Stellar magnitudes: how to measure stars’ brightness

What is stellar magnitude, and how do you measure a star's brightness?

Stars in Southern Hemisphere night sky, looking towards the centre of the Milky Way. Credit: Kim Westerskov / Getty Images

If you could be teleported back to the time of the ancient Greeks, one of the best people to seek out would be astronomer and mathematician Hipparchus. His initial thoughts about the night sky were probably the same as many of ours: not all stars and astronomical objects are the same brightness.

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Hipparchus called this varying in brightness ‘magnitude’, a term that is often abbreviated to just ‘mag.’ He catalogued the brightness of stars into six magnitudes.

The 20 brightest stars were put in magnitude 1, or the ‘first magnitude’. Slightly fainter stars fell into magnitude 2, and so on right down to magnitude 6, which were the faintest stars Hipparchus could see with his eyes.

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Our modern system is more accurate, with the mathematical difference between one magnitude and the next being about 2.5 times in brightness.

This means a first magnitude star is around 100 times brighter than a sixth magnitude star.

Useful magnitude references

  • +3.0 Faintest stars visible in a light-polluted urban sky
  • +6.5 Faintest stars visible at a dark-sky site
  • +9.5 Faintest stars visible with 10×50 binoculars
  • +30 Faintest stars visible with the Hubble Space Telescope
The stars of the Summer Triangle asterism. Star Vega (top left), is the 5th brightest star in the night sky. Altair can be seen below in the middle and Deneb is far left. Credit: A. Fujii
The distinct stars of the Summer Triangle asterism. Star Vega (top left), is the 5th brightest star in the night sky. Altair can be seen below in the middle and Deneb is far left. Credit: A. Fujii

Today we include all objects in the sky, not just those visible to the naked eye, and have shuffled some of the stars around.

For instance, a few stars have jumped into a higher brightness magnitude than 1. Indeed, the star Vega has the magical magnitude of zero.

And the confusion does not end there, for stars can pass zero into brighter magnitudes where the numbers become negative. Take Sirius, the brightest star in the night sky, which is mag. –1.4.

All this magnitude information only tells us how bright an object looks to us in the sky: its apparent, or visual, magnitude.

This doesn’t tell us anything about how bright the object really is, because all the things in space are at various distances from us. So a very dim star might be just a very long way away and actually be very bright, and vice versa.

Sirius is the brightest star in the night sky. Photographic by Tommy Hsu / Getty Images
Sirius is the brightest star in the night sky. Photographic by Tommy Hsu / Getty Images

Ancient Greeks like Hipparchus knew nothing about these great distances, and indeed neither do we when we stargaze – everything looks like it sits on a great ‘celestial sphere’ around us.

If you could line up all the stars at the same distance away from us then we could see exactly how they differ in brightness.

Our modern technology has actually allowed us to do this, and we can now theoretically line up all objects at a certain distance (32.6 lightyears) to get their true or absolute magnitude.

Once you start comparing the difference between a star’s absolute and apparent magnitudes, you’ll be even more amazed just how big space is.

10 brightest objects in the sky

  1. The Sun mag -26.7 as seen from Earth
  2. The Moon mag. -12.6 when at full Moon
  3. Venus mag. -4.7 when at its brightest
  4. Mars mag. -2.9 when at its brightest
  5. Jupiter mag. -2.9 when at its brightest
  6. Mercury mag. -1.9 when at its brightest
  7. Sirius mag. -1.4 the brightest star in the night sky
  8. Canopus mag. -0.7 the second brightest star in the night sky
  9. Saturn mag. -0.3 when at its brightest
  10. Alpha Centauri mag. -0.01 leading star in the constellation Centaurus
Magnitude is represented on star charts by size: Polaris is brighter than Beta (β) Ursae Majoris. Credit: Pete Lawrence
Magnitude is represented on star charts by size: Polaris is brighter than Beta (β) Ursae Majoris. Credit: Pete Lawrence

How to judge stellar magnitudes

Sometimes people say that a star is really big. Of course all stars are so far away that they only ever appear as points of light to your eye. It’s what your eye does with that dot that can make them seem bigger.

In fact, drawing bigger dots is the only way of showing the difference in stars’ brightnesses on a star chart. The brighter stars have the biggest dots.

There will generally be a key to the dots and what magnitude they represent nearby.

It’s a good idea to get acquainted with the faintest stars you can see in your usual night sky: your magnitude limit.

Take a star chart and look for the smallest dots (the faintest stars) close to, or in a recognisable constellation and see if they’re visible in the sky.

Then you’ll know how much your viewing pleasure is affected by light pollution.

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This guide originally appeared in the June 2009 issue of BBC Sky at Night Magazine.