1
00:00:00,929 --> 00:00:02,267
In the last video we learned that there are

2
00:00:02,267 --> 00:00:06,277
a class of stars called Cepheid variables.

3
00:00:06,277 --> 00:00:07,867
And these are these super-giant stars,

4
00:00:07,867 --> 00:00:10,600
as much as thirty thousand times as bright as the sun,

5
00:00:10,600 --> 00:00:14,420
with a mass as much as twenty times the mass of the sun.

6
00:00:14,420 --> 00:00:16,000
And what's neat about them is

7
00:00:16,000 --> 00:00:17,670
one, because they're so large and so bright

8
00:00:17,670 --> 00:00:19,760
you can see them really, really far away.

9
00:00:19,760 --> 00:00:21,502
And what's even neater about them is

10
00:00:21,502 --> 00:00:23,684
that they're variable, that they pulsate.

11
00:00:23,684 --> 00:00:27,748
And because their pulsations are related to their actual luminosity,

12
00:00:27,748 --> 00:00:31,765
you know, if you see a Cepheid variable star in some distant galaxy,

13
00:00:31,765 --> 00:00:34,800
you know what it's luminosity actually is

14
00:00:34,800 --> 00:00:36,867
if you were kind of at the star.

15
00:00:36,867 --> 00:00:41,099
Because you can see its period of pulsation.

16
00:00:41,099 --> 00:00:42,934
And so if you know its actual luminosity

17
00:00:42,934 --> 00:00:45,267
and then you know, obviously, its apparent luminosity,

18
00:00:45,267 --> 00:00:47,333
you know how much it's gotten dim.

19
00:00:47,333 --> 00:00:50,133
And the more dim it's gotten from it's actual state,

20
00:00:50,133 --> 00:00:51,850
you know the farther away it is.

21
00:00:51,850 --> 00:00:53,067
So that's the actual value of them.

22
00:00:53,067 --> 00:00:55,000
What I want to do in this video

23
00:00:55,000 --> 00:00:57,733
is to try to explain why they're variable,

24
00:00:57,733 --> 00:00:59,867
why they pulsate.

25
00:00:59,867 --> 00:01:03,759
And to do that, what we're going to think about is

26
00:01:03,759 --> 00:01:08,081
doubly and singly ionized heliums.

27
00:01:08,081 --> 00:01:09,533
And just to review helium.

28
00:01:09,533 --> 00:01:13,004
So neutral helium (let me draw neutral helium) . . .

29
00:01:13,004 --> 00:01:14,733
Neutral helium's got two protons.

30
00:01:14,733 --> 00:01:16,579
It's got two protons,

31
00:01:16,579 --> 00:01:19,815
two neutrons,

32
00:01:19,815 --> 00:01:20,733
and then two electrons.

33
00:01:20,733 --> 00:01:23,081
And obviously this is not drawn to scale.

34
00:01:23,081 --> 00:01:25,800
So this is neutral helium, right over here.

35
00:01:25,800 --> 00:01:30,133
Now, if you singly ionize helium, you knock off one of these electrons.

36
00:01:30,133 --> 00:01:32,299
And these type of things happen in stars.

37
00:01:32,299 --> 00:01:34,366
When you have a lot of heat, easier to ionize things.

38
00:01:34,366 --> 00:01:37,067
So singly ionized helium will look like this.

39
00:01:37,067 --> 00:01:39,341
It'll have the same nucleus,

40
00:01:39,341 --> 00:01:41,378
two protons, two neutrons,

41
00:01:41,378 --> 00:01:44,954
one of the electrons get knocked off, so now you only have one electron.

42
00:01:44,954 --> 00:01:47,467
And now you have a net positive charge.

43
00:01:47,467 --> 00:01:50,620
So here (let me do this in a different color)

44
00:01:50,620 --> 00:01:53,641
this helium now has a net charge.

45
00:01:53,641 --> 00:01:56,000
We could write "1+" here, but if you just write a "+"

46
00:01:56,000 --> 00:01:58,533
you implicitly mean a positive charge of one.

47
00:01:58,533 --> 00:02:05,071
Now, you can also doubly ionize helium if the environment is hot enough.

48
00:02:05,071 --> 00:02:09,759
And doubly ionizing helium is essentially knocking off both of the electrons.

