Light — The Cosmic Messenger
The electromagnetic spectrum, photons, the speed of light, and redshift
Why Light Is Everything
In astrophysics, we cannot reach out and touch the objects we study. We cannot bring a star into a laboratory. Almost everything we know about the universe — the temperature of a star, the composition of a nebula, the speed at which a galaxy is receding — we know because of light. Light is not merely what we see with our eyes. It is an entire family of electromagnetic radiation, and learning to read its properties is the fundamental skill of astronomy.
What Is Light?
Light is an electromagnetic wave — a coupled oscillation of electric and magnetic fields propagating through space. Unlike sound, it requires no medium and travels freely through the vacuum of space. In 1905, Einstein showed that light also behaves as discrete packets of energy called photons. This wave-particle duality is one of the stranger facts of quantum mechanics.
E = hf = hc/λ — where h is Planck’s constant, f is frequency, c is the speed of light, and λ (lambda) is wavelength. Higher frequency means higher energy.
The Speed of Light
In a vacuum, light travels at exactly 299,792,458 metres per second — approximately 300,000 km/s. This is denoted by the letter c. It is not simply the fastest thing we know of; it is a fundamental constant of the universe. According to special relativity, nothing with mass can reach this speed, and information cannot travel faster than light.
This finite speed is both a limitation and a gift. Every time you look at a distant object, you are looking back in time. The Moon as we see it is 1.3 seconds old. The Sun: 8.3 minutes. Andromeda: 2.5 million years. The most distant galaxies visible to the James Webb Space Telescope show us light from over 13 billion years ago.
A telescope is not just a device for seeing far — it is a device for seeing back in time. The further away we look, the deeper into the past we see. The night sky is a mosaic of different historical moments, all arriving at your eye simultaneously.
The Electromagnetic Spectrum
Visible light — the narrow band our eyes detect — is a tiny sliver of the full electromagnetic spectrum. Astrophysicists study the universe across all wavelengths, each revealing different phenomena invisible to the others.
This is why modern observatories span such different technologies — radio dishes in the desert, infrared mirrors in space, X-ray satellites in orbit. Each wavelength window requires different instruments and is blocked to different degrees by Earth’s atmosphere.
Spectroscopy — Reading the Light
When light from a star passes through a prism or diffraction grating, it splits into a spectrum. This spectrum is not a smooth rainbow — it contains dark lines at specific wavelengths, called absorption lines. These lines are the fingerprints of atoms: each element absorbs light at specific, characteristic wavelengths.
By reading these lines, astronomers can determine the chemical composition of a star without ever touching it. Hydrogen, helium, iron, calcium — all identified from the pattern of dark lines in the spectrum. This technique, called spectroscopy, is arguably the single most powerful tool in astrophysics.
Redshift and Blueshift
When a light source moves relative to an observer, its wavelengths are stretched or compressed. This is the electromagnetic equivalent of the Doppler effect you hear when an ambulance siren shifts pitch as it passes.
- A source moving away: light is stretched to longer (redder) wavelengths — redshift
- A source moving toward us: light is compressed to shorter (bluer) wavelengths — blueshift
Redshift is one of the most important measurements in all of cosmology. In 1929, Edwin Hubble found that nearly all galaxies are redshifted, and the further away they are, the greater their redshift. This was the key evidence that the universe is expanding. We will return to this in Lesson 14.
The redshift of distant galaxies is not (primarily) caused by their motion through space. It is caused by the expansion of space itself stretching the wavelength of light as it travels. This cosmological redshift is distinct from the Doppler effect, though mathematically similar at low redshifts.
Blackbody Radiation
Every object with a temperature above absolute zero emits electromagnetic radiation. A blackbody is an idealised perfect emitter, and stars approximate blackbodies closely. The peak wavelength of emission is determined solely by temperature, described by Wien’s Displacement Law:
This is why hot stars appear blue-white and cool stars appear orange-red. The Sun, at about 5,778 K, peaks in the yellow-green range — which is precisely why our eyes evolved to be most sensitive to that part of the spectrum.
1. An astronomer observes absorption lines in a star’s spectrum shifted toward longer wavelengths. What does this indicate?
2. Why can’t we observe X-ray emissions from stars using ground-based optical telescopes?
3. A star appears blue-white in colour. According to Wien’s Law, what can we infer?