How bright is that star? Magnitude explained

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Understanding Magnitude

How bright is that star? is actually a very good question and one I am asked many times on our courses and when we have visitors outside here at Astrofarm gazing at our night sky. To answer, the first thing we need to establish is “compared to what?” This may be another star or a planet.  Then we need to look at the scale on which star brightness is measured. This scale is called the “Magnitude Scale” and is counter intuitive when it comes to brightness because the scale has a middle point which is ‘0’ and goes up into + figures and down into – figures from zero with minus figures being brighter and + figures being fainter. The Apparent magnitude (m) of a celestial object is a number which is a measure of brightness as seen by an observer on Earth.

The table shows the magnitude scale for stars and planets which can be mistaken for stars because of their brightness.  The star Vega in the constellation of Lyra is our 0.0 star. This is helpful in the summer when Lyra is visible but not in the winter, so we use other stars in the winter such as Sirius (the brightest star in the northern hemisphere, next to our own star “The Sun”)

Sirius is -1.5 while the full moon, for example is -12.5 the Sun is -26.8 and naked eye limit, from a rural location is +6  or 6th magnitude and the space telescopes limit optically is +32.

The system started with Hipparchus and Ptolemy when they divided the stars into six magnitudes. About 20 of the brightest stars that they could observe from their location were assigned to the first magnitude. The next set of bright stars was assigned to second magnitude and so on. Sixth magnitude stars were assigned to stars that were barely visible to the unaided eye under good conditions. It was then theoretically determined that the ratio of first magnitude to sixth magnitude was 100 to 1.

Then a logarithmic scale of 2.512 between magnitude levels is to be implemented. So for example, a first magnitude star is 100 brighter than a sixth magnitude star or the sixth magnitude star is 1/100 or 100 times dimmer that a first magnitude star.

A star is 2.512 times brighter than a star one magnitude less. To make it slightly less complicated the table below shows only to 6th magnitude as we can only see up to 6th magnitude with the naked eye.

 

1 100 Times 2.51 x 2.51 x 2.51 x 2.51 x 2.51
2 39.8 Times 2.51 x 2.51 x 2.51 x 2.51
3 15.8 Times 2.51 x 2.51 x 2.51
4 6.3 Times 2.51 x 2.51
5 2.51 Times 2.51 x
6

 

With the advancement of photography and CCD astronomy this table is normally much longer.

Several factors affect your ability to see faint stars:

  • Your eyesight (Do you still have 20/20 vision?)
  • Dark adaption (Time given to let your eyes get used to the dark)
  • Clouds, haze
  • Nearby lights (Street lights, houses, security lights, villages)
  • Distant city lights (light pollution)

In order to see faint stars, you must allow your eyes to adapt to the dark. When you go outside at night your eyes will slowly become more sensitive to faint objects. Your eyes adjust to darkness by dilating their pupils: they may become about 7 millimetres wide, much larger than their normal size. It takes ten to twenty minutes for your pupils to open fully.

So let us take a constellation and look at the different stars within it. I am going to use Ursa Minor (Little Bear or Little Dipper) for a couple of reasons. Firstly, it is always visible from the Northern Hemisphere all year round. You can see from all the labelled magnitudes, in the image below, is that there is a good range of brightness’s from 2.0 (Polaris) the pole star to 6.5

measuring magnitude

Image courtesy of SPA  www.popastro.com

This is another reason why I used this constellation, as it is used by astronomers to check the quality of the sky. The fainter the star that can be seen the better the quality of sky and lack of light pollution. Next time you go out at night when it is clear, have a look for the constellation of Ursa Minor and see what magnitude you can see.

If you don’t know how to find the constellation, look for the Plough (Big Dipper) or Ursa Major, The Big Bear. Take the furthest two right hand side stars in the bowl and make a straight line away from the, what would be the open end, of the bowl until you come to a star. That is Polaris at the tip of the handle on the little dipper. (see below).

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Summary

The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its value. The Sun, has an apparent magnitude of −27 and it is the brightest object in the sky. We have fantastically clear, dark skies here at Astrofarm and so you can see many more stars – especially the fainter ones that you wont see in much of the UK. It is a great place to really see the difference in the brightness of the stars in our night sky.

 

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Andrew Davies
Astronomer at Astrofarm
Andrew is the resident astronomer at Astrofarm residential astronomy centre in the South of France. He has been hooked on the night skies since a schoolboy and has never tired of both solar and night time observing as often as he is able. As a teacher of both astronomy and photography classes Andrew has a wealth of knowledge and likes nothing better than to share with others.

He founded both the Mid Cheshire and Runcorn & Widnes Astronomy clubs and organises and presents the annual North West Astronomy Festival. He excels in outreach and inspiring beginners and loves to share his knowledge as well as to learn from others. In addition, Andrew is a keen photographer of both the night sky and the world around - you will see examples of his interest and skill throughout our website www.astrofarmfrance.com