49
00:02:09,759 --> 00:02:15,130
So then it's really just a helium nucleus.

50
00:02:15,130 --> 00:02:20,467
Like this. This right here is doubly ionized helium.

51
00:02:20,467 --> 00:02:22,667
Now, I just said, in order to do this,

52
00:02:22,667 --> 00:02:24,400
you have to have a hotter environment.

53
00:02:24,400 --> 00:02:28,533
There has to be a hotter environment in order to knock off both.

54
00:02:28,533 --> 00:02:31,133
This electron really doesn't want to leave.

55
00:02:31,133 --> 00:02:32,694
I mean, to take an electron off of something

56
00:02:32,694 --> 00:02:34,133
that's already positive is difficult.

57
00:02:34,133 --> 00:02:36,133
So you have to have a lot of,

58
00:02:36,133 --> 00:02:37,867
really pressure and temperature.

59
00:02:37,867 --> 00:02:39,133
This is cooler.

60
00:02:39,133 --> 00:02:40,333
Now, this is all relative.

61
00:02:40,333 --> 00:02:42,958
We're talking about the insides of stars.

62
00:02:42,958 --> 00:02:47,349
So, you know, this is a hotter part of the star vs. a cooler part of the star,

63
00:02:47,364 --> 00:02:48,425
I guess is the way to think about it.

64
00:02:48,425 --> 00:02:49,691
This is still a very hot environment

65
00:02:49,691 --> 00:02:52,687
by our traditional, "everyday" standards.

66
00:02:52,687 --> 00:02:55,882
Now, the other thing about the doubly ionized helium

67
00:02:55,897 --> 00:03:00,585
is that it is more opaque,

68
00:03:00,585 --> 00:03:04,778
which means it doesn't allow light to go through it;

69
00:03:04,778 --> 00:03:05,995
it actually absorbs light.

70
00:03:05,995 --> 00:03:12,077
It is more opaque. It absorbs light.

71
00:03:12,092 --> 00:03:14,595
Or another way, it absorbs that light energy,

72
00:03:14,595 --> 00:03:16,487
that energy will make it even hotter.

73
00:03:16,487 --> 00:03:18,267
So that's just something to think about.

74
00:03:18,267 --> 00:03:26,421
Now, the singly ionized helium is more transparent.

75
00:03:26,421 --> 00:03:28,590
It allows the light to pass through it,

76
00:03:28,590 --> 00:03:32,467
so it doesn't get heated as much by photons

77
00:03:32,467 --> 00:03:35,133
that are kind of going near it,

78
00:03:35,133 --> 00:03:36,200
or through it, or whatever.

79
00:03:36,200 --> 00:03:37,338
It allows them to go through it.

80
00:03:37,338 --> 00:03:40,800
Here, the photons are going to actually heat up the ion.

81
00:03:40,800 --> 00:03:44,119
So let's think about how this might cause a Cepheid variable

82
00:03:44,119 --> 00:03:45,200
to pulsate.

83
00:03:45,200 --> 00:03:50,800
So assuming that Cepheid variables have a large enough quantity

84
00:03:50,800 --> 00:03:53,133
of these ions, we can imagine

85
00:03:53,133 --> 00:03:56,333
that when a Cepheid variable is dim

86
00:03:56,333 --> 00:03:58,800
(So let me draw a dim Cepheid variable.

87
00:03:58,800 --> 00:04:07,400
So I'll draw this in a dim color. So this is a dim Cepheid variable right here.)

88
00:04:07,400 --> 00:04:10,867
In it's dim state, just like this,

89
00:04:10,867 --> 00:04:15,267
you have a lot of the doubly ionized helium.

90
00:04:15,267 --> 00:04:19,088
You have a lot of doubly ionized helium in the star,

91
00:04:19,088 --> 00:04:21,333
or at least kind of the outer surface of the star.

92
00:04:21,333 --> 00:04:23,600
Doubly ionized helium.

93
00:04:23,600 --> 00:04:26,933
And so this does not allow a lot of light to pass through.

94
00:04:26,933 --> 00:04:31,673
So this is the dim part of the pulsation of the Cepheid variable.

95
00:04:31,673 --> 00:04:37,084
Now, because this doubly ionized helium is opaque,

96
00:04:37,084 --> 00:04:38,756
it is absorbing the light.

97
00:04:38,756 --> 00:04:40,200
It is getting heated.

