M 42 — the stellar nursery next door

About 1,350 light-years — by the standards of this gallery that is just around the corner. The Orion Nebula is the closest large stellar nursery, the only object of its kind that the naked eye sees as a patch of nebulosity, and the most thoroughly studied piece of star formation in the entire sky. For this gallery it is also a time capsule: the exposure behind it was taken in 2016 with an 8-inch Newtonian and a Canon DSLR — half a year of astrophotography, and then the mount gave up. Ten years later the image was dug out again, reprocessed and measured.

A cavity, seen from the inside

The most important thing about M 42 is not directly visible in the image: its geometry. The nebula is not a freely floating cloud but the broken-open front face of the huge Orion A molecular cloud. The four young Trapezium stars — above all θ¹ Orionis C, an O star — have burned a bubble into the face of the cloud, and that bubble happens to be open toward us. So we are looking into an illuminated cavity whose back wall is the molecular cloud:

The blister geometry. The famous Orion Bar is the edge of the cavity: an ionization front seen almost exactly edge-on — the same limb brightening as at the Cygnus Wall, only six times closer. And deep in the cloud behind it, completely hidden at visible wavelengths, the Becklin-Neugebauer Object marks the next generation of embedded protostars.

The most crowded nursery in the neighborhood

More than two thousand young stars, barely a million years old, are packed into the Orion Nebula Cluster — the complete cast of the star formation article in the smallest of spaces. Its most famous result came from Hubble in 1993: the proplyds, dozens of protoplanetary disks photographed directly, as silhouettes and as glowing droplets. Many of them are being boiled away by θ¹ Ori C — one watches emerging planetary systems while the radiation of a neighboring star eats into their building material. Birth and destruction in the same cubic light-year, as everywhere in this gallery — only here with an address for every single disk.

Where to find it

The finder chart comes for free: with Betelgeuse in the upper left and Rigel in the lower right, Orion carries both ends of the temperature ladder from the stellar lifetimes article — distinguishable with the naked eye. M 42 is the blurred middle “star” of the sword.

A few things worth knowing about it:

M 42 was the very first nebula ever photographed. Henry Draper exposed it in 1880 through an 11-inch refractor — 51 minutes on a photographic plate. That of all objects this one turns up here on a ten-year-old DSLR exposure is therefore a tradition: for 145 years the Orion Nebula has been the astrophotographer's first piece of work.
Spectroscopy settled a century-old dispute here. In 1864 William Huggins pointed his spectroscope at the nebula and found emission lines instead of starlight: M 42 is glowing gas, not an unresolved star cluster. It was the first direct proof that genuine gaseous nebulae exist — the birth of the nebular physics of the hydrogen article.
Visually it is green. In the telescope M 42 is one of the very few nebulae that actually shows color to the eye — a pale gray-green, the [O III] line, to which the dark-adapted eye is most sensitive (why, is explained in the colors article). The photographic red and the visual green are both real; they simply come from different lines.
M 43 is a small bubble of its own. The round nebula northeast of the main nebula has its own ionizing star in NU Orionis and is separated from the large M 42 by a dust lane — the same physics in pocket format, right next door.
The nebula is an expiration date with a sequel. In about a hundred thousand years the Trapezium will have blown its cavity wide open and dispersed the gas. What remains is a young open cluster without a nebula — the stage the star cluster article lists as “blown clear.” The Pleiades of the day after tomorrow hang in Orion's sword today.

In our own sky

All of the images come from the 2016 exposure — the oldest image in this gallery, taken with equipment that no longer exists:

M 42 as a whole — an open bubble in front of the Orion A cloud — M 42 as a whole: the open bubble in front of the Orion A cloud, with the bright central region around the Trapezium and the wide, sweeping wings. The blister geometry from the diagram appears here in projection — bright where we look into the cavity, dark where the face of the cloud is still closed.My photograph · M42 · 42.9′ × 38.1′ · view gallery entry

Huygens Region — Trapezium, Orion Bar and M 43 along one boundary — The Huygens Region, the heart of the nebula: the Trapezium, the razor-sharp Orion Bar as an ionization front seen edge-on — and right beside it M 43 as a small bubble of its own behind its dust lane. Almost the entire subject matter of this knowledge section stands within this one square degree.My photograph · M42 · 24.1′ × 19.2′ · view gallery entry

NGC 1977 · Running Man — a dark figure in reflected light — NGC 1977, the Running Man: not an emission nebula but a reflection nebula — bluish reflected starlight with a dark figure of foreground dust cut into it. The immediate neighbor shows how different the same region looks when no O star ionizes the gas.
My photograph · M42 · 14.1′ × 25.3′ · view gallery entry

M 43 and the northeastern dark lane — a nebular space behind dust — M 43 and the northeastern dark lane: the small neighboring bubble, separated by a band of dust that merely lies in front of it. A lesson in miniature — its own lamp, its own front, its own cast shadow.My photograph · M42 · 20.8′ × 26.4′ · view gallery entry

A cavity, seen from the inside

The most crowded nursery in the neighborhood

Where to find it

In our own sky

Further reading