The Orion Nebula

The Belt is your signpost; the thing you want hangs off it. Drop south from Alnilam, the middle belt star — at right angles to the Belt, not along it — and a short line of fainter stars runs down to Hatysa. That is Orion's Sword, and it is 4.7° long.

Count the three "stars" strung down the Sword. The middle one is 4.2° below Alnilam, and it is not a star: it will not sharpen into a point, and at any magnification it grows instead of shrinking. Everything else in the Sword behaves like a star. This one does not.

M42, the Orion Nebula: a cloud of ionised hydrogen 1,350 light-years away, measured to ±2% by radio parallax on the young stars inside it. The ring is its visual extent — 90 arcminutes, nearly three Moon-widths — which makes it one of the very few deep-sky objects that is genuinely large as well as genuinely bright.

It glows for the same reason a neon tube does. Ultraviolet from the hot young stars buried in the cloud strips electrons off the surrounding hydrogen; when the electrons fall back, the gas fluoresces. That means the nebula is not reflecting starlight, it is emitting — and it is only lit at all because the stars that light it were made here, out of it, within the last million years. The nebula is the leftovers, and it is still glowing from the heat of the meal.

This is the argument for M42 as a beginner's first nebula: it survives light pollution. At Bortle 6 — bright suburban sky, where the Andromeda Galaxy has already given up — the core is still obvious as a haze around the middle Sword star, because its light is concentrated rather than spread thin. Binoculars pull out the two wings. A small telescope shows the dark notch of the Fish's Mouth cutting into it.

At the heart of the glow sit four stars in a lopsided quadrilateral spanning 47 arcseconds: the Trapezium. The brightest, θ¹ Orionis C, is an O6 star putting out roughly 200,000 Suns and supplying most of the ultraviolet that lights the entire nebula. Any telescope over 60 mm splits all four. They are under a million years old — younger than Homo sapiens.

M42 sits at declination −5°, so it rides with the Belt: visible from almost everywhere on Earth, and above 30° altitude for several hours on a winter night from mid-northern latitudes. Watch the altitude readout for the window — and go out in the middle of it, not at the edge, because the last 10° of altitude costs you a whole extra atmosphere of haze.