What Do We Mean By Magnitude?

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.”

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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.

Using this math when it comes to telescopes, one can see that

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).

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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 at 33.72 light years away, our Sun appears as a dim star barely visible to the naked eye to an exo-astronomer living on an exoplanet orbiting that star – and would be washed out by light pollution from a city on that exoplanet.

As soon as one travels beyond 50 light years away, our Sun would not only need a telescope to bring it into view, but our sun would look just like any dim background star and thus be hard to identify under the best conditions!

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!

Even if this quasar replaced Sagittarius A* at the center of our galaxy at 23.000 light years away, it would STILL be so bright that it would feel like having a second but dimmer “sun” in the sky. Oh… and if that were also to happen, life on earth wouldn’t be able to survive the x ray and gamma radiation!

What Are Some Other Examples?

This list shows you that there are objects that are thousands to millions of light years away that are brighter than nearby dwarf planets too small and too dim to see in a small telescope.

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 steady air and enough contrast.

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 as dim as mag. +14, light pollution severely hinders that ability. Even if I was under a perfectly dark sky, then the transparency of the sky and seeing conditions determine how dim I can actually see.

With your naked eye limit typically being limited to +6 under the best conditions, and an 8″ Telescope having a theoretical magnification limit at +14, that means such telescopes can theoretically see things about 1500-1600 times dimmer than what your naked eye can see!

But with that said, anything dimmer than +14 means no amount of long exposure will bring the object into view. So to see those dimmer objects, one needs a bigger telescope!

Note how there are more stars visible in the famous “Pillars of Creation” in the James Webb image versus the famous Hubble image. It’s very simple… Besides the differences in wavelengths between infrared and visible, The James Webb telescope is nearly three times wider, and collects over seven times more light!

The Magnitude Figure in Brightness is THERORETICALLY… Not Necessarily Absolutely….

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!

Objects like the Orion Nebula or Andromeda Galaxy are said to be third to fourth (apparent) magnitude objects. But that applies to the source of light being sharp points, not spread out over arc minutes to degrees across. As such… seeing them through a telescope one can see how much dimmer the large structures actually appear compared to the foreground/background stars – They’re easier to see despite calculated to be dimmer in apparent magnitude!

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 with more contrast, but the comet still would not have been anything to write home about.