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
Saturn's interior, artwork
Saturn's interior. Computer artwork showing the interior structure of the planet Saturn. Usually, Saturn and Jupiter are known as gas giants, but they are not made entirely of gas. Saturn is composed of 96% hydrogen, 3% helium and 1% other elements, but these are just gaseous in the atmosphere (beige), which is about 1000 km thick. Below the atmosphere is a layer of liquid hydrogen and helium about 30, 000 km thick (light blue), located above another layer of metallic hydrogen (dark blue) about 15, 000 km thick. There is thought to be a rocky core (sphere, centre), with a radius of about 12, 000 km, at the centre of the planet.
© CARLOS CLARIVAN/SCIENCE PHOTO LIBRARY
Artwork of proton-proton chain reaction
The proton-proton chain is an important nuclear reaction which occurs inside the core of main-sequence stars such as the Sun. It involves the conversion of hydrogen nuclei (protons, or 1H) into helium nuclei (4He) via several steps. First, two protons come together and stick. One of them is converted into a neutron, and debris is emitted in the form of a positron and a neutrino. The result is deuterium - a nucleus consisting of one proton and one neutron. With the addition of another proton and the subsequent emission of a gamma-ray, a new product is formed - an isotope of helium called helium-3 or tralphium. Finally, two of these helium-3 nuclei collide, and two protons are ejected, to make hydrogen. On average, a typical proton can wait up to a billion years in the nucleus of a star before it collides with another to initiate this whole process.
© MARK GARLICK/SCIENCE PHOTO LIBRARY