Temperature Gallery

Diagram showing the spectral class and luminosity of stars

Diagram showing the spectral class and luminosity of stars. The diagram was named The Hertzsprung-Russell Diagram, or the H-R diagram for short. At the beginning of the 20th century two astronomers found that if stars were plotted on a diagram with their luminosity (brightness) on one axis, and their spectral class on the other, that stars formed three distinct groups. The largest group, the Main Sequence Stars, is where 90% of the stars are found. These stars are fusing hydrogen into helium in their cores. This group propagates diagonally from the upper left corner, down to the right corner. The group below the main sequence is the White Dwarfs, which is a group of small, earth-sized stellar remnants. The third group, which is found above the main sequence is that of the giants.
Stars appear on a specific place on the main sequence depending on their mass and age. The mass determines when it will leave the main sequence.The H-R diagram is a significant tool for astronomers, when it comes to understanding stellar evolution

© Fahad Sulehria/Stocktrek Images

Female Nude Seated by Fireside, c.1850-1900 (mantochrome, paper-taped)

990443 Female Nude Seated by Fireside, c.1850-1900 (mantochrome, paper-taped) by Mante, Louis Amedee (1826-1913) & Goldschmidt, Edmond (1863-1934); 17.78x12.7 cm; Private Collection; (add.info.: Female Nude Seated by Fireside. Louis Amedee Mante (1826-1913) and Edmond Goldschmidt (1863-1934). Mantochrome, paper-taped. Photographed late 19th century. 17.78 x 12.7cm.); Photo A© Christie's Images; out of copyright

© © Christie's Images / Bridgeman Images

Hydrometers and Hygrometers C017/3433

Plate from 18th century encyclopedia showing illustrations of hydrometers and hygrometers. A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water. A hygrometer is an instrument used for measuring the moisture content in the environment. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity

© DAVID PARKER/SCIENCE PHOTO LIBRARY