M 31 and M 33 take us beyond the Milky Way: their light has travelled for roughly two and a half and three million years respectively. NGC 891, NGC 6946 and NGC 6951 now extend the gallery farther out. These two nearby spirals remain particularly instructive because a garden telescope can reveal many structures in their disks.
The neighborhood
The Milky Way belongs to the Local Group, an association spanning several million light-years. M 31 and the Milky Way are its two most massive members; M 33 is the third large spiral. Numerous smaller galaxies complete the group. The exact membership depends partly on which very faint dwarfs have been discovered and securely assigned.
Distances are approximate; arrangement and sizes are schematic. The arrow shows M 31 approaching along our line of sight. Whether and when a merger follows also depends on sideways motion, masses and the other Local Group members.
What you are actually looking at
A spiral galaxy is a layered object, and the layers have different inhabitants — the color theme from the article on stellar lives, moved up one order of magnitude: yellow old stars in the center, blue young ones in the arms, and the H II regions from the hydrogen article as red markers of ongoing star formation.
Bulge, disk and halo, plus arms with glowing gas regions. M 31 is highly inclined; M 33 presents its disk more openly. Viewing angle changes which structures stand out.
A few things worth knowing about them:
M 31 is visible to the unaided eye. A dark sky reveals its bright inner region as a faint patch. The photographic extent is much larger; naked-eye visibility strongly depends on the sky and the observer.
Our view of the Universe changed here. Hubble’s observations of Cepheids in M 31 helped establish that the “Andromeda Nebula” is a stellar system far outside the Milky Way.
M 33 lacks a prominent classical bulge. Its disk and star-forming regions dominate; NGC 604 is a particularly large example.
The future remains open. Recent orbital simulations also allow outcomes without a Milky Way–M 31 merger within the next ten billion years. A certain collision in four to five billion years cannot be claimed.
“Bigger” needs a definition. Visible extent, star count and total mass including dark matter are different quantities and need not give the same ranking.
The neighbors as a portrait of types: on the left the M 31 type — inclined, yellow core, dust lanes, companions; on the right the M 33 type — loose arms, hardly any core, strewn with red star factories. This image is AI-generated — not a photograph but an illustration.
In our own sky
Both neighbors and their entourage, photographed from the garden:
23 objects — showM 31 as a whole: a highly inclined disk, dust lanes and two companions in the field. Light from M 31 has travelled for about two and a half million years; the point-like foreground stars lie in the Milky Way.My photograph · M 31 · view gallery entry
1 object — showM 110 is a dwarf elliptical companion of M 31. Its smooth appearance should not be equated with a complete absence of dust; dark structures can occur in these systems too.My photograph · M 31 · 33.3′ × 25.4′ · view gallery entry
1 object — showM 32, compact to the point of absurdity: a lot of mass in a tiny space. One common reading sees it as the leftover core of a galaxy whose envelope M 31 stripped away during a close encounter — tides as a sculptor, one size up from the nebulae.My photograph · M 31 · 18.8′ × 19.2′ · view gallery entry
M 33 with a more open view of its disk: a weak central concentration, loose arms and reddish H II regions. The photograph gives access to star formation in another galaxy without supplying a complete map of its age.
8 objects — showThe northern arm of M 33, where the disk frays out: individual Hα knots all the way to the visible edge. That individual star-forming regions in another galaxy can be resolved from a suburban garden is perhaps the quietest wonder in this gallery.My photograph · M 33 · 27.1′ × 28.4′ · view gallery entry
