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How To Understand Magnitude in Astronomy

How To Understand Magnitude in Astronomy

AstroTelescopium Team |

In astronomy, magnitude (sometimes referred to as stellar magnitude), is simply the measure of a celestial object's brightness.

If you're interested in stargazing or astrophotography, understanding the concept of stellar magnitude is important. Particularly when deciding which objects you would like to observe, as well as which objects your telescope has the capability to observe.

In general, magnitude provides a standardized method for determining the overall brightness and dimness of the range of celestial objects in our night sky.

In this article, we will cover:


What is the Magnitude Scale?

When trying to determine the specific brightness of a star or other celestial object, a method of measurement is needed. That's where the magnitude scale comes in.

The magnitude scale is a numerical spectrum of integers that represent a 2.512 increase or decrease in brightness for each point change.

For example, a magnitude 1 star is 2.512 times brighter than a magnitude 2 star. Additionally, that same magnitude 1 star would be 100 times brighter than a magnitude 6 star. The calculation for this would be as follows: 2.512 x 2.512 x 2.512 x 2.512 x 2.512 = 100.

Although, the inverted number rating may seem counter-intuitive initially, keep in mind that the lower the value of an object's magnitude, the brighter the object appears. In fact, the brightest objects have negative values on the magnitude scale.

It is also important to note that the magnitude scale assumes no atmospheric effects. These calculations are based on clear observation of the object without any interference such as local light pollution.


Origins of Magnitude

In the second century BCE, the Greek astronomer Hipparchus was believed to have first cataloged the apparent brightness of stars. However, Claudius Ptolemy, an Alexandrian astronomer, coined the term magnitude and classified the stars on a formalized six point scale.

This six point scale ranged from very bright 'first magnitude' stars to the most faint 'sixth magnitude' stars.

However, the logarithmic magnitude scale currently being used was proposed in 1856 by the English astronomer Norman Robert Pogson. He standardized the Pogson's Ratio, which is the fifth root of 100 calculation that describes how a first magnitude star is 2.512 times brighter than a second magnitude star.

In order to create a reference point for the scale, it was necessary to choose a star to represent magnitude 0.0. The star chosen initially was Polaris (the north star). However, since Polaris was a variable star (i.e its brightness varied), the star Vega was selected instead.

Therefore, any object with a positive apparent magnitude on the scale would be considered dimmer than Vega. Conversely, any object with a negative apparent magnitude on the scale would be considered brighter than Vega.

Today, however, due to more precise measurements, the star Vega is actually measured at magnitude +0.03.


Magnitude Types - Apparent vs Absolute

There are two main types of magnitudes referred to in astronomy. Apparent magnitude and absolute magnitude. Here, we will describe the differences.

Apparent Magnitude, represented as m, is the brightness of an object as it appears in the sky from Earth's surface.

This is the type of magnitude that is most often referenced when discussing a celestial object's brightness.

Absolute Magnitude, represented at M, is the brightness of an object as seen from a standard distance of 10 parsecs which equates to 32.6 light years (one light-year is a unit of length equivalent to 5.88 trillion miles or 9.46 trillion kilometers).

By providing a standard distance for each object, absolute magnitude is more representative of how bright an object actually is. This makes it useful in stellar astrophysics.

Since we are located here on Earth, apparent magnitude is more relevant to our experience with observational astronomy and every day stargazing.


Brightest Magnitudes of The Planets and Stars

Below, we've provided a chart that displays the apparent magnitudes of many of the most popular celestial objects in our night sky.

Object Apparent Magnitude (m)
Sun -26.74
Full Moon -12.90
International Space Station -5.90
Venus -4.92
Jupiter -2.94
Mars -2.94
Mercury -2.48
Sirius -1.47
Canopus -0.72
Saturn -0.55
Arcturus -0.04
Alpha Centauri A -0.01
Vega +0.03
Capella +0.08
Rigel +0.12
Procyon +0.34
Achemar +0.46
Betelgeuse +0.50
Antares +0.60
Hadar +0.61
Acrux +0.76
Altair +0.77
Aldebaran +0.85
Spica +1.04
Pollux +1.14
Formalhaut +1.16
Becrux +1.25
Deneb +1.25
Regulus +1.35
Adhara +1.50
Castor +1.57
Gacrux +1.63
Shaula +1.63
Polaris +1.98
Andromeda Galaxy +3.44
Orion Nebula +4.00
M41 +4.50
Vesta +5.20
Uranus +5.38
M33 +5.72
Naked Eye Limit +6.00
Pallas +6.49
Ceres +6.64
M81 +6.90
Neptune +7.67
Hygiea +8.94
Typical 7x50 Binoculars Limit +9.50
Pluto +13.65
Chiron +15.40
Eris +18.70
Hubble Space Telescope Limit +31.50
James Webb Space Telescope Limit +34.00


Start Observing

With a much clearer understanding of how to determine a celestial object's brightness, it's time to go ahead and begin observing.

While many objects listed on the chart above can be viewed with the naked eye, the more exciting observations require a good pair of binoculars or a quality telescope.

If you're interested in learning more about how to observe our celestial neighbors in the sky, you can take a look at How to Observe the Planets in 2023.

Or, if you're considering purchasing or upgrading your astronomy gear, we carry a curated selection of binoculars, telescopes and accessories from industry-leading brands.

Feel free to browse our telescope collection or browse our binoculars.


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