If the Sun has an atmosphere then it has 
weather and understanding solar weather 
is going to be fun, because it involves 
interesting physics. 
It's going to be important because it 
actually effects life on earth so let's 
try to understand how, in what way solar 
weather is very, very different from the 
weather here on earth. 
And so the indication of solar weather 
are these blemishes, these spots on the 
sun. 
first known recording of these is in the 
writings of Chinese astronomer whose name 
I can only pronounce as Gan De. 
Galileo observed the sun spots and used 
them to find the rotation period around 
the sun, 
the sun rotates about its axis moving 
from west to east, every 25.4 days. 
This is a medieval depiction of two sun 
spots on the surface of the sun. 
these sun spots appear as dark spots on 
the sun. 
Here, you see a modern image of the sun 
with a few regions of sunspots. 
They appear dark, as we said, because 
they're cooler than the surrounding at 
the center of a sunspot, the temperature 
can dip as low as 4,000 Kelvin. 
Because of T4, to the fourth they emit 
less. 
And if you remember that the sun is 
approximately 100 times the radius of 
Earth. 
Then the typical size of a sunspot is 
about the size of Earth. 
And it was the observation of Wilson in 
1769 that in fact, by watching the 
behavior of sunspots as they approach the 
edge of the visible disc of the sun. 
That they're in fact depressions in the 
solar surface. 
This is what a sunspot looks like when 
magnified. 
This is an Earth like region which is 
cooler than the surrounding, the center 
of, part of the sunspot is called the 
umbra. 
The less dark exteriors called the 
penumbra of those channels have nothing 
to do with it, 
the words are completely borrowed. 
So, what are these things and hints to 
the fact to the origin of these things 
come by observing the patterns of their 
formation. 
It turns out that some spark number 
varies with a very nice eleven year 
cyclicity, this famous butterfly graph 
here on the right indicates, well on the 
bottom you see the number of, 
sunspots or the total fraction of the 
solar disk's area covered by sunspots as 
a function of years from somewhere in the 
eighteen hundreds all the way through the 
to, the present day. 
And we see there's this periodic 
oscillation. 
Lot's of sunspots there, none, and lot's 
of sunspots there, none. 
And when you look more closely at where 
along the sun they're generated that's 
what butterfly graph above shows us. 
This is sort of a picture of 
The, the vertical axis is latitude along 
the sun. 
And you see that early in a cycle 
sunspots are created at mid latitudes, 
and then as the cycle proceeds their 
formed closer and closer to the equator. 
Then there's a whole bunch of then at the 
equator, then they all disappear. 
And then there's a break, and then some 
more sun spots. 
Form at mid latitudes, and slowly they 
move towards the equator. 
And the pattern repeats. 
it's hard to see in this graph. 
But in fact 2010 was the end of a very 
abnormally long solar minimum in which 
there were no sun spots. 
And nobody understands exactly, as far as 
I can tell what the cause was for this 
reason. 
Now. 
An important a discovery is that you can 
associate sunspots to magnetic free 
regions of strong vertical magnetic 
fields of the sun. 
So magnetic field poking out of the 
surface of the sun and the way you 
measure this is because it turns out that 
a magnetic field has an effect on the 
spectrum of 
Atoms in the presence of a magnetic 
field. 
You can measure the shift in the 
absorption lines that we saw in the Sun's 
spectrum. 
It's all by looking. 
And we can therefore measure the 
magnitude of a magnetic field. 
And this is a magnetic field map. 
Again we have the lines of latitude. 
And longitude along the sun. 
As, as a function of time, we see that 
when there are lots of sun spots, there 
are a lot of these regions of high 
magnetic field. 
Notice that yellow is one polarity and 
blue, the opposite polarity. 
So magnetic field pointing in and out of 
the sun. 
And sunspots appear in pairs. 
You see regions of blue preceded by 
yellow in this rotation. 
So there is 
blue dot and then to the west of it, a 
yellow dot in the northern hemisphere and 
the polarity is reversed in the southern 
hemisphere. 
And this is consistent throughout a cycle 
and would then have in the, between 
cycles the polarity reverses so that if 
the blue dots, we don't have two cycles 
here, but if the blue dots were ahead of 
the yellow dots in the north in this 
cycle and the yellow ahead of the blue in 
the north, in the south in this cycle 
this would reverse from one cycle to the 
other. 
And this is our hint. 
That what drives these sunspots is in 
fact the magnetic field. 
It turns out that what makes them cool is 
that, remember we talked about, when we 
talked about the Van Allen radiation 
belts, we said that regions of strong 
magnetic field repel charged particles, 
the [INAUDIBLE] 
gas coming up to the photosphere from the 
interior regions is charged. 
Magnetic field intense magnetic fields, 
like in the sunspots choke off this 
convection. 
And for that reason, the interior of a 
sunspot is cooler than the surrounding 
part of the sun. 
Down, as I said, to 4,000 Kelvin. 
And. 
So we understand sun spots is a magnetic 
phenomenon. 
And the fact that the particles on the in 
the atmosphere are charged and therefore 
interact strongly with the magnetic 
field. 
As are, by the way, all of the particles 
in the interior of the sun plays an 
important role in understanding what the 
causes of this phenomenon and some of its 
consequences. 
So we're led to look to the solar 
magnetic field to try to understand that 
and see how that drives the weather. 
the magnetic field of the sun is not 
quite as that of the Earth. 
It's not well described. 
Most times. 
Like a bar man in the North and the South 
pole. 
Though at any time, there is roughly a 
north and a south pole. 
