Spiral galaxies like the Milky Way are full of light-emitting gas clouds, called emission nebulae, that eventually condense to form clusters of stars. These swirls of ionized hydrogen, other elements, and dust are stellar nurseries, resembling the Sun’s birthplace some 4.5 billion years ago.
We see the nearest emission nebula, the Orion Nebula (M42), because it’s illuminated by the Trapezium, a tight group of luminous, hot stars that have formed within it. Their intense radiation makes the gas glow, like a fluorescent lightbulb.
Observing tip: M42, located in the sword of Orion, is visible to the naked eye. If you view through a large telescope, the nebula may appear greenish. The vibrant colors in beautiful photos of this object are too faint for the human eye to register.