Celestial Objects

Point a telescope at the sky and you will find, depending on where you look and what wavelength you observe in, an enormous diversity of objects: balls of fusing plasma, dead stellar cores compressed to nuclear density, clouds of molecular gas collapsing under their own gravity, rocky worlds with magma oceans, ice moons hiding liquid water, and structures so large that light takes billions of years to cross them. Astronomy classifies this zoo into categories that reflect physical processes, not just appearances. Understanding the classification is the first step toward understanding the universe.

This page provides the taxonomy. The deep physics, history, and observational evidence live in the dedicated pages linked throughout.

Stars

Stars are self-gravitating spheres of plasma sustained by nuclear fusion. They range from red dwarfs (0.08 solar masses, surface temperatures around 3,000 K, lifetimes exceeding a trillion years) to blue supergiants (tens of solar masses, 30,000+ K, lifetimes of a few million years). The Hertzsprung-Russell diagram organizes stars by luminosity and temperature, revealing that most stars fall along a "main sequence" determined by mass. How stars form, evolve, and die is covered in detail in the Stellar Evolution page. The key insight: a star's mass at birth determines nearly everything about its life and death.

Stellar Remnants

When stars exhaust their nuclear fuel, they leave behind compact objects whose nature depends on the progenitor's mass. White dwarfs (below the Chandrasekhar limit of ~1.4 solar masses) are electron-degenerate carbon-oxygen cores roughly the size of Earth. Neutron stars (formed in core-collapse supernovae from progenitors above ~8 solar masses) pack 1.4-2 solar masses into a sphere 20 kilometers across, with matter at nuclear density. Black holes (formed from the most massive stellar collapses, or grown to millions/billions of solar masses at galaxy centers) are regions where spacetime curvature prevents anything from escaping. The Event Horizon Telescope imaged the shadows of supermassive black holes in M87 and Sagittarius A*, confirming general relativistic predictions. The Chandra X-ray Observatory page covers how X-ray observations reveal accretion and jet physics around these objects.

Planets

Planets are bodies massive enough to be roughly spherical and to have cleared their orbital neighborhoods, but not massive enough for fusion. Our solar system divides them into rocky terrestrials (Mercury, Venus, Earth, Mars) and gas/ice giants (Jupiter, Saturn, Uranus, Neptune). The Solar System page covers these in detail, including their moons, ring systems, and what missions like Cassini, Juno, and New Horizons have revealed.

Beyond our system, over 5,700 confirmed exoplanets have revealed planetary types with no solar system analog: hot Jupiters in multi-day orbits, super-Earths and sub-Neptunes dominating the galactic census, lava worlds with magma oceans, and circumbinary planets orbiting two stars. Detection methods, atmospheric characterization, and the search for habitable worlds are covered in the Exoplanetology page.

Small Bodies

Asteroids, comets, and dwarf planets are the solar system's construction debris, preserved from the epoch of planet formation 4.6 billion years ago. Asteroids (rocky, concentrated in the main belt between Mars and Jupiter) and comets (icy, originating from the Kuiper Belt and Oort Cloud) carry pristine material from the early solar system. OSIRIS-REx returned samples from asteroid Bennu in 2023, revealing hydrated minerals and organic compounds. Dwarf planets (Pluto, Eris, Ceres, Haumea, Makemake) occupy the boundary between planets and small bodies, with New Horizons revealing Pluto as a geologically active world with nitrogen glaciers and water-ice mountains. The Solar System page covers these in their orbital and compositional context.

Nebulae

Nebulae are extended clouds of gas and dust serving as both stellar nurseries and graveyards. Molecular clouds (cold, dense regions of molecular hydrogen, typically 10-50 K) are where stars form; ALMA observes their internal structure and chemistry at millimeter wavelengths. H II regions (hot, ionized gas surrounding young massive stars) glow in visible light, with the Orion Nebula as the nearest prominent example. Planetary nebulae (expelled outer layers of dying low-mass stars, ionized by the hot remnant core) produce structures like the Ring Nebula and Cat's Eye Nebula. Supernova remnants (expanding shells of shock-heated gas from stellar explosions) are observed in X-rays by Chandra, with Cassiopeia A and the Crab Nebula as the best-studied examples.

Galaxies

Galaxies are gravitationally bound systems of stars, gas, dust, and dark matter, ranging from dwarf galaxies with a few billion stars to giant ellipticals with trillions. Edwin Hubble's morphological classification (spirals, ellipticals, irregulars) remains useful shorthand, though modern understanding emphasizes that galaxy properties form continuous distributions shaped by mass, environment, and merger history. Our Milky Way is a barred spiral containing 200-400 billion stars across a disk roughly 100,000 light-years in diameter. The Hubble Ultra Deep Field, a pencil-beam image of a tiny patch of sky, revealed roughly 10,000 galaxies in various stages of evolution spanning 13 billion years of cosmic history. Galaxy formation and evolution are among the central problems in cosmology, with simulations like IllustrisTNG (covered in the Computational Astrophysics page) now reproducing observed galaxy populations from first principles.

Large-Scale Structure

Galaxies are not randomly distributed. They cluster into groups (tens of galaxies), clusters (hundreds to thousands), and superclusters connected by filaments of dark matter and gas, forming a "cosmic web" with vast voids between the filaments. This structure grew from tiny density fluctuations in the early universe (imprinted in the cosmic microwave background, covered in the CMB page) amplified by gravity over 13.8 billion years. The largest coherent structures, like the Sloan Great Wall (~1.4 billion light-years long), approach the scale at which the universe appears statistically homogeneous. Mapping this structure through galaxy surveys (SDSS, DESI, and the upcoming Vera Rubin Observatory LSST) constrains dark matter, dark energy, and the geometry of the universe.

Dark Matter and Dark Energy

The most humbling entry in the celestial taxonomy: roughly 95% of the universe's energy content is in forms we cannot directly observe. Dark matter (~27%) reveals itself through gravitational effects on visible matter, gravitational lensing, and its role as the scaffolding for cosmic structure formation. Dark energy (~68%) drives the accelerating expansion of the universe, discovered through Type Ia supernova observations in 1998. Neither has been directly detected or identified with known particles or fields. The Dark Matter and Dark Energy page covers the evidence, candidates, and ongoing searches. The Hubble Tension page covers how disagreements in measuring the expansion rate may point toward new physics.