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With such a wide range of wavelengths, not all radio waves interact with Earth’s atmosphere in the same way. FM and TV waves are not absorbed and can travel easily through our atmosphere. AM radio waves are absorbed or reflected by a layer in Earth’s atmosphere called the ionosphere (the ionosphere is a layer of charged particles at the top of our atmosphere, produced by interactions with sunlight and charged particles that are ejected from the Sun).
We hope this brief survey has left you with one strong impression: although visible light is what most people associate with astronomy, the light that our eyes can see is only a tiny fraction of the broad range of waves generated in the universe. Today, we understand that judging some astronomical phenomenon by using only the light we can see is like hiding under the table at a big dinner party and judging all the guests by nothing but their shoes. There’s a lot more to each person than meets our eye under the table. It is very important for those who study astronomy today to avoid being “visible light chauvinists”—to respect only the information seen by their eyes while ignoring the information gathered by instruments sensitive to other bands of the electromagnetic spectrum.
[link] summarizes the bands of the electromagnetic spectrum and indicates the temperatures and typical astronomical objects that emit each kind of electromagnetic radiation. While at first, some of the types of radiation listed in the table may seem unfamiliar, you will get to know them better as your astronomy course continues. You can return to this table as you learn more about the types of objects astronomers study.
| Types of Electromagnetic Radiation | |||
|---|---|---|---|
| Type of Radiation | Wavelength Range (nm) | Radiated by Objects at This Temperature | Typical Sources |
| Gamma rays | Less than 0.01 | More than 10 8 K | Produced in nuclear reactions; require very high-energy processes |
| X-rays | 0.01–20 | 10 6 –10 8 K | Gas in clusters of galaxies, supernova remnants, solar corona |
| Ultraviolet | 20–400 | 10 4 –10 6 K | Supernova remnants, very hot stars |
| Visible | 400–700 | 10 3 –10 4 K | Stars |
| Infrared | 10 3 –10 6 | 10–10 3 K | Cool clouds of dust and gas, planets, moons |
| Microwave | 10 6 –10 9 | Less than 10 K | Active galaxies, pulsars, cosmic background radiation |
| Radio | More than 10 9 | Less than 10 K | Supernova remnants, pulsars, cold gas |
Some astronomical objects emit mostly infrared radiation, others mostly visible light, and still others mostly ultraviolet radiation. What determines the type of electromagnetic radiation emitted by the Sun, stars, and other dense astronomical objects? The answer often turns out to be their temperature .
At the microscopic level, everything in nature is in motion. A solid is composed of molecules and atoms in continuous vibration: they move back and forth in place, but their motion is much too small for our eyes to make out. A gas consists of atoms and/or molecules that are flying about freely at high speed, continually bumping into one another and bombarding the surrounding matter. The hotter the solid or gas, the more rapid the motion of its molecules or atoms. The temperature of something is thus a measure of the average motion energy of the particles that make it up.
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