The Sun


Your goal with this lab is learn about the general visible features of the Sun, as well as the multi-year sun spot cycle.

This lab will use images taken by NASA/ESA's SOHO spacecraft.

Pay close attention to the wavelength of light that was imaged, assume nothing about the pictures' origins!

Part 1:  Surface Activity

The Sun is a star, and stars are giant burning orbs plopped in the void of space.  They are very dynamic objects.


The surface of the Sun shows granulation, caused by convection zones just below the Sun's apparent surface. The following GIF is a time-lapsed view of the Sun's boiling surface over the period of an hour or so.


The following image shows what's going on under the surface to cause granulation:  Hot gas rises to the surface, and as it cools falls back down in columns shared by neighboring cells.  This leads to the surface being broken up into cells.

Sun Spots

The Sun sometimes forms dark spots on its surface.  These are aptly named Sun Spots, and they appear dark because they are about 2000 K cooler than the 5772 K average temperature of the surface of the Sun.  Here is what they look like in the visible spectrum.  (Note the granulation around the sunspot.)


Sunspots form when magnetic field lines originating deep within the Sun penetrate through the surface, limiting the rate of convection and keeping that part of the Sun cooler than its surroundings.

Take a look:


In different wavelengths, Sunspots can actually appear bright!  Even though they result in cooler surface temperatures, those powerful magnetic fields create very high energy disturbances in the chromosphere, resulting in very bright UV emissions.  Take a look at the following image to see how sunspots appear dark in light from the surface, but bright in light from the chromosphere.

The image on the left is 400-800 nm (visible) light.  The image on the right is the same Sun but the light captured has a wavelength of 30.4 nm, which is deep UV.


These outbursts of energy lead to our next topic, prominences and filaments, but to understand those, here is another example of what's going on:


Prominences, Solar Flares, and Filaments

When magnetic field lines undergo what is called magnetic reconnection, a huge blast of energy is shot out away from the Sun.  These are known as Solar Flares.  Here is the image of a solar flare taken on August 31st, 2012:


A solar prominence occurs when the Sun's magnetic field manages to hold onto the material, extending out into space thousands of kilometers.  There is no ejection of material like in a flare.  When one of these large arcs is viewed coming off the side of the Sun, it is called a solar prominence.  When the arc is viewed over the Sun itself, it appears to create a shadow and is renamed a filament.  So a filament is just a prominence when viewed across the Sun's surface.  Take a look at the following image of a massive solar prominence.


Solar and Heliospheric Observatory

NASA and the European Space Agency  teamed up to put a probe into space.  The probe is located at the Earth-Sun L1 Lagrange Point.  (Remember what Lagrange Points are from the Asteroids lab?)

Every 12 hours a new set of images are taken and published to this website.

Once you get there, you can click on "About these images" to learn more about the instruments that take those 8 images.  

You can also select "More 512x512" underneath each image to see the last 2 weeks of images.

Part 2: Differential Solar Rotation

For the first half of Part 2: Slow Your Roll, Sun, you will estimate how long it takes the Sun to rotate. In other words: what is the length of a solar day.  This will be approximate, but reasonably close.  You have two images of the Sun taken 24 hours apart.  You will measure the distance that a Sunspot near the center of the Sun moves over that 24 hour period.

Recall that:

Rate x Time = Distance.

Your rate is going to be

Rate = ∆X/24 hours

Your first step is to measure ∆X, that's the distance the sunspot traveled in 24 hours.

The distance is going to be π x Diameter.

Distance = πD

The diameter here is the diameter of the Sun on your paper - measure its width of the Sun at the latitude the Sunspot is moving.  (Why is the distance going to be πD?  Is it because the Sun is a sphere?)

Once you have those two measurements, you can solve for Time.  This time will be the time it takes to travel around the Sun.  For example, if the sunspot travels 100 pixels per day and needs to travel 4000 pixels, then 4000pixels* 1 day/100 pixels = 400 days.

After estimating the time it takes the Sun to rotate near its equator, you'll find images from SOHO to look for a spot near the poles of the Sun.  You should find that the Sun rotates faster near the equator, and more slowly near the poles.

Part 3:  The Solar Cycle and Sunspots 

The Sun goes through a 22 year solar cycle. Every 11 years the Sun's magnetic field flips poles.  

N to S, then S to N, then back to N to S.

It's unknown exactly why the Sun flips its magnetic field every 11 years, and some think it could be because every 11 years Jupiter, Earth, and Venus are all lined up.  When this happens, there might be enough tidal disruption from the planets' gravity to cause the Sun's dynamo to freak out and flip around.

In any case, take a look at the following images, from 1996 - 2006 to answer your questions in Part 3.