Star Brightness, Luminosity, and Magnitude. What’s Apparent?

Brightness, luminosity, and magnitude are terms that describe stars. But what do they mean? How do they relate to your night sky viewing experience? As a newbie, I found these terms a little confusing. So here I explore star brightness, luminosity and magnitude and what they mean for newbie backyard astronomers [+ diagrams and tables].

Why this is important: When looking to buy a telescope or astronomy binoculars, you might come across ‘magnitude’ in the specifications. Often this is written as ‘stellar magnitude‘ — stellar meaning star. It’s worth knowing what it relates to and what it means for your sky viewing experience.

Star brightness: Refers to object’s visibility from the perspective of where it’s viewed (apparent magnitude, V)

Luminosity: The word ‘luminosity’ means the state of reflecting bright light (Source: Cambridge dictionary). In astronomy, it refers to energy emitted from the celestial object’s surface, often as per unit of time.

Luminosity is the rate at which a star radiates energy into space.

Professor Barbara Ryden

Magnitude: In astronomy, it refers to apparent magnitude, which indicates a star’s brightness from a viewing perspective; absolute magnitude indicates a star’s brightness from a standard distance; and stellar magnitude is another way of expressing apparent magnitude.

star brightness expressed as apparent magnitude, also referred to as stellar magnitude
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Is luminosity and brightness the same thing?

Luminosity and brightness are not the same it seems. Luminosity represents how much energy a star emits from its surface, whereas a star’s brightness, expressed as apparent magnitude, relates to its visibility from a viewing perspective, which, for us, is Earth.

Star brightness is what you would care about as a backyard astronomer, but it’s good to know about luminosity, because it’s a factor in star brightness. Still confused? Read on…

Star brightness vs luminosity

What is the difference between luminosity and star brightness? Stargazers consider star brightness as how visible the star is from Earth. It’s expressed as apparent magnitude. This differs to a star’s luminosity, which tells us of the power of the star, the energy emission from its surface, measured in watts, joules per second, or per luminosity of the Sun.

The observed star brightness does not correspond to luminosity (with one hypothetical exception that I explain later).

Luminosity and absolute magnitude

Luminosity correlates with absolute magnitude, but on a reversed scale, meaning as luminosity goes up, magnitude moves down. The Hertzspring-Russell diagram below shows this relationship best. See how the positive high luminosity (>10000 Suns) of the red giants Betelgeuse and Antares corresponds to negative absolute magnitudes.

star types, Hertzspring-Russell-Diagram
Chart showing star luminosity as a measure of the Sun’s, and the corresponding absolute magnitude

When querying star brightness against luminosity and absolute magnitude, you’re probably wondering how apparent magnitude fits into the picture.

This has to do with distance…

Star brightness and distance

Star brightness expressed as apparent magnitude is the brightness of the star from a viewing perspective. In which case, the distance will vary.

As a star’s light emitted at its surface travels the distance to where it’s viewed, the light spreads out and its visual brightness diminishes.

Drawing to show how as light travels toward you, it spreads out to cover a larger area.
Inverse square law demonstrates how light spreads out and covers a larger area as it travels towards you. Image source: Wikimedia Commons

With absolute magnitude, the observing distance is confined to 10 par secs (aka 32.6 light years).

Here’s the hypothetical exception I mentioned earlier…if a star’s distance from Earth or any other viewing perspective is 32.6 light-years, then its star brightness (as apparent magnitude) would equal its absolute magnitude and indicate its luminosity.

The apparent magnitude of the star from a distance of 32.6 light-years equals its absolute magnitude.

Or, think of it like this…These two measures: apparent magnitude and absolute magnitude, are the same in a situation where the star’s distance from the viewing perspective is 32.6 light years.

The beauty of absolute magnitude is that it standardises the observing distance so you can compare stars according to their actual or absolute brightness rather than their apparent one.

Why do stars differ in brightness?

There are two reasons why stars differ in brightness to us: luminosity and distance from Earth.

We know that stars emit energy, and that this luminosity varies among them. Some are more luminous than others because they’re larger or hotter than others.

