Hertzsprung-Russell Diagram - Stellar Classification And The H-r Diagram, The Main Sequence, Giant Stars, The H-r Diagram And Stellar Evolution - The nature of the H-R diagram
luminosity classes graph luminosities
A Hertzsprung-Russell diagram, or H-R diagram, is a graph of stellar temperatures (plotted on the horizontal axis) and luminosities, or brightnesses (plotted on the vertical axis). H-R diagrams are valuable because they reveal important information about the stars plotted on them. After constructing an H-R diagram for a group of stars, an astronomer can make estimates of many important stellar properties including diameter, mass, age, and evolutionary state. Our understanding of the processes at work in the stars depends on knowing these parameters, so H-R diagrams have been essential tools in twentieth-century astronomical research.
Figure 1. The spectral classes and corresponding surface temperatures are given at bottom, while the luminosities are given at left. (The luminosities are in solar units, meaning that "1" equals the luminosity of the Sun, while "10" means ten times the luminosity of the Sun, and so forth.) Clearly the stars are not randomly distributed on this graph. They fall in several well-defined areas, with most stars on a narrow strip running from upper left to lower right. This graph, the H-R diagram, was a fundamental advance in astronomy. Illustration by Hans & Cassidy. Courtesy of Gale Group.
Figure 1 shows all the important features of the H-R diagram. The stars fall into several relatively narrow strips which W. W. Morgan, another famous classifier of stellar spectra, called luminosity classes. Luminosity classes are denoted by Roman numerals.
Additional Topics
The nineteenth century saw the development of a powerful technique called spectroscopy. This technique involves the use of an instrument called a spectrograph, which disperses light passing through it into its component colors in the same way that an ordinary prism does. Indeed, many spectrographs in use today have prisms as one or more of their components. When sunlight or starlight passes throug…
Luminosity class V is the long, narrow strip running diagonally across the diagram, and it is called the main sequence. The Sun lies on the main sequence, as do 90% of all stars. Stars on the main sequence are stable and healthy, shining as a result of nuclear fusion reactions in their cores that convert their hydrogen to helium. Stars
On the H-R diagram, the Sun is a main sequence star. Main…
Main sequence stars are, by definition, normal. The other luminosity classes, of which the main ones are III and I, contain stars that are very different. Consider class III stars. They are fairly cool since they lie near the right side of the H-R diagram. But they are also much brighter than any normal K and M star should be—perhaps 100 times as luminous as the Sun. We know that luminosity…
One of the most important properties of the H-R diagram is that it lets us trace the lives of the stars. A ball of gas officially becomes a star at the moment that nuclear fusion reactions begin in its core, converting hydrogen to helium. At the point the star is a brand-new main sequence object, and lies at the lower boundary of the main sequence strip. Sensibly enough, this is called the zero-ag…
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