In astronomy, a deep sky object (DSO) is any celestial object that is located outside our solar system and beyond our Milky Way galaxy. This includes galaxies, nebulae, star clusters, and other objects that are too faint to be seen with the naked eye.

Deep sky objects are a popular target for amateur astronomers and astrophotographers because they provide a glimpse into the vastness and complexity of the universe. Unlike planets and stars, which can be observed individually and in detail, deep sky objects require telescopes and long exposure times to capture their faint light.

Examples of deep sky objects include the Andromeda Galaxy, the Orion Nebula, the Pleiades star cluster, and the Whirlpool Galaxy. They are typically categorized as emission, reflection, or dark nebulae, globular or open clusters, or galaxies based on their structure, composition, and location in the sky.

Deep sky objects are generally classified as emission or reflection. The main difference between an emission and a reflection lies in how they produce visible light.

Emission nebulae are clouds of ionized gas that emit light at specific wavelengths as a result of high-energy photons from nearby stars. These nebulae appear reddish, pinkish or bluish in color depending on the specific gases present, such as hydrogen, oxygen, and sulfur. Examples of emission nebulae include the Orion Nebula and the Lagoon Nebula.

On the other hand, reflection nebulae are clouds of dust and gas that do not produce their own light, but instead reflect the light of nearby stars. They typically appear as bluish-gray or silvery-white regions against a dark background. The bluish color is due to the scattering of blue light by the dust grains in the nebula. Examples of reflection nebulae include the Pleiades star cluster and the Horsehead Nebula.

Both emission and reflection nebulae are commonly observed and imaged in astrophotography, and both can provide important clues about the structure and composition of the universe.