1
00:00:00,000 --> 00:00:01,020
[MUSIC PLAYING]

2
00:00:01,020 --> 00:00:02,478
ANNA FREBEL: Have
you ever wondered

3
00:00:02,478 --> 00:00:06,120
how all the chemical
elements are made?

4
00:00:06,120 --> 00:00:08,640
Then join me as we are
lifting all these data

5
00:00:08,640 --> 00:00:11,630
secrets to understand the
cosmic origin of the chemical

6
00:00:11,630 --> 00:00:12,130
elements.

7
00:00:16,129 --> 00:00:17,480
Let's talk about spectroscopy.

8
00:00:17,480 --> 00:00:20,570
This is the technique
we use to observe stars

9
00:00:20,570 --> 00:00:22,870
in order to figure out
their chemical composition.

10
00:00:22,870 --> 00:00:25,870
[MUSIC PLAYING]

11
00:00:30,870 --> 00:00:33,170
Now you've probably
all seen a rainbow.

12
00:00:33,170 --> 00:00:35,060
I really hope you have.

13
00:00:35,060 --> 00:00:37,130
And what happens in a rainbow?

14
00:00:37,130 --> 00:00:40,550
Well, white light comes
through a little water droplet

15
00:00:40,550 --> 00:00:43,400
and it gets split up
into the rainbow colors.

16
00:00:43,400 --> 00:00:45,980
And we do the same thing
with a spectrograph

17
00:00:45,980 --> 00:00:47,810
mounted at a telescope.

18
00:00:47,810 --> 00:00:50,000
We take the starlight
and we split it up

19
00:00:50,000 --> 00:00:52,550
into its rainbow colors.

20
00:00:52,550 --> 00:00:57,380
Now what we see when we do
this is not just the rainbow.

21
00:00:57,380 --> 00:00:59,480
Actually, we see less
than the rainbow,

22
00:00:59,480 --> 00:01:03,110
because there are certain
colors of the rainbow missing.

23
00:01:03,110 --> 00:01:08,420
So if I draw this schematically
here, I have a rainbow.

24
00:01:08,420 --> 00:01:17,510
And let's say I have blue here
and then green and yellow,

25
00:01:17,510 --> 00:01:20,390
what I will see
also is that there

26
00:01:20,390 --> 00:01:23,790
is a big line, something
like this, missing here

27
00:01:23,790 --> 00:01:25,340
and it's black.

28
00:01:25,340 --> 00:01:30,770
And then there will be a few
things here and a couple there

29
00:01:30,770 --> 00:01:32,900
and many really,
really thin ones

30
00:01:32,900 --> 00:01:37,160
in between that are hard to see.

31
00:01:37,160 --> 00:01:42,320
And that missing part, or
those missing parts here,

32
00:01:42,320 --> 00:01:45,230
they contain all the
information that we want.

33
00:01:45,230 --> 00:01:46,820
It's actually not
the colors and such,

34
00:01:46,820 --> 00:01:49,730
it's what's missing from there.

35
00:01:49,730 --> 00:01:51,830
Now how can we understand that?

36
00:01:51,830 --> 00:01:56,690
If we come back to our stars
and look at stellar surface,

37
00:01:56,690 --> 00:02:01,070
let's draw a surface layer
here, and the core is here.

38
00:02:03,613 --> 00:02:05,780
We know that nuclear fusion
is going on in the core,

39
00:02:05,780 --> 00:02:09,199
so it's really hot there,
and energy comes out

40
00:02:09,199 --> 00:02:12,870
of the core in the
form of hot photons.

41
00:02:12,870 --> 00:02:15,680
So we have these photons
escaping from the core.

42
00:02:15,680 --> 00:02:19,340
And they come, they pass
through this outer layer.

43
00:02:19,340 --> 00:02:24,620
Of course, we are sitting here
with our telescope observing

44
00:02:24,620 --> 00:02:26,560
the stellar surface.

45
00:02:26,560 --> 00:02:29,180
All right, as I mentioned
in a previous section,

46
00:02:29,180 --> 00:02:32,120
that we can't look
into the core,

47
00:02:32,120 --> 00:02:34,628
we can only observe
the surface here.

48
00:02:34,628 --> 00:02:36,170
And specifically
what we're observing

49
00:02:36,170 --> 00:02:37,795
is we're observing
all the photons that

50
00:02:37,795 --> 00:02:39,260
come off the surface.

51
00:02:42,160 --> 00:02:46,830
So in this outer layer we have
hydrogen and helium atoms,

52
00:02:46,830 --> 00:02:49,470
because that's what the
stars are mostly made of,

53
00:02:49,470 --> 00:02:50,395
hydrogen, helium.

54
00:02:53,040 --> 00:02:56,070
But, of course, there are--
unless we're talking about

55
00:02:56,070 --> 00:02:59,940
the very first stars, but
that's a separate story--

56
00:02:59,940 --> 00:03:04,650
there will be other atoms in
here, iron, magnesium, carbon,

57
00:03:04,650 --> 00:03:09,420
oxygen. And so what happens
is that all elements, hydrogen

58
00:03:09,420 --> 00:03:12,660
and helium as well,
plus iron, magnesium,

59
00:03:12,660 --> 00:03:14,972
and so forth, they absorb.

