Julian Date: 2459040.16
2019-2020: CLXXIV
THE DAILY ASTRONOMER
Thursday, July 9, 2020
Remote Planetarium 62: Interstellar Extinction/Reddening
So often in astronomy the theory is simple, but the practice proves difficult. During our discussion about stars so far we have often mentioned apparent magnitude, the star's apparent brightness. This concept appears at first blush to be beautifully straight forward: look, measure and then write down the number. This method assumes that the star's light is passing through a perfect vacuum. Unfortunately, it isn't. The galaxy contains substantial amounts of gas and dust, most of which is naturally concentrated in the galactic plane. While the average density of interstellar material is only about one atom per cubic centimeter, the cumulative effect of all the dust grains over the vast stretches of space is sufficient to cause both interstellar extinction (the dimming of incoming light) and interstellar reddening (the apparent reddening of incoming light cause by the scattering of blue light).
Below we see an image of an interstellar dust grain. The white line at the lower right equals one micron (one millionth of a meter). The core of such grains generally consists of silicates, iron or carbon while ices of carbon dioxide, water, methane and ammonia comprise the outer "mantle." The surface will often be coated with simple organic compounds.
As the diameters of these dust grains tend to be comparable to the wavelength of blue light, they will tend to either absorb the blue light entirely to scatter it away from the light's pathway.
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Reminder: the visual section of the electromagnetic spectrum.
The visible band represents a minuscule segment of the EM spectrum, which extends from the long wavelength radio waves to the very short wavelength gamma rays. As we can see below, the visible band range extends from the 400 nm (nanometer = one billionth of a meter) to 700 nm. Blue light waves are of higher energy than red waves and so their wavelength is shorter.
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As a consequence of this scattering and absorption, the incoming light from a celestial object will be reduced or reddened. The extent of this extinction is distance dependent. Within the galactic plane, the brightness reduction equals about 1.8 magnitudes per kiloparsec. (A kiloparsec equals 1000 parsecs, or about 3,260 light years.)
The longer wavelength light, toward the reddish end of the spectrum, doesn't interact with these dust grains as directly and so will tend not to be affected by them unless they encounter a local region of unusually high density. Astronomers who observe astronomical objects in the optical region must take these effects into account when measuring the brightness of stars and other celestial objects.
The other complication is that dust concentrations vary along the various directions within the galaxy as well as above and below the galactic plane, where the extinction is at maximum. Fortunately, through infrared observations, astronomers have developed a "dust map" of the Milky Way Galaxy. Infrared observations also enable astronomers to see regions beyond dust clouds. Below, for instance, we see six different images of the dark nebula Barnard 68. The top three images were captured in visible light, the lower three in infrared. We can notice immediately that the infrared images permit us to see objects behind this dark nebula.
As we'll discover, dust is just one of many complications we'll encounter as we proceed. Tomorrow, the next quiz.
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