We can never visit a star. But its light carries a hidden message about what it's made of, how hot it is, and how fast it's moving. Your mission: learn to read that message.
Units cheat sheet
Tip: tap any underlined number on this page to translate it.
nm
nanometer, a billionth of a meter. Visible light runs from about 400 nm (violet) to 700 nm (red). A human hair is about 70,000 nm wide.
K
kelvin, a temperature scale that starts at absolute zero. Add 273 to °C. The Sun's surface, 5,800 K, is about 10,000 °F.
km/s
kilometers per second. 1 km/s is about 2,200 mph.
eV
electron-volt, a tiny unit of energy used for atoms and light.
spectrum
light spread out by color (wavelength). Plural: spectra.
1
Hidden rainbows
White light is a mix of every color. A diffraction grating spreads the mix out so you can see it.
SimulationAnything hot glows: a stove burner, a light bulb, a star. Slide the temperature and watch the glow change.
Predict first: as something gets hotter, what color does it glow?
Glows
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Brightest color
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Light per m² vs Sun
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Notice: a hot glowing thing makes a continuous rainbow with no gaps. Hotter objects pump out more blue light. That's why the hottest stars look blue-white and cooler stars look orange-red.
For the curious: how the math works
The color where the glow is brightest moves with temperature (Wien's law):
peak wavelength = 2,898,000 nm·K ÷ temperature
And hotter objects give off far more light per square meter: double the temperature means 2⁴ = 16× more light (Stefan–Boltzmann law).
light per m² ∝ temperature⁴
2
Where colors come from
Glowing gas is different. It only gives off a few exact colors. Why?
ModelElectrons in an atom can only sit on certain "steps" (energy levels), like a ladder. When an electron jumps down, the atom shoots out a bit of light (a photon). Bigger jump = more energy = bluer light.
Start on step
Jump down to
Pick a start step and an end step, then make the jump.
Your hydrogen spectrum
Challenge: find all 4 jumps that make visible light. 0 / 4
You built hydrogen's fingerprint!
These 4 lines are the same ones you see in the hydrogen tube. Every hydrogen atom in the universe makes exactly these colors. Other elements have different ladders, so they make different patterns.
For the curious: how the math works
Hydrogen's steps have energies E = −13.6 eV ÷ n². The light's energy is the gap between the steps, and its wavelength is:
wavelength (nm) = 1240 ÷ energy gap (eV)
Make a jump to see your numbers.
3
Measure it like a scientist
Turn your phone into a spectrometer with phyphox.
Your eye sees the colors. A phone camera behind the grating can measure them. phyphox's Spectroscopy experiment averages the picture into a graph: brightness vs. position on the camera chip (in pixels). But pixels aren't colors yet. You have to calibrate: teach the graph which pixel is which wavelength, using lines you already know.
The trick: use the hydrogen lines you already built in Mission 2. Hydrogen's aqua Hβ line is at 486 nm and its bright red Hα line is at 656 nm. Find them, tell the graph, and every other pixel falls into place.
Simulated phone screenThe hydrogen tube seen through a grating: the four Balmer lines you made in Mission 2. Hα is red, Hβ is aqua, Hγ and Hδ are violet. The graph only knows pixels, not colors. Tap a peak, then pick which Balmer line it is. (Tip: use the two brightest lines; they're far apart, so your scale comes out most accurate.)
Tap a peak on the graph.
Point 1
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Point 2
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nm per pixel
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Check yourself: after calibrating with the red and aqua lines, the small violet peak just left of the aqua one lands at about…
Real phyphox tips (for your phone)
Open phyphox → find the Spectroscopy experiment (camera section). Allow camera access.
Rubber-band the phone so the main camera sits flat against the grating, looking down the tube at the slit.
Rotate until the rainbow runs across the screen, then lock it in place. If the phone moves, recalibrate.
Point at the hydrogen tube, tap its red and aqua peaks, and enter 656 and 486. (The violet lines are faint and may not show up on a phone. That's fine; you only need two.)
Backup: the mercury tube's blue 436 nm and green 546 nm lines work too. Old-style fluorescent lights contain mercury; LED lights don't.
Now point at other tubes and write down the wavelengths of the peaks. Bring them to Mission 4!
No phone? No problem. Use your eyes and your worksheet, or pair up.
For the curious: how a grating works
A grating has about 1,000 tiny grooves per millimeter, so the grooves are 1,000 nm apart. Light bends by an angle that depends on its wavelength:
sin(angle) = wavelength ÷ groove spacing
Red bends more than violet, and that spreads the colors apart.
4
Element fingerprints
Every element makes its own barcode of bright lines. Match the barcode, name the gas.
Lab wavelengthsLine positions are lab-measured values from the NIST Atomic Spectra Database. Brightness is approximate.
The reference tubes
Mystery signals
Three unknown glowing gases. Turn on reference fingerprints to compare. When every bright line is explained, pick your answer. Careful: one signal is a mix of two gases.
Overlay:
My answer:
Your phone's numbers
Type the wavelengths (in nm) of the peaks your phone found, separated by commas. The matcher compares them with every fingerprint.
Phone calibrations are usually good to about ±5–10 nm, so the matcher allows some wiggle room.
5
Reading starlight
Now point it at the sky. These are real spectra of stars.
A. Why stars have dark lines
A star is a hot glowing ball (rainbow) wrapped in a thinner, cooler gas layer. That gas absorbs its own fingerprint colors, leaving dark lines in exactly the spots where it would make bright ones.
B. What is the Sun made of?
Real dataSpectrum of a star just like the Sun (type G2). Turn on each element's fingerprint. If its lines land on dark dips, that element is in the star!
The element found in the Sun first
In 1868, astronomers watching a solar eclipse saw a bright yellow line at 587.6 nm that matched no element known on Earth. They named the mystery element after the Greek word for the Sun, helios. Helium wasn't found on Earth until 1895. Try the helium fingerprint above: it doesn't show up as dips in ordinary sunlight, which is why it hid for so long.
C. Sort the stars by temperature
Real dataSeven real star spectra, each scaled so its brightest point reaches the top. Hotter stars give off more blue; cooler stars give off more red. Tap the stars in order from hottest to coolest.
D. A star that breaks the rules
Real data · SDSSThis is SDSS J215018.83+003547.8, a dwarf carbon star. Normal stars have more oxygen than carbon. This one has more carbon, which forms molecules like C₂ that carve wide, sawtooth bands into the spectrum. Dwarf carbon stars probably got their carbon from a companion star that has since died, which makes them clues to how pairs of stars live and die together.
The wide dips at 474, 517 and 564 nm come from…
E. Is that star moving?
Simulated shift of real dataWhen a star moves away from us, all its lines slide toward red. Moving toward us, they slide toward blue (the Doppler effect, like a passing siren). We took a real hot star and gave it a secret speed. Slide the lab fingerprint until the hydrogen lines line up with the dips.
For the curious: how the math works
speed = speed of light × (shift ÷ lab wavelength)
The speed of light is 300,000 km/s.
★
My detective report
Fills in as you go.
Closing thought
We can't visit a star, but its light tells us what it's made of, how hot it is, and how fast it's moving.