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THE BRIGHTNESS AND TEMPERATURE OF STARS

Distance To Stars

QUESTIONS:-

1. Find out about the RANGEFINDER. Briefly describe how it worked. What was its main use?

2. The distance from the Earth to the Sun is 150 million kilometres. What is the biggest base length of the triangle used to calculate stellar distances?

3. The speed of light is 300,000 km per second. What distance does light travel in

a. 1 second
b. 10 seconds
c. 1 minute
d. 1 hour of 3,600's
e. 1 week of 7 days
f. 1 year of 52 weeks

THE TRUE BRIGHTNESS OF STARS

Even the most casual glance at the night time sky shows that there are bright stars and there are dim stars. A star that looks bright in the night time sky might really be a very dim star that happens to be nearby; conversely, a faint star might really be a very luminous star that is extremely far away. Astronomers need to know the true brightness of stars; the true brightness tells us how much power is emitted from the surface in the form of starlight, and it can be calculated once the distance to the star is known.

Once the distance to the star is known, it is quite straight forward to calculate its true brightness or luminosity. For example, the North Pole Star, Polaris, is not especially bright, but Polaris is 680 light years away. Since Polaris is so remote yet can still be seen with the naked eye, it must be extremely luminous. In fact, the true brightness of Polaris is ten thousand Suns.

When stating the true brightness, or luminosity, of stars, it is convenient to compare them with the luminosity of our own Sun. Our sun shines with a power output of 4 x 1026 Watts; for convenience, we say that this amount of power equals one Sun. A star that has only half the power output of the Sun is said to have a luminosity of 1/2. Another measure of a stars brightness is called its magnitude. The larger the number, the fainter the star. The brightest stars are about magnitude zero, and the faintest stars that can be seen with the naked eye are about magnitude 5. The luminosity of stars cover an enormous range: the brightest shine with a brilliance of a million Suns, and the dimmest stars emit only 1 millionth of a Sun.

Our Sun lies in the middle of the range of stellar luminosity's, it is a very ordinary star.

THE TEMPERATURES OF STARS

Only a few hundred years ago it was believed that stars were so incredibly remote that we had no hope of ever discovering their true nature. At the same time that astronomers began to measure the distances to the stars, physicists discovered important facts about light. These discoveries showed that a beam of light can contain an enormous amount of information.

In the 1660's Isaac Newton discovered that when a beam of white light is passed through a prism, it breaks into the colours of the rainbow; called a spectrum. The spectrum of a star tells us the temperature of the stars' surface. If you look carefully at the stars in the sky, you will notice that they are not all the same colour; most are white, but some are a little bluish and others are slightly reddish.

Figure 3 shows the spectra of three hot objects. These spectra run from the ultra violet through the visible wavelengths and on through the infra red; although the general shape of the spectrum is the same for all hot objects, hotter objects emit more of their energy towards the blue and ultra violet end of the spectrum.

Measurements by astronomers tell us that stellar temperatures lie in the range 3,000 to 30,000ºC. Like the Sun with a surface temperature of 5,500ºC, most stars' surface temperatures lie near the cool end of the range, but there are a few very hot stars such as Rigel, Beta Centauri and Spica which shine with a brilliant blue light.

Figure 1 Temperatures and Colours

Blackbody curves for three temperatures are shown. Notice that the relative amounts of light emitted in various colours are directly related to the temperature. Since the actual distribution of energy emitted by a star closely approximates a blackbody curve, astronomers can determine the surface temperature of a star by accurately measuring the colours of the light it radiates.

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The EIA Team / Tel:+44 (0)1274 384070 / 25 Sept 1995