Multiwavelength Sun Exercise

In this exercise, you will be presented with background information on the topic of interest, including lecture notes. After reviewing that material, you should download and print out the PDF of the problem. Finally with the problem in front of you, open up the image tool and use it to analyse the images in order to answer the questions on the PDF. Write on the printout of the problem and bring it to class.




The Sun that dominates our daytime sky and provides the thermal (heat) energy necessary to support life on Earth is of course -- a star. Its proximity to Earth, at about 150 million kilometers, allows us to study its surface and to characterize this average star with unprecedented detail. In terms of the speed of light, the Sun is about 8-1/3 light minutes from us. The next nearest star is about 4 light years away! The Suns diameter is about half a degree, the same as the full moon.

Because of its high luminosity, astronomers often use specialized telescopes to study the Sun. Hence, the sources of images for this particular gallery are different from most of the others in our online Museum.

Multiwavelength Sun: Radio Wavelengths

Sun in Radio (NMA)

Microwave and radio wave are the names given to light with wavelengths from about 1 millimeter (1 million nanometers) to more than 10 meters. (Compare that to the wavelengths of X-rays.) The Sun is the brightest source of radio waves in the sky. Radio waves penetrate through the outer layers of solar gas, called the "chromosphere" and "corona". The depth to which the radio waves and microwaves can penetrate depends on their exact wavelength. The image here is constructed from microwaves with wavelength 1.7 centimeters. It shows us the structure of the Sun's atmosphere near the "transition region" between the chromosphere and the corona, about 2000-2200 km above the photosphere. If the Sun has spots today, you might be able to see bright active regions in this microwave image (active regions are associated with sunspots). Also you can sometimes see prominences -- great strands of gas that extend above the edge of the Sun.

This radio image comes from the Nobeyama Radio Observatory, in Japan.

Sun in Radio (VLA)

When most people think about the Sun, they usually don't have any idea that it is a strong source of radio waves. The picture shows an image of the full disk of the Sun at a frequency of 4.6 GHz made with the Very Large Array radiotelescope in New Mexico. The resolution of the image is 12 arcseconds, or about 8400 km on the surface of the Sun. The brightest features (red) in this image have a temperature of 1 million degrees (the radio brightness temperature measures a true temperature in the Sun's atmosphere) and show where very strong magnetic fields exist in the Sun's atmosphere. An optical image on this day shows sunspots under these features. The green features are not as hot, but show where the Sun's atmosphere is very dense. The disk of the Sun is at a temperature of 30000 degrees, and the dark blue features are cooler yet. The giant slash across the bottom of the disk in this image is a feature called a filament channel, where the Sun's atmosphere is very thin: it marks the boundary of the South Pole of the Sun on this day. Another interesting feature of the radio Sun is that it is bigger than the optical Sun: the solar limb in this image is about 20000 km above the optical limb.

Credit: Image courtesy of NRAO/AUI and Image courtesy of Stephen White, University of Maryland, and of NRAO/AUI.

Multiwavelength Sun: Infrared Wavelengths

Sun in Infrared

Did you know your body emits infrared light? Infrared light is heat, the same as the heat your body gives off. More than half the Sun's power output is in the form of infrared light, though much of it is absorbed by the Earth's atmosphere. The picture here is made from light with a wavelength of 1083 nanometers. (That's a little more than a thousandth of a millimeter.) It shows some features of the Sun's chromosphere, and some features in the corona.

Infrared pictures often show dark markings on the Sun that are caused by absorption of the infrared light. Some of the light is absorbed wherever it collides with gas in the Sun's atmosphere, so the darker features in an infrared picture show where the gas is more dense. If there are filaments on the Sun, or loops near active regions, they typically show up dark. The coronal holes in the north and south poles typically show up as slightly brighter than the rest of the solar disk. Compare this to the appearance of the north and south poles in the X-ray pictures.

This picture comes from the National Solar Observatory at Kitt Peak, in Arizona.

Multiwavelength Sun: Optical Wavelengths

Sun in Optical

This image was taken in "White Light." White light pictures show how the Sun appears to the naked eye, when all the colors of the rainbow are collected by the camera. In a white light photo, the part of the Sun that we see is called the "photosphere". It has a temperature of about 6000 degrees Celsius, much cooler than the corona. Sometimes there are notable dark spots. These "sunspots" come and go, so look at the Sun again in a few days to see how its appearance has changed.

The white-light image comes from Big Bear Solar Observatory in California, which is operated by the New Jersey Institute of Technology.

Multiwavelength Sun: Extreme UV (304-A)

Sun in UV (304 A)

This image was made from light with a wavelength of 304-Angstrom. These are in the realm of the Extreme UltraViolet (EUV). The 304-A picture shows a portion of the layer of the Sun's atmosphere known as the chromosphere. In the 304-A image we can sometimes see large prominences rising high above the surface of the Sun. At the north and south poles of the Sun, less EUV light is emitted -- these regions often end up looking dark in the pictures, giving rise to the term "coronal holes." The darkness of these polar regions can sometimes also be seen in images made with X-rays. If you do not see these features in the images above, check back in a few days to see how the Sun's appearance has changed.

These images come from the Extreme ultraviolet Imaging Telescope (EIT), an instrument on the SOlar and Heliospheric Observatory (SOHO). SOHO is a spacecraft orbiting a million miles above the Earth, at the point where the Earth's and the Sun's gravity cancel each other out.

Multiwavelength Sun: Extreme UV (195-A)

Sun in UV (195 A)

This image was taken in light with a wavelength of 195 Angstrom, in the realm of the Extreme UltraViolet (EUV). The 195-A picture shows a part of the chromosphere higher up than what is seen in the 304-A picture. Most of the 195-A light comes from "active regions", where we can sometimes see loops.

This image come from the Extreme ultraviolet Imaging Telescope (EIT), an instrument on the SOlar and Heliospheric Observatory (SOHO). SOHO is a spacecraft orbiting a million miles above the Earth, at the point where the Earth's and the Sun's gravity cancel each other out.

Multiwavelength Sun: X-Rays

Sun in X-rays

The term "X-Rays" refers to light with wavelengths shorter than about 10 nanometers. (That's only 10 billionths of a meter! Compare this to the wavelengths of radio waves.) This picture was taken with a camera that sees light with wavelengths between about 0.3 and 4.5 nanometers, the so-called "Soft X-rays." The X-rays we see all come from the corona, the outermost and hottest visible layer of the Sun's atmosphere.

Not all the corona emits the same amount of X-rays. We often see structures called "loops" and "arches" and "streamers". Some of these are found around active regions, while some are occasionally found in more quiet parts of the corona. Movies made from X-ray pictures show that the corona is a very stormy place, constantly changing and erupting. Look at the Sun again a few days from now to see how it has changed. In X-ray images you can sometimes see the "coronal holes," regions of low brightness at the north and south poles. The coronal holes are also sometimes seen in the Extreme Ultraviolet images.

This picture of the Sun (taken Dec. 2001) comes from the Soft X-Ray Telescope on the Yohkoh spacecraft, orbiting about 630 kilometers above the Earth. NOTE: The Yohkoh solar observatory is currently offline.

PDF of Assignment

Print out the following PDF file, print out and answer using the interactive tool below.

PDF of Assignment

Multiwavelength Sun: Interactive Comparison

Open Image Analysis Tool