Apparent Magnitude

By Steve Mohr

One of the most important things I wish I knew about at the beginning of my astronomy voyage, was “apparent magnitude”, and how this determines what you can see with the equipment you have.

As per the Wikipedia encyclopaedia, the apparent magnitude (m) of a star, planet or other celestial body is a measure of its apparent brightness as seen by an observer on Earth. The brighter the object appears, the lower the numerical value of its magnitude. Here the brightest stars are said to be of first magnitude (m = 1), while the faintest were of sixth magnitude (m = 6), the limit of human visual perception (without the aid of a telescope). Each grade of magnitude was considered to be twice the brightness of the following grade (a logarithmic scale).

Assuming you have a nice and dark location reasonably free of city lighting (referred to as “light pollution”), the scale to the right provides a best case situation on what can be seen with what level of equipment you have.

Obviously, few of us will ever be in control of a 200″ scope, or have our own Hubble, but a 3″ to 8″ scope is very much in the price range of most hobbyists. And as you can see from the scale on the right, objects up to a Visual limit of about +14 will be capable for those with an 8″ telescope. Now remember though, this limit presumes the best conditions for viewing.

What could you expect to see when within 10km’s of a major city (say 3-4 million people). Well, conditions do vary from location to location, but assume that if you can get yourself at least 10km’s away from the city skirts/boarder, then firstly consider objects up to +6 apparent magnitude as being good possibilities.

When considering Messier objects, the following are good possibilities for first time viewing:

M45: magnitude +1.37

M7: magnitude +3.50

M44: magnitude +4.00

M6: magnitude +4.50

M47: magnitude +4.50

M31: magnitude +4.50

M25: magnitude +4.90

Now this is a short list of some of the brighter Messier objects, there are 110 objects in this list. One thing to remember, your viewing site and atmospheric conditions will greatly effect your viewing quality. From night to night, objects will appear differently, and things like upper atmosphere wind conditions and heat emissions will alter viewing conditions. Ideally, cool, calm and stable conditions will provide optimum viewing.

What are these Messier objects?

Well the Wikipedia encyclopaedia describes it as follows:

The Messier objects are a set of astronomical objects catalogued by Charles Messier in his catalogue of Nebulae and Star Clusters first published in 1774. The original motivation behind the catalogue was that Messier was a comet hunter, and was frustrated by objects which resembled but were not comets. He therefore compiled a list of these objects.

The first edition covered 45 objects numbered M1 to M45. The total list consists of 110 objects, ranging from M1 to M110. The final catalogue was published in 1781 and printed in the Connaissance des Temps in 1784. Many of these objects are still known by their Messier number.

Because Messier lived and worked at astronomy in France in the Northern Hemisphere, the list he compiled contains only objects from the north celestial pole to a celestial latitude of about –35°. Many impressive Southern objects, such as the Large and Small Magellanic Clouds are excluded from the list. Because all of the Messier objects are visible with binoculars or small telescopes (under favorable conditions), they are popular viewing objects for amateur astronomers. In early spring, astronomers sometimes gather for “Messier marathons”, when all of the objects can be viewed over a single night.