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.
Lets study some of the images displayed in the above matrix.
Visible: White Light: and Visible: Calcium-K
The two images above were obtained at California's Big Bear Solar Observatory. The white light image (above left) is the result of collecting all of the visible light waves (that is, all of the colors in the rainbow). The black-and-white image shows the solar photosphere, with a diameter of about 1.4 million kilometers. [Photos is the Greek word for light.] While the Sun is essentially a very large ball of glowing gas, the photosphere is normally taken to define the solar surface, the region where most of the light originates. The temperature of the photosphere is about 5600-5800 Kelvin (K), or degrees (Celsius) above absolute zero.
The dark splotches on the leftmost (eastern) limb, and scattered elsewhere in the image, are sunspots. These features are cooler than elsewhere in the image, are sunspots. These features are cooler than the surrounding photosphere and are associated with regions of high magnetic fields. Sunspots can be tracked across the face of the Sun as it rotates (with proper viewing precautions, of course!). Whereas the earth rotates every 24 hours, the Sun has a differential rotation period. The rotation periods vary from 25 days at the equator to 36 days at the poles. Sunspot appearances vary on timescales ranging from days to weeks. For a close-up look at a sunspot, click here.
The other black-and-white photograph (above right) is the result of passing the Suns visible light through a narrow-band filter, to selectively detect only light of a particular wavelength. In this case, the Calcium-K filter measures blue light at a wavelength of 393 nanometers (nm). The source of this light is the chromosphere, and is hotter (6000 K to 20,000 K) than the underlying solar photosphere. [Chromos is the Greek word for color.] The (dark) graininess of the photo is due to a solar feature called supergranulation. These convection cells are larger than the diameter of the Earth, and signify that hot gas is being vertically transported within the chromosphere, much like bubbles in a boiling pot of water.
To ascertain what is happening within the white portions of the Calcium-K image, compare their positions on the disk to those of the sunspots seen in the other photograph above (taken a short time earlier). The positions are similar! Apparently, the strong magnetic fields associated with sunspots tend to inhibit the formation of supergranules in the chromosphere.
The color image (above) is taken through another visible-light filter, and was obtained at the Learmonth Solar Observatory in Western Australia. The hydrogen-alpha filter admits red light at a wavelength of 656 nanometers, also originating in the solar chromosphere. The lighter regions again correspond to regions with sunspot activity.
The near-infrared image (above) of our Sun was obtained at a wavelength of 1083 nm (or 1.083 microns) at the National Solar Observatory atop Kitt Peak in Arizona. The darker regions are areas where the gas is cooler and denser, and where some of the IR light is absorbed. If you look closely at the southwest (lower right) limb, you may be able to see a solar prominence. This is an eruption of gas from within the solar corona, the outermost layer of the Sun. In this region, the temperatures reach two million degrees!
UV: Soho: and EUV: SOHO
The ultraviolet (UV) and extreme-UV (EUV) photographs (above), at wavelengths of 19.5 nm and 30.4 nm respectively, were obtained by the Extreme Ultraviolet Imaging Telescope aboard the space-borne Solar and Heliospheric Observatory (SOHO), a collaboration between the European Space Agency and NASA. The UV light originates from the upper chromosphere and lower corona, whereas the EUV light comes from lower regions in the chromosphere. At these wavelengths, we are starting to observe active regions, denoting some of the higher energy phenomena associated with the Sun. These include such features as flares and coronal mass ejections. Lighter regions correspond to the hottest, or most energetic, regions.
The spectacular x-ray image of the Sun was obtained by the Japanese observatory Yohkoh (Sunbeam), a collaborative effort with the US and UK. Launched in 1991, Yohkoh took the above image at wavelengths corresponding to a few nanometers, and these x-rays originate from the Suns corona. Note the amazing variety of coronal loops and streamers, seen in edge-on projections along the solar limb. Darker regions denote cooler areas where the gas is more quiescent (less active) and denser.
We close our solar journey with a quick look at a radio image (above) taken from Japan's Nobeyama Radio Observatory at a wavelength of 1.7 centimeters. As in the other photos, the most active regions are the most luminous.
Would you like to know more about the Sun, see additional images and movies, and try out some interesting classroom activities? Then we suggest you visit YPOP, the Yohkoh Public Outreach Project. This excellent NASA-supported educational site is a collaborative effort between Lockheed Martin Solar and Astrophysics Lab in Palo Alto, California and Montana State University in Bozeman.
Would you like to see the latest images of the Sun? Then visit the SOHO Web site!