If you look at the SKY at NIGHT you will see BRIGHT stars and DIM stars. The BRIGHTNESS of stars depends upon

(a) - How much POWER the star gives out and

(b) - How FAR away the star is from us.

A star that appears BRIGHT to us may be a dim star that is NEAR to us or a VERY bright star that is FAR away. To overcome this problem astronomers use the term TRUE BRIGHTNESS (or LUMINOSITY) to describe how bright a star is. This terms describes the POWER output of the star and is therefore INDEPENDENT of distance away from us.

The TRUE BRIGHTNESS of a star is often stated with reference to how much power our own SUN gives out (it has a power output = 1 SUN). For example, the NORTH POLE STAR (Polaris) which gives out 10,000 times the power of our own Sun is said to have a true brightness of 10,000 suns. The true brightness of stars varies enormously from about a million suns to one-millionth of a sun. Our own Sun is a very ORDINARY star!

If you look CAREFULLY at the stars at night you will notice that although most are WHITE. some look a little BLUISH and some slightly REDDISH in colour. This is because the WAVELENGTH of light given out by any object (including stars) depends upon its TEMPERATURE. A star with a temperature of:-

(a) 3,000 ºC looks slightly REDDISH

(b) 6,000 ºC looks WHITE

(c) 10,000 ºC looks BLUISH.

Answer the following questions on the BRIGHTNESS and TEMPERATURE of stars.

1 - Which of the following stars would appear brightest to someone looking at them from the Earth?

STAR A --- has a TRUE BRIGHTNESS of 2 suns and is 10 light years away

STAR B --- has a TRUE BRIGHTNESS of 2 suns and is 5 light years away

STAR C --- has a TRUE BRIGHTNESS of 1 sun and is 5 light years away

2 - When light passes through a PRISM it is split up into a number of different COLOURS (wavelengths).

a. What is this set of colours (which make up white light) called?

b. What are the different colours white light can be split up into?

c. Which colour has the SHORTEST wavelength?

3 - What colour would a star be if its surface temperature was 3,150 C?

The following two tables contain a RANDOM sample of stars; those which are CLOSEST to Earth and those which are BRIGHTEST in the sky.


Name of Star      Distance Away      True              Surface  
                  from the Earth     Brightness        Temperature of 
                  (light years)      (Our Sun = 1)     Star
Sun                    -             1.0               5,530   
Proxima               4.2            0.1               2,530   

Alpha Centauri A      4.3            1.03              5,530   
Alpha Centauri B      4.3            0.93              4,480   
Barnard's Star        6.0            0.30              2,730   

Wolf  359             7.7            0.01              2,430   

BD  36                8.2            0.53              2,930   

UV  Ceti  A           8.4            0.10              2,430   

UV  Ceti  B           8.4            0.07              2,430   

Sirius  A             8.6            1.28              8,830   

Sirius  B             8.6            0.47              8,730   

Ross  154             9.4            0.31              2,730   

Ross  248             10.4           0.17              2,530   

Eta Eri               10.8           0.89              4,330   

Ross  128             10.9           0.28              2,630   

Cygnus  A             11.1           0.77              4,130   

Cygnus  B             11.1           0.70              3,930   

Eta Ind               11.2           0.82              4,180   

BD  43  A             11.2           0.53              3,030   

BD  43  B             11.2           0.28              2,730   

Procym  A             11.4           1.2               6,330   

Procym  B             11.4           0.32              7,130   

BD  59  A             11.6           0.47              2,830   

BD  59  B             11.6           0.41              2,730   

COD                   11.7           0.60              3,030   

N.B. (a) There are seven sets of BINARY stars in the above table
(b) The BRIGHTNESSES are in fact log (true brightness)

1 - Plot a SCATTER graph of the TEMPERATURE of the star (on x-axis going up to about 27,000 C) against TRUE BRIGHTNESS (on y-axis going up to 2.0) Plot each star as a HEAVY dot. Also plot the stars on page 3.

a. All but EIGHT of the stars lie almost on a SMOOTH curve. Draw a smooth curve through MOST of the stars
b. Label on this curve our SUN; the HOT, BRIGHT stars and the COOL, DIM stars


Name of Star    Distance away        True Brightness      Surface   
                from the Earth       Our Sun = 1)         Temperature of  
                (light years)                             Star (C>

Canopus                74               1.61              7,330   

Vega                   25               1.35              9,630   

Archernar              69               1.51             20,230    

Betelgeuse          1,400               2.00              3,930   

Hadar                 320               1.77             25,230    

Acrux                 510               1.78             26,480    

Altair                 16               1.21              7,830   

Aldebaran              60               1.43              3,860   

Antares               520               1.83              3,010   

Spica                 220               1.67             25,230    

Fomalhaut              22               1.23              8,780   

Becrux                460               1.79             26,680    

Deneb               1,500               2.00              9,070   

Regulus                69               1.43             13,180    

Adhara                570               1.80             22,730    

Castor                 49               1.36              9,380   

Gacrux                120               1.50              2,730   

Shaula                330               1.69             23,980    

The stars lying on or near this curve are all about the same MASS and SIZE just like our Sun.

2 Look at the stars on the curve.

a. Explain the way in which it shows that stars' temperature and true brightness are related
b. Where on the curve would you expect to find the
1. RED stars?
2. BLUE stars?

3 a. Label the two "WHITE DWARFS" on the curve
b. Write down the NAMES of the two "white dwarfs".
c. White dwarfs are SMALL stars (only about the size of the Earth). Which THREE of the following words describes a white dwarf?

Hot Dim Large Cool Bright Small

4 a. Label the six "RED GIANTS" on the curve.
b. Write down the names of the six "red giants".
c. Red giants are the LARGEST stars in the Universe (many thousands of times the size of our Sun). Which THREE of the following words describe a red giant?

Hot Dim Large Cool Bright Small


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