Red dwarf star
Red dwarf star. Artwork representing the intense activity of a red dwarf star. These are among the least massive stars in the universe ranging from 0.1 to 0.7 solar masses. The star shown here belongs to a subclass of red dwarf stars, known as the flare stars, which are characterised by frequent (3-4 per hour) flares. These are seen as sudden brightenings with a typical rising time between 1s and 1min and a slower fading time between 1min and 1 hour. During a flare, filaments of gas in the atmosphere are accelerated outwards with speeds up to 1000km/s. Such speeds are greater than the escape velocity and therefore the matter ejected will be lost from the parent star.
© JULIAN BAUM/SCIENCE PHOTO LIBRARY
Artwork of an AM Herculis binary star
Polar binary star. Artwork of a binary star system known as a polar. Polars are short period variable stars which emit X-rays. They consist of a normal star (left) in mutual orbit with a white dwarf (right). Their periods of variation are typically around 2 hours, and correlate with the orbital period of the white dwarf. Their light is strongly polarised. The gravitational field of the white dwarf pulls material off the larger star, and the dwarf's strong magnetic field channels it towards its poles, as seen at lower right. This material emits X-rays as it nears the white dwarf. Polars are also known as AM Her stars, after the first such star to be discovered, AM Herculis.
© MARK GARLICK/SCIENCE PHOTO LIBRARY
Artwork of a brown dwarf star
Brown dwarf. Artwork of a planet-like brown dwarf "star" on the edge of a globular cluster of stars, seen from a nearby moon. Brown dwarves may be part of dark matter, the invisible material that makes up about 90% of the universe's mass. Brown dwarves are classed as MACHOs (Massive Compact Halo Objects) because they are likely to have formed on the periphery (halo) of a galaxy where material is scarce. A brown dwarf is an object which began to accumulate material like a star but which never collected enough for nuclear reactions to begin in its core. Brown dwarves are difficult to detect because, like a planet, they can only be seen by the light they reflect from nearby stars.
© JOE TUCCIARONE/SCIENCE PHOTO LIBRARY