The North America Nebula

Last weekend I awoke in the middle of the night to find the sky unexpectedly clear and superbly transparent. So I grabbed my 10×30 and 15×70 binoculars and spent two delightful hours outside before dawn interrupted my observing session.

I viewed many fine objects, but one that particularly interested me was NGC 7000, the North America Nebula. This patch of cloudy light near the brilliant star Deneb in Cygnus is many times bigger than the full Moon. It's famously hard to see in the presence of light pollution, but this night was as good as it ever gets at my country house. (For those who are interested in these things, my Sky Quality Meter read 21.35.) And the nebula was almost directly overhead.

The North America Nebula shines primarily in the hydrogen-alpha wavelength. This reddish light shows well in photographs, but human night vision is very insensitive to it. Instead, we most see the much fainter greenish light emitted by oxygen atoms.
Adam Block / NOAO / AURA / NSF
So even with no optical aid, I easily spotted a triangular patch of light in the correct location. And it was even more prominent through my binoculars. Indeed, the North America Nebula is often said to be easier to see with binoculars than a telescope. But I've always wondered on such occasions exactly what I'm seeing. As the photo at right shows, the nebulosity is all mixed in with an extremely rich star field. Are we really seeing the combined light of all those faint stars or the glowing gas that surrounds them?

One way to answer that is with nebula filters, which block most of the starlight while dimming the nebulosity very little. Whenever I've viewed the North America with filters held in front of each lens of my eyeglasses, its shape has always changed dramatically, indicating that what I'm seeing without the filters is indeed mostly starlight.

But the view through my 15×70 binoculars made the entire situation quite clear. Mexico and Florida, which include the brightest nebulosity but very few stars, looked milky smooth through that instrument. The rest of the bright patch looked grainy, just like the rest of the Milky Way in Cygnus. So I conclude that in most of the bright patch, I'm just seeing stars — many of which are bright enough for 70-mm binoculars to resolve. But around the Gulf of Mexico, I'm seeing true nebulosity.

Switching to the 10×30 binoculars, Florida and Mexico were still visible, though faint. But with just my unaided eyes, I couldn't make this part of the bright patch out at all — it was simply too small and faint, and swamped by the nearby star cloud.

So that's the answer — for me, anyway. For all practical purposes, the bright patch that I see north of Deneb with my unaided eyes is entirely composed of the merged light of faint stars, with negligible contribution from the true nebulosity. And that also explains why the view changes so dramatically when I hold nebula filters in front of my eyeglasses.