Chapter 6 Notes & Questions

Required reading: All Sections

Some (but not all) Important Concepts:

Some Questions from Students (note these are question from previous classes and so include material that we may not have covered - but if you have some more questions, ask, and I can add them to this page!):

Question 1:
How did we find out all this about the sun? Was it through radio telescopes, or what? How can they possibly know how dense the atmosphere of the sun is and that 99.5% of the photons are escaping from the sun? How do they know that light is emitted and reabsorbed? OR... did they tell me and I was spacing out and missed it?

By looking at the light, both spectral lines and overall colors. You've already seen a bit of this now, and we're only scratching the surface (but we really don't have time to go into all the details in just this one semester). But the basic idea is that most physical conditions will affect some aspect of the light that the Sun gives off. Try this one: if the photosphere of the Sun was more transparent, we'd be able to see further into it, to hotter layers beneath what we see now. But because we'd be getting light from hotter gas, we'd see slight differences in the spectral lines from what we see today (maybe more ions).

Your second question is a bit more subtle, we know the light is emitted and reabsorbed beacuse that's exactly what would happen if it took place here on Earth. It's like if I told you a kid in Indiana was going to drop a basketball. You'd be very surprised to hear that instead of falling, it floated into the air and off into space. In fact, I bet you wouldn't beleive me. It's because you know that physics in Indiana works just like the physics here - same laws, same behavior. And as far as that goes, there's no real difference between here and the Sun either.

Question 2:
What, in plain English, is the Zeeman effect?

Oooh, plain english...a challenge. If it didn't make sense in class, try thinking of it this way: electrons are stuck in very specific and precsise orbits (energy levels) around the nuclei of atoms. Magnetic fields can take the energy levels and split them - pull them apart (often into two very close levels). So now instead of just one orbit for electrons to absorb photons from, there are two (or more). And instead of one spectral line, you see two (or more). Keep in mind that one atom will have its electron in one of the two (or more "sub levels"), while another atom might have its electron in the other. It's the whole bunch of atoms, with electrons in the two energy levels that create the two lines.

Question 3:
What are they talking about with the Density compared to room temperature of the Sun and pressure at sea level?

They are comparing the density of air here on Earth with the density of gas in the Sun's photosphere. The point they are trying to make is that the gas in the photosphere is much much less dense than the air in your room. Likewise, they are comparing the pressure in the photosphere with the air pressure here on Earth (at sea level), which is again much much less.

Question 4:
In the section labled "The Composition of the Sun," it states, "The atoms and molecules contained in the Sun are all in the form of gases." Does this mean that stars have absolutely no matter in the form of a solid?

Yes. No way solids are going to be able to exist (except near the outer layers of the very coolest known stars). The temperatures are just too high.

Question 5:
I do not understand the sunspot portion in the text. How is a sun spot detected and formed? What determines a region on the sun to active or not?

Sunspots form because magnetic fields (which have popped through the surface of the Sun because they've been wound too tight) keep the material at the bottom of the photosphere (the granules) from convecting. No convection, no energy flow, no heat transport, and that part of the photosphere becomes cooler. You detect sunspots by looking at the Sun through a serious filter or a special telescope set-up. (Remeber, there's a reason Galileo, the discoverer of sunspots, went blind.)

Question 6:
If over the past million or so years the sun has been losing about 10 million tons per year. Why can't we see a change in the sun and why isn't it affected by this?

The mass of the Sun is about 2,000,000,000,000,000,000,000,000,000,000 kg. Loosing 9,000,000,000 kg per year is nothing to the Sun (sort of like Bill Gates loosing a dollar).

Question 7:
I once read that Saturn was possibly a forming star that never worked out and that it's composition has alot of hydrogen and helium like the sun, what happened?

You're thinking of Jupiter, although the composition of Saturn is also similar - but it has much less mass than Jupiter. We will cover this later when we talk about how stars form and start generating energy.

Question 8:
Didn't understand what thet meant when they said that the transition region may be wrapped around the spicules like a thin cloak.

Yes, I wasn't really happy with this either, but its hard to describe. You have the spicules, these "spikes" of gas rising up through the chromoshpere and into the corona. Somewhere in there, the temperature also skyrockets, and its thought that the spicules have a lot to do with it, almost as if the energy (heat) was coming from the spicules heating up the space around them.

Question 9:
The book talks about sun spots. It says that there weren't as many sun spots back in the 1700's as there are now. It then goes on to say that the Sun's activity is related to the number of spots the sun has. Why would the Sun be more active now than before? And, is this a bad thing that the Sun is more active than it used to be?

Nobody knows for sure why there are long term differences in the activity level (and therefore the number of sunspots). What is important are how the luminosity (energy output of the Sun) also changes. Just a 1% difference is very noticable to us and our climate, and 1% variations are observed among other Sun-like stars which gives us reason to worry (as if there's not already enough stuff that can go wrong). Perhaps someday we'll be able to figure out the cause of the small changes in luminosity, but right now the problem is just too complicated for us (you think weather forecasts are bad, the Earth's weather is pretty simple compared to the Sun).

