Visible light is a thin slice of the electromagnetic spectrum. Cold gas clouds glow in radio waves, dust-wrapped newborn stars in infrared, and the hottest gas around black holes in X-rays. To see the whole universe, astronomers build telescopes for every band, and where they put them depends on what our air lets through.
Why it works
Earth's atmosphere absorbs most wavelengths. The thin upper air soaks up gamma rays and X-rays, and the ozone layer stops most ultraviolet (which is good for living things). Water vapor and carbon dioxide absorb much of the infrared. Only two broad "windows" stay open: visible light (with a little near infrared and ultraviolet around it) and radio waves from about a centimeter to around ten meters. So optical and radio telescopes can sit on the ground, while X-ray, gamma-ray, most ultraviolet and far-infrared telescopes must fly above the atmosphere. Infrared telescopes also need to be cold, or their own warmth glows in the very light they are trying to see.
Key idea
The atmosphere is open to visible light and radio waves, so those telescopes work from the ground. Gamma-ray, X-ray, most ultraviolet and far-infrared telescopes must go to space (some infrared work is possible from high, dry mountains).
| Band | Reaches the ground? | Example telescope | What it sees |
|---|---|---|---|
| Radio | yes | Very Large Array (New Mexico) | cold hydrogen gas, pulsars, jets from black holes |
| Infrared | partly | James Webb Space Telescope (in space) | dusty star nurseries, the first galaxies |
| Visible | yes | Keck telescopes (Hawaii) | stars, galaxies, planets |
| Ultraviolet | no | Hubble (in space) | hot young stars |
| X-ray | no | Chandra X-ray Observatory (in space) | million-degree gas, matter near black holes |
| Gamma ray | no | Fermi Gamma-ray Space Telescope (in space) | exploding stars, gamma-ray bursts |
Worked example
Worked example: Tuning in to hydrogen
Cold hydrogen gas gives off radio waves with wavelength $21$ cm. What is their frequency?
- $f = \frac{c}{\lambda} = \frac{3.00 \times 10^{8}\ \text{m/s}}{0.21\ \text{m}}$.
- $f = 1.4 \times 10^{9}$ Hz $= 1400$ MHz (more precisely, $1420$ MHz).
- Radio telescopes tuned near this frequency map hydrogen gas across our galaxy, right through the dust that blocks visible light. (Waves this short are at the microwave end of the radio band.)
Answer: About $1.4 \times 10^{9}$ Hz.
Common mistake
Two opposite slips: thinking every telescope collects visible light, and thinking radio telescopes need to be in space. Radio waves pass through the air (and through clouds) easily, so radio dishes work on the ground, day and night. It is the short-wavelength telescopes, X-ray and gamma ray, that must fly. Check: your car radio works indoors and in the rain, but you need no protection from the Sun's X-rays, because the air stops them.