60
00:03:14,972 --> 00:03:17,310
So let's draw this here.

61
00:03:17,310 --> 00:03:21,510
They absorb photons with
their very specific energy

62
00:03:21,510 --> 00:03:24,310
or wavelength that's equivalent.

63
00:03:24,310 --> 00:03:29,167
And so what comes out of here,
here is one that gets absorbed,

64
00:03:29,167 --> 00:03:30,750
all these get absorbed,
and then there

65
00:03:30,750 --> 00:03:32,400
are some that pass through.

66
00:03:35,910 --> 00:03:37,565
So what we see here
is all the ones

67
00:03:37,565 --> 00:03:38,940
that came through
and, of course,

68
00:03:38,940 --> 00:03:43,140
not the ones that were
absorbed by these atoms.

69
00:03:43,140 --> 00:03:45,180
And so that's exactly
what we see here.

70
00:03:45,180 --> 00:03:47,760
The colors is everything
that came through,

71
00:03:47,760 --> 00:03:52,740
and then the black lines here
are the ones that are missing.

72
00:03:52,740 --> 00:03:54,690
So we can see what's missing.

73
00:03:54,690 --> 00:03:58,860
All the iron atoms here, they
have absorbed all the photons

74
00:03:58,860 --> 00:04:02,400
at a specific color, at
a specific wavelength.

75
00:04:02,400 --> 00:04:04,890
And so that's missing.

76
00:04:04,890 --> 00:04:07,920
However, this is actually
not entirely black black.

77
00:04:07,920 --> 00:04:11,790
It has only a certain amount
being absorbed, perhaps not

78
00:04:11,790 --> 00:04:12,780
completely.

79
00:04:12,780 --> 00:04:14,250
And so what we can
measure is when

80
00:04:14,250 --> 00:04:16,529
we take a cross
cut through this,

81
00:04:16,529 --> 00:04:18,840
we are going to get something
that looks like this.

82
00:04:23,540 --> 00:04:27,540
And so there is a
strong absorption here,

83
00:04:27,540 --> 00:04:28,490
less absorption here.

84
00:04:28,490 --> 00:04:30,430
Let's say that this
is our calcium.

85
00:04:30,430 --> 00:04:32,090
That's a calcium line here.

86
00:04:32,090 --> 00:04:34,000
And these are three
magnesium lines.

87
00:04:34,000 --> 00:04:36,310
These are two sodium lines.

88
00:04:36,310 --> 00:04:38,230
Then we can see from
these line strength

89
00:04:38,230 --> 00:04:43,980
here what the abundance of
the magnesium atoms here--

90
00:04:43,980 --> 00:04:46,630
here is another one--

91
00:04:46,630 --> 00:04:47,270
is.

92
00:04:47,270 --> 00:04:49,060
So line strength
here corresponds

93
00:04:49,060 --> 00:04:54,640
to abundance of magnesium
atoms in the outer atmosphere.

94
00:04:54,640 --> 00:04:57,040
And the nice thing,
of course, is

95
00:04:57,040 --> 00:04:59,830
that when we want
to find the most

96
00:04:59,830 --> 00:05:02,260
metal poor stars, or
the oldest stars, then

97
00:05:02,260 --> 00:05:07,360
we want to look for stars whose
spectra have very weak lines.

98
00:05:07,360 --> 00:05:08,485
Let's say like this.

99
00:05:12,040 --> 00:05:15,110
Because that means that only
a little calcium, magnesium,

100
00:05:15,110 --> 00:05:19,402
and sodium actually are
present in the star,

101
00:05:19,402 --> 00:05:20,860
which means that
the star must have

102
00:05:20,860 --> 00:05:22,900
formed at a really
early time when

103
00:05:22,900 --> 00:05:25,480
the cycle of chemical
enrichment had only

104
00:05:25,480 --> 00:05:27,580
gone around a few times.

105
00:05:27,580 --> 00:05:32,710
So this is the secret of
spectroscopy, absorption line

106
00:05:32,710 --> 00:05:33,990
spectroscopy.

107
00:05:33,990 --> 00:05:36,110
We take these kinds
of data here and we

108
00:05:36,110 --> 00:05:37,510
measure the line strength.

109
00:05:37,510 --> 00:05:42,550
We measure how much
is present here.

110
00:05:42,550 --> 00:05:46,480
And with the help of
computer programs,

111
00:05:46,480 --> 00:05:49,450
and a whole bunch of physics, we
can turn these lines strengths

112
00:05:49,450 --> 00:05:52,840
here into an abundance
in the stellar surface,

113
00:05:52,840 --> 00:05:57,100
and that tells us about the
formation time of these stars.

114
00:05:57,100 --> 00:06:00,150
[MUSIC PLAYING]