Light, the messenger
Almost everything we know about the universe arrived as light. Learn to read it: what a spectrum is, how dark lines reveal what a star is made of, and how a shift in those lines shows motion.
ONE FAMILY
Light is much more than what you see
Light is a wave, and its wavelength decides what kind it is. Visible light runs from violet (about 400 nanometres) to red (about 700 nanometres). Stretch the wave longer and you get infrared, microwaves and radio. Squeeze it shorter and you get ultraviolet, X-rays and gamma rays. It is all the same thing, travelling at the same speed.
Your Wi-Fi router, a hospital X-ray machine and a candle all make light. Your eyes only pick up the narrow slice in the middle, so astronomers build different telescopes for the other slices.
Spectrum
Light spread out in order of wavelength, so you can see how much of each colour it contains. A prism does this, and so do raindrops.
A rainbow is the spectrum of sunlight. An instrument called a spectrograph does the same job on the light of a single star, in far finer detail.
Check yourself
Light, or something else?
- Radio waves carrying a football match
- The sound of thunder
- X-rays at the dentist
- Warmth you feel from a heater across the room
- Ocean waves
- Microwaves in a microwave oven
Show the answer
A form of light: Radio waves carrying a football match, X-rays at the dentist, Warmth you feel from a heater across the room, Microwaves in a microwave oven
Not light: The sound of thunder, Ocean waves
FINGERPRINTS
Every element has its own lines
Atoms are picky. Each element absorbs and gives off light only at its own exact set of wavelengths. Hydrogen has one set, sodium another, iron another. When light from a star's hot interior passes through the cooler gas of its outer layers, the atoms there remove their wavelengths. The star's spectrum arrives with thin dark lines in it, and the pattern of lines says which elements are there.
Drop a pinch of table salt into a gas flame and it flares yellow-orange. That is sodium giving off its own wavelength. The same yellow line appears, as a dark gap, in the spectrum of the Sun.
A barcode printed on starlight
A shop scanner doesn't open the box. It reads the pattern of dark bars and knows what's inside. A star's spectrum works the same way: measure where the dark lines fall, compare with patterns recorded in laboratories on Earth, and you have the ingredients. Where the analogy breaks: a star carries dozens of barcodes printed on top of each other, and its temperature changes how strongly each one shows, so reading it takes care.
Check yourself
Shirin spreads out the light of a distant star. She finds dark lines at exactly the wavelengths that hydrogen and sodium produce in the lab. What is the sensible conclusion?
- The star is made of sodium only
- The star has no hydrogen or sodium, since those colours are missing
- The star's outer layers contain hydrogen and sodium
- Dust on the telescope mirror blocked those colours
Show the answer
The star's outer layers contain hydrogen and sodium
Right. The colours are missing because hydrogen and sodium atoms in the star's outer gas absorbed them. A gap in the light is the sign of an element being present.
MOTION
The Doppler shift
When an ambulance passes you, its siren drops in pitch. Coming toward you, the sound waves are squeezed; going away, they are stretched. Light does the same. If a star or galaxy is moving away, its whole pattern of lines slides toward longer, redder wavelengths: a redshift. If it is coming toward us, the pattern slides toward blue. The bigger the slide, the faster the motion.
The pattern keeps its shape, so astronomers still recognise hydrogen. It is just sitting in the wrong place, and how far it has moved gives the speed in kilometres per second.
Check yourself
- Galaxy A: a hydrogen line that sits at 656 nm in the lab shows up at 670 nm.
- Galaxy B: the same hydrogen line shows up at 655 nm.
What is the difference between the two galaxies?
- A is moving away from us, B is moving toward us
- A is much hotter than B
- A contains a different kind of hydrogen
- B has no hydrogen at all
Show the answer
A is moving away from us, B is moving toward us
Yes. 670 is longer than 656, so A's light is stretched: a redshift, moving away. 655 is slightly shorter: a small blueshift, moving toward us. A's shift is also much bigger, so it is moving faster.
Two different messages in the same light
Overall colour
Tells you temperature. A cool star glows red, around 3,000 °C at its surface. The Sun is yellow-white at about 5,500 °C. The hottest stars are blue-white, well above 10,000 °C. It is the same order a piece of metal goes through as you heat it.
The thin lines
Tell you ingredients and motion. Which lines are there says which elements are present. Where they sit, compared with the lab, says whether the star is approaching or receding, and how fast.
Check yourself
On a winter night Arash finds Orion. One shoulder star, Betelgeuse, is clearly orange-red. The foot star, Rigel, is blue-white. Which has the hotter surface?
- Betelgeuse, because red is the colour of heat
- Rigel, because blue-white means a hotter surface than red
- They are the same, since colour depends only on distance
- You cannot tell anything from colour alone
Show the answer
Rigel, because blue-white means a hotter surface than red
Right. Heated objects go from red to yellow to white to blue as they get hotter. Rigel's surface is roughly three times hotter than Betelgeuse's.
Check yourself
The spectral lines of the Andromeda galaxy are shifted slightly toward blue. This tells astronomers that Andromeda is hotter than other galaxies.
Show the answer
False
False. A shift in the position of the lines is about motion, not temperature. Andromeda's blueshift means it is moving toward us, at roughly 110 km every second. Temperature shows in the overall colour and in which lines are strong.
Lesson recap
- Radio, infrared, visible light, X-rays and gamma rays are all light; only the wavelength differs.
- Each element absorbs its own exact wavelengths, so the dark lines in a star's spectrum list its ingredients.
- A star's overall colour gives its surface temperature: red is cool, blue-white is hot.
- Lines shifted toward red mean motion away from us; toward blue, motion toward us. The size of the shift gives the speed.
- Read this way, starlight shows that stars are mostly hydrogen and helium.