Reddening and Extinction

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The air also absorbs and scatters electromagnetic radiation by an amount that varies with the wavelength. Redder (longer wavelength) light is scattered less by atmosphere molecules and dust than bluer (shorter wavelength) light. This effect is known as reddening. This effect explains why the Sun appears orange or red when it is close to the horizon. The other colors of sunlight are scattered out of your line of sight so that only the orange and red colors make it through the atmosphere to your eyes. This effect also explains why the sky is blue. Since blue light is scattered more, you will see more blue light scattered back to your eyes when you look in a direction away from the Sun.

why the 
sky is blue and sunsets are orange-red

All wavelengths of light are scattered or absorbed by some amount. This effect is called extinction. Some wavelength bands suffer more extinction than others. Some of the infrared band can be observed from mountains above 2750 meters elevation, because the telescopes are above most of the water vapor in the air that absorbs much of the infrared energy from space. Carbon dioxide also absorbs a lesser amount of the infrared energy. Gamma-rays and X-rays are absorbed by oxygen and nitrogen molecules very high above the surface, so none of this very short wavelength radiation makes it to within 100 kilometers of the surface. The ultraviolet light is absorbed by the oxygen and ozone molecules at altitudes of about 60 kilometers. The longest wavelengths of the radio band are blocked by electrons at altitudes around 200 kilometers.

The atmosphere also scatters light coming from the ground to wash out a lot of the fainter stars and planets in what is called light pollution. As more people move to the cities and the cities get larger, an increasing percentage of people are missing out on the beauty of a star-filled night sky. The increasing light pollution is also threatening the amount and quality of research that can be done at many of the major astronomical observatories. The image below shows how much of the world is now cut off from the night sky. Select the image to bring up a larger version from NASA's Earth Observatory website. Visit the International Dark-Sky Association website for more about light pollution and ways to bring back the night sky.

city lights as seen from space

The Hubble Space Telescope (HST) is able to observe in the ultraviolet, something that ground-based research telescopes cannot do. This is one advantage that HST will always have over ground-based telescopes, even those with adaptive optics. Even though HST has a smaller objective than many ground-based telescopes, its ability to observe in shorter wavelengths will keep its resolving power very competitive with the largest ground-based telescopes with the best adaptive optics. The Hubble Space Telescope can also observe in a broader swath of the infrared than can be done from the ground. Furthermore, HST has no sky glow background (the sky background random noise contamination is less in space) so it is able to easily detect very faint objects against a truly black background---HST can see fainter things than the ground-based telescopes. Another ultraviolet space telescope was the Galaxy Evolution Explorer (GALEX).

Chandra X-ray satellite telescope

Select the image to go to the Chandra X-ray Observatory Center

XMM-Newton X-ray telescope

Select the image to go to the XMM-Newton site

Telescopes used to observe in the high-energy end of the electromagnetic spectrum, like the Chandra X-ray Observatory and XMM-Newton above and NuSTAR at the high-energy end of X-rays, must be put above the atmosphere and require special arrangements of their reflecting surfaces. The extreme ultraviolet and X-rays cannot be focussed using an ordinary mirror because the high-energy photons would bury themselves into the mirror. But if they hit the reflecting surface at a very shallow angle, they will bounce off. Using a series of concentric cone-shaped metal plates, high energy ultraviolet and X-ray photons can be focussed to make an image.

grazing incidence mirrors for high-energy photons

Gamma rays have too high an energy to be focussed with even the shallow angle reflecting technique, so gamma ray telescopes simply point in a desired direction and count the number of photons coming from that direction. Some examples of gamma-ray space observatories are shown below. Clicking on the images will take you to sites describing the telescopes in greater detail.

Swift spacecraft

Swift (above) has a gamma-ray burst detector (BAT) plus a X-ray telescope (XRT) and an ultra-violet/optical telescope (UVOT) to study the gamma-ray bursts in other wavelength bands. NuSTAR (below) is the first orbiting telescope to focus light in the high energy X-ray (6 - 79 keV) region.

NuSTAR telescope can focus the highest energy X-rays

Fermi Gamma-ray Spacecraft in orbit around Earth

Fermi Gamma-ray Space Telescope

On the long wavelength end are the infrared space telescopes such as Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE). The future James Webb Space Telescope (JWST) will be the largest infrared telescope ever put in space with a 6.5-meter primary mirror (it will launch in 2018). Infrared space telescopes are cooled to very low temperatures (just a few degrees above absolute zero) and they observe from behind a sun shield so that the telescope's own internal heat will not interfere with the observations (JWST's sun shield will be the size of a tennis court!). Infrared observations are especially good for studying objects hidden behind thick dust clouds such as forming stars and planets, cool objects such as asteroids, dim stars, and exoplanets, and very distant galaxies, including the first galaxies that formed in the universe.

Spitzer Space Telescope in orbit trailing far from the warm Earth

Spitzer Space Telescope trails far behind the warm Earth and its sun shield (on the left side) blocks the warm sunlight.

WISE against a mid-infrared Milky Way

WISE against a view of the Milky Way as seen in the mid-infrared wavelengths.

Atmospheric lines

Gases in the Earth's atmosphere can introduce extra absorption lines into the spectra of celestial objects. The atmospheric spectral lines must be removed from the spectroscopy data, otherwise astronomers will find a hot star with molecular nitrogen, oxygen and water lines! Such lines are only produced by gases much cooler than that in stars.

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last updated: June 6, 2013

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Author of original content: Nick Strobel