1
00:00:00,090 --> 00:00:02,490
The following content is
provided under a Creative

2
00:00:02,490 --> 00:00:04,030
Commons license.

3
00:00:04,030 --> 00:00:06,330
Your support will help
MIT OpenCourseWare

4
00:00:06,330 --> 00:00:10,690
continue to offer high quality
educational resources for free.

5
00:00:10,690 --> 00:00:13,320
To make a donation or
view additional materials

6
00:00:13,320 --> 00:00:17,250
from hundreds of MIT courses,
visit MIT OpenCourseWare

7
00:00:17,250 --> 00:00:19,615
at ocw.mit.edu.

8
00:00:19,615 --> 00:00:20,906
MARK HARTMAN: So let's hear it.

9
00:00:20,906 --> 00:00:24,136
What was the most important
difference between these two

10
00:00:24,136 --> 00:00:25,132
types of galaxies?

11
00:00:25,132 --> 00:00:28,120
Let's see, Bianca,
Steve, and [? Nicki. ?]

12
00:00:28,120 --> 00:00:29,116
You raised your hand?

13
00:00:29,116 --> 00:00:30,610
AUDIENCE: [INAUDIBLE].

14
00:00:30,610 --> 00:00:32,353
MARK HARTMAN: All right, let's
start with Bianca and Steve,

15
00:00:32,353 --> 00:00:33,769
and somebody else
has to be brave.

16
00:00:33,769 --> 00:00:36,586
AUDIENCE: So in the false
color [INAUDIBLE] light image,

17
00:00:36,586 --> 00:00:39,076
you can actually--

18
00:00:39,076 --> 00:00:41,566
in the spiral galaxy,
you can actually

19
00:00:41,566 --> 00:00:44,511
see the form of the spiral
galaxy, see the arms,

20
00:00:44,511 --> 00:00:45,052
how it looks.

21
00:00:45,052 --> 00:00:47,044
But in the X-ray
[INAUDIBLE] image,

22
00:00:47,044 --> 00:00:49,534
you only see a
scattering of stars,

23
00:00:49,534 --> 00:00:53,020
so you can't really actually
see the form of the galaxy.

24
00:00:53,020 --> 00:00:56,506
And in the elliptical
galaxy, you

25
00:00:56,506 --> 00:01:00,488
can see the [INAUDIBLE] galaxy
in the false color [INAUDIBLE]

26
00:01:00,488 --> 00:01:00,988
light.

27
00:01:00,988 --> 00:01:04,142
And in an X-ray image, you
can also see the center,

28
00:01:04,142 --> 00:01:06,964
so you know where it
is you're [INAUDIBLE]..

29
00:01:06,964 --> 00:01:07,980
MARK HARTMAN: All right.

30
00:01:07,980 --> 00:01:09,021
Steve, what do you think?

31
00:01:09,021 --> 00:01:10,945
AUDIENCE: [INAUDIBLE]

32
00:01:10,945 --> 00:01:12,336
MARK HARTMAN: What about it?

33
00:01:12,336 --> 00:01:28,887
AUDIENCE: [INAUDIBLE]

34
00:01:28,887 --> 00:01:30,970
MARK HARTMAN: So angular
size is a big difference.

35
00:01:30,970 --> 00:01:34,440
How about somebody else from
this group, group number three?

36
00:01:37,397 --> 00:01:38,480
Anybody want to volunteer?

37
00:01:38,480 --> 00:01:44,060
[INAUDIBLE] What's the
thing that you wrote down?

38
00:01:44,060 --> 00:01:45,440
AUDIENCE: [INAUDIBLE]

39
00:01:45,440 --> 00:01:48,357
MARK HARTMAN: So everybody
listen up over here.

40
00:01:48,357 --> 00:01:53,846
AUDIENCE: [INAUDIBLE]
galaxy [INAUDIBLE]

41
00:01:53,846 --> 00:01:58,087
actually see the arms of
the galaxy [INAUDIBLE] But

42
00:01:58,087 --> 00:02:15,303
when you [INAUDIBLE] you
can see all the [INAUDIBLE]

43
00:02:15,303 --> 00:02:16,925
MARK HARTMAN: In which one?

44
00:02:16,925 --> 00:02:19,044
In spiral galaxies or
elliptical galaxies?

45
00:02:19,044 --> 00:02:20,466
AUDIENCE: [INAUDIBLE]

46
00:02:20,466 --> 00:02:22,591
MARK HARTMAN: Right, you
can't see individual stars

47
00:02:22,591 --> 00:02:24,560
in [INAUDIBLE] OK?