…we may say luminosity is the function of temperature and radius, and magnitude is the function of luminosity and distance.

Jim Tubb, amateur astronomer

Stars with the same luminosity will differ in brightness to us because of their distance from Earth. The human eye perceives objects as brighter when they are closer to us because there is more light available for our eyes.

This means that stars that are far away will appear dimmer than those not so far (with equivalent luminosity).

So luminosity and distance from Earth account for a star’s brightness in our sky.

It’s also important to note that the brightness of a star changes as it ages, from when it’s born to when it’s extinguished. I cover this further in my article on star types.

Why is star color and brightness important?

When we observe a star, its brightness and color can help astronomers understand the star’s distance from Earth, life span and size, as well as other features (sometimes).

How do we measure the brightness of stars?

Brightness of stars is measured by a scale of apparent magnitude, where stars with negative values reign brighter than those with positive ones. So bigger, meaning more positive magnitudes equate to dimmer stars.

planet apparent magnitude list, star apparent magnitude list, table of brightness for stars and planets to discover for backyard astronomers
*mean brightness from Earth. Source: Wikipedia “apparent magnitude”

The apparent magnitude has more relevance to us backyard astronomers as it is a scale of the brightness viewed from Earth. The units range from -30 to 30 on this magnitude scale.

The list alongside shows the apparent magnitude of stars you’re able to see in the night sky with astronomy binoculars (-1 and up to +9) as well as Proxima Centauri, the closest star to Earth, after the Sun.

Even with its proximity to us, Proxima Centauri is less bright than the other stars in the list. We can concur that this small red dwarf’s luminosity is low relative to the others.

Planet apparent magnitude

The planets are close in distance to Earth relative to the stars, apart from the Sun of course. You’ll find the mean planet magnitudes from Earth included in the list alongside.

From Greek times to now

It’s amazing to realise that this started long ago, in the era of emerging Greek philosophy.

Around 2100 years ago, Hipparchus, a Greek astronomer, decided to classify the stars by brightness. As told by Terence Dickinson (author of NightWatch: A Practical Guide to Viewing the Universe and some the best books for beginner astronomers) the Greek astronomer, Hipparchus, devised a system dividing stars into six magnitude levels.

“He designated the brightest as first magnitude and the faintest as sixth magnitude, with the others scattered in between,” Dickinson wrote.

A 19th century astronomer, Sir Norman Robert Pogson, tweaked it and formalised today’s system back in 1856.

It extends the earlier scale of Hipparchus’ to include stars apparent beyond those only seen by the naked eye and at the other end of the scale, objects brighter than the original 1st magnitude stars.

change in magnitude and ratio of brightness for stars seen from Earth
Star brightness ratio and magnitude difference

Our modern system for measuring magnitude rates some of Hipparchus’ 1 magnitude stars as zero and objects brighter with negative magnitudes.

Today’s magnitude scale charts objects between -30 (bright end) and +30 (faint end) with the Sun a magnitude of -27, the full moon a magnitude of -13, and Sirius, one of the brightest stars in the sky, an apparent magnitude of -1.

In it, a first magnitude star is 100x the brightness of a 6th, the limit of the human eye unaided (change in magnitude = 5). And, the brightness of the Sun (-27) is 16 trillion times that of a +6 star (naked-eye limit) — The difference between their magnitudes is 33, so times brighter is (2.512)33.

The accompanying chart shows how change in magnitude relates to ratio of brightness.

This magnitude scale is logarithmic. Each step to the next magnitude in the scale indicates an average change in brightness by a factor of 2.512. This compares to Hipparchus’ scale, which was designed with a factor of 2.

Summing it

This article is not exhaustive on all things related to star luminosity or magnitude but it will give you some concept of these terms and hopefully spur your interest in learning more.

Info sources

Nasa: Brightest Stars; Swin Edu: Absolute vs apparent; Jim Tubbs amatuer astronomer: on luminosity

  1. Dickinson, T. 2019. NightWatch: A Practical Guide to Viewing the Universe. Firefly Books.