Scale 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

Map of the City of Dublin, 1797

Map of the City of Dublin, Ireland. Published in 1797, this map includes details of the canals being built at the time. Canal Harbour (lower right) connects across bottom with the Grand Canal (lower left). The Royal Canal (upper right and a branch at upper centre) was a competing canal. Running across centre is the River Liffey. Borders of the city's wards are marked in coloured lines, with a key at lower right, next to the city coat of arms. The scale at lower left is in both English and Irish miles. In 1797, Ireland was ruled by Britain, and this map was published by the geographer to King George III and the Prince of Wales (the future King George IV).

© LIBRARY OF CONGRESS, GEOGRAPHY AND MAP DIVISION/SCIENCE PHOTO LIBRARY

Forth Bridge

March 1890: Pictures showing various aspects of building the Forth Bridge: its size compared with the Eiffel Tower; a demonstration of the cantilever process; a cassion being towed into position and a general view of the Inchgarvie cantilever. The Forth Bridge spans the Firth of Forth at Queensferry in Lothian and is over a mile long. The steel cantilever construction was designed by John Fowler and Benjamin Baker to carry a double-track railway line and a road over the river. When it was completed in 1890 it was the longest bridge of its kind and considered a great achievement in engineering. Graphic - pub. 1890 (Photo by Hulton Archive/Getty Images)