --> 00:06:39,894
If you're beyond that boundary, you might be able to escape the black hole.

149
00:06:39,894 --> 00:06:41,891
So this guy could escape.

150
00:06:41,891 --> 00:06:43,458
This guy over here, no matter what he does,

151
00:06:43,458 --> 00:06:46,052
is going to have to go back into the black hole.

152
00:06:46,052 --> 00:06:49,267
This boundary right here is called the event horizon.

153
00:06:49,267 --> 00:06:52,141
This right here is the event horizon.

154
00:06:52,141 --> 00:06:55,594
Another word used in a lot of science fiction movies.

155
00:06:55,594 --> 00:06:56,664
And for good reason,

156
00:06:56,664 --> 00:06:57,937
because it's fascinating.

157
00:06:57,937 --> 00:07:01,400
And we'll actually learn in future videos, hopefully, about Hawking radiation.

158
00:07:01,400 --> 00:07:04,139
We'll see that it's not radiation from the black hole itself,

159
00:07:04,139 --> 00:07:07,063
it's the byproduct of quantum effects

160
00:07:07,063 --> 00:07:09,801
that occur at the event horizon.

161
00:07:09,801 --> 00:07:13,536
But the event horizon is just this point in space

162
00:07:13,536 --> 00:07:16,459
or the sphere in space, or this boundary in space.

163
00:07:16,459 --> 00:07:19,647
Anything closer than or within the event horizon

164
00:07:19,647 --> 00:07:22,295
has to eventually end up in the singularity,

165
00:07:22,295 --> 00:07:23,244
contributing to that mass.

166
00:07:23,244 --> 00:07:26,533
Anything on the outside has a chance of escaping.

167
00:07:26,533 --> 00:07:28,116
So what does a black hole look like?

168
00:07:28,116 --> 00:07:30,035
Well, not even light can escape from it,

169
00:07:30,035 --> 00:07:31,894
so it will be black.

170
00:07:31,894 --> 00:07:34,711
It will be black in the purest sense.

171
00:07:34,711 --> 00:07:38,372
It will not emit any type of radiation from the black hole itself,

172
00:07:38,372 --> 00:07:40,426
from that mass.

173
00:07:40,426 --> 00:07:45,353
So here are some depictions I got from NASA of black holes.

174
00:07:45,353 --> 00:07:47,346
And so just to be clear what's happening here,

175
00:07:47,346 --> 00:07:49,076
what you're seeing here as black,

176
00:07:49,076 --> 00:07:51,462
that is not... You can view that as the black hole.

177
00:07:51,462 --> 00:07:52,908
And when people talk about the black hole,

178
00:07:52,908 --> 00:07:54,133
that's often what they're talking about.

179
00:07:54,133 --> 00:07:56,756
But there's a point of infinite density

180
00:07:56,756 --> 00:07:59,884
at the center of this black sphere right here.

181
00:07:59,884 --> 00:08:01,799
And what you see as that black sphere

182
00:08:01,799 --> 00:08:04,499
that really is the boundary of the event horizon.

183
00:08:04,730 --> 00:08:09,369
So this right here is the boundary of the event horizon.

184
00:08:09,369 --> 00:08:13,222
And what we're seeing right here is the accretion disc around the black hole.

185
00:08:13,563 --> 00:08:16,721
As all of this matter gets closer and closer to it

186
00:08:16,721 --> 00:08:18,704
it's being squeezed more and more.

187
00:08:18,704 --> 00:08:20,007
It's moving faster and faster

188
00:08:20,007 --> 00:08:21,075
and getting hotter and hotter.

189
00:08:21,075 --> 00:08:22,748
And that's why the way the artist depicted...

190
00:08:22,748 --> 00:08:26,475
It looks like this stuff over here is redder and hotter than the stuff further out.

191
00:08:26,475 --> 00:08:28,766
It's just accelerating as it approaches that event horizon.

192
00:08:28,766 --> 00:08:30,410
Once it approaches that...

193
00:08:30,410 --> 00:08:32,158
Once it's IN the event horizon

194
00:08:32,158 --> 00:08:33,953
we cannot even see the light that it's emitting,

195
00:08:33,953 --> 00:08:37,794
even though it would be starting to become unbelievably energetic.

196
00:08:37,794 --> 00:08:39,541
Here's some other pictures.

197
00:08:39,541 --> 00:08:42,050
This is a picture of a star being ripped apart.

198
00:08:42,050 --> 00:08:43,980
Not a picture, it's actually an artist depiction.

199
00:08:43,980 --> 00:08:45,019
All of these are artist depictions.

200
00:08:45,019 --> 00:08:47,965
We would never be able to get such good pictures

201
00:08:47,965 --> 00:08:50,419
of actual action occuring near black holes.

202
00:08:50,419 --> 00:08:51,380
These are artist depictions.

203
00:08:51,380 --> 00:08:54,265
This is a star being ripped apart by a black hole.

204
00:08:54,265 --> 00:08:58,427
So this star is getting pretty close to this black hole.

205
00:08:58,427 --> 00:09:01,803
Already out here various... where the star is

206
00:09:01,803 --> 00:09:04,103
its very strong gravitational attraction,

207
00:09:04,103 --> 00:09:06,788
so any mass that's being emitted from the star

208
00:09:06,788 --> 00:09:10,103
in that direction is slowly being pulled into the black hole.

209
00:09:10,103 --> 00:09:13,157
So this star is kind of being ripped apart by the black hole.

210
00:09:13,157 --> 00:09:15,396
This is maybe a better depiction of it.

211
00:09:15,396 --> 00:09:16,680
This is the star at first.

212
00:09:16,680 --> 00:09:19,257
And once it gets under the influence

213
00:09:19,257 --> 00:09:21,819
of the black hole's gravitation

214
00:09:21,819 --> 00:09:23,073
it starts to kind of elongate

215
00:09:23,073 --> 00:09:25,111
and it gets ripped apart,

216
00:09:25,111 --> 00:09:27,169
and its matter starts spiraling in

217
00:09:27,169 --> 00:09:29,207
closer and closer to the black hole.

218
00:09:29,207 --> 00:09:30,938
And once it's IN the event horizon,

219
00:09:30,938 --> 00:09:32,392
we won't even see it any more,

220
00:09:32,392 --> 00:09:34,876
because even the light from that matter,

221
00:09:34,876 --> 00:09:38,315
from that intensely hot matter that's entering into the black hole

222
00:09:38,315 --> 00:09:40,999
cannot even escape the black hole itself.

223
00:09:40,999 --> 00:09:43,392
Anyway, hopefully you found that interesting.

224
00:09:43,407 --> 00:09:47,922
And I want to be clear: we still don't understand a lot about black holes.

225
00:09:47,922 --> 00:09:51,053
In fact, this whole notion of a singularity,

226
00:09:51,053 --> 00:09:55,092
the fact that all the math and all the theory breaks down at the singularity

227
00:09:55,092 --> 00:09:57,976
is a pretty good sign that our theory isn't complete.

228
00:09:57,976 --> 00:09:59,430
Because if our theory was complete,

229
00:09:59,430 --> 00:10:01,853
we would maybe get something a little more sensible

230
00:10:01,853 --> 00:10:06,461
than just all of our equations not making sense at that infinitely dense point.

231
00:10:06,461 --> 00:10:08,346
Anyway, hopefully you found that interesting.