Using a Hertzsprung Russell diagram to estimate distance

The scientists noticed that large bright stars had a colour near the blue end of the spectrum. Because they were large the gravitational forces are great and that makes the fusion reaction to go faster. With the higher temperature more radiation of shorter wavelength is emitted.

A cluster of stars showing colours







H&R H&R produced a chart relating the colour of a star to its absolute magnitude or brightness.

A Hertzsprung Russell diagram

Hertsprung Russell diagrams

The especially useful part id the narrow band of “main sequence stars” of similar type and size as our own sun. These stars can be used as markers to estimate distance.

Simplified HR diagram

The star, in this example Sirius, is examined and its colour is determined as well as the apparent brightness. From the colour it is placed on the graph and then the absolute magnitude is assessed by drawing a horizontal line to the vertical scale.

A calculation can then be made to assess the distance of the star from the Earth. The method gives a reasonable estimate but it is not highly accurate.

Using an HR diagram


Other pages of notes and video on astronomy which may be useful are:

Units of distance notes and video    Measuring distance by parallax/triangulation notes and video     Life cycle of stars    Geostationary and polar satellites notes and video     Big Bang theory and evidence     Development of the Universe after the Big Bang      Real and apparent magnitude     Hubble's Law and measuring distance notes and video     The age of the universe notes and video       Cepheid variable stars      Type 1A supernova