ESA Science & Technology - The Hertzsprung-Russell diagram

All-sky view of 58 000 stars from the second data release of ESA's Gaia satellite, located within 2200 light years of the Sun. The animation shows how astronomers use measurements of star brightness, distance and colour to create a Hertzsprung-Russell diagram.

Named after the astronomers who devised it in the early twentieth century, the Hertzsprung-Russell diagram is a fundamental tool to study populations of stars and their evolution.

Stars are first sorted on the basis of their colour, with bluer stars, which have hotter surfaces, on the left, and redder stars, with cooler surfaces, on the right. Then, they are sorted on the basis of their brightness, with brighter stars shown in the top part of the diagram, and fainter stars in the lower part. Information about stellar distances is fundamental to calculate the true brightness, or absolute magnitude, of stars.

The colour scale in this image represents the colour of stars, and the size of the disc represents their brightness.

The large diagonal stripe across the centre of the graph is known as the main sequence. This is where fully-fledged stars that are generating energy by fusing hydrogen into helium are found. Massive stars, which have bluer or whiter colours, are found in the upper left end of the main sequence, while intermediate-mass stars like our Sun, characterised by yellow colours, are located mid-way. Redder, low-mass stars are found towards the lower right.

As stars age they swell up, becoming brighter and redder. Stars experiencing this are shown on the diagram as the vertical arm leading off the main sequence and turning to the right. This is known as the red giant branch.

While the most massive stars swell into red giants and explode as powerful supernovae, stars like our Sun end their days in a less spectacular fashion, eventually turning into white dwarfs – the hot cores of dead stars. These are found in the lower left of the diagram.

Acknowledgement: Gaia Data Processing and Analysis Consortium (DPAC); K. Nienartowicz / L. Eyer / L. Rimoldini, Observatory of Geneva, Switzerland

 
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