Magnitude when it comes to astronomy refers to the brightness of an object. However, unlike earthquakes for example where the higher the number means bigger, in astronomy, the LOWER number means the brighter. The brightest objects in the sky are actually in the negative numerical values!
Long before modern astronomy took over, the ancient Greeks noticed the different levels of brightness in the stars, and measured them by their size as seen with the naked eye.
Later on, when astronomers figured out the stars’ brightness had to do with their distances, they classified them in an order of “classes.” The brightest stars would be the “first class” and then the next level set would be “second class” and so on until the dimmest stars seen were labeled “sixth class.”
With the help of mathematics using more logarithmic scales, they were able to figure out that first magnitude stars were 100 times brighter than sixth magnitude. This scale also made it possible to see that some stars were brighter than first magnitude, such as Sirius being mag. -1.46 and Vega being a magnitude 0 star.
There are two types of magnitude:
Apparent Magnitude – how bright the object appears from your location.
The Sun has an apparent magnitude of -27 as seen from Earth. That number increases the more the brightness decreases, as you get further away from the Sun. From the Kuiper Belt, the Sun’s apparent magnitude is anywhere between -18 to -16. From Alpha Centauri, the Sun is as bright as a magnitude 0.5 star!
Absolute Magnitude – has to do with the level of luminosity or reflected light of an object. It’s essentially how bright the object would be if it was 10 parsecs away (32.6 light years).
Pollux in Gemini is a star that’s ~10 pc away, so we can use that as an example.
The Sun’s absolute magnitude would be 4.83. That means from Pollux, our Sun appears rather dim, almost washed out if there was a hypothetical civilization living near Pollux.
Quasar 3C-273 is 2.6 billion light years away and appears as a magnitude 12.9 point of light. However, it’s actually so luminous that if this quasar was the same distance as Pollux is from Earth, it would appear as bright as our Sun!
What Are Some Other Examples?
The Moon| -12.9 (full)/ -10.5 (half)/ -8.4 (crescent)/ – 2.5 (new)
Venus|–4.9 max brightness/ -2.99 min. brightness
Jupiter| –2.94 max brightness/ –1.66 min. brightness
Mars|–2.94 max brightness/ +1.86 min. brightness
Sirius (Brightest Star in Night Sky)| -1.47
Saturn| –0.55 max brightness/ +1.17 min. brightness
Uranus| +5.2 max brightness/ +6.03 min. brightness
(Objects dimmer that mag +6 need a telescope to see!)
Neptune| +7.67 max brightness/ +8.0 min. brightness
Proxima Centauri (closest star outside our Solar System)| +11.05
Quasar 3C-273| +12.9
Pluto| +13.65 max brightness/ +16.3 min. brightness
Your Eye and Your Telescope Have Limits!
Everything in the sky has an apparent magnitude value, and when we are saying things like “I can see a mag. 4 star,” we are almost always talking apparent magnitude.
Every optical device, including your naked eye has limits in the apparent magnitude brightness. Your naked eye under good dark conditions has a limit of 6. Everything dimmer than mag. 6 needs a telescope to be viewed.
However, one must take into account the amount of light pollution in the sky, AND the conditions of the sky itself such as seeing and transparency.
Under an inner city sky, your naked eye limit can go as low as second magnitude depending on how brightly lit your surroundings are. Over Griffith Park in Los Angeles, the limit is around 4, but that’s IF the conditions are clear, with decent enough contrast and steady air.
And don’t forget, while your telescope can collect more light from celestial objects, it also collects the light pollution as well! So while my 8″ telescope can theoretically pick up anything mag. 14 and brighter, light pollution severely hinders that ability.
The other thing to note is that for deep sky objects, especially nebulae, globular clusters, and galaxies, the book value doesn’t mean it will appear as bright as you may think! M42 (Orion Nebula) is a fourth magnitude object and can actually be glimpsed with the naked eye, but it doesn’t appear as bright as a star of the same magnitude because the light is spread out.
A great example was when Comet 46P/Wirtanen made a close approach in December 2018. It was estimated to be a fourth magnitude object, but when I saw it through my telescope under a bright suburban sky, it looked nowhere near as bright as M42 usually does from my home. In fact, I could easily see some mag. 7 and 8 stars near it, but the comet was so widespread that it appeared very dim and ghostly. Had I gone to a darker location, I would’ve had a better view.