Why Magnitude Lies
Why a magnitude 8.4 galaxy can be easier than a magnitude 5.7 one
Surface brightness, not total magnitude, determines whether a deep-sky object will survive Bortle 9. Magnitude is for stars; surface brightness is for sensors.
When I started astrophotography, I made the same mistake most beginners make: I picked targets by magnitude.
Magnitude 5.7? That’s brighter than magnitude 8.4, so it should be easier to image. Right?
Wrong.
M33, the Triangulum Galaxy, has a listed magnitude of 5.7. M82, the Cigar Galaxy, is magnitude 8.4. Yet M82 is dramatically easier to capture from light‑polluted skies. The Cigar Galaxy reveals its edge‑on profile and dust lanes in a reasonable integration time. Triangulum? It hides, even after an hour.
The problem isn’t the data. The problem is using the wrong number.
The Magnitude Trap
Magnitude measures total brightness — all the light from an object summed into one value. That works fine for stars: point sources whose light falls onto essentially a single pixel.
But deep‑sky objects are extended sources. Their light is spread across thousands of pixels. A galaxy may have the total brightness of a fifth‑magnitude star, but that light is smeared across an area half the size of the full Moon.
What your camera actually records at any given pixel isn’t total magnitude. It’s surface brightness — the amount of light arriving per unit area.
The Math Behind Surface Brightness
Surface brightness is measured in magnitudes per square arcsecond (mag/arcsec²). The formula is:
SB = m + 2.5 × log₁₀(A)Where m is total magnitude and A is the object’s area in square arcseconds.
That 2.5 factor isn’t arbitrary. It comes from Norman Pogson’s 1850s definition that a 5‑magnitude difference corresponds to exactly 100× in brightness. The math works out to 2.5 being the link between brightness ratios and magnitude differences.
A Worked Example
Take M31, the Andromeda Galaxy:
- Visual magnitude: 3.4
- Size: 3.3° × 1.2° (11,880" × 4,320")
- Area (ellipse): π × 5,940 × 2,160 ≈ 40.3 million square arcseconds
Applying the formula:
SB = 3.4 + 2.5 × log₁₀(40,300,000)
SB = 3.4 + 2.5 × 7.6
SB = 3.4 + 19.0
SB ≈ 22.4 mag/arcsec²Despite being one of the brightest objects in the sky by total magnitude, Andromeda’s surface brightness is a relatively dim 22.4 mag/arcsec² — right at the edge of what’s comfortable from Bortle 9 skies.
You don’t have to calculate this by hand:
For most deep‑sky objects, you can look up surface brightness directly on Telescopius.com — just search for your target and check the “Surf. Brightness” value. The Andromeda Galaxy, for example, shows 22.4 mag/arcsec², matching our calculation.
A Rule of Thumb for Light‑Polluted Imaging
Based on my experience and the data I’m gathering for the Bortle‑9 Imaging Index, here’s a rough guide:
| Surface Brightness | Expectation from Bortle 9 |
|---|---|
| < 21 mag/arcsec² | Should work well in one hour |
| 21–23 mag/arcsec² | Challenging—depends on conditions and filters |
| > 23 mag/arcsec² | Likely needs dark skies or many hours of integration |
M82, the Cigar Galaxy? Surface brightness around 21.2 mag/arcsec² — challenging but doable.
M33, the Triangulum Galaxy? Around 23.0 mag/arcsec² — a struggle from urban skies.
Same object class, similar techniques — but nearly 2 mag/arcsec² difference. The “dimmer” galaxy by total magnitude is actually the easier one to image.
When Surface Brightness Doesn’t Tell the Whole Story
Surface brightness is most predictive for broadband targets — galaxies, reflection nebulae, and clusters. But emission nebulae follow different rules.
Take the Rosette Nebula: magnitude 9.0, surface brightness 27.1 mag/arcsec². On paper, it should be nearly impossible from Bortle 9. Yet with a dual‑band filter and just 16 minutes of integration, it’s surprisingly workable.
Why?
Emission nebulae emit light at specific wavelengths (primarily Ha at 656nm). A dual‑band or narrowband filter blocks most light pollution while letting those key wavelengths through. The listed surface brightness is based on broadband light — not what your filtered sensor sees. In Ha, the Rosette’s effective surface brightness relative to your sky background is far better.
The pattern emerging:
- Galaxies & reflection nebulae: surface brightness is the key predictor
- Emission nebulae: filter response dominates; SB becomes secondary
- Planetary nebulae: often bright in OIII, making them easier than their size suggests
This is why the Bortle‑9 Imaging Index tracks object type alongside surface brightness. Emission targets likely need a different predictive model than broadband objects.
Quick Field Reference: Reading ASIAIR’s “Tonight’s Best”
ASIAIR shows magnitude and size but not surface brightness. For broadband targets, here’s how to read the list realistically:
| ASIAIR Shows | Surface Brightness | Likely Tier |
|---|---|---|
| Low mag (bright) + small size | Low SB number (easy) | 4-5 (Go for it) |
| Low mag (bright) + large size | High SB number (hard) | 2-3 (Caution) |
| High mag (dim) + small size | Moderate SB | 3-4 (Worth a try) |
| High mag (dim) + large size | High SB number (hard) | 1-2 (Likely frustrating) |
For emission nebulae, be more optimistic — filters change the equation in your favor.
Remember: a lower surface brightness number means more light per pixel—which is what your sensor actually cares about. The magnitude scale runs backwards from intuition.
Why This Matters
One goal of the Bortle‑9 Imaging Index is to test how reliably surface brightness predicts imaging difficulty from urban skies — and when other factors take over. I expect SB to be one of the strongest correlates for broadband targets, but emission nebulae will likely follow different curves.
What’s already clear is this: total magnitude alone is misleading. For urban imaging, surface brightness gives you a far better first read. It won’t tell you everything — object type, filter choice, altitude, seeing, and gradients all matter — but it gets you much closer to the truth than a number designed for naked‑eye observers.
Conclusion
In the end, magnitude isn’t lying to you — it’s just telling the part of the story that matters least under Bortle 9 skies. What determines whether a target will survive a one‑hour session is surface brightness relative to your sky background. That ratio sets the contrast your sensor actually records. When you start evaluating targets this way — not by total magnitude, but by how well their surface brightness stands up to your skyglow — everything clicks. You stop blaming your gear, stop chasing misleading numbers, and start choosing objects your sky will actually let you reveal. It’s a small shift in thinking, but a freeing one.
Clear skies,
Pete