And it is roughly aligned with the axis 
of rotation. 
The big difference, is that in the 
interior of the sun, the roiling 
convecting fluid that presumably 
generates the magnetic field. 
Is not just conducting but is charged. 
It's a plasma. 
And you recall, when we talked about the 
interaction of the solar wind with the 
Earth's magnetic field. 
That I said that because a stream of 
charge particles both is influenced by a 
magnetic field but also creates a 
magnetic field. 
You can imagine that we have a situation 
where magnetic fields and charged 
particles are strongly interacting. 
And we can. 
Model this by imagining that the magnetic 
field lines and the particles are tied to 
each other. 
So if we have a weak current that we had 
in our tube, the current is trapped 
around the magnetic field. 
But if we have a stronger current, it can 
drag the magnetic field around, which is 
what the solar wind did to Earth's 
magnetic field and is what the rotating. 
Plasma in the interior of the Sun does to 
the magnetic field. 
And a crucial ingredient in this is that 
the rotation period of the Sun is not 
uniform like in, in the case of Jupiter. 
The Sun experiences a fluid body, that 
experiences differential rotation. 
The sun's rotation period is 25 and a bit 
days at the equator, but 29 days at the 
pole so that the equator rotates faster 
than the pole. 
This has the following effect upon the 
sun's magnetic field. 
So here's the rotating sun, it's 
undergoing differential rotation. 
You start with sort of a dipolar magnetic 
field, but the magnetic field lines are 
trapped by the plasma, and because the 
equator rotates faster than the poles, 
the magnetic field lines are stretched. 
So that you get first you started with a 
north south magnetic field, then you 
ended up with an east west magnetic 
field. 
And these field lines are stretched and 
these stretch field lines are kind of 
like rubber bands. 
They contain large amounts of magnetic 
energy and this releases by the field 
sort of reconnecting, popping out of the 
sun's surface, creating all these 
bubbles. 
And these might remind you some of the 
structures we saw when we looked at 
Coronagraph remember that if magnetic 
field lines poke out of the sun then the 
charged particles trapped along them will 
spiral along the magnetic fields and form 
loops of charged particles and these are 
the 
prominences that we saw and what happens 
is that this process of winding up the 
magnetic field until its energetically 
favorable to reconnect and the sun's 
field becomes a mess for a while takes 
about eleven years. 
And so every eleven years the sun's 
polarity in fact reverses. 
You start with the nice polar magnetic 
field that we started with and then you 
wind it up. 
You produce a mess, and 
the mass reinstates a polar magnetic 
field with the opposite North South 
polarity after about eleven years and the 
pattern repeats itself. 
This is the cause of the magnetic cycle, 
and of the creation of the sunspots. 
So sunspots are these regions where the 
magnetic field lines poke out of the sun 
and in and their orientation in each 
hemisphere is due to the direction of 
rotation relative to the direction of the 
magnetic field and 
This reverses between cycles because the 
polarity reverses and we get a big 
vertical component where these arches 
poke out and at the bottom of the two 
base points of each of these arches sits 
a sun spot. 
So the most active times in the sun's 
magnetic history are the times when the 
field is in the process of reversing. 
These reconnection events release the 
magnetic energy stored in these magnetic 
fields. 
I told you that you could treat them as 
rubber bands. 
They snap back. 
They release a lot of energy 
the local release of energy in a 
recollection event. 
Can be on the order of ten to the 25 
joules. 
This leads to rather dramatic solar 
flares where one region of the Sun 
suddenly becomes very bright, and amidst 
a lot of ultra-violet and x-ray light. 
And these beautiful ultra-violet images, 
you can actually trace the outlines of 
those magnetic field arches that I spoke 
about. 
That's not because we can see the 
magnetic fields, but because we see the 
ultraviolet emissions, from the charged 
particles that are trapped along the 
magnetic fields. 
It's very pre, kind of these particles to 
give such an indication of the fields. 
so solar flares 
Are energetic events. 
There are far more violent events that go 
on. 
they're called prominences or coronal 
mass ejections and here is a, a 
collection of views from many NASA 
observatories of a coronal mass ejection 
that occurred a few months ago. 
this is an event in which an amount of 
hydrogen and charged particles in the 
order of the mass of the earth is ejected 
at high speed as magnetic pressure 
basically builds up and ejects these 
charged particles at high speed. 
this one was not directed towards earth. 
When such events happen and are directed 
towards earth this is, these periodic 
sudden increases in the intensity of the 
solar wind that cause beautiful aurora 
but also 
can cause disruption of communications as 
the electronics and communication 
satellites is injured by this stream of 
charged particles. 
And so, this is why we have so many 
different observatories observing the Sun 
at many wave lengths, from many angles in 
order to be able to understand this 
phenomena, if possible predict them and 
certainly give us the three day warning. 
three days is approximately the time it 
takes one of these coronal mass ejections 
to get from the sun to Earth. 
And getting the three day warning, allows 
satellite operators to sort of shutdown 
operations in an orderly fashion and not 
allow damage to happen to their 
satellites. 
And so, we've developed a pretty decent 
understanding of how the sun works as a 
star both in terms of the internal 
energetic and energy production and, in 
terms of the structure of the sun itself. 
Our job now is going to be try to extend 
that to nearby starts and the first 
question we're going to have to ask is, 
how far are these things because anything 
we see needs to be interpreted 
differently depending on how far it is. 
And so our first chore is going to be to 
measure distances to stars. 
And it's to that, that we're going to 
turn next.