Radio, heat, colour, X-rays: all of it is light, the same ripple of electricity and magnetism, moving at the same speed. The only difference is the length of one wave. Each numbered step on this dial is ten times shorter than the one before. There are twenty steps.
A shorter wave beats faster, and carries more energy in each packet (a photon). So the dial has three scales that always move together: the length of a wave, its frequency in beats per second (hertz), and the energy of one photon (electron-volts). Halve the first and the other two double.
The sky up there starts black on purpose. The thin gas streaming out of the Sun turns back any radio wave from space longer than about 10 km, so no telescope has ever mapped it. On Earth these waves are handy: they follow the curve of the planet and reach submarines under the sea.
The ionosphere, a layer of electrically charged air that begins about 60 km up, reflects radio waves longer than roughly 20 m. That is why AM radio carries beyond the horizon, especially at night. It is also why nothing this long reaches the ground from space.
In 1932 Karl Jansky, hunting the source of static on telephone links, found a hiss at 14.6 m that rose and set with the stars. It came from the centre of the Galaxy. What glows up there: electrons moving at almost the speed of light, steered in spirals by the Galaxy’s magnetic field.
FM radio sits near 3 m. All the stations arrive at your aerial together; the radio picks one frequency out. This is a “waterfall”, the way engineers watch a band: frequency across, time running down, colour for strength. The radio sky above is painted in the same colours.
How sharply a telescope sees depends on how many waves fit across it. Your eye’s pupil is about 10,000 waves of light wide. The 76 m dish that helped make the best-known whole-sky radio map, at 73 cm, is about 100 waves wide. The sky above is blurred the way that dish blurs it.
Cold hydrogen atoms, the commonest thing in the universe, give off radio waves at exactly 21.1 cm. Found in 1951, that single line let astronomers trace the Galaxy’s spiral arms straight through the dust that hides them from ordinary telescopes.
A microwave oven and a Wi-Fi router use nearly the same waves, about 12 cm long. The oven is some 10,000 times stronger and shut in a metal box. You can see in because the holes in the door, about 1 mm wide, are far too small for a 12 cm wave to squeeze through. Light is 2,000 times smaller than the holes.
From about 20 m down to about 1 cm the air is almost perfectly clear to radio, by day and through cloud. Shorter than that, water vapour and oxygen start to absorb. Weather radar works just inside the clear part: raindrops scatter centimetre waves straight back.
This mottled glow comes from every direction. It was set free about 380,000 years after the Big Bang, when the universe first turned clear, and has been stretched about 1,100 times since. The colours are differences of a few parts in 100,000. The band across the middle is our own Galaxy, in the way.
Anything warmer than absolute zero gives off light, and the cooler it is, the longer the waves. Dust between the stars sits near −255 °C and glows at about a sixth of a millimetre. The dark lanes of the visible Milky Way are the brightest thing in this sky: the same dust, seen by its own warmth.
The S-shaped ribbon is not in the Galaxy at all. It is dust inside the solar system, shed by comets and asteroids, lying in the plane the planets orbit in and warmed by the Sun. It looks bent only because this map is laid out along the Milky Way, which is tilted 60 degrees to that plane.
You shine too, most strongly near 10 µm, and a thermal camera reads that glow as temperature. William Herschel found infrared in 1800 by laying a thermometer just past the red end of a rainbow and watching it warm. Glass blocks these waves, which is why spectacles look cold and black.
Just beyond red, waves slip past the dust grains that stop visible light. This is the Galaxy as it is built: a thin disc of stars and a swollen middle, seen from inside. A TV remote flashes at 0.94 µm, and the internet runs through glass fibre at 1.55 µm.
Every colour you know fits in the thin rainbow stripe on the dial. It is where the Sun shines brightest and where air and water are both clear, so it is the light that eyes evolved to use.
Animals cut the stripe differently. Bees see ultraviolet but not red. Many birds have a fourth kind of colour cell, tuned to ultraviolet. Dogs have two kinds where we have three. Pit vipers sense infrared heat with pits on their faces: a second kind of sight altogether.
Each photon now carries enough energy to snap a chemical bond, which is how ultraviolet burns skin. Ozone high in the air soaks up nearly everything shorter than about 300 nm. Some materials swallow ultraviolet and hand the energy back as visible light: the glow under a blacklight.
Only stars much hotter than the Sun shine here. They burn out fast, so this sky shows where stars were born lately. Then, at 91 nm, a photon can knock the electron off a hydrogen atom, and hydrogen is everywhere. Space turns foggy: the light is still made, but swallowed on the way.
