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The Hertzsprung – Russell Diagram

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In this laboratory a random sample of stars will be used to create a HR Diagram. From the diagram it will be determined which category certain stars belong to: Super Giants, Giants, and White Dwarfs. The properties of each category will be discussed as well as the Main Sequence, apparent and absolute magnitudes and spectral types and surface temperatures.

Background:

If total energy outputs, (absolute magnitude or luminosity), are plotted against surface temperatures, (spectral type), a Hertzsprung-Russell Diagram (HR Diagram) will be obtained.

Apparent (Visual) and Absolute Magnitude

Apparent magnitude, m, is the relative brightness of a star (or other stellar object) as viewed from the Earth; the lower the magnitude, the brighter the star. Sirius, the brightest star, has an apparent magnitude of -1.46, while the faintest stars visible to the naked eye have magnitudes of about 6. A difference of five magnitudes between two objects is a factor of 100 in brightness. The apparent brightness of a star depends on two factors: its actual brightness, and its distance from the Earth.

Absolute magnitude, M, measures the actual brightness of a star, so the distance of the star from the Earth is needed. The absolute magnitude reflects the true amount of light, and therefore energy, emitted by the star. Often this value is expressed as the luminosity of the star.

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Spectral Types and Surface Temperatures

Spectral types are a measure of the surface temperatures of stars and are letter designations: O, B, A, F, G, K, and M. Type O stars have the highest surface temperatures and can be as hot as 30,000 Kelvins. On the other extreme, type M stars have the lowest surface temperatures and can be as cool as 3,000 K.

Hertzsprung-Russell Diagram

The most significant feature of the HR Diagram is that the stars are not distributed in it at random (showing any combinations of absolute magnitudes and spectral types), but rather group themselves only into certain parts or regions of the diagram thus forming several groups.

Main Sequence

Most stars are located in a narrow band running from the upper left corner (high surface temperature and high luminosity) of the diagram diagonally toward the lower right corner (low temperature and low luminosity); this band of stars is called the Main Sequence. These stars are like the Sun fusing hydrogen into helium during the fusion reactions in their core. The larger stars on the Main Sequence are the brighter stars with high luminosities and high surface temperatures. These stars fuse the hydrogen into helium at a very rapid rate. Therefore the higher up on the Main Sequence the star is, the shorter it lives. The smaller main sequence stars have low luminosities and low surface temperatures. These stars take a very long time to fuse hydrogen into helium and will therefore live a very long life.

Giants

A much smaller number of stars can be found above the Main Sequence in the upper right hand part (low temperature and high luminosity) of the diagram. The stars of this group have a large radius, low mass, and their cool surface temperatures mean they appear red in color. These stars have burned all of the hydrogen in their core into helium and have entered the final part of their life. As this happens the star begins to swell and becomes brighter. Although its surface has become cooler, its core has become hotter. At this stage the helium begins to fuse into carbon and it has become a Red Giant. A Red Giant will not be able to fuse carbon so when the helium is gone, the star will begin to die.

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Super Giants

At the top part of the diagram, above the Main Sequence and above the Red Giants/Giants, are scattered a few stars of highest luminosities and any (from high to low) surface temperatures. These stars are named Super Giantsbecause their radii are even larger than those of the Red Giants. At one time these massive stars were on the upper Main Sequence. After the hydrogen in their core has been fused into helium they begin to fuse helium into carbon. Because of their mass this happens much quicker than it does in low mass stars. As a result these stars never become Red Giants and instead become Super Giants. Super Giants can fuse past carbon; in fact they can fuse carbon into oxygen, oxygen into neon, neon into magnesium, magnesium into silicon, and finally, silicon into iron. Once an iron core is reached the super giant will die in a supernova explosion. Depending on the original mass of the star the result will be a neutron star or a black hole.

White Dwarfs

Finally, there are also few stars found below the Main Sequence in the lower left (high temperature and low luminosity) corner. The stars of this group are known as White Dwarfs, and they have low luminosities because their radii are in general quite small, about the same as the Earth’s radius. Stars like our Sun, will not be able to fuse further than carbon and once the reactions in their core stop the star begins to cool, the outer layers begin to dissipate and the core begins to shrink. Eventually there will be no evidence of the outer layers and all that remains will be the small cooling core of the star, a white dwarf.

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