Question 10:
When scientist figure out the mass of the earth, do they just use the solid ground or do they include man made objects?

Interesting question! Think about this - human made objects are made out of the Earth (metals, plastics from petrolium, bricks from rocks - everything around us is), so the mass of the Earth includes all the stuff on it too and it has not changed due to our activities.

Question 11:
Because the sun is hot many "things" are vaporized even before they reach the sun. How close can an object get to the sun before it is vaporized, and what is the closet anything has come to the sun?

I'm not sure what the record is. Certainly comets have been observed being destroyed by the Sun (long before they reach it). As far as human-made objects, there's never been a space mission to the Sun for obvious reasons (and we can study it just fine from here anyways).

Question 12:
Could you please elaborate on why the "Northern Lights" are brighter in the northern regions of the world?

Because they are due to charged particles "following" the Earth's magnetic field lines towards the magnetic poles, the Northern Lights are more visible the closer to the pole you get (within about 20 degrees). The same is true for the southern hemisphere too. The "Southern Lights" are just as spectacular.

Question 13:
Is the sun considered a star and what determines if it is or is not?

We will talk more about this later. But basically, if it is able to produce its own energy via nuclear reactions, it's a star. The Sun is a star, just much closer to us than the others are.

Question 14:
You mentioned stars being born earlier in class. What are the chances of a star being born close to earth like our sun? Could we then have two suns?

We almost did! (If Jupiter had been about 100 times more massive, we would have two stars in our solar system). We will talk more about this later, but stars are born in large clouds of dust and gas. There really aren't any clouds that close to us, so there's no chance (we'd see it). But back when the Sun first formed, it was probably born with about 20 or so other stars - so somewhere up there are our brothers and sisters, we've just all wandered away from each other. More about this later.

Question 15:
Why when I see pictures for space, they always look dark. Should they not be bright from the sun?

No (this would be easier to describe in person) but the reason the sky looks bright here on Earth during the day is that particles in the air scatter the light all over the place (so even if you are looking away from the Sun, some of the photons have bounced clear over to the other half of the sky before getting scattered into your eye). In space, there is no air, and no scattering of light, so unless you are looking right at the light source, you will see no light (and it will look black).

Question 16:
The book described how solar winds from the sun carries material outward into our solar system. Is it possible that this material could reach Earth and be identified as coming from the sun?

In principle, yes, by looking at its composition, but you would have to go above the atmosphere to collect it. Also, the solar wind material is pretty much just protons and electrons (ionized hydrogen) because the more massive atoms have more trouble escaping the Sun. So if you tried it, you'd pretty much just get a bunch of hydrogen (and there are certainly cheaper/easier ways to do it than this!).

Question 17:
How often do Solar Eclipses occur, and whats the difference between a solar and lunar eclipse?

A solar eclipse is when the Moon passes in front of the Sun and eclipses the Sun. A lunar eclipse is when the Moon is eclipsed by the Earth's shadow. Think of it this way: solar eclipse - Sun disapears, lunar eclipse - Moon disapears. There are usually one or two eclipses of both types per year (see the back of your book for a nice table of upcoming eclipses).

Question 18:
If the moon blocks out the sun during an eclipse, than why is it bad for our eyes?

The problem is that not all the light is always blocked off. With the Sun mostly hidden, it becomes darker, and your iris opens up to allow more light to enter. But the parts of the Sun that are visible, are still emitting a lot of light, which your lens will focus right onto your retina, which will cause damage. This is the same reason why lasers are dangerous.

Question 19:
Have astronomers yet discovered a planet with an atmosphere that would be capable of supporting life?

None have been found that will support life as we know it (although Mars comes close). We have not been able to actually resolve any planets around stars other than our Sun, so little is known about the planets around other stars.

Question 20:
Could you explain more about why the temp. of the sun is lower in the interior and hotter as you move toward the exterior.

This isn't quite correct. As you go down in the photosphere, the temperatures increase. By the time you get to the center of the Sun, the temperature is about 15 million Kelvin. As you go out through the photosphere, once you hit the chromosphere, temperatures start to increase again. By the time you hit the corona, the temperature can be as high as 1 million Kelvin.
The reason for the increase is thought to be due to the magnetic field of the Sun. Remember, the atmosphere of the Sun contains many ions (because of the high temperature). These ions and the free electrons can be accelerated to high speeds by the magentic fields. This is almost like the in your washing machine where the aggitator moves back and forth, stirring up the water. Remember, in the corona, the high temperatures really just mean the particles are moving very fast.

Question 21:
There is a hole in the corona because radiation is missing. Why?

