doodlebright

Fact sheet · updated

HOW STAR BRIGHTNESS IS MEASURED

Smaller number,
brighter star.

Astronomers count brightness backwards.

Check your sky
A clear night sky packed with stars above a dark rolling hillside, with the band of the Milky Way rising on the right
AI-generated image, not a photograph of a real person or place.

MAG 6the naked-eye limit

The magnitude scale began as a simple 1 to 6 ranking of the stars over two thousand years ago. Today it runs from the Sun at -26.7 to the faintest things Hubble can detect, at about +30.

At a glance

100× brighter

Brightness across 5 magnitudes

A difference of 5 magnitudes is defined as exactly 100 times in brightness, so one magnitude is about 2.5 times. Venus and Sirius differ by about 3 magnitudes: Venus looks about 15 times brighter.

THE SCALE · AAVSO

61%

Could see 10 or fewer stars in Orion

In CPRE's 2020 Star Count, 61% of about 2,500 people could see no more than ten stars within Orion, a sign of severe light pollution and the highest figure since the counts began.

STAR COUNT 2020 · CPRE

TWO NUMBERS WORTH KNOWING

Five steps of brightness,
and a sky going grey.

One figure from the scale itself,
one from Britain's own star count.

The picture has since improved a little: the British Astronomical Association reports that in CPRE's 2022 count, 49% of participants recorded severe light pollution, down from the 2020 peak.

A QUICK SKY CHECK

How faint can
you see from here?

Pick where you are looking from. The value is the faintest magnitude you can expect to reach; the bar shows how deep into the sky that takes you.

Typical figures for people with normal eyesight on a clear night. Your own limit depends on your eyes, the Moon and the weather.

Sky check

Choose your view
An urban neighbourhoodAbout mag 3.0

Street lights and sky glow hide the fainter stars. Anything brighter than about magnitude 3, such as Sirius at -1.46 or Venus, still shows through.

A dark, moonless siteAbout mag 6.0

Roughly 9,000 stars in the whole sky are brighter than magnitude 6, and about 2,000 of them are above the horizon and visible at any one time.

Very keen eyes, dark siteAbout mag 6.8

A few observers with exceptional eyesight have reliably seen the spiral galaxy M81, which shines at magnitude 6.8.

With binocularsAbout mag 9.5

The faintest objects visible with binoculars are around magnitude 9.5, well past Uranus at its brightest (+5.5) and far beyond the naked-eye limit.

Sources: Las Cumbres Observatory, apparent magnitude table; Scientific American, What's the faintest star you can see in the sky? (2023).

THE SHORT ANSWER

Brightness is a
number you can read.

The scale looks odd at first.
Three ideas make it click.

THE SCALE

It runs backwards

The ancient scale gave the brightest stars magnitude 1 and the faintest magnitude 6. Today the brighter an object appears, the lower its magnitude, so the brightest go negative.

See the numbers

THE SKY

Lights wash it out

In an urban neighbourhood the naked-eye limit is about magnitude 3.0. At a dark, moonless site most people with normal eyesight reach magnitude 6.

Check your sky

YOUR EYES

Give them time

Your pupils widen in a few minutes, but eye sensitivity can take up to 20 minutes to build. White light spoils it; red light leaves it largely intact.

How to look
A pair of binoculars and a small torch glowing red resting on a wooden fence post at dusk, with the first stars appearing above a field
AI-generated image, not a photograph of a real person or place.

FOUR STEPS FOR TONIGHT

See fainter stars
without a telescope.

Nothing here needs special kit. A clear night, a little patience and a darker spot do most of the work.

  1. Pick a moonless night

    Choose a dark site and make sure the Moon is not up. That is where most people with normal eyesight reach magnitude 6.

  2. Wait twenty minutes

    Let your eyes adapt. Sensitivity can take up to 20 minutes to reach its full potential.

  3. Use red light only

    White light quickly spoils your night vision. Use a red torch, or the red-light setting on a star app.

  4. Count the stars in Orion

    In winter, count the stars you can see inside Orion's main shape. Ten or fewer means severe light pollution by CPRE's measure; 30 or more means truly dark skies.

APPARENT AND ABSOLUTE

Bright is not
the same as close.

Apparent magnitude only says how bright something looks from Earth. Venus outshines every star in our sky, yet it is far less luminous than any of them: it is just very close.

To compare true brightness, astronomers use absolute magnitude: how bright an object would look from 10 parsecs away. From there, our Sun would shine at only +4.7, about as bright as Jupiter's moon Ganymede looks to us.

A city skyline of apartment blocks at night under a thick orange glow, with almost no stars visible above the rooftops
AI-generated image, not a photograph of a real person or place.

Questions

Questions people ask

Why do brighter stars have lower magnitudes?

The scale started as a ranking: the brightest stars were first magnitude and the faintest sixth. Astronomers kept that order when they made the scale precise, so the brightest objects now have negative numbers.

What is the brightest star in the night sky?

Sirius, at magnitude -1.46. The next brightest are Canopus at -0.72 and Alpha Centauri, which looks like one star to the eye, at -0.3. The Sun, at -26.74, is far brighter than any of them.

How many stars can I see with the naked eye?

Roughly 9,000 stars are brighter than magnitude 6, the usual naked-eye limit from a dark site. Only about 2,000 are above the horizon and visible at any one time.

What is magnitude 0 based on?

In the 1850s astronomers chose the bright star Vega as the reference point, giving it magnitude 0.0. Fainter stars have positive magnitudes and the handful of brighter ones are negative.

Why do astronomers use red torches?

In the dark your eyes slowly become more sensitive, which can take up to 20 minutes. White light quickly undoes this, while red light leaves your night vision relatively intact.

Sources

Where the facts come from

  1. AAVSO: Magnitudes, measuring the brightness of stars
  2. Las Cumbres Observatory: Apparent magnitude
  3. Las Cumbres Observatory: What is absolute magnitude?
  4. Scientific American: What's the faintest star you can see in the sky? (2023)
  5. CPRE: Star Count 2020, explore what we learned
  6. British Astronomical Association: CPRE 2022 Star Count results

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