Milky Way vs Andromeda Galaxy: What Are the Differences?

Differences Between the Milky Way and the Andromeda Galaxy

Two Dominant Spirals of the Local Group

The Milky Way and the Andromeda Galaxy (M31) are both dominant spiral galaxies of the Local Group, which contains more than 60 galaxies. They nevertheless exhibit significant differences in the size of their disks, their central bulges, and their central supermassive black holes.

Size of Galactic Disks

Milky Way:

• The diameter of the galactic disk of the Milky Way is estimated at about 100,000 light-years.
• Its total mass is about 1 trillion solar masses, with most of the mass in the form of dark matter.
• The thickness of the disk is about 1,000 light-years.

Andromeda (M31):

• The disk of Andromeda is larger, with a diameter of about 220,000 light-years, roughly twice as extended as that of the Milky Way.
• Its total mass is estimated to be slightly greater than that of the Milky Way, at about 1.2 trillion solar masses.
• Its disk is also thicker, notably due to past merger activity with other galaxies.

Size of Central Bulges

Milky Way:

• The central galactic bulge (the spheroidal region at the center of the galaxy) has a diameter of about 10,000 light-years.
• It contains about 10 billion solar masses, largely old stars and globular star clusters.

Andromeda (M31):

• The central bulge of M31 is much larger, with a diameter estimated at about 30,000 light-years, making it one of the most imposing bulges among known spiral galaxies.
• Its mass is far greater, reaching about 100 billion solar masses.

Supermassive Black Holes at the Center

Both galaxies harbor a supermassive black hole at their center, but their size and mass differ considerably.

Milky Way (Sagittarius A*):

• The central black hole of the Milky Way, Sagittarius A*, has a mass of about 4.1 million solar masses.
• It is relatively calm, absorbing little matter at present, which makes it difficult to observe directly.

Andromeda (M31):

• M31 also possesses a massive central black hole, whose mass is estimated at about 100 million solar masses, i.e., 25 times more massive than that of the Milky Way.
• This black hole is surrounded by a more active disk of matter, although it remains moderately calm compared to those of active galaxies.

Summary Comparison

Synthesis of Structural Differences

The following table summarizes the main structural differences and mass gaps between the two galaxies.

Physical Remark

Role of Galactic Mergers

The differences in mass and size are partly explained by past interactions and galactic mergers.

Growth of Central Black Holes

The Milky Way has experienced a period of relative stability for several billion years, unlike Andromeda, which probably merged with several satellite galaxies over the past 3 billion years, increasing its mass and the size of its bulge. These processes also fueled the growth of its central black hole, far more massive than that of the Milky Way.