98
00:04:40,200 --> 00:04:46,113
It is getting heated.

99
00:04:46,113 --> 00:04:49,605
And because it's getting heated, it'll cause the star to expand.

100
00:04:49,621 --> 00:04:53,067
So because it's getting heated, it'll become more energetic

101
00:04:53,067 --> 00:04:54,867
and the star will actually expand.

102
00:04:54,867 --> 00:04:58,867
The star will actually expand.

103
00:04:58,867 --> 00:05:03,000
Now, as the star expands because this doubly ionized helium is getting heated,

104
00:05:03,000 --> 00:05:04,200
what's going to happen?

105
00:05:04,200 --> 00:05:07,400
The further away you are from the core of the star, the cooler it gets.

106
00:05:07,400 --> 00:05:10,200
So this expanded because it was getting heated.

107
00:05:10,200 --> 00:05:11,933
But then because it expanded,

108
00:05:11,933 --> 00:05:14,533
the outer layers of the star become cooler.

109
00:05:14,533 --> 00:05:18,600
And since they're cooler, helium won't be doubly ionized anyomre.

110
00:05:18,600 --> 00:05:25,934
Each helium atom can now get an electron from the plasma, I guess we could say,

111
00:05:25,934 --> 00:05:27,533
to become singly ionized helium.

112
00:05:27,533 --> 00:05:30,420
So now, we have singly ionized helium.

113
00:05:30,420 --> 00:05:33,764
We have singly ionized helium.

114
00:05:33,764 --> 00:05:36,933
And now the star is going to be more transparent,

115
00:05:36,933 --> 00:05:38,867
it's going to allow more light to pass through it.

116
00:05:38,867 --> 00:05:43,307
So now this is the bright part of the pulsation.

117
00:05:43,307 --> 00:05:49,467
It's going to allow more light through, so now the star is bright.

118
00:05:49,467 --> 00:05:51,124
But what's happening now?

119
00:05:51,124 --> 00:05:53,333
Because the light is no longer --

120
00:05:53,333 --> 00:05:55,358
or it's not being absorbed as well by the helium

121
00:05:55,358 --> 00:05:57,867
as when it was a doubly ionized helium,

122
00:05:57,867 --> 00:06:00,768
now it's letting most of the light, or a lot more of the light, get through,

123
00:06:00,768 --> 00:06:03,439
it's not going to get heated as much.

124
00:06:03,439 --> 00:06:05,854
And so it won't have the kinetic energy to kind of

125
00:06:05,854 --> 00:06:07,846
keep pushing out, to keep moving outward,

126
00:06:07,846 --> 00:06:11,200
and so it'll collapse back into the star.

127
00:06:11,200 --> 00:06:14,097
And so that this will cool down and collapse back in.

128
00:06:14,097 --> 00:06:17,752
And when it collapses back in, what's going to happen?

129
00:06:17,768 --> 00:06:21,159
When it collapses back in,

130
00:06:21,175 --> 00:06:24,600
when these helium atoms get closer to the center of the star,

131
00:06:24,600 --> 00:06:25,776
to the core of the star,

132
00:06:25,776 --> 00:06:28,886
they're going to be heated again, because they're closer now to the core.

133
00:06:28,886 --> 00:06:30,467
And when they get heated

134
00:06:30,467 --> 00:06:34,200
they're going to become doubly ionized,

135
00:06:34,200 --> 00:06:39,574
so that we have doubly ionized helium again.

136
00:06:39,574 --> 00:06:40,933
And then the cycle will go again:

137
00:06:40,933 --> 00:06:43,533
It is now opaque; it will now absorb more energy;

138
00:06:43,533 --> 00:06:47,023
that will cause it to have more kinetic energy to expand;

139
00:06:47,023 --> 00:06:51,109
once it expands it'll get cool again, and transparent, and bright.

140
00:06:51,109 --> 00:06:54,685
So this is the current best theory of why

141
00:06:54,685 --> 00:06:58,516
Cepheid variable stars are vaiable to begin with.

142
00:06:58,516 --> 00:07:01,767
It's this whole notion of having the doubly ionized helium

143
00:07:01,767 --> 00:07:03,972
vs. the singly ionized helium

144
00:07:04,003 --> 99:59:59,999
in kind of the outer layers of the star itself.