Stars
The Sun is an ordinary star 109 Earths wide, and Betelgeuse would hold hundreds of millions of Suns. Compare real star sizes in 3D, place stars on a Hertzsprung-Russell diagram, and follow a star from a gas cloud to a white dwarf, neutron star or black hole.
About this course
The Sun is an ordinary star 109 Earths wide, and Betelgeuse is about 700 times wider than the Sun. The word "star" once covered almost anything bright in the sky, including planets, which the Greeks called wandering stars, and the idea that the Sun is a star took centuries to settle. Sunlight travels 149.6 million km at 299,792 km/s, and the course has you work out how long that takes. Sirius B, by contrast, is a whole star yet slightly smaller than Earth.
The seven units are what a star is, how big stars are, colour and temperature, brightness and distance, the Hertzsprung-Russell diagram, the life of a Sun-like star, and massive stars and what they leave. Brightness and distance separates apparent brightness, how bright a star looks from here, from luminosity, how much light it gives off, and has you compare Rigel at about 860 light-years with Sirius at 8.6. The diagram unit starts in the early 1900s, when Ejnar Hertzsprung in Denmark and Henry Norris Russell in the United States separately plotted stars by brightness and colour. The Sun is 4.6 billion years old with about 5 billion years left on the main sequence. You compare real star sizes in 3D, place stars on the diagram and follow a star from a gas cloud to a white dwarf, neutron star or black hole.
In late 2019 Betelgeuse dimmed by about 60% within months, an event now called the Great Dimming, and the course asks what Hubble studies found caused it. For stars heavier than about 20 Suns, NASA says the collapse of an iron core produces a stellar-mass black hole.
Before you start
No physics is needed. Since the course compares stars with the Sun and with Earth, it helps to have a feel for their sizes from The solar system. Large numbers and temperatures in kelvin appear often; the course explains light-years and parsecs in the brightness unit.
When you finish
Afterwards you can explain how the Sun turns 4.26 million tonnes of mass into light each second, compare Sirius B with Betelgeuse, and read a star's temperature from its colour using O, B, A, F, G, K, M. You can tell apparent from true brightness, place stars on a Hertzsprung-Russell diagram, and follow a star to a white dwarf, neutron star or black hole.
- 1What a star isEvery second the Sun turns about 4.26 million tonnes of its mass into light and heat.
- 2How big stars areSirius B is a whole star yet slightly smaller than Earth; Betelgeuse is about 700 times wider than the Sun.
- 3Star colour and temperatureBetelgeuse looks orange-red at about 3,600 K; Rigel looks blue-white at about 12,100 K.
- 4Brightness and distanceSirius looks brighter than Rigel, yet Rigel gives off thousands of times more light; it is about 100 times farther away.
- 5The Hertzsprung-Russell diagramESA's Gaia satellite placed more than four million stars on one chart of colour against brightness.
- 6The life of a Sun-like starIn about 5 billion years the Sun will swell into a red giant large enough to engulf Mercury and Venus.
- 7Massive stars, supernovae and what they leaveWhen a massive star's core turns to iron, it runs out of fuel within days and collapses.
Sources for this course (14)
- NASA Sun Fact Sheet (nssdc.gsfc.nasa.gov/planetary/factsheet/sunfact.html)
- NASA Sun facts (science.nasa.gov/sun/facts)
- NASA Stars (science.nasa.gov/universe/stars)
- NASA Star types (science.nasa.gov/universe/stars/types)
- NASA What is Betelgeuse (science.nasa.gov/universe/what-is-betelgeuse-inside-the-strange-volatile-star)
- NASA Hubble Betelgeuse dimming (science.nasa.gov/missions/hubble/hubble-finds-that-betelgeuses-mysterious-dimming-is-due-to-a-traumatic-outburst)
- NASA Marshall Watch the Skies Betelgeuse (nasa.gov/blogs/watch-the-skies/2024/09/06)
- ESA/Hubble Sirius B (esahubble.org/news/heic0516)
- NASA Hubble Sirius asset (science.nasa.gov/asset/hubble/the-dog-star-sirius-and-its-tiny-companion)
- NASA SVS Alpha Centauri (svs.gsfc.nasa.gov/20377)
- ESA Gaia HR diagram (sci.esa.int/web/gaia/-/60198-gaia-hertzsprung-russell-diagram)
- NASA Imagine the Universe Life Cycles of Stars (imagine.gsfc.nasa.gov/educators/lifecycles/LC_main3.html)
- NASA Chandra HR diagram (chandra.harvard.edu/edu/formal/variable_stars/bg_info.html)
- NASA Space Math spectral classes (spacemath.gsfc.nasa.gov stars 5Page43).
Checked: 2026-10-17