Home space resource The colour of stars

The colour of stars

Learn why stars have different colours and how colour relates to temperature. A curriculum-aligned astronomy resource for NZ students and teachers.

Years 11-13

This activity is part of our live SpacewardBound sessions delivered in schools across New Zealand.

Earth & Space Science

Astrophysics

20–30 mins reading or 30–45 mins with discussion

Article

Our resources are built from real questions asked by over 40,000 students during our visits to 200+ New Zealand schools.

Why is this good to know

Learning outcomes

1. Understand that star colour is related to temperature

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2. Explain how light and energy are linked to the behaviour of stars

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3. Recognise black body radiation as a model used to describe how stars emit light

Prior knowledge

Students should understand basic concepts of light, including spectra and how light carries energy, as well as the idea that stars emit light.

If you’ve looked up at stars, you may have noticed their colour varies. Some stars appear to be slightly orange, others blue, and some appear to be white. The colour variation is quite subtle to our eyes, and we see the combined emission across the visible spectrum of light from that star. You would see a distribution if you measured light intensity at different wavelengths and graphed it. Wilhelm Wien discovered a relationship between light intensity at different wavelengths for an object and its temperature. Assuming that an object is a perfect emitter that doesn’t absorb any radiation, we can make a nice curve on a graph for any given temperature. We call this a black body radiation curve. Of course, objects are not perfect emitters, and depending on what they are made of, there will be bumps in the black body radiation curve where certain atoms and molecules absorb the radiation and even absorb and re-emit the radiation at different wavelengths.

Black body distribution of radiation

The graph below depicts the blackbody emission for a star at the temperature in the title. It starts with the Sun at 5777K. At that temperature, we see the peak emission at about 502nm in the green part of the spectrum. The curve is coloured to show where the visible part of the electromagnetic spectrum is. You can adjust the temperature with the slider above the graph; moving it to the right will increase the temperature, and moving it to the left will decrease the temperature.

To the right of the graph is a coloured circle that simulates what a star would look like to a human eye. It averages the intensity of light in the visible spectrum for that temperature. This will take on a blue hue for higher temperatures, and for cooler temperatures, it will be redder in hue.

The graph uses Planck’s Law of Black Body Radiation:

A deep sky photo of cluster NGC 5617, representing stars — This is the Open Cluster NGC 5617 showing some of the subtle variation in star colour typical of an Open Cluster. The image was taken by the author using the Chile Slooh telescope.

Examples of various stellar regions visible from the Dark Sky Reserve of Wairarapa

Photos taken at our Star Safari Observatory in Wairarapa.

Camera: Canon Ra, with Sigma Art 24 lens, 1.4, exposure 20 sec, 3200 ISO Images are unprocessed.

We take concepts like this and turn them into interactive, hands-on learning experiences students actually understand.

We’ve worked with 200+ schools and 40,000+ students,
helping make complex space science understandable and engaging.

Want to bring this experience into your classroom?

This activity is part of our live SpacewardBound sessions delivered directly in schools. We run it as a full interactive experience using our mobile planetarium and hands-on activities.

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