Because the Sun is such a close star, we can easily see features on its surface.  One of the things this allows us to measure directly is the Sun's rotation period, as Galileo did in the 17th century.

In this lab you will determine the Sun's rotation period using data from an Earth-orbiting observatory. Other stars, on the other hand, are much further away than the Sun. Only in exceptional circumstances can we study surface features on other stars. In the vast majority of the cases, stars are just points of light to our instruments because of their tremendous distances from the Earth. Consequently, the only way to study these stars is by analyzing the light they give off. Using atomic spectral lines, for example, we can determine the spectral types of stars (and therefore their colors and temperatures). We can also measure how much light they give off and from that determine their brightness.

To understand why most stars cannot be studied like the Sun, let us consider what would happen if we moved the Sun to the distance of the next nearest star. The next nearest star, Proxima Centauri, is just over a parsec away, so let us use 1 parsec for the sake of simplicity. You may recall from the parallax lab that there are 206265 astronomical units (AU) in a parsec. Therefore, by moving the Sun to that distance we are making it appear 206265 times smaller than it appears now. At that size, you can well imagine that the Sun would appear only as a point of light.

The Sun would also be a lot fainter and would appear as just another star in the sky. In this lab we will calculate the rotation period of the Sun and examine its properties when viewed from a distance (as just another star). In the latter case we will compare the Sun's properties with those of other stars. In the process, we will learn how stars live out their lives.

Much solar research is carried out at the National Solar Observatories.  One of their telescopes is located in the Sacramento Mountains of southern New Mexico.  To see the latest solar images:  Link to NSO at Sunspot New Mexico

Image of Dunn Solar Telescope provided by: NSO/AURA/NSF

The Rotation Period of the Sun

In this portion of the lab we will calculate how long it takes the Sun to rotate once on its axis. We will do this by using observations of the Sun made by a very sophisticated telescope named SOHO (The Solar and Heliospheric Observatory). SOHO is orbiting the Earth and taking images of the Sun at optical and ultraviolet portions of the spectrum. SOHO's images are downloaded to an archive about four times a day. Click on the SOHO link below to begin. You will see a vertical strip of solar images taken at different times. Click on the top image and then print it. You will also need to print a second image from the same strip. Choose an image that shows the sunspot traveling half way or all the way across the face of the sun to make it easier.    If that is not possible use an image that was taken several days earlier.  We will find the time that it takes the sunspot to travel this far around the Sun, and then extrapolate to find the time for it to go around the Sun once. (NOTE: Instead of printing the images, you can open them in new windows and then use the JRuler, which you can find in the START menu.)

SOHO LINK (If there are no visible sunspots go to ARCHIVE)

Use the following recipe if you weren't able to find a convenient (1/4 rotation or 1/2 rotation) sun spot movement.

1. Measure the diameter (D) of the Sun. Choose your own units and stick with them.
   
   
2. Estimate the Sun's circumference (S) in the same units using the formula S = π x D.
   
   
3. Estimate the time interval (T) between the two images.
   
   
4. Now measure the distance (L) that the sunspots have traveled during the above time interval.
   
   
5. Calculate the rotation period (P) using the above numbers and the formula P = S x T / L. Remember to use consistent units.
   
   
6. Think about the answer you obtained and compare it to the rotation period of the Earth. Does the answer surprise you?
   
   As we stated earlier, the Sun is unique in that we can study its atmospheric features quite easily owing to its proximity. The rotation period is an example of what can be learned. Next we will study and compare with the Sun two observable properties of stars that offer tremendous insight into their structure and evolution: their spectral type and luminosity.