The fog clears once photons are too energetic for hydrogen to catch. It takes gas at a million degrees or more to glow in X-rays: the wreckage of exploded stars, and a hot halo round the Galaxy. The two great shells above and below the centre were mapped in full in 2020. Cold gas in the plane casts a shadow.
These waves are smaller than atoms. They pass between the light atoms of skin and muscle and are stopped by heavier ones, such as the calcium in bone, so a shadow picture shows the skeleton. Wilhelm Röntgen made the first in 1895: his wife’s hand, with her ring.
The X-ray sky is points. Almost every one is gas spiralling onto a neutron star or a black hole, heated to tens of millions of degrees on the way down. The brightest, Scorpius X-1, was found in 1962 by a rocket that spent about five minutes above the air.
Gamma rays come from atomic nuclei and from violent collisions. One exact wavelength, 2.43 picometres, is what you get when an electron meets its antimatter twin and both vanish. The middle of the Galaxy glows at precisely that wavelength, and nobody has settled where the antimatter comes from.
Nothing focuses a gamma ray: it goes straight through any mirror or lens. A gamma-ray telescope is a block of detector that records each photon as a single hit and works out roughly where it came from. The picture above is turning into those hits, and it is blurry again.
The bright stripe is the Galaxy’s gas, lit from within. Protons flying at almost the speed of light, called cosmic rays, strike hydrogen between the stars, and each collision ends in gamma rays. The sharp points are spinning neutron stars, and distant galaxies firing jets straight at us.
One wave is now a fraction of a proton across, and one photon carries billions of times the energy of a photon of visible light. They are so few that the sky breaks into single dots. Two bubbles show above and below the Galaxy’s centre, each about 25,000 light-years tall, found in 2010.
One sky, the whole Milky Way, drawn in every kind of light: radio on the left, gamma rays on the right. The thin bright line is all that human eyes can see.
All of this light arrives all the time, day and night. Eyes catch the stripe. The rest took instruments, and nearly all of them were built in the last hundred years.
Each picture is the whole sky with the Milky Way across the middle and the centre of the Galaxy in the centre. Nothing moves between them. Only the wavelength changes.
The sky on this page is a model, not a photograph. A simplified Milky Way (a disc of stars, a central bar, spiral arms, dust, glowing gas, hot gas, cosmic rays, about 150 named objects) is built in three dimensions and viewed from where the Sun sits. Each stretch of the dial then lights the parts that really shine there, in the colours the real instruments’ maps are usually printed in. It is drawn to resemble those maps; the real ones are linked below and are better.
Each stretch has its own exposure, as every real map does, so brightness cannot be compared between bands. The radio sky is blurred by the true ratio of wavelength to dish width. The curve for what the air blocks is a sketch for a clear, dry sky at sea level.
| Band | One wave | What makes it | What catches it | Through the air? | Everyday |
|---|---|---|---|---|---|
| Radio | longer than 1 m | Electric currents; fast electrons in magnetic fields | Aerials and dishes | Yes, from about 20 m down | AM and FM radio, TV, radio-set clocks |
| Microwave | 1 m to 1 mm | Circuits; cold hydrogen; the early universe | Dishes and horn aerials | Yes to about 1 cm, then patchy | Phones, Wi-Fi, GPS, radar, ovens |
| Infrared | 1 mm to 750 nm | Anything warm; cool stars; dust | Chilled electronic sensors | Mostly no; a few gaps | Thermal cameras, remotes, fibre internet |
| Visible | 750 to 380 nm | Things at thousands of degrees; atoms | Eyes, film, camera chips | Yes | Everything you see |
| Ultraviolet | 380 to 10 nm | Very hot stars; electric arcs | Coated camera chips, in orbit | Only the longest part | Sunburn, blacklights, germ-killing lamps |
| X-ray | 10 nm to 10 pm | Million-degree gas; electrons stopped hard | Nested grazing mirrors, in orbit | No | Hospital and airport scanners |
| Gamma ray | shorter than 10 pm | Atomic nuclei; antimatter; cosmic rays | Detector blocks that count single photons | No | Cancer treatment, PET scans, sterilising |
Where one band stops and the next starts is a habit, not a law of nature: different books draw the lines in different places.
A model sky for teaching, not for measurement. Positions of named objects are approximate. Schoolbook figures (wavelengths, dates, sizes) are given to the precision a diagram can show.