Doppler Shift of a Moving Star (Earth & Space)

Lesson notes

Overview

A star’s radial velocity shifts its spectral lines by Δλ/λ = v/c. This topic is rated intermediate and is number 73 of 98 in Earth & Space. Builds on the basics with relationships, methods and trade-offs.

What you are seeing

The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

When a star moves toward or away from us, the wavelengths of its spectral lines shift by Δλ/λ = v/c, blue when approaching and red when receding. Measuring the shift gives its radial velocity. A star with a planet wobbles around the centre of mass, so a regular periodic shift of a few metres per second reveals the planet and its mass, which is how the first exoplanet around a Sun-like star, 51 Pegasi b, was found in 1995.

Reading the relationship

The expression "Δλ/λ = v/c" is a compact summary of the idea. It uses the symbols Δ, λ, v, c; name what each one stands for before you start. Notice that it contains a ratio, so increasing the denominator makes the result smaller. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.

Try this

Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.

Common pitfalls

Earth and space systems work on very different scales of time and distance. Check which scale a number refers to before comparing it with another.

How it connects

It builds on “Orbital Decay of a Satellite”, the topic before it in Earth & Space. Once you are comfortable, continue with “Cepheid Period–Luminosity Relation”.

Why it matters

Earth and space science helps us understand our planet, its climate and the wider universe.

Where you might use it

Revising “Doppler Shift of a Moving Star” before a class, exam or certification
Forecasting weather and understanding climate records and their uncertainty
Planning for earthquakes, floods and other natural hazards in cities
Interpreting satellite images, space missions and the age of rocks and stars

Frequently asked questions

What is Doppler Shift of a Moving Star?

A star’s radial velocity shifts its spectral lines by Δλ/λ = v/c. It is an intermediate topic in Earth & Space.

How do I use the Doppler Shift of a Moving Star simulation?

The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.

What does the formula mean?

The expression Δλ/λ = v/c packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.

Who is this topic for?

Builds on the basics with relationships, methods and trade-offs. If the quiz feels hard, reread the lesson notes and try the previous topic first.

What should I learn before this?

Start with “Orbital Decay of a Satellite”, the previous topic in Earth & Space. Each topic in a subject builds on the ones before it.

How do I track my progress?

Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.