48
00:02:24,560 --> 00:02:26,880
So here's a question for
you, and I think you actually

49
00:02:26,880 --> 00:02:32,780
swapped around [INAUDIBLE]
But we've got--

50
00:02:32,780 --> 00:02:33,960
they're very different.

51
00:02:33,960 --> 00:02:36,410
Elliptical galaxies
show a smudge

52
00:02:36,410 --> 00:02:39,390
in both visible light
and in X-ray light,

53
00:02:39,390 --> 00:02:42,380
but they're not the same shape
in visible light and X-ray

54
00:02:42,380 --> 00:02:43,790
light, at least this galaxy.

55
00:02:43,790 --> 00:02:48,046
It's not a nice
smooth X-ray image.

56
00:02:48,046 --> 00:02:51,209
Whereas over here, you've
got this nice spiral galaxy,

57
00:02:51,209 --> 00:02:52,625
but do you see
that spiral pattern

58
00:02:52,625 --> 00:02:54,826
when you look at
it in X-ray light?

59
00:02:54,826 --> 00:02:55,325
No.

60
00:02:55,325 --> 00:02:58,270
You just see these
individual little dots.

61
00:02:58,270 --> 00:03:04,160
And Bianca was saying, well, you
can't see the spiral pattern.

62
00:03:04,160 --> 00:03:06,840
For these people on this side,
are the dots in the spiral

63
00:03:06,840 --> 00:03:09,178
arms or not?

64
00:03:09,178 --> 00:03:10,044
AUDIENCE: Yeah.

65
00:03:10,044 --> 00:03:10,669
AUDIENCE: Some.

66
00:03:10,669 --> 00:03:11,794
MARK HARTMAN: Some of them?

67
00:03:11,794 --> 00:03:12,809
Are they all in?

68
00:03:12,809 --> 00:03:13,350
AUDIENCE: No.

69
00:03:13,350 --> 00:03:14,670
AUDIENCE: Some of [INAUDIBLE]

70
00:03:14,670 --> 00:03:17,300
MARK HARTMAN: Some are
in the gaps in between,

71
00:03:17,300 --> 00:03:21,410
the places where there's low
flux in the visible light.

72
00:03:21,410 --> 00:03:23,750
OK, so you guys have done
a lot of observations.

73
00:03:23,750 --> 00:03:27,950
You learned a little bit about
things that give off X-rays.

74
00:03:27,950 --> 00:03:29,070
What's going on?

75
00:03:29,070 --> 00:03:32,926
What's our model for
why are these different?

76
00:03:32,926 --> 00:03:35,884
[RING]

77
00:03:36,384 --> 00:03:37,866
You guys want to grab that?

78
00:03:37,866 --> 00:03:39,348
AUDIENCE: The arm itself--

79
00:03:39,348 --> 00:03:41,818
[RING]

80
00:03:41,818 --> 00:03:44,288
MARK HARTMAN: The arm itself
gives off X-ray light.

81
00:03:44,288 --> 00:03:51,698
AUDIENCE: The arm [INAUDIBLE]

82
00:03:51,698 --> 00:03:54,052
MARK HARTMAN: OK, does it?

83
00:03:54,052 --> 00:03:56,266
AUDIENCE: [INAUDIBLE]
could be because it's not

84
00:03:56,266 --> 00:03:58,480
a hot enough temperature.

85
00:03:58,480 --> 00:03:59,939
MARK HARTMAN: OK,
so the gas is not

86
00:03:59,939 --> 00:04:01,188
hot enough to give off X-rays.

87
00:04:01,188 --> 00:04:03,060
So we're not seeing
the spiral arm.

88
00:04:03,060 --> 00:04:05,193
We're only seeing the
dots in the spiral arm

89
00:04:05,193 --> 00:04:06,984
and they're not even
all in the spiral arm.

90
00:04:06,984 --> 00:04:07,978
[RING]

91
00:04:07,978 --> 00:04:10,374
AUDIENCE: Gosh!

92
00:04:10,374 --> 00:04:11,457
MARK HARTMAN: Check again.

93
00:04:11,457 --> 00:04:14,439
And if it's still the same,
say please don't call back.

94
00:04:14,439 --> 00:04:16,924
[RING]

95
00:04:16,924 --> 00:04:19,409
AUDIENCE: They asked
for Joanna last time.

