Getting Started
On a dark, clear night, far from cities or other sources of light, the Andromeda galaxy (M31), can be seen with the naked eye as a faint, fuzzy patch on the sky, comparable in diameter to the full Moon. Yet the fact that it is visible from Earth belies this galaxy’s enormous distance from us: It lies roughly 2.5 million light-years away.
An object at such a distance is truly inaccessible in any realistic human sense. Even if a space probe could miraculously travel at the speed of light, it would need 2.5 million years to reach this galaxy and 2.5 million more to return with its findings. Considering that civilization has existed on Earth for less than 10,000 years, and its prospects for the next 10,000 are far from certain, even this unattainable technological feat would not provide us with a practical means of exploring other galaxies. Even the farthest reaches of our own galaxy, “only” a few tens of thousands of light-years distant, are effectively off limits to visitors from Earth, at least for the foreseeable future.
Given the practical impossibility of traveling to such remote parts of the universe, how do astronomers know anything about objects far from Earth? How do we obtain detailed information about planets, stars, or galaxies too distant for a personal visit or any kind of controlled experiment? The answer is that we use the laws of physics, as we know them here on Earth, to interpret the electromagnetic radiation emitted by those objects.
Light and Radiation
Radiation is any way in which energy is transmitted through space from one point to another without the need for any physical connection between the two locations. The term electromagnetic just means that the energy is carried in the form of rapidly fluctuating electric and magnetic fields.
Virtually all we know about the universe beyond Earth’s atmosphere has been gleaned from painstaking analysis of electromagnetic radiation received from afar. Our understanding depends completely on our ability to decipher this steady stream of data reaching us from space.
Electromagnetic Spectrum
These are all electromagnetic waves. From low-frequency radio, for communication and microwave for, well, microwaves all the way up to high-frequency ultraviolet for suntans (and burns), X-rays, for medical purposes and gamma rays which are usually formed by radioactive elements and damage any living things nearby.
Note
Despite the different names, the words light, rays, radiation, and (electromagnetic) waves all really refer to the same thing. The names are just historical accidents, reflecting the fact that it took many years for scientists to realize that these apparently very different types of radiation are in reality one and the same physical phenomenon.
Remember that, despite the wavelengths being vastly different, they are really all the same thing, and all move at the same speed (the speed of light \(c\))
Only a small fraction of the radiation produced by astronomical objects actually reaches Earth's surface, because of the opacity of Earth's atmosphere. Opacity is the extent to which radiation is blocked by the material that it is travelling through. As you can see, the atmosphere is opaque to most wavelengths, with a window in the visible, near-infrared and radio wavelengths. The atmosphere is also partially transparent to some microwave wavelengths.
Questions
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Compared to UV, infrared has a greater:
a. wavelength
b. amplitude
c. frequency
d. energy
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Compared with red light, blue wavelengths of visible light travel:
a. faster
b. slower
c. at the same speed.
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An X-ray telescope located in Antarctica would not work well because of
a. the extreme cold
b. the ozone hole
c. continuous daylight
d. Earth’s atmosphere
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a
-
c
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d