Emission nebula
Ultraviolet radiation from hot stars ionizes surrounding gas. When the gas recombines, it emits light—often with the characteristic red glow of hydrogen.
Gas and dust
Nebulae
Nebulae are interstellar clouds. Some shine because energetic stars ionize their gas, some scatter nearby starlight, and others appear as dark silhouettes. Several nebula types mark different stages in the lives of stars.
Reflection nebula
Dust scatters light from nearby stars without becoming ionized. These nebulae often appear blue because shorter wavelengths scatter efficiently.
Dark nebula
A cold, dense cloud of dust blocks visible light from stars and glowing nebulae behind it. Dark nebulae are often sites of future star formation.
Planetary nebula
An ageing, Sun-like star sheds its outer layers, which glow around the exposed hot core. Despite the historical name, it has nothing to do with planets.
Star-forming region
A large molecular cloud in which gravity is producing new stars. One region can contain emission, reflection, and dark nebulae at the same time.
Herbig–Haro object
A compact glowing patch created when a fast jet from a newborn star collides with surrounding gas. These objects trace the violent early lives of stars.
Supernova remnant
The expanding material and shock wave left by an exploded star. The remnant enriches interstellar space with elements produced inside the star.
Variable nebula
A reflection nebula whose apparent brightness or shape changes as moving dust and changing illumination alter the light reaching us.
Families of stars
Star clusters
Star clusters contain stars connected by a shared origin, gravity, or both. They allow astronomers to compare stars with similar ages and chemical composition.
Open cluster
A loose group of tens to thousands of comparatively young stars born from the same molecular cloud. Encounters and Galactic tides gradually disperse it.
Globular cluster
A dense, nearly spherical system containing tens of thousands to millions of mostly old stars. Globular clusters orbit the outer regions of a galaxy.
Star cloud
A rich-looking patch of the Milky Way seen through a relatively clear line of sight. Its stars may lie at many distances and need not form one bound cluster.
Super star cluster
An exceptionally massive, compact young cluster. It contains many luminous stars and may resemble a young precursor of a globular cluster.
Islands of stars
Galaxies
A galaxy is a gravitationally bound system of stars, stellar remnants, gas, dust, and dark matter. Its appearance reflects both its history and its environment.
Spiral galaxy
A flattened rotating disc with spiral arms, gas, dust, and ongoing star formation, usually surrounding a central bulge.
Barred spiral galaxy
A spiral galaxy with a straight bar of stars crossing its centre. Spiral arms extend from the ends of the bar.
Elliptical galaxy
A smooth, rounded or elongated galaxy dominated by older stars, with little cold gas and usually little current star formation.
Lenticular galaxy
A disc galaxy with a central bulge but no prominent spiral arms. It bridges some characteristics of spiral and elliptical galaxies.
Irregular galaxy
A galaxy without a clear spiral or elliptical form. Its shape may reflect strong star formation or gravitational disturbance.
Dwarf galaxy
A relatively small galaxy containing far fewer stars than a large system such as the Milky Way. Dwarfs are the most numerous kind of galaxy.
Interacting galaxies
Galaxies close enough for their gravity to distort their shapes, trigger star formation, exchange material, or eventually cause a merger.
Ring galaxy
A galaxy dominated by a ring of stars and gas, sometimes produced when another galaxy passes through its disc.
Peculiar galaxy
A broad observational label for a galaxy with an unusual shape, structure, or composition, often caused by interaction or merger activity.
Seyfert galaxy
A galaxy with a bright active nucleus powered by matter falling toward a supermassive black hole, while the surrounding host galaxy remains visible.
Radio galaxy
An active galaxy producing enormous radio-emitting jets and lobes, powered by activity around its central supermassive black hole.
Quasar
An extremely luminous active galactic nucleus seen across immense distances. Its energy comes from matter heated as it falls toward a supermassive black hole.
Blazar
An active galactic nucleus whose relativistic jet points nearly toward Earth, making it appear bright, variable, and energetic.
The large-scale universe
Galaxy systems
Galaxies are not distributed independently. Gravity gathers them into groups and clusters, which in turn trace even larger patterns in the cosmic web.
Galaxy group
A relatively small gravitationally bound collection of galaxies. The Milky Way belongs to the Local Group.
Galaxy cluster
A massive bound system containing hundreds or thousands of galaxies, hot intracluster gas, and a large amount of dark matter.
Supercluster
An enormous association of galaxy groups and clusters. Unlike a galaxy cluster, an entire supercluster is generally not bound as one single object.
Galaxy merger
The long process in which interacting galaxies combine. Stars rarely collide directly, but gas clouds, orbits, and galactic structure change dramatically.