96
00:04:19,409 --> 00:04:20,900
Hello?

97
00:04:20,900 --> 00:04:24,876
AUDIENCE: That the spiral arms
are made up of more visible--

98
00:04:28,852 --> 00:04:30,880
of more objects that
give off visible light

99
00:04:30,880 --> 00:04:33,570
than objects that
give off x-rays.

100
00:04:33,570 --> 00:04:35,070
MARK HARTMAN: OK,
so the spiral arms

101
00:04:35,070 --> 00:04:37,610
are made of objects that give
off lots of visible light

102
00:04:37,610 --> 00:04:39,625
and not made of a lot--
in the spiral arms,

103
00:04:39,625 --> 00:04:41,750
there's lots of objects
that give off visible light

104
00:04:41,750 --> 00:04:44,690
but only a few objects
that give off x-ray light.

105
00:04:44,690 --> 00:04:47,484
What do you think those objects
that give off visible light

106
00:04:47,484 --> 00:04:47,984
are?

107
00:04:47,984 --> 00:04:49,940
AUDIENCE: Stars.

108
00:04:49,940 --> 00:04:51,287
MARK HARTMAN: Stars.

109
00:04:51,287 --> 00:04:52,870
What do you think
are the objects that

110
00:04:52,870 --> 00:04:54,824
are giving off x-ray light?

111
00:04:54,824 --> 00:04:57,240
AUDIENCE: Super stars?

112
00:04:57,240 --> 00:04:59,280
MARK HARTMAN: Super stars!

113
00:04:59,280 --> 00:05:01,170
What do we call super stars?

114
00:05:01,170 --> 00:05:03,450
AUDIENCE: Galaxies?

115
00:05:03,450 --> 00:05:05,410
MARK HARTMAN: No, this
is only one galaxy.

116
00:05:05,410 --> 00:05:06,810
What could those be?

117
00:05:06,810 --> 00:05:07,810
AUDIENCE: Neutron stars?

118
00:05:07,810 --> 00:05:10,355
MARK HARTMAN: What
kind of neutron stars?

119
00:05:10,355 --> 00:05:12,457
AUDIENCE: Binary systems.

120
00:05:12,457 --> 00:05:14,290
MARK HARTMAN: Those are
x-ray binary systems

121
00:05:14,290 --> 00:05:19,020
in another galaxy because
they're tiny little dots.

122
00:05:19,020 --> 00:05:22,120
They're point
sources Why can't we

123
00:05:22,120 --> 00:05:25,485
see individual stars in
the visible light image

124
00:05:25,485 --> 00:05:29,616
but we seem to be able to see
those dots in the x-ray light

125
00:05:29,616 --> 00:05:30,116
image?

126
00:05:36,424 --> 00:05:37,840
OK, I want you to
think about that

127
00:05:37,840 --> 00:05:39,840
because I'm going to ask
you to write about that

128
00:05:39,840 --> 00:05:40,765
in just a minute, OK?

129
00:05:40,765 --> 00:05:43,100
Thinking back to a
bunch of other things,

130
00:05:43,100 --> 00:05:45,270
what is it that is
giving off X-ray light?

131
00:05:45,270 --> 00:05:49,205
If over here, the only thing
that we're seeing are these

132
00:05:49,205 --> 00:05:52,030
little dots which we think are
super stars or X-ray emitting

133
00:05:52,030 --> 00:05:54,010
stars--

134
00:05:54,010 --> 00:05:57,550
neutron stars, X-ray
binaries, black hole binaries.

135
00:05:57,550 --> 00:06:00,823
You know, X-ray binaries include
black hole binaries as well.

136
00:06:00,823 --> 00:06:03,190
AUDIENCE: [INAUDIBLE] all stars?

137
00:06:03,190 --> 00:06:06,290
MARK HARTMAN: Possibly,
but a good thing

138
00:06:06,290 --> 00:06:09,330
check there is how luminous
would you expect a B

139
00:06:09,330 --> 00:06:10,290
or an O star to be?

140
00:06:10,290 --> 00:06:12,559
You can actually
calculate the flux there.

141
00:06:12,559 --> 00:06:14,600
We can give you the distance
and you can find out

142
00:06:14,600 --> 00:06:17,500
how luminous are these objects?

143
00:06:17,500 --> 00:06:20,241
I don't think you can find
that they're as luminous--

144
00:06:20,241 --> 00:06:21,870
or an O star is
not luminous enough

145
00:06:21,870 --> 00:06:24,580
in x-rays to be able to show up.

