Star Magnitude – Just What Can You See

Star Magnitude – Just What Can You See in the Night Sky?

How do you know if you are even able to see a deep space object in the night sky? You could be pointing your telescope right at the object you are looking for and never know it, simply because it is too dim for you to see. You can waste countless hours searching for a night sky object that is just too dim for the gear you are using or the location you are observing from. Read on to learn more about how to recognise which objects you should be able to see to avoid wasting your time trying to observe the dimmest of objects.

What is magnitude?


A simple description of magnitude is that it is a measure of how bright an object is in the sky. The higher the number the dimmer the object is. This is a logarithmic scale so a magnitude one star is 100 times brighter than a magnitude six star. Knowing the magnitude of a star will guide you in deciding the stars you should spend your time trying to observe. The lower the number, generally, the easier it will be to see. When it comes to deep space objects the surface brightness is more important.

Why is Surface Brightness Important?

When it comes to observing deep space objects the surface brightness is more important than the magnitude of the object. A simple way to understand the magnitude is to imagine all the light from the object being collected together to a point, like a star. The magnitude is the brightness of this point. 

The problem is deep space objects are larger in the sky and so spread the light out. Generally speaking the bigger the object the more it's light is spread out and so the dimmer it will actually be. This is why you are unable to see say a magnitude 6 deep space object. As the light is spread over a larger distance meaning that practically speaking the magnitude is much higher. 

Triangulum galaxy.jpg

The Triangulum galaxy is visible to the naked eye but is notoriously difficult to observe under minimal light pollution (even with a telescope) due to it being a diffuse object. (Image by Hewholooks)

Surface brightness attempts to provide an average magnitude for each unit of area the deep space object occupies. When viewing deep space objects it is important to look at the surface brightness to ascertain if you'll be able to see the object with your telescope and location. However, this is an average of the magnitude across the whole object. Some large objects (like M42, Orion's nebula) are very large with a bright centre. Orion's nebula can be seen with the naked eye but it has a surface brightness of 11. So, you can't rely on the magnitude or surface brightness to determine if you'll be able to see an object. Use the image, the magnitude and the surface brightness to see how much light the object has, how far the light is spread out and  how uniform the light is in the object.


What magnitude can I see?


There are many factors that will affect your ability to see dim objects. These include your dark vision adaptation, telescope, gear, light pollution, atmospheric changes (seeing), distance from the zenith and your ability and experience using a telescope.


Your telescope improves your ability to see objects. How much a telescope can increase your ability to see dim objects can be calculated. There are many complicated algorithms to work out your limiting magnitude.


You can obtain a rough idea of the magnitude you can see with your telescope by adding the darkness of your night sky (see below) to the number next to your telescope aperture size in the table. This will give you an estimate of the dimmest object you could theoretically see, termed your limiting magnitude.


4 inch telescope (102mm) – 6 magnitude

5 inch telescope (127mm) – 6.5 magnitude

6 inch telescope (150mm) – 7 magnitude

8 inch telescope (200mm) – 7.5 magnitude

10 inch telescope (250mm) – 8 magnitude

Magnitude you can see with a telescope

Dimmest star seen by naked eye + Your telescope magnitude


How dark is my night sky?


Under a dark sky the highest magnitude stars visible to the naked eye are around magnitude 6.5.

Effect of light pollution on clouds.jpg

Image comparing a dark sky with a light polluted sky. Note the clouds look black under a dark sky. Image by: Christopher Kyba and Ray Stinson

Most suburban night skies range between a magnitude of three to five. You can determine the limit of magnitude by looking at the sky with your naked eye and observing the faintest star you can see, then consult a sky chart to see what magnitude it is. Alternatively, there are charts that you can use that have been invented by meteor observers. These charts take an area of a constellation and ask you to count how many stars you can see in the area. The number of stars you can see gives you an indication of the limiting magnitude of the sky to the naked eye.

Light pollution is the orange glow from lighting in suburban areas. It can severely reduce the number of stars that you can see and hence, the number of deep space objects you can see too. Light pollution tends to be worst near the horizon and at it’s least near to the zenith (directly overhead). So the darkness of your night sky changes depending on where in the night sky you are trying to observe. You have a much better chance of seeing dim deep space objects near to the zenith because of this.


This is a simplified explanation of astronomical magnitude with the aim to give amateur astronomers a working method to help to know what they can observe in the night sky. More detailed information on apparent magnitude, limiting magnitude and absolute magnitude can be found by following the external links.

Related Posts:  Observing

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