10. Light, Spectra & Telescopes
High SchoolNearly everything we know about distant objects comes from analyzing their light. This chapter shows how astronomers read the fingerprints hidden in starlight.
The electromagnetic spectrum and blackbody light
Visible light is just one narrow slice of the electromagnetic spectrum, which also includes radio, infrared, ultraviolet, X-ray and gamma-ray light, all travelling at the same speed c but differing in wavelength. Hot objects like stars radiate a smooth blackbody spectrum whose peak wavelength shifts with temperature, described by Wien's Law:
Passing starlight through a prism or spectrograph splits it into a spectrum crossed by dark absorption lines — wavelengths absorbed by specific elements in the star's outer layers. Every element has a unique pattern of lines, like a fingerprint, letting astronomers identify a star's chemical composition from light alone.
Reading motion from colour: the Doppler shift
If a light source moves toward us, its wavelengths compress, shifting toward blue (blueshift); moving away, wavelengths stretch, shifting toward red (redshift). Measuring exactly how far a star's absorption lines have shifted from their known laboratory wavelength reveals its velocity toward or away from us — the same principle used across all of cosmology (Chapter 16).
Worked example — a star's surface temperature
A star's spectrum peaks at wavelength λmax = 500 nm = 5.00×10⁻⁷ m. Find its surface temperature using Wien's Law.
- Rearrange Wien's Law: T = b / λmax.
- Substitute: T = 2.898×10⁻³ / 5.00×10⁻⁷.
- Compute: T ≈ 5 796 K.
Answer: about 5 800 K — very close to the Sun's actual surface temperature of 5 778 K, and consistent with the Sun's peak emission in yellow-green visible light.
Test your understanding
Which is hotter: a blue star or a red star?
The blue star. By Wien's Law, a shorter peak wavelength (blue) corresponds to a higher temperature, while a longer peak wavelength (red) corresponds to a cooler surface.
How do absorption lines reveal a star's composition?
Each element absorbs light only at very specific, characteristic wavelengths, leaving a unique pattern of dark lines in the star's spectrum. Matching these patterns to known laboratory spectra identifies exactly which elements are present in the star's outer layers.
A galaxy's spectral lines are shifted toward longer (redder) wavelengths. What does that tell us?
The galaxy is redshifted, meaning it is moving away from us — this is the Doppler effect applied to light, and it's the key evidence behind the expanding universe (Chapter 16).
Why do large telescopes use mirrors instead of lenses?
A large lens can only be supported around its edge, so gravity makes it sag and distort under its own weight, and glass that thick also absorbs some light. A mirror, by contrast, can be supported across its entire back surface, letting it stay rigid and precise at enormous sizes.
How the ideas connect
Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.
The key facts, visualised
Worked problems, step by step
Follow each solution line by line, then try to reproduce it on paper before moving on.
Example 1One star peaks in blue light, another in red. Which is hotter?
- By Wien's law peak wavelength is inversely related to temperature
- Blue is a shorter wavelength than red
- Shorter peak means higher temperature
Example 2A star's spectral lines are shifted to longer wavelengths. What does that tell us?
- Longer wavelength means redshift
- Redshift indicates the source is moving away from us
Now you try
Work each one out first, then tap to reveal the worked answer.