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
Evolution of the Universe, artwork
Evolution of the Universe. Computer artwork showing the evolution of the Universe from the Big Bang (far left) 12-15 billion years ago to the present day (far right). Immediately after the Big Bang was the Plank Era (yellow) during which all four forces were unified. This symmetry then breaks, leading to the inflation period (orange) a rapid time of exponential expansion. The universe begins to cool and atomic nuclei can form (nucleosynthesis, red). After 300, 000 years (pink) atoms start to form and after 1 billion years (purple) galaxies start to form.
© JOSE ANTONIO PE'AS/SCIENCE PHOTO LIBRARY
Forearms. Thermogram of forearms. The thermogram shows the temperature of the skin surface by recording its emission of infrared radiation. This heat radiation is displayed with each temperature in a different colour. Here, the temperature scale runs from white (over 34 degrees Celsius) through red (33 degrees) and yellow (32 degrees) to green (30-31 degrees). Areas of highest blood flow, such as the hands, are warmest.
© Dr.Arthur Tucker/Science Photo Library