Image description: The Andromeda Galaxy is visible to the naked eye on a dark night. In the bottom right, you can see its companion, the dwarf galaxy NGC 205 or M110. The bright spot at 10 o'clock very close to the edge of Andromeda's disk is M32, another companion galaxy. Image source: NASA.

Differences between the Milky Way and the Andromeda Galaxy

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

1. Size of the Galactic Disks

Milky Way:

The diameter of the Milky Way's galactic disk is estimated to be about 100,000 light-years.
Its total mass is about 1,000 billion 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):

Andromeda's disk is larger, with a diameter of about 220,000 light-years, roughly twice the extent of the Milky Way's disk.
Its total mass is estimated to be slightly greater than that of the Milky Way, at about 1,200 billion solar masses.
Its disk is also thicker, partly due to past merger activity with other galaxies.

2. Size of the Central Bulges

Milky Way:

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

Andromeda (M31):

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

3. Supermassive Black Holes at the Center

Both galaxies host a supermassive black hole at their center, but their sizes and masses differ significantly.

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 quiet, accreting little matter at present, making it difficult to observe directly.

Andromeda (M31):

M31 also has a massive central black hole, with a mass estimated to be about 100 million solar masses, making it 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 quiet compared to those in active galaxies.

Summary Comparison

Characteristic	Milky Way	Andromeda (M31)	Main Difference
Disk Diameter	~100,000 light-years	~220,000 light-years	M31 has a disk 2x wider.
Disk Mass	~1,000 billion M☉	~1,200 billion M☉	M31 is slightly more massive.
Bulge Diameter	~10,000 light-years	~30,000 light-years	The bulge of M31 is 3x larger.
Bulge Mass	~10 billion M☉	~100 billion M☉	The bulge of M31 is ~10x more massive.
Central Black Hole Mass	~4.1 million M☉	~100 million M☉	The black hole of M31 is ~25x more massive.

Physical Note

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

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