Introducing the Sun!

The Sun in Mythology• The Greeks and

Romans believed the Sun was the god Apollo driving a chariot across the sky.

• Egyptians also worshiped their Sun God.

Modern Views of the Sun

By viewing the Sun with different parts of the electromagnetic spectrum, we can see different properties of our star.

The Copernican System• The Sun became the center of the

known universe. • The universe and the Sun proved to be

much larger than once believed.• Then it was discovered that the Sun is

not in the middle of our own galaxy.• It was thought be constant, made of a

pure substance with no internal structures.

The Wobbling Sun

• Newton’s Laws of Motion and Gravitation force us to modify the Copernican viewpoint.

• The Sun and the planets actually revolve around a common center of mass, which in all cases is inside the Sun itself.

• This causes the Sun to wobble a small amount.

Galileo discovered the sun had impurities…sunspots

The sunspots move across the sun’s surface. It rotates!

This layer of the Sun, which we see is the photosphere.

March 25, 2008

Close up of a Sunspot…

UMBRA

PENUMBRA

GRANULES

PhotospherePhotosphere

The Sunspot Cycle

• Soon after Galileo’s discovery, records were kept of sunspots.

• It was discovered that they increase and decrease in a more-or-less orderly fashion.

A BIG Sunspot Graph• Assemble the graphs together to make one

large plot of sunspots from 1700 to 2008. • Choose any five maxima. When were they?

How many years separate them on average?• Do the same for the minima. • How often do you see exceptionally high

maxima? Are they followed by very low minima?

• Predict what the graph will look like for the next six years (2009 - 2018).

How is your prediction?The next sunspot minimum is now ending in 2008 - a very low minimum! There were over 200 days last year without any sunspots! And currently there are NO sunspots

The next maximum is scheduled for 2012, and it should be large.

March 2009

The Butterfly Effect

• Sunspots in both hemispheres of the sun move toward the equator in mirror images.

The Magnetic SunCharged particles flowing through the sun produces magnetic fields. Areas of strong magnetism produce sunspots on the surface.

Horace Babcock

How do we detect magnetic fields?

• Spectral lines are split into triplets in the presence of strong magnetic fields.

• This is known as the Zeeman Effect.

• Sunspots show the Zeeman Effect most strongly.

The Magnetic Cycle matches the Sunspot Cycle

Minimum

Maximum

Differential Rotation

Differential Rotation• The sun rotates faster on the equator than on

the poles.• This is due to the fact that the sun is a fluid.• This differential rotation distorts the magnetic

fields.• When fields are aligned, the sunspots are at

a minimum. When distorted, the number of spots increases.

• This is the basis behind the Babcock Model.

Relationship of Sunspots to the Sun’s Magnetic Field…

Sunspot Groups Act Like Bar Magnets

• Sunspots often form pairs, with one a “North Pole” and the other the “South.”

• The magnetic field connects these two groupings with an arch.

• This is often accompanied by a solar prominence.

Solar Flare 2002

Close-up of the Photosphere

GRANULES

Granulation Animation

What Causes the Granules?

• Hot gases rise in the middle of the granule.

• Cooler gases sink along the edges.

• These are convection currents heated from below.

The Sun’s

Interior:Intense heat in

the core fuels solar

activity.

Hydrogen fusion to Helium

• The sun’s core is 10 million °K.

• At this temperature, hydrogen nuclei fuse into helium

• Huge amounts of energy are released.

Solar Storms can extend thousands of miles into space.

Coronal Mass Ejections• During times of solar

maxima, the sun ejects gas into space.

• They are called coronal mass ejections (CMEs).

• These electrically charged gases can travel far beyond the Sun…solar wind.

Effect on the Earth’s Magnetic Field

AURORA BOREALIS

The ChromosphereThis thin layer just above the photosphere is best seen during a total solar eclipse.

Solar flares, spicules, and prominences are found in this thin gaseous layer.

The Corona: the outermost layer of the Sun

The Corona’s shape is dependent on solar activity below

• The corona is stretched out during sunspot minima.

• Sunspot maxima occur during times of great solar storms.

12

3 45

1 23 4 5

Babcock Model: a SummarySunspot Maximum

Mag-netic fields twisted and dis-torted.

Solar Storms frequent.Watch for auroras!

Corona extends in all directions

Solar wind is strong.

Sunspot Minimum

Mag- netic fields aligned north to south.

Few solar storms

Corona found only near solar equator

Solar wind is calm.

The Solar Spectrum

• A spectroscope pointed at the sun reveals an absorption spectrum.

• The dark lines identify elements, especially hydrogen (above) and helium.

Hot sun

Cool thin gas

How Do We Know that Helium is in the Sun?

• A glowing He fluorescent tube creates the emission spectrum shown on the right.

• These same spectral lines are found on the Sun as well, in fact they were actually seen there first.

• This is why helium means “sun element.”

Hot glowing thin gases give off emission spectra.

The peak of the sun’s radiation is 500 nm500 nm, so…

The Sun is yellow. This means its surface temperature is 5800 °K or 10,000 F.

The General Theory of Relativity

• Albert Einstein proposed his General Theory of Relativity in 1918.

• It stated that gravity is a “dimple” in the space-time continuum.

• Light bends as it goes by the sun, and Mercury’s orbit is also affected.

SUN

Quiz!!• What are the three major layers of the

Sun and what properties do they have?• Explain briefly what Babcock stated

about the role of magnetism in the sunspot and solar storm cycles.

• What is meant by the term differential rotation?

• What phenomenon proved Einstein’s Theory of General Relativity?