The Sun is Still News
So what is a sunspot?
Scientists use bigger and better telescopes than have been available in the past
to learn more about what sunspots are. George Fisher, a solar astronomer at
the University of California, describes how scientists think of sunspots today: "A
sunspot is a dark part of the sun's surface that is cooler than the surrounding
area. It turns out it is cooler because of a strong magnetic field there that
inhibits the transport of heat via convection in the sun. The magnetic field is
formed below the sun's surface, and continues out into the sun's extended
outer atmosphere, or "corona."
1. Define Sunspot.
2. Why is a Sunspot cooler than the rest of the surface of the sun?
Sunspot anatomy
Sunspots are made up of two parts: a dark, roughly circular central disk called
the umbra, and a lighter outer area called the penumbra. The term "umbra"
means "shade" in Latin, "penumbra" means "almost shade." A single sunspot
has a finite lifetime in which it appears, grows, and gradually disappears. Of
course, we may not observe the disappearance if it happens after the sun has
rotated the sunspot to the sun's opposite side. An average-sized sunspot is as
large as the earth!
3. Define Umbra. What shape is the Umbra generally?
4. Define Penumbra.
The sun is made of about 90% hydrogen and 8% helium, with traces of many
other elements. Over time, the nuclear fusion reactions that fuel the sun's core
are converting hydrogen into helium, changing the ratio of these two elements.
These reactions create a tremendous amount of energy, which appears as light
and heat.
This moving of heat in a flow of matter is convection; it works the same way in
a pot of water on the stove, where a current of water heated at the bottom
carries energy toward the surface. After some of the energy is released in
steam, the cooled water sinks back down, is re-heated, and starts up again in a
circular pattern. The circulating plasma flows in the Sun's convection zone
"rise" (move outward) all over the sun to release heat in the solar atmosphere,
then sinks back inward to be re-heated.
5. How do we know the Sun is producing energy?
6. What material is flowing in the sun’s convective region?
The circulating motion of the convection layer at the sun's surface, or
photosphere, creates a granular pattern in the area outside of the sunspots.
Each granule is a separate flow of hot plasma from the interior. These granules
make the surface of the sun look almost like a patchwork quilt.
The sun, like the earth, generates a magnetic field that extends out into space.
However, the Sun's magnetic field changes both its shape and intensity over
the surface, and over time, much more rapidly.
7. What causes the granules to form?
8. How does the magnetic field of the Earth compare to the magnetic fields
of the Sun?
Magnetic fields from electrons
Have you ever made a magnet by winding wire around a nail and hooking the
ends to a battery? This works because moving electrons, like the electric
current in the wire, generate a magnetic field. The magnetic field lines go
through loops of wire and the nail, and the nail becomes a magnet. Something
similar happens in the Sun.
Coronal loops are places where the magnetic fields of the Sun have broken
through the surface. These magnetic fields are produced in the convective
region of the Sun by flowing layers of plasma. Each loop is a dense bundle of
field lines. Sunspots mark the places where the ends emerge and reenter into
the photosphere.
9. What process starts the formation of Coronal loops?
10. How is the Sun Similar to a nail?
David Dearborn refers to the flow of plasma in the convective region as the
"solar dynamo," saying: "So there you have mechanical motion, and that
mechanical motion is involved in generating the magnetic fields that cause
sunspots." Scientists like Dearborn believe that convection creates the varying
magnetic field at the sun's surface, but the ultimate reasons for each
fluctuation in the flows and fields are not well understood. The sun's rotation is
also an important force. The next generation of solar scientists may unravel the
workings of this immense powerhouse.
11. What do you think the word dynamo means?
12. Describe what goes on during convection and how it possibly relates
to sunspots.
Sunspots are regions of very strong magnetic field, where the field lines get so
crowded together that they push up through the surface, bringing some of the
hot plasma with them in a spectacular arc, or loop. We see the end of the loop
as a sunspot on the sun's visible surface, or photosphere. This dense bundle of
field lines creates huge magnetic pressures. What is magnetic pressure? We
know what pressure is in a gas: if you compress some gas, like squeezing a
balloon, it tries to push out again.
David Dearborn explains, "If you take those places where there are
concentrations of magnetic field and put them together, they have pressure of
their own. You can feel magnetic pressure when you take two magnets and
take the ends of the same polarity and try to put them together. They just don't
want to go together. That's magnetic pressure." The more the magnetic field
lines are scrunched together, the more they want to push apart again.
13. How could we use magnetic pressure to predict when and where solar
flares might occur?
Balance pressures to keep cool
Think of a sunspot as a bubble, surrounded by the gas pressure of the
photosphere, the surface layer that produces light. For the sunspot to exist, its
pressure outward must balance the inward pressure of the region outside.
David Dearborn elaborates on how magnetic fields keep sunspots cooler:
"Outside a sunspot, you have only gas pressure, which depends on the
temperature. In the sunspot you have both gas pressure and magnetic field
pressure combined." Since the pressure must be in balance, magnetic pressure
inside the sunspot allows the gas pressure to remain lower than the areas
outside. This actually slows the convective motion which ordinarily brings hot
matter up from the interior of the sun, making the sunspot cooler.
14. Which two pressures combine to make sunspots cooler than their
surroundings?
15. Describe how this happens?
Fisher says sunspots are still quite hot: "Instead of being about 4300 Kelvin like
the rest of the photosphere, the temperature of a sunspot is more like 3500
Kelvin. But that is still very hot, compared to anything here on earth." This
means the magnetic pressure inside the sunspot is so strong it can cool down a
part of the surface of the sun by almost one third!
Since sunspots are about one third cooler than the surrounding materials, they
are much darker. Dearborn explains, "If you have a piece of gas or iron and you
heat it up and ask how much light it emits, you can measure it. If you then
double the temperature, the amount of light that's emitted is almost eight
times as much." In other words: Small change in temperature = Big change in
brightness. Reverse this process for a hot object being cooled, and it follows
that while sunspots are moderately cooler they are much darker than other
parts of the sun's surface.
16. What is the relationship between light and heat in this paragraph?
17. Is it possible to add only a small amount of heat to an object and
greatly change how bright it is? Why or why not?