STARS...
OBSERVING STARS
The colours, brightness, and size of stars allow astronomers to discover more about their evolution and the Universe they live in.
BRIGHTNESS AND COLOUR
When we look up at the stars they do not all appear the same colour, and they have varying degrees of brightness. A star's perceived colour depends on its surface temperature, so some glow with a reddish hue (the cooler ones) while others sparkle a brilliant blue or white (the hottest ones). A star's perceived brightness depends on its actual brightness as well as its distance from us. Some extremely bright stars look faint because they are very distant. Conversely, a faint star can be close to us in space and so appear relatively bright in the sky. Astronomers compare star brightness by establishing each star's "absolute magnitude" This is the magnitude (or brightness) a star would have if it were placed at a set distance from us (usually 32.6 light-years). "Apparent magnitude" is the magnitude the star appears to have in our night sky. So it is the differing apparent magnitudes of stars that are of real interest to amateur stargazers.
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BINARY AND MULTIPLE STARS
It is thought that most stars in the Universe are accompanied by nearby companions. A pair of stars that are near to each other in space and that orbit a common centre of mass (called a barycentre) is known as a binary star system. A star may even have more than one other companion in space. Many of the lovely multiple star systems in the night sky can be seen with even a small telescope, such as the gorgeous blue-and-yellow Albireo system and Epsilon Lyrae (the "Double-Double" star system of four stars) in the constellation of Lyra. Optical double stars are stars that appear close to each other in the night sky (due to our line of sight) but which are not really bound to each other by gravity. A classic example of an optical double is the pair of stars Alcor and Mizar in the handle of the asterism of the Plough. They appear to be near to each other but are in fact widely separated in space.
VARIABLE STARS
VARIABLE STARS
Some stars in our night sky do not always shine at a constant brightness. This is not always obvious, and can be difficult to observe, because the brightness (apparent magnitude) of these variable stars can fluctuate over the course of days, months, or even years. There are several possible reasons for this varying magnitude, from changes within the star itself to the dimming effect of a companion star passing in front of it.
Amateur and professional astronomers study variable stars for many reasons, but especially because they can provide clues as to how certain stars behave during key stages in their evolution. Variable star research is one of the areas of astronomy where amateurs, who make regular observations of variable stars, can really help the professionals. By recording the brightness of a variable star over an extended period, astronomers can then plot a graph known as a
"light curve". This is simply a curving line on a graph, mapping light levels from the dimmest point of a star to its brightest and then back to its dimmest. A light curve shows how the star's brightness fluctuates over time and helps determine the nature of the star's variability. The length of time between successive peaks or troughs on a light curve is known as a variable star's period.
CATACLYSMIC VARIABLES
Cataclysmic variables occur in a binary star system of a white dwarf and a cool star in its main sequence phase. Typically the white dwarf rips material off the other star onto itself, and this ignites in a large thermonuclear explosion, causing its brightness to rise spectacularly and creating a "nova" - a newly visible star. This image captures the explosion of Nova Cygni in 1992, which exploded so brightly that it was visible to the naked eye.