146
00:06:24,580 --> 00:06:27,170
But one of these X-ray
binaries might be.

147
00:06:27,170 --> 00:06:29,670
So let's got back to the
other side of the room.

148
00:06:29,670 --> 00:06:31,576
If we have a model
for what we think

149
00:06:31,576 --> 00:06:35,740
these dots are in
the spiral galaxy,

150
00:06:35,740 --> 00:06:40,110
what few things giving off x-ray
light in the elliptical galaxy?

151
00:06:40,110 --> 00:06:42,408
Stars don't give
off X-ray light?

152
00:06:42,408 --> 00:06:44,868
What do you think, David?

153
00:06:44,868 --> 00:06:45,857
DAVID: The nucleus?

154
00:06:45,857 --> 00:06:46,940
MARK HARTMAN: The nucleus?

155
00:06:46,940 --> 00:06:47,773
What is the nucleus?

156
00:06:47,773 --> 00:06:48,875
DAVID: The central region.

157
00:06:48,875 --> 00:06:50,250
MARK HARTMAN: The
central region.

158
00:06:50,250 --> 00:06:53,154
Why would the central region
maybe give off X-ray light?

159
00:06:53,154 --> 00:06:54,820
DAVID: Maybe there's
a black hole there.

160
00:06:54,820 --> 00:06:57,620
MARK HARTMAN: OK, maybe
that's why there's that bright

161
00:06:57,620 --> 00:07:01,170
or that high flux spot
in the middle whereas

162
00:07:01,170 --> 00:07:04,330
here was there a high flux
spot in the middle in this one?

163
00:07:04,330 --> 00:07:05,700
I honestly don't know.

164
00:07:05,700 --> 00:07:07,132
AUDIENCE: [INAUDIBLE]

165
00:07:07,132 --> 00:07:08,590
MARK HARTMAN: In
the visible light?

166
00:07:08,590 --> 00:07:09,370
In X-rays?

167
00:07:09,370 --> 00:07:10,382
AUDIENCE: [INAUDIBLE]

168
00:07:10,382 --> 00:07:11,090
MARK HARTMAN: No.

169
00:07:11,090 --> 00:07:12,923
So what does that tell
us about this galaxy?

170
00:07:15,815 --> 00:07:18,004
There's no nucleus?

171
00:07:18,004 --> 00:07:18,950
AUDIENCE: It's weird.

172
00:07:18,950 --> 00:07:20,366
MARK HARTMAN: It's
a weird galaxy?

173
00:07:20,366 --> 00:07:25,926
AUDIENCE: It's a
joke [INAUDIBLE]

174
00:07:25,926 --> 00:07:28,914
AUDIENCE: There's
not a black hole?

175
00:07:28,914 --> 00:07:34,765
MARK HARTMAN: If you're going to
something, say it to all of us.

176
00:07:34,765 --> 00:07:37,060
AUDIENCE: What
does it even mean?

177
00:07:37,060 --> 00:07:41,010
MARK HARTMAN: So David
says, in the x-ray image,

178
00:07:41,010 --> 00:07:43,910
this is the bright in the center
because a model could be maybe

179
00:07:43,910 --> 00:07:46,940
there's a supermassive
black hole there.

180
00:07:46,940 --> 00:07:51,770
But if there's no bright spot
at center in the spiral galaxy

181
00:07:51,770 --> 00:07:53,959
in X-rays, what does that mean?

182
00:07:53,959 --> 00:07:54,854
Lauren?

183
00:07:54,854 --> 00:07:57,270
LAUREN: There's probably no
supermassive black hole there.

184
00:07:57,270 --> 00:07:59,644
MARK HARTMAN: There's probably
no supermassive black hole

185
00:07:59,644 --> 00:08:00,600
there.

186
00:08:00,600 --> 00:08:02,016
We could say, oh
yeah, it's there.

187
00:08:02,016 --> 00:08:05,020
But maybe there's a
cloud covering it up.

188
00:08:05,020 --> 00:08:07,400
Could be, we'd need to do
some other observations.

189
00:08:07,400 --> 00:08:09,525
And if you looked at
it for a long time,

190
00:08:09,525 --> 00:08:11,970
we'd see it wink back.

191
00:08:11,970 --> 00:08:16,470
So what did David tell
us us about this morning

192
00:08:16,470 --> 00:08:20,885
that also gives off X-rays?

