Not all stars in the sky shine at the same brightness.  Some are obviously brighter than others.  Astronomers refer to the brightness of an object as it's magnitude. At first the scale in current use may seem a bit odd, with negative magnitudes being much brighter than positive one but it is an open-ended scale that suits us quite well.

The magnitude scale has a historical background in ancient times, with the first mapper of the stars Hipparchus.  He devised a scale from 1 to 6 to describe the differences he saw.  Magnitude 1 stars were the brightest stars and magnitude 6 stars were those he could just barely see. 

The magnitude scale used today was first proposed by N. R. Pogson, in 1856.  He proposed that stars which were separated by 5 magnitudes of brightness (as Hipparchus originally had) should have a difference of 1 to 100.  This means that a star one magnitude brighter than another is brighter by 2.516 times (which is the fifth root of 100).  This is not easy for most people to picture! 

In practise a few general guidelines to understanding magnitudes apply.  As mentioned before the lower the magnitude the brighter the object.  Venus shines at a steady magnitude of -4, which is bright enough to be seen during the day.  Jupiter shines at mag. -2.  The brightest star in the sky is Sirius, the Dog Star, which shines at mag. -1.4.  The closest star we can see unaided, alpha Centauri, shines at only -0.2.  There are a small number of stars about mag. 1.0, a few more than that shine at mag. 2.0, and so on.  As more and more stars are registered as we go to fainter magnitudes.  The nominal limit for the naked eye is considered to be magnitude 6.0 but depending on where you are situated this will vary widely.  In the inner suburbs you should be able to see 4th magnitude stars.  Away from the city on a steady night you may see as faint as 7th magnitude and it is possible to go fainter than that again!.

Binoculars and telescopes will help you to see fainter magnitudes than this.  A pair of 10x50 binoculars will let you see around 8th magnitude.  Most small telescopes will help you see to 10th magnitude and beyond, depending on their size.  Telescopes are designed to be light collectors rather than light magnifiers, so the bigger the size of the aperture the fainter you will be able to see.

Most star charts will represent the difference in magnitude by having different size dots for the stars.  The brighter stars have bigger dots, mimicking what you see in the sky. 

There are a couple of other terms related to the magnitude of star brightness.  Absolute Magnitude is used for comparing the brightness of one star to another if they were the same distance away from the Earth.  This set distance is 10 parsecs, or 32.6 light years.  Limiting Magnitude refers to the faintest magnitude detectable by a given means.  This may be with the eye looking at the sky, on a photograph or the faintest star detectable by a telescope. 

Below is a photograph of the Magellanic Clouds, small, irregular companion galaxies to our own Milky Way.  In it you can see stars of different magnitudes.  The brightest "star" in the picture is a globular cluster known as 47 Tucanae.  It's listed magnitude is 4.0.  Next to it is the Small Magellanic Cloud (SMC) which has a magnitude of 2.3.  The Large Magellanic Cloud (LMC) on the left has a magnitude of 0.1.   How does that work out?  When describing the magnitude of such spread out objects like galaxies and nebulae it is given as if it were a defocused star.  So in the case of the SMC, if you defocused a mag. 2.3 star so that it covered the same area as as the galaxy does they should match in brightness.  A similar method  is used to estimate the brightness of comets, as they are often faint and diffuse objects. 

The Large and Small Magellanic Clouds
The Large (LMC) is on the left, the Small (SMC) is on the right.
Just to the right of the SMC is the globular cluster 47 Tucanae.
The bright knot in the lower left of the LMC is the Tarantula Nebula (NGC 2070)

Surface Brightness is a term you may hear used when describing galaxies and nebulae.  A low surface brightness means that although the galaxy is listed as bright, when you try to find it in a telescope it looks very dim (that's if you manage to find it)  as it is spread out over a large area.  If this is the case with your telescope the only solution to get a better image is to get a bigger telescope to collect more light.  Magnifying the image will not make it brighter - the image will in fact get dimmer as you are only looking at a portion of the original image.  Examples of this effect are the Magellanic Clouds, discussed above, M83 in Hydra and the Helix Nebula in Aquarius.  In contrast a galaxy with high surface brightness would be M104, the Sombrero galaxy in Virgo.  Most of these objects can be seen with binoculars too.



 © Stargazers Astronomy,  2001.