Солнечная система и ее тайны

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Absorption

In the Spectroscopy section, we discussed how various elements absorb different frequencies of light. We talked about how useful that is for us, in that it allows us to determine what distant objects are made from without needing to travel to them. Now it's time to talk about the down side of spectral lines: they block our view.

Light from distant stars has a broad range of colors. Try out the simulation below to see two examples, hydrogen and oxygen. You can see how much light is absorbed in each color, and also see the change in the electron energy levels.

We are examining medium yellow light shining through hydrogen gas in an interstellar cloud.

The energy of yellow light does not correspond to an energy level difference in hydrogen atoms, so the electrons in the atoms do not react. The light passes through with no loss of intensity.

Earth's atmosphere, interstellar clouds, even the tenuous gas floating in space, all of these can block and absorb light of different frequencies. And more processes than just absorption are at play: light can also "scatter" from atoms and molecules, making our view fuzzier and reducing the amount of light that reaches us.

Our atmosphere allows visible light and some radio waves through very well. It scatters and absorbs high-energy ultraviolet light, X-rays, and gamma rays (which is definitely good for our health!). On the other side, it also absorbs much of the infrared spectrum and long-wavelength radio waves. You can see that a lot of different kinds of light can't make it through our atmosphere.

Each sort of interstellar cloud also has its own profile of light that it blocks, which explains the dark clouds on the image shown below. Looking in the right frequencies can sometimes let us see right through these clouds. Take a look at the image below, which shows the same nebula in a different color of light. The dark clouds are almost transparent, and the brighter green dust is completely see-through in the infrared.

Orbiting satellites are very important for this purpose. We are no longer limited to those kinds of light that can pass through our atmosphere. When looking at distant stars and nebulae we can concentrate on avoiding the colors that are blocked by interstellar clouds, or we can specifically look in those frequencies to better understand the clouds themselves. Orbiting telescopes open up a lot of opportunities that aren't available to Earth-bound telescopes.

Солнечная система и ее тайны