LIFE OF THE STARS
MEASURE OF THE STARSAs we cannot yet travel outside the Solar System, we have to learn as much as we can about the stars by studying them at a distance. Astronomers can tell the brightness, colour, and temperature of a star by analysing the light it gives out. By splitting starlight into its constituent colours, they can find out what the stars are made of and how fast they are moving. And with accurate measurements of potion, astronomers can predict where stars will wander through the sky thousands of years from now.
On a dark starry night, we can see perhaps 2,500 stars. To our eyes, they appear as little more than twinkling points of light. Some are brighter than others, some are grouped in clusters, and here and there a red or blue star stands out. It may seem hard to believe, but studying starlight has learned everything we understand about the stars. We know that they are suns and, like our Sun, they are powered by nuclear energy. We know how they are born, how they live their lives, and how they die. Astronomers classify stars according the their brightness (magnitude) and colour.
Astronomers measure brightness in magnitudes. The smaller the magnitude number, the brighter the star. The very brightest stars have negative magnitudes. On a dark night, the faintest stars visible to the naked eye are about magnitude 6, each step on the magnitude scale represents and increase or decrease in brightness of 2.5 times.
A stars colour depends on its temperature: the hottest stars are blue-white and the coolest are orange-red. Astronomers classify stars into seven spectral types: O, B, A, F, G, K, and M, where O is the hottest and M the coolest. Each spectral type has 10 subdivisions numbered 0 to 9 (hotter to cooler). The Sun is type G2.