Clusters, Nebulae, and Galaxies — Telling Deep-Sky Objects Apart
Those hazy smudges in the night sky — how do clusters, nebulae, and galaxies actually differ? Here's how to tell deep-sky objects apart, with the story behind the Messier catalog.
What is a "deep-sky object"?
Look at the night sky long enough and you'll spot things that aren't single stars — knots of many stars bunched together, or hazy smudges of light. Collectively, these objects beyond our Solar System, inside and beyond our own galaxy, are called "deep-sky objects." They fall into three broad categories — clusters, nebulae, and galaxies — which can look superficially alike even though they're utterly different in nature.
The simplest way to tell them apart is to ask what they're made of. A cluster is a crowd of individual stars. A nebula isn't stars at all — it's a cloud of gas and dust. A galaxy is an entire universe unto itself, binding stars, gas, and dark matter together by gravity.
Clusters — open and globular
A cluster is a group of stars born together, around the same time, from the same nebula. They come in two shapes. An "open cluster" is a loose scatter of a few dozen to a few hundred young stars — the Pleiades (M45) is the classic example. Gravity between the stars is weak, so open clusters gradually drift apart and dissolve over time.
A "globular cluster" is the opposite: tens of thousands to hundreds of thousands of old stars packed tightly into a ball by strong mutual gravity. They mostly orbit in the halo around our galaxy's center, and even a small telescope shows them as a round glow that grows denser toward the middle. M13 in Hercules is the most famous globular cluster in the northern sky.
Nebulae — pictures painted in gas and dust
A nebula isn't made of stars — it's a cloud of gas, mostly hydrogen, and dust. Nebulae are classified by how they shine: an "emission nebula" (like the Orion Nebula, M42) glows red as its gas is lit up by ultraviolet light from nearby hot stars; a "reflection nebula" glows blue by simply scattering starlight back at us; and a "dark nebula" appears black, blocking the glow of whatever lies behind it.
Some nebulae mark a star's final moments. A "planetary nebula" (like the Ring Nebula, M57, in Lyra) forms when a Sun-like star gently sheds its outer layers at the end of its life — it's called "planetary" only because it looks round like a planet through a small telescope, with no actual connection to planets. The wreckage left behind when a massive star explodes as a supernova is called a "supernova remnant" (like the Crab Nebula, M1, in Taurus).
Galaxies — universes unto themselves
A galaxy operates on an entirely different scale than a cluster or nebula. It's a vast "island universe" of hundreds of billions of stars, gas, dust, and unseen dark matter, all bound together by gravity — and our own Milky Way is one such galaxy. The nearest neighbor visible to the naked eye is the Andromeda Galaxy (M31), estimated to hold anywhere from 250 billion to a trillion stars.
Through a telescope, galaxies show up in several forms: "spiral galaxies" with sweeping arms, "elliptical galaxies" that are round or oval with no arms, and "irregular galaxies" with no defined shape at all. Though they can look star-like at a glance, galaxies sit hundreds to thousands of times farther away than clusters or nebulae — their light carries a picture of what they looked like millions of years in the past.
The story of the Messier catalog
The "M" and number that often precede a deep-sky object's name — M31, M42, M45, and so on — come from a catalog compiled by the 18th-century French astronomer Charles Messier. Messier was actually a comet hunter, and he kept confusing fixed, comet-like smudges in the sky for real comets. So he compiled a list of exactly the objects that were "not comets," to rule them out.
That list, the Messier catalog, ended up containing 110 clusters, nebulae, and galaxies — and today, ironically, it's used not for hunting comets but as the checklist for amateur astronomers running the "Messier Marathon," ticking off deep-sky objects one by one. Working through the Messier catalog with binoculars or a small telescope is a great way to move beyond memorizing constellation names and get a feel for the actual structure of the universe.