![]() ![]() ![]() This was also the form of the diagram using apparent magnitudes of a cluster of stars all at the same distance. Hertzsprung had already been working with this type of diagram, but his first publications showing it were not until 1911. The same type of diagram is still used today as a means of showing the stars in clusters without having to initially know their distance and luminosity. The apparent magnitude of stars in the same cluster is equivalent to their absolute magnitude and so this early diagram was effectively a plot of luminosity against temperature. These spectral lines serve as a proxy for the temperature of the star, an early form of spectral classification. In 1910 Hans Oswald Rosenberg published a diagram plotting the apparent magnitude of stars in the Pleiades cluster against the strengths of the calcium K line and two hydrogen Balmer lines. He took this as an indication of greater luminosity for the narrow-line stars, and computed secular parallaxes for several groups of these, allowing him to estimate their absolute magnitude. Hertzsprung noted that stars described with narrow lines tended to have smaller proper motions than the others of the same spectral classification. In one segment of this work Antonia Maury included divisions of the stars by the width of their spectral lines. In the nineteenth century large-scale photographic spectroscopic surveys of stars were performed at Harvard College Observatory, producing spectral classifications for tens of thousands of stars, culminating ultimately in the Henry Draper Catalogue. The diagram was created independently in 1911 by Ejnar Hertzsprung and by Henry Norris Russell in 1913, and represented a major step towards an understanding of stellar evolution. ![]() The Hertzsprung–Russell diagram (abbreviated as H–R diagram, HR diagram or HRD) is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective temperatures. The Sun is found on the main sequence at luminosity 1 ( absolute magnitude 4.8) and B−V color index 0.66 (temperature 5780 K, spectral type G2V). In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. Stars tend to fall only into certain regions of the diagram. Astronomers call them red giants or red supergiants, depending upon their exact size.Scatter plot of stars showing the relationship of luminosity to stellar classification An observational Hertzsprung–Russell diagram with 22,000 stars plotted from the Hipparcos Catalogue and 1,000 from the Gliese Catalogue of nearby stars. ![]() Old stars are bloated stellar objects, often hundreds of times the radius of the Sun. Astronomers still use this scheme today.Īre older stars part of the classification? Yes, even though they have left the main sequence. The scheme had O, B, A, F, G, K, M, with O for the hottest stars and M indicating the coolest. It grouped stars into seven classes, each designated with a letter, according to their surface temperature. In 1918, astronomers and their assistants from Harvard University, United States, proposed a system of classification known as the Harvard Spectral Classification. They are called blue supergiant stars and have extreme temperatures of more than 10 000K. At the other end of the scale, are stars larger than the Sun. Called red dwarf stars, they have temperatures of just a few thousand Kelvin. About 90% of the stars are smaller than the Sun. The Sun is a yellow star with a temperature of about 6000K. The temperature of the star determines its colour. The more massive the star, the hotter its surface and the shorter its lifetime. The mass of the star determines how fast its nuclear reactions take place and therefore how much energy it releases. Sometimes there are more extreme examples, of course. When stars form, they have different masses, mostly somewhere between one-tenth and ten times the mass of our Sun. However, even between main sequence stars, there are large differences, usually determined by their mass. Astronomers refer to these as 'on the main sequence', which refers to their position on a chart invented by Ejnar Hertzsprung and Henry Norris Russell. Most are steadily converting hydrogen into helium. ![]()
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