The fundamentals of astronomy for beginners
EXPLAINER
Brightness vs luminosity
All brightness is relative. Scott Levine reveals how we categorise our favourite stars
One of the first things many of us learn when we start stargazing is that Sirius (Alpha Canis Majoris) is night’s brightest star. But what does it mean to say that something is bright, and what’s the difference between brightness and luminosity?
Astronomers use different terms to talk about how much light we see from celestial objects and how much light those objects actually produce. But first, let’s talk about magnitude.
What is magnitude?
We generally talk about celestial objects in terms of magnitude. This doesn’t measure how much light exists; rather it describes objects’ brightness relative to each other. The Greek astronomer Hipparchus devised a scale in the second century BC, grouping stars by their brightness: the brightest stars at 1 and the dimmest stars at 6.
Today, objects with lower magnitudes are still listed as brighter than objects with higher magnitudes, and we use negative numbers for the brightest objects. So a star of mag. –1 is brighter than a star of mag. +2. Objects of about magnitude +1.5 or brighter are ‘first-magnitude’, objects from +1.5 to +2.5 are second, +2.5 to +3.5 are third, and so on.
Apparent magnitude, visual magnitude or simply magnitude, is the brightness we see when we observe our favourite stars. Hipparchus set Vega (Alpha Lyrae) at 0.0 on this scale and all other magnitudes are generally based on it (Vega’s has since been refined to +0.03). Under dark skies, stars as dim as +6.5 are visible with the naked eye. The Sun’s apparent magnitude is –26.74. It’s the brightest object in our sky by far. The average full Moon’s is about –12.75, and at its brightest Venus’s apparent magnitude reaches about –4.9.
But just as a neighbour’s window lamp appears brighter than a distant streetlight, what we see is a combination of how much light an object creates, and its distance. So two identical stars at different distances can have different apparent magnitudes.
While apparent magnitude gives us an idea of how we see stars, no matter their distance, absolute magnitude puts them on equal footing. Instead of saying, “I think that one’s brighter,” we compare how bright objects would be if they were all 10 parsecs (32.6 lightyears) away (about the same distance as Arcturus (Alpha Boötis).
We can then talk about objects in terms of how much light they produce: how luminous they are. Some very distant stars create so much energy that we see them as among the night’s brightest, despite them being so far away.
Distance and luminosity
With this in mind, let’s go back to the Sun. It’s bright enough to bring us daylight, but its absolute magnitude is only about +4.83. That’s about the same as the apparent magnitudes of Delta Ursae Minoris at +4.34, and Eta Ursae Minoris at +4.95. Both are more luminous than the Sun. From Earth, though, they’re quite faint, just like most of Ursa Minor’s stars, and they give us an idea of what our Sun would look like if it lay near Arcturus.
Two of the sky’s brightest stars, Sirius and Rigel (Beta Orionis), appear near each other and are a spectacular sight on a dark night. But Sirius (apparent magnitude –1.46) is about 8.6 lightyears away, while Rigel (apparent magnitude +0.13) is far behind, around 700 lightyears.
Sirius is about twice the size of the Sun and has an absolute magnitude of +1.43. Meanwhile, Rigel a supergiant 18–24 times more massive than the Sun, has an absolute magnitude of –7.84. So while nearby Sirius appears brighter, the absolute magnitudes tell us distant Rigel is more luminous, and far outshines the brightest star in the night sky.
Next time you look up at the stars, consider absolute and apparent magnitude and imagine just how bright the stars you’re observing actually are.
How bright are the brightest stars?
The sky’s brightest stars are not always its most luminous. Here are the five brightest stars visible from the UK, sorted by their apparent magnitude (brightness) and absolute magnitude (luminosity)
Scott Levine is an astronomy writer and naked-eye observer based in New York’s Hudson Valley.