It’s one of the most common words used by astronomers—and two of them featured in the James Webb Space Telescope’s debut image package—but do you know what a nebula is?

If you think you do then you probably don’t.

That’s because the word nebula actually has several different meanings. Annoying, right? A symptom of “fuzzy blobs” in the night sky gradually being revealed by better and better telescopes, the history of astronomy has left us with several completely different kinds of nebula.

Here’s everything you need to know about this most beautiful word and the often weird and wonderful objects it refers to:

Why there are several types of nebula

A nebula—which comes from the Latin word for cloud—historically refers to something in the night sky that is not a planet or a comet. In short, a fuzzy blob in telescopes until the 20th century.

Many of those fuzzy blobs turned out to be something else. For example, the Andromeda nebula become the Andromeda galaxy. However, the term nebula still refers to a cloud, specifically of interstellar dust and gas.

1. Planetary nebula

The leftovers of a star that went supernova are called planetary nebula. Why? Their blue-green look is reminiscent of Uranus and Neptune. In a planetary nebula the remaining core of a collapsed giant star—now a white dwarf—is producing enough energy to make its surrounding shell of gas glow.

Famous examples of planetary nebula include:

2. Diffuse nebula

Diffuse nebulas are one stage in the circle of life in the Universe. They are areas of cosmic gas and dust that cause stars to be born. These star-forming regions are among the most exciting objects to look at in the night sky and are often the targets for space telescopes. Since they emit their own light these clouds of dust and gas are often also called emission nebulae.

Famous examples of diffuse nebula include:

3. Reflection nebula

Clouds of dust and gas that are lit-up by nearby stars are called reflection nebula. That scattering of light is sometimes called “nebulosity” and it’s most famously seen in the Pleiades open star cluster—which is easily visible to the naked eye from September through March.

Famous examples of reflection nebulae include:

• The Pleiades (M45) in the constellation of Taurus.
• Witch Head Nebula (IC 2118) in the constellation of Orion.
• Ghost Nebula (IC 63) in the constellation of Cassiopeia.

4. Dark nebula

These are dark, dense clouds of interstellar dust that completely blocks out visible wavelengths of light from objects behind it—including stars. The spaces between the stars can tell us much about the night sky as the stars themselves, and many cultures have developed constellations based around these dark patches.

Famous examples of dark nebula include:

• Coalsack Nebula (TGU H1867) in the constellation of Crux.
• North American Nebula (NGC 7000) in the constellation of Cygnus.
• Horsehead Nebula (Barnard 33) in the constellation of Orion.

Wishing you clear skies and wide eyes.