HINTS ON OBSERVING

This article featured in the December 2003 Beginners Magazine

STAR MAGNITUDES AND COLOURS

STAR MAGNITUDES

When we look up into the night sky on a very clear night we can see up to about 5000 stars with the naked eye. There are of course many more stars up there but they are too faint to see. On the very clearest night at a very dark location away from any town lights a person with good eyesight can see stars as faint as the 6 th magnitude but what does this mean.

Astronomers measure stars in units called magnitudes but this is not a unit like a meter or a kilogram. Each magnitude is about 2½ times brighter than the previous magnitude that is in turn is 2½ times brighter than the previous magnitude. The larger the magnitude number the dimmer the star will appear. Very bright stars have negative (minus) numbers. There are two kinds of magnitude measurements used :-

APPARENT MAGNITUDE

This is how bright a star appears to be in our sky. Although this measurement tells us how bright each star appears it does not tell us how bright a star actually is. This is because all stars are at different distances from us. Those nearer appear brighter and those further away appear not so bright.

ABSOLUTE MAGNITUDE

This is how bright stars would appear if they were all the same distance away from us. Absolute magnitude is therefore a measure of the real brightness of a star. The standard distance for measuring absolute magnitude is 10 parsecs or 32.6 Light Years. Polaris the Pole Star has a magnitude of 5 absolute and is therefore 10,000 times brighter than our Sun would be at the same distance. The nearest star to our Sun is about 4 light years.

ABSOLUTE MAGNITUDES

-8   156250 x as bright as our Sun   (7 Cass -8)

-7   62500 x as bright as our Sun   (Rigel -7.1)

-6   25000 x as bright as our Sun   (Betelgeuse -5.6)

-5   10000 x as bright as our Sun   (Polaris -4.6)

-4   3800 x as bright as our Sun   (Antares -4.4)

-3   1525 x as bright as our Sun   (Spica -3.5)

-2   610 x as bright as our Sun   (Alcyone -1.6)

-1   244 x as bright as our Sun   (Regulus -0.6)

0   97 x as bright as our Sun   (Arcturus -0.2)

1   39 x as bright as our Sun   (Sirius 1.4)

2   15.6 x as bright as our Sun   (Alcor 2.1)

3   6.25 x as bright as our Sun   

4   2.5 x as bright as our Sun   (Alpha Cent 4.4)

5   1 Our Sun has an Absolute magnitude of 4.8

6   2.5 x less bright than our Sun

7   6.25 x less bright than our Sun

8   15.6 x less bright than our Sun

It can be seen that a star, two magnitudes brighter than another star will be 2.5 x 2.5 = 6.25 times brighter. Three magnitudes will be 2.5 x 2.5 x 2.5 = 15.6 times brighter. So a star with a magnitude of 13 will be 156250 times fainter than a star of magnitude 0. Very bright stars have a magnitude less than 0 and therefore have negative magnitudes for example Sirius in Canis Major is the brightest star visible from Britain and has an apparent magnitude of 1.47. Venus has a maximum apparent magnitude of 4.5 and the Sun is -27.

stronomers studying variable stars, use stars of a known magnitude to estimate the changing brightness of the star they are studying. Experts in this field can judge the brightness of a variable star to less than a tenth of a magnitude.

 

STAR COLOURS

When we look up into the night sky the stars look much the same. Some stars appear brighter than other stars but they all look white. If the stars are looked at through a pair of binoculars some appear to be different colours. Many stars look quite orange in colour and some have a blue / white tint. The colour of a star depends on its surface temperature and indirectly on their size.

Generally the more massive a star is, the hotter and more powerful it will be. This is not the same a physical diameter mass refers to the amount of material in a star rather than how large the star is. If we can tell what the colour of a star is we can estimate how hot it is and therefore how large that star is.

The energy and heat produced by a star is directly proportional to the mass of that star. However a graph plotting mass against energy produced does not produce a straight line. The energy output of a star sharply increases with increasing mass. This produces a graph with a sharply increasing slope as the mass increases. A star that is perhaps ten times the mass of another star may be a million times more powerful.

 

GREEN WHITE   ............Type W & O    ..36000+ o C      ..... Giant very hot and active stars

BLUE   ............................Type B       ....... 28600+ o C   .........Very hot Helium stars

WHITE   ..........................Type A       ....... 10700+ o C    ....... Large hot stars

YELLOW WHITE   ........Type F       ......... 7500+ o C   ..........Stars larger and brighter than our Sun

YELLOW   .....................Type G       ......... 6000+ o C  .......... Like our Sun

ORANGE   .....................Type K       ........ 4800+ o C   ..........Cooler smaller stars

ORANGE RED   ...... ......Type M       ....... 3400+ o C   ..........Old dying stars

RED  .............................. Type N & S       ..2500+ o C    ..    ...Cool Carbon stars

 

When medium sized stars like our Sun are very young they tend to be very active and hot. Giant stars burn their fuel very fast and are also very hot both these types of star will shine white or blue or sometimes even green. Old stars become bloated into giants so the heat they produce and spread over a large surface area so they appear cooler and shine with a red colour much like when an electric fire element is cooling down. Very small stars don't produce so much heat so they also appear red and cooler.

By knowing the colour of a star the mass can be calculated. Then the absolute magnitude can be calculated. If the Absolute Magnitude is then compared to the observed Apparent Magnitude the distance that the star is from the observed point Earth can be calculated. By carrying out a simple analysis of the colour and brightness of a star a lot can be learnt about it.

The Type letter referred to in the table above, comes from a system that categorised stars in a sequence denoted by letters in the alphabet. However as knowledge grew the sequence was shuffled to reflect the true nature of the stars. Astronomers had however become used to the labels so the letters are not now in order.

 

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