Messier 31 revisited

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Messier 31 The Andromeda Galaxy Group. Altair Lightwave 66mm ED refractor and Canon 600d DSLR.
Image Credit Kurt Thrust.

"Our nearest spiral galaxy neighbour the Andromeda Galaxy is riding high in the Northern Hemisphere autumn sky. The weather has been far from kind and the JPO team has been laid low by a rather nasty virus which may be the latest variant of  Covid. Anyway, as the team has been wrapped up warm for a while, Kurt decided to reprocess this data captured in a previous year.

The above image shows the three galaxies M31 (the large central inclined spiral), M32 the elliptical galaxy (appears as a fuzzy spot on the upper edge of the M31 spiral) and M110 a dwarf elliptical galaxy ( just below and to the centre right of M31). 

So let Professor G.P.T. Chat, our visiting astrophysicist, compare and contrast these nearby galaxies (approximately 2.5 million light years distant) with our home Milky Way galaxy". - Joel Caio CEO of the Jodrell Plank Observatory.

Milky Way – Our Home Galaxy

The Milky Way is itself a barred spiral galaxy, somewhat smaller than M31. Its disk extends ~100,000–120,000 light-years, with a mass of about 1 trillion solar masses and a few hundred billion stars. Structurally, it resembles M31: both have stellar halos, bulges, bars, spiral arms, and satellite galaxies. In cosmic terms, the Milky Way is the second major member of the Local Group, and the future collision and merger of the Milky Way and M31 will reshape them into a single giant elliptical galaxy in several billion years.

M31 – The Andromeda Galaxy

Andromeda is the giant of this quartet. With a disk spanning about 220,000 light-years, it is roughly twice the diameter of the Milky Way. Its stellar population approaches one trillion, compared with the Milky Way’s 200–400 billion. In both size and luminosity, M31 slightly outclasses our own Galaxy, and it exerts enough gravitational pull to dominate the Local Group, which also contains the Milky Way and dozens of smaller galaxies.

• Diameter: ~220,000 light-years (about twice the Milky Way’s diameter).
• Mass: ~1.5 trillion solar masses.
• Stars: ~1 trillion.
• Luminosity: ~2.6 × 10¹⁰ solar luminosities.

Notes: A vast spiral galaxy with an extensive stellar halo and a large, bright disk. It dominates the Local Group both in size and gravitational influence.

M32 – Compact Elliptical

Placed against the scale of the Milky Way, M32 looks minuscule. With only ~3 billion stars in a body just 6,500 light-years across, it is smaller than even some of the Milky Way’s largest globular clusters when measured by diameter. Where the Milky Way’s spiral disk is rich in gas and dust and actively forming stars, M32 is stripped bare, almost entirely quiescent. Its compact, blazing core makes it bright for its size, but compared with the Milky Way, it is less than 1% as luminous.

• Diameter: ~6,500 light-years (tiny compared with M31).
• Mass: ~3 × 10⁹ solar masses.
• Stars: ~3 billion.
• Luminosity: ~3 × 10⁸ solar luminosities.

Notes: Dense and bright, especially in its central regions. Its outer stars and gas are thought to have been stripped by M31, leaving only the compact core we see today.

M110 – Dwarf Elliptical

M110 sits somewhere between M32 and the Milky Way in scale. Its 15,000–17,000 light-year span is still tiny compared with the Milky Way’s disk, and its ~10 billion stars are a mere fraction of the Milky Way’s population. Unlike M32, however, M110 retains some irregular features, dust, and evidence of star formation. It is a faint satellite, thousands of times less luminous than the Milky Way, but still large enough to stand as one of the more substantial dwarf galaxies in the Local Group.

• Diameter: ~15,000–17,000 light-years.
• Mass: ~1–2 × 10⁹ solar masses.
• Stars: ~10 billion.
• Luminosity: ~9 × 10⁸ solar luminosities.

Notes: More diffuse than M32, with some evidence of dust lanes and past star formation. Its structure is irregular compared with typical smooth ellipticals, likely reflecting tidal interaction with M31.

Comparative Perspective

M31 and the Milky Way are the two great spirals of the Local Group, differing mainly in scale (M31 being the larger).

M32 is a stripped-down remnant, a tiny elliptical companion of M31, utterly dwarfed by both the Milky Way and Andromeda.

M110 is a diffuse dwarf elliptical, more extended than M32 but still only a faint shadow of the Milky Way’s scale and richness.

Together, these four galaxies illustrate the hierarchy of galactic forms: two massive spirals shaping the Local Group, orbited by much smaller companions that bear the scars of their gravitational relationship with the giants.

The Approaching Collision

• The Milky Way and M31 are moving toward each other at about 110 km/s.

• In roughly 4–5 billion years, their outer halos will begin to overlap, triggering the first close passage.

• The collision will not be like two solid bodies smashing together; instead, stars will mostly pass by one another because of the vast spaces between them. But gas, dust, and dark matter halos will interact strongly, producing shocks, bursts of star formation, and tidal distortions.

The Merger

• After a series of close encounters, the Milky Way and M31 will merge into a single giant elliptical galaxy — sometimes nicknamed “Milkomeda” or “Milkdromeda.”

• This process will take several billion years to settle into a stable form. The final product will likely resemble today’s giant elliptical galaxies, with a vast stellar halo and little organized spiral structure.

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The Fate of M32

• M32, already stripped and compact, is tightly bound to M31.

• During the merger, it will almost certainly be swallowed whole by the combined Milky Way–Andromeda system.

• Its compact nature means it will survive tidal forces fairly well, likely ending as a dense nucleus or central star cluster within the merged galaxy.

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The Fate of M110

• M110, being larger and more diffuse, will fare differently.

• It may be torn apart by tidal forces during the merger, with its stars spread out into long stellar streams and absorbed into the halo of the new galaxy.

• Some fraction of its stars may survive as a remnant dwarf core, but it will be much more disrupted than M32.

The Long-Term Result

• The Local Group will transform from a system dominated by two spirals into a single elliptical super-galaxy, containing perhaps 2 trillion stars.

• The smaller companions, including M32, M110, and the Milky Way’s own satellites (like the Magellanic Clouds), will either be absorbed into the giant remnant or left orbiting as faint shells and streams.

• From a cosmic distance, the Local Group will eventually resemble a single luminous elliptical galaxy, drifting in relative isolation as the universe continues to expand.

 In short: M31 and the Milky Way will merge into one great elliptical galaxy; M32 will likely become part of its core, while M110 will be torn apart and assimilated into its halo.