The Sun in Visible Light


White (unfiltered) light	Calcium-K (filtered) light	Hydrogen-alpha (filtered) light

What we call "visible light" can be broken into two categories: white light, and filtered 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. Filtered light refers to the selection of specific colors by using a filter. This is sometimes done to reveal features of the Sun that get lost in white light pictures. For more discussion, including a hands-on activity, check out the "Image Filtering" lesson in the YPOP Solar Classroom.

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.

A filtered image often can show features on the Sun that are harder to see in the glare of unfiltered light. Two common filters for observing the Sun are the "Hydrogen-alpha" and the "Calcium-K" filters. They allow light that is within the visible part of the spectrum, but is tough to see without filters. H-alpha filters allow the camera to see only red light with a wavelength of 656.3 nanometers; and Calcium-K filters allow a camera to see only blue light with a wavelength of 393.4 nanometers. These particular colors of sunlight come not from the Sun's photosphere, but from the next highest layer, called the "chromosphere." We normally can't see the chromosphere in white light pictures, because it is so much fainter than the photosphere that the photosphere "washes it out." It's like trying to see a candle held in front of a much brighter spotlight. The temperature of the chromosphere varies between 6000 and 20000 degrees Celsius. Sometimes long, thin, dark ribbon-like structures can be seen -- these are called "filaments" when they're seen on the disk part of the Sun, and "prominences" when they're seen extending beyond the edge of the limb.

The white-light and Calcium-K images comes from Big Bear Solar Observatory in California, which is operated by the New Jersey Institute of Technology. The H-alpha image comes from Learmonth Solar Observatory in Western Australia.

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