You are asking about coronal holes - places in the corona where there is not much high temperature gas. Again, this gets into the Sun's magnetic field (hard to beleive how important it is!). In most of the corona, the hot gas is held there by magnetic fields (the corona is very hot, lots of ions, lots of charged particles, which stick to magnetic fields). Coronal holes then are places where the magnetic field is weaker. Without the strong magentic field to hold the ionzied coronal gas in place, the hot ions (and electrons) will leave the Sun (they are moving fast enough for this), creating the solar wind.

Question 22:
I don't understand the sunspot cycle. I understand how long it lasts and the solar maximum/solar minimum concepts, but I guess what I didn't understand from the reading is what the sun spots actually are and how the cycle works - especially the Zeeman effect (I didn't get that at all!). I hope you will explain this a lot in class! :)

Well, if the explanation of the Zeeman effect from class (and above, see Question 2) doesn't make sense, see me and we can talk more about it.
Its the Zeeman effect that then tells us that the sunspots are associated with strong magnetic fields. What happens is that the differential rotation of the Sun "winds up" the magnetic fields to the point where they start "popping" through the surface. The magnetic fields also inhibit convection, and without the convection (hot gas rising to the top), that part of the photosphere cools - instant sunspot. The sunspot cycle is 11 years (really 22 years) because that is about how long it takes the differential rotation of the Sun to wind the magnetic feilds up to the point where they start popping through the surface.

Question 23:
Could you talk more about why magnetic energy causes the corona to be so hot.

The Sun has a magnetic field, it is also rotating, and the magnetic field rotates with the Sun. Remember electrically charged partciles like to stick to magnetic fields. So, as the Sun rotates, the protons and electrons in the corona are carried along with the field. By the time you get far from the Sun, the fields are moving pretty fast, which accelerates the charges.
Its exactly like inthose ice skating shows when they do the "crack the whip." Think of the line of skaters as the magnetic field. By the time you get far from the surface, out in the corona, the field is moving very fast, like the last person on the whip. And speed = energy of motion = heat.

Questiton 24:
Why does an explosion on the sun cause power outages here on Earth? The book refered to this happening, but it didn't refer to why.

The power outages are caused by all the charged particles from the Sun flodding the Earth. These are moving electrical charges, which create a magnetic field. This magnetic field then pushes/pulls on the Earth's magnetic field, causing it to change. But power lines are nothing but really long wires sitting in the Earth's magnetic field. The changing magnetic field of the Earth causes current (electricity) to be generated in the lines.
If the change in the Earth's magnetic field (due to a flood of charged particles) is big enough, so much current can be generated in the power lines that it overloads the equipment. This is exactly what happened in Canada (and could happen here!).

Question 25:
What holds all the suns elements together?

All of the material that makes up the Sun is in the gas state - it's just too hot for solids and liquids to exist. This gas is held there by the Sun's gravity (which is much much stronger than the Earth's).

Question 26:
In the chapter, it was mentioned that the sun hit a maxima in the year 2001. It also stated that the sun, when it hits it's maxima, will have the effect of increasing temperatures on Earth. Could this be one of the causes for the recent trend in Global warming?

Yes, but not really. At the peak of activity, the Sun is about 0.1% more luminous than during sunspot minimum. The climate changes we may be seening are greater than this. Plus, people would talk about previous "hot cycles" - the last peak was around 1980, and nobody talks about "them hot early 80's" or hot summers and werid weather in the late 1960's - remember this cycle is only 11 years long.
Something else is going on with our recent weather. It's just worrisome because a runaway greenhouse effect is an irreversable process, not a path you want to go down.

Question 27:
When the sun absorbs/emits photons, does it do it simutaneously or is there a pattern?
When I was at UW-Fox Valley astronomers were looking at sunspots through a telescope. How can they not damage their eyes?

Nope, its random. The best you can do is predict a probability that it will happen at a certain color and at a certain depth. The photon that is emitted leaves in a random direction too and the whole process takes a few millionths of a second or so.
There are two ways to do this. One is to use special filters which block out a lot of the Sun's light (they're very very dark filters). The second is to have the sunlight pass through a pin hole and project that light on a screen - the pin hole only lets a tiny amount of light through. This is discussed in your book on page 147.

Question 28:
Do solar bursts increase the heat on the earth any that we as humans can notice the difference?
Also, how were the gentlemen looking at the sunspots so long ago with such limited technology?

No, the bursts to not appreciably change temperatures here on the surface,but they can heat the upper layers of out atmosphere (the ionoshpere) and cause significant changes in it. The worst problem is when the ionosphere is heated, it expands (like any good gas). The trouble is that our sattelities really aren't that high up and when the ionosphere expands. the "air resistance" on them increases, causing them to slow down (and fall to Earth sooner than they otherwise would). The is widely regarded by the people who own and depend on the sattelites as a bad thing.
The gentlemen looking at sunspots (like Galelieo) went blind a lot. But it didn't take people long to realize they could look at an image of the Sun projected through a small pin hole and save their eyes.