193
00:08:20,885 --> 00:08:21,867
Chris?

194
00:08:21,867 --> 00:08:23,840
CHRIS: The hot gas.

195
00:08:23,840 --> 00:08:27,400
MARK HARTMAN: OK, so maybe
the x-ray stuff over here

196
00:08:27,400 --> 00:08:29,890
is not x-ray binaries.

197
00:08:29,890 --> 00:08:33,500
But does it look like points?

198
00:08:33,500 --> 00:08:37,880
Like the x-ray stuff over here,
does that look like points?

199
00:08:37,880 --> 00:08:38,980
How did you describe it?

200
00:08:38,980 --> 00:08:41,764
Let's have a word from
the people over here.

201
00:08:41,764 --> 00:08:43,740
We're talking about the
texture or the shape.

202
00:08:43,740 --> 00:08:46,956
How did you guys describe your
x-ray image of this galaxy?

203
00:08:52,930 --> 00:08:55,612
I can't hear a word
that you're saying.

204
00:08:55,612 --> 00:08:57,748
That doesn't mean that
you should be quiet.

205
00:08:57,748 --> 00:08:59,242
That means you should be louder.

206
00:08:59,242 --> 00:09:10,198
AUDIENCE: That you see
all these [INAUDIBLE]

207
00:09:10,198 --> 00:09:12,190
MARK HARTMAN: OK.

208
00:09:12,190 --> 00:09:13,186
Lauren?

209
00:09:13,186 --> 00:09:19,660
LAUREN: In the center of where
the galaxy is supposed to be,

210
00:09:19,660 --> 00:09:23,146
you'll see that there's
a lot of light there

211
00:09:23,146 --> 00:09:26,134
and then there's the
shape around it that's not

212
00:09:26,134 --> 00:09:28,126
[INAUDIBLE].

213
00:09:28,126 --> 00:09:33,106
So that could be [INAUDIBLE]

214
00:09:33,106 --> 00:09:35,130
MARK HARTMAN: OK, but
the important point

215
00:09:35,130 --> 00:09:37,428
is it's not dots.

216
00:09:37,428 --> 00:09:39,740
It's not a compact dot.

217
00:09:39,740 --> 00:09:42,240
It's not a point source object.

218
00:09:42,240 --> 00:09:44,130
It's an extended source.

219
00:09:44,130 --> 00:09:45,868
I think David described
one that I think

220
00:09:45,868 --> 00:09:47,034
we say up there as extended.

221
00:09:47,034 --> 00:09:47,904
Right?

222
00:09:47,904 --> 00:09:49,620
Steve also had an
interesting question

223
00:09:49,620 --> 00:09:51,780
about how can I tell
something is a galaxy

224
00:09:51,780 --> 00:09:53,650
or if something is a star?

225
00:09:53,650 --> 00:09:55,716
And what did we decide?

226
00:09:55,716 --> 00:09:59,500
AUDIENCE: [INAUDIBLE]
they're stars.

227
00:09:59,500 --> 00:10:01,787
When they're [INAUDIBLE]
they're galaxies.

228
00:10:01,787 --> 00:10:03,870
MARK HARTMAN: OK, what was
the other thing that we

229
00:10:03,870 --> 00:10:06,160
use to describe galaxies?

230
00:10:06,160 --> 00:10:09,590
AUDIENCE: [INAUDIBLE]

231
00:10:09,590 --> 00:10:12,265
MARK HARTMAN: OK,
galaxies are fluffier.

232
00:10:12,265 --> 00:10:14,122
They're oval shaped.

233
00:10:14,122 --> 00:10:17,120
If you have something that's
round, it could be a galaxy

234
00:10:17,120 --> 00:10:19,460
or it could be a star.

235
00:10:19,460 --> 00:10:22,710
But bright stars normally
have little spikes on them.

236
00:10:22,710 --> 00:10:24,940
And you guys will see
on the right hand side,

237
00:10:24,940 --> 00:10:28,991
you'll see a bright object,
but it has spikes on it.

238
00:10:28,991 --> 00:10:33,526
That actually comes from when
light from a point source

239
00:10:33,526 --> 00:10:37,563
gets changed by the
telescope just a little bit.

240
00:10:37,563 --> 00:10:38,062
OK?

241
00:10:38,062 --> 00:10:41,010
Those spikes actually
have to do with the way

242
00:10:41,010 --> 00:10:43,030
that the mirrors are set up.

243
00:10:43,030 --> 00:10:44,580
So--