From Giants to Dwarfs Sorting the Stars by Size

The radius of a star can be determined from the luminosity of the star (which can be determined if the

Star Words

Spectral classification is a system for classifying stars according to their surface temperature. The presence or absence of certain spectral lines is used to place stars in a spectral class.

Star Words

Spectral classification is a system for classifying stars according to their surface temperature. The presence or absence of certain spectral lines is used to place stars in a spectral class.

distance is known) and its surface temperature (from its spectral type). It turns out that stars fall into several distinct classes. In sorting the stars by size, astronomers use a vocabulary that sounds as if it came from a fairytale:

> A giant is a star whose radius is between 10 and 100 times that of the sun.

> A supergiant is a star whose radius is more than 100 times that of the sun. Stars of up to 1,000 solar radii are known.

> A dwarf star has a radius similar to or smaller than the sun.

Working independently, two astronomers, Ejnar Hertsprung (1873-1967) of Denmark and Henry Norris Russell (1877-1957) of the United States studied the relationship between the luminosity of stars and their surface temperatures. Their work (Hertsprung began about 1911) was built on the classification scheme of another woman from the Harvard College Observatory, Antonia Maury. She first classified stars both by the lines observed and the width or shape of the lines. Her scheme was an important step toward realizing that stars of the same temperature could have different luminosity. Plotting the relationship between temperature and luminosity graphically (in what is now known as a Hertzsprung-Russell diagram or H-R diagram), these two men discovered that most stars fall into a well-defined region of the graph. That is, the hotter stars tend to be the most luminous, while the cooler stars are the least luminous.

Telescopes Mastery

Telescopes Mastery

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