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

Megalodon shark and great white

Megalodon shark (Carcharodon megalodon), computer artwork. A great white shark (Carcharodon carcharias) is shown below it at the same scale. This enormous shark lived between around 20 and 1.2 million years ago, and is known from fossils. It was related to the modern great white shark, and is thought to have fed on whales, as this is the only source of food that could have provided it with enough meat. Megalodon was thought to attain lengths exceeding 20 metres, and possibly significantly larger. It is thought that it died out after the seas cooled, allowing warm-blooded whales to escape the sharks for much of the year by swimming to waters too cold for them to survive in

© CHRISTIAN DARKIN/SCIENCE PHOTO LIBRARY

Foot of a chameleon

Foot of a chameleon. Coloured scanning electron micrograph (SEM) of the foot of a chameleon. Chameleons are the only lizards with zygodactyle feet, or with toes in a pincer arrangement. This gives them a strong grasp for climbing. On the front limb, the foot has two toes on the outside and three on the inside; on the hind limb this is reversed. Chameleons are found mainly in Africa and Madagascar, though some species also inhabit southern Europe and Asia

© STEVE GSCHMEISSNER/SCIENCE PHOTO LIBRARY