Large and Small Galaxies

Spatial Layer

Table of Contents

The largest galaxy to date is IC 1101

The smallest one is Segue 2

IC 1101 is approximately 6 million light-years across. That’s 60 times wider than our galaxy!

Size: If it replaced the Milky Way, its halo would swallow up the Andromeda Galaxy (our nearest major neighbor) and everything in between.
Star Population: An estimated 100 trillion stars. The Milky Way has about 200-400 billion. This galaxy hosts more stars than most galaxy clusters.
Location: Roughly 1.07 billion light-years away, in the center of the Abell 2029 galaxy cluster.
Type: Supergiant elliptical galaxy. It has no spiral arms, no central bar, and very little gas or dust.

Key Features:

Brightness: It is one of the most luminous galaxies ever found, shining with the power of nearly 2 trillion Suns.
The Core: Its central region is massive and diffuse, containing an ultra-massive black hole estimated at 40–100 billion solar masses.
Color: Appears golden-yellow, dominated by old, cool, red giant stars. Star formation here has largely ceased because it has almost no cold gas left.

Size: Approximately 200 light-years across. That’s tiny—about 1/500th the diameter of the Milky Way. You could fit 500 Segue 2s across the width of the Large Magellanic Cloud.
Star Population: A mere 1,000 stars (some estimates go as low as 500). The Milky Way has more stars in a single small star cluster.
Location: About 114,000 light-years from Earth, in the constellation Aries. It’s a satellite of the Milky Way.
Type: Dwarf spheroidal galaxy (or possibly an ultra-faint dwarf).

Key Features:

Dimness: Its total luminosity is only about 800 times that of our Sun. That’s incredibly faint. For comparison, a single large globular cluster (like Omega Centauri) is 50 times brighter.
The Dark Matter Mystery: Despite having almost no visible mass from stars, Segue 2 moves as if it has a significant amount of dark matter binding it together. In fact, it is one of the most “dark-matter-dominated” objects known.
Survival: It’s a miracle Segue 2 still exists. Orbiting the Milky Way, it should have been torn apart by our galaxy’s gravitational tides long ago. Its strong dark matter halo is likely the only thing holding it together.

Feature	IC 1101 (The Titan)	Segue 2 (The Dwarf)
Diameter	~6 million light-years	~200 light-years
Star Count	~100 trillion	~1,000
Mass	~2 quadrillion solar masses	~550,000 solar masses
Luminosity	~2 trillion Suns	~800 Suns
Shape	Smooth, featureless ellipse	Irregular, smudgy blob
Gas & Dust	Almost none (old galaxy)	Almost none (stripped away)
Black Hole	Ultra-massive (40-100 billion Suns)	None detected
Environment	Center of a rich galaxy cluster	Lone satellite of the Milky Way

Despite their dramatic differences, these two extremes share three important features:

Both Are Mostly Dead (Stellar Cemeteries): Neither galaxy is actively forming many new stars. IC 1101 used up or blew away its gas billions of years ago. Segue 2 had its gas stripped away by the Milky Way’s gravity. Both are populated almost entirely by old, metal-poor stars.
Both Are Dark Matter Dominated: You might think the larger galaxy has less dark matter because it’s so bright. Wrong. In both cases, the visible stars account for only a tiny fraction of the total gravitational mass. Over 90% of the mass in both IC 1101 and Segue 2 is invisible dark matter holding them together.
Both Are Remnants: Neither is a “normal” galaxy like our Milky Way or Andromeda. IC 1101 grew to its monstrous size by cannibalizing dozens of smaller galaxies in the Abell cluster. Segue 2 is what’s left over after a larger galaxy was mostly destroyed—a tiny, dim remnant barely holding on.