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Star colours  (Article No: 113)

We know that stars are made of many of the same chemical elements found on Earth. We can measure their size, mass and temperature and have some understanding of their internal structures. We also know that they come in a range of beautiful colours and that there is a relationship between a star’s vital statistics and its colour. So, how can we know so much about the nature of stars that lie so achingly far away in the remoteness of space that their light can take years, or even centuries to reach us?

Let’s have a closer look. Next time you look at the night sky (allow 20 minutes to dark adapt your eyes) let your gaze shift between the stars and you will notice a slight difference in their colours. At first you will think that the colours are very subtle. With a little practice and some knowledge of “what’s up in the night sky” (feel free to contact the Club if you need a little guidance) you will see some marked differences in star colours. For example, in the constellation of Orion the red colour of Betelgeuse and the blue tint of Bellatrix are easily seen. Because our colour vision is not great at low light levels the colours of the brightest stars are more evident.

A star’s colour is directly related to its surface temperature. The intensity of light from a cool star peaks at long wavelengths making the star look red and a hot star’s intensity of light peaks at short wavelengths making it look blue. Our Sun with an intermediate surface temperature, looks yellow.

How do astronomers measure the colour of a star? The light from a star is collected by a telescope and passed through one of a set of differently coloured filters. The filtered light is then collected by a light sensitive device called a CCD. This process is repeated through each filter. The image of the star will have different brightness’s through each filter and by comparing these astronomers can find the wavelength of the peak light intensity --- and hence the star’s temperature. Interstellar dust and gas can cause a star to look redder than it actually is (just as dust particles in Earth’s atmosphere can make a setting Sun seem redder) and this is taken into account to determine a star’s surface temperature from its colour ratios. A star’s spectrum, measured with a spectrograph, gives a more precise measurement but is a slower process.

Astronomers have classified stars into a sequence of spectral types according to their temperature and assign a letter to each spectral type. The letters used are OBAFGKM. This sequence can be remembered with the mnemonic: “Oh, Be A Fine Girl (or Guy) Kiss Me”. The bluest and hottest stars would have surface temperatures of 40,000 to 25,000 C and the coolest stars would be red, with surface temperatures of 3,500 to 3,000 C. This is further refined by attaching an integer from 0 to 9 to each letter. Our Sun, with a surface temperature of 5,800 C, would be a G2 yellow star. Clear skies!

Visit www.shannonsideastronomy.com or call Conn at 061 301493