Christopher Intagliata reports. The harmful properties of lunar dust are not well known. This is because lunar dust is more chemically reactive and has larger surface areas composed of sharper jagged edges than Earth dust. If the chemical reactive particles are deposited in the lungs, they may cause respiratory disease. The dust grains absorb the ionising UV radiation and protect molecules that have already formed from being destroyed by the radiation field.
Create your free OpenLearn profile. Course content Course content. Comparing stars Start this free course now. Free course Comparing stars. View larger image. Figure 20 The effect of interstellar dust on radiation from a star. Spectrum b is seen by an observer looking at the star through the dust cloud, while spectrum c is seen by an observer looking at the dust cloud against a star-less background.
Figure 21 Extinction by interstellar dust: the top curve is the sum of the other two. The extinction is measured in magnitudes per unit distance usually per kiloparsec. Question 6 A star like our Sun is located in a star cluster at a known large distance and is subject to significant interstellar extinction. Previous 2.
Next 2. Print Print. Take your learning further Making the decision to study can be a big step, which is why you'll want a trusted University. OpenLearn Search website Back to top. Our partners OpenLearn works with other organisations by providing free courses and resources that support our mission of opening up educational opportunities to more people in more places. Find out more Support us. Such a cloud of dust, illuminated by starlight, is called a reflection nebula , since the light we see is starlight reflected off the grains of dust.
One of the best-known examples is the nebulosity around each of the brightest stars in the Pleiades cluster. The dust grains are small, and such small particles turn out to scatter light with blue wavelengths more efficiently than light at red wavelengths. A reflection nebula, therefore, usually appears bluer than its illuminating star Figure 5. Gas and dust are generally intermixed in space, although the proportions are not exactly the same everywhere.
The presence of dust is apparent in many photographs of emission nebulae in the constellation of Sagittarius, where we see an H II region surrounded by a blue reflection nebula. Which type of nebula appears brighter depends on the kinds of stars that cause the gas and dust to glow. Stars cooler than about 25, K have so little ultraviolet radiation of wavelengths shorter than Stars hotter than 25, K emit enough ultraviolet energy that the emission nebulae produced around them generally outshine the reflection nebulae.
The tiny interstellar dust grains absorb some of the starlight they intercept. But at least half of the starlight that interacts with a grain is merely scattered, that is, it is redirected rather than absorbed. Since neither the absorbed nor the scattered starlight reaches us directly, both absorption and scattering make stars look dimmer. The effects of both processes are called interstellar extinction Figure 6.
Astronomers first came to understand interstellar extinction around the early s, as the explanation of a puzzling observation.
In the early part of the twentieth century, astronomers discovered that some stars look red even though their spectral lines indicate that they must be extremely hot and thus should look blue. The solution to this seeming contradiction turned out to be that the light from these hot stars is not only dimmed but also reddened by interstellar dust, a phenomenon known as interstellar reddening. Figure 6. Barnard 68 in Infrared: In this image, we see Barnard 68, the same object shown in [link].
The difference is that, in the previous image, the blue, green, and red channels showed light in the visible or very nearly visible part of the spectrum. In this image, the red color shows radiation emitted in the infrared at a wavelength of 2. Interstellar extinction is much smaller at infrared than at visible wavelengths, so the stars behind the cloud become visible in the infrared channel.
Scattering of Light by Dust: Interstellar dust scatters blue light more efficiently than red light, thereby making distant stars appear redder and giving clouds of dust near stars a bluish hue. Here, a red ray of light from a star comes straight through to the observer, whereas a blue ray is shown scattering. Dust does not interact with all the colors of light the same way. Much of the violet, blue, and green light from these stars has been scattered or absorbed by dust, so it does not reach Earth.
Some of their orange and red light, with longer wavelengths, on the other hand, more easily penetrates the intervening dust and completes its long journey through space to enter Earth-based telescopes Figure 7. Thus, the star looks redder from Earth than it would if you could see it from nearby. We have all seen an example of reddening on Earth. The Sun appears much redder at sunset than it does at noon.
The lower the Sun is in the sky, the longer the path its light must travel through the atmosphere. Over this greater distance, there is a greater chance that sunlight will be scattered. Since red light is less likely to be scattered than blue light, the Sun appears more and more red as it approaches the horizon.
As sunlight comes in, it scatters from the molecules of air. We therefore see these objects as dimmer and redder than they really are. These effects are known as extinction and interstellar reddening respectively. Dust grains in the interstellar medium have a typical size that is comparable to the wavelength of blue light.
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