The Milky Way is a barred spiral galaxy comprising a stellar disk over 100,000 light‑years across with several hundred billion stars, including the Sun, embedded in spiral structure and surrounded by a massive dark matter halo, as summarized by NASA Science and Encyclopaedia Britannica. From Earth, it appears as a milky band of light across the night sky because we view the galaxy from within its disk, a perspective noted in NASA Science.

Structure and components

Astronomers describe six principal components: nucleus, central bulge, bar, disk (thin and thick), spiral arms, and halo, with the luminous parts embedded in a much larger halo of Dark matter inferred from galaxy rotation and other gravitational phenomena, according to Britannica and Hubble/NASA. Infrared mapping with NASA’s Spitzer telescope indicates the Milky Way’s stellar spiral pattern is dominated by two major arms (Scutum–Centaurus and Perseus) attached to a central bar, while the Sagittarius and Norma features are less star‑rich, a pattern consistent with barred spirals (JPL/Spitzer). The Sun resides in a minor feature between major arms commonly called the Orion Spur, as illustrated by NASA.

The bulk of the Milky Way’s mass is nonluminous: combining Hubble and ESA’s Gaia measurements yields a total mass of roughly 1.5 trillion solar masses within ~129,000 light‑years, implying a dominant, extended dark matter halo (ESA/Hubble; Hubble/NASA). The stellar halo contains about 150 known globular clusters, which trace ancient star formation episodes (Hubble/NASA; NASA Science).

Galactic center and Sagittarius A*

The Galactic Center is located in Sagittarius about 26,000 light‑years from the Sun and hosts Sagittarius A* (Sgr A*), a supermassive black hole of about four million solar masses, as characterized by X‑ray and infrared observations and highlighted when the Event Horizon Telescope collaboration released the first image of Sgr A* in 2022 (Hubble/NASA; Event Horizon Telescope; NSF). The central regions also drive bipolar outflows known as the Fermi Bubbles, with Hubble spectroscopy showing material moving at roughly 2 million miles per hour, evidence of episodic nuclear activity or central starbursts (Hubble/NASA).

Solar location and motion

The Solar System orbits the Galactic Center at a distance near 26,000 light‑years and completes one revolution in roughly 230–250 million years, values communicated in public summaries by NASA Science and by Hubble/ESA+Gaia distance measurements to the center (Hubble/NASA). Radio astrometry and model fits suggest a circular speed for the Sun of order ~220–250 km/s, consistent with recent revisions to Milky Way parameters reported by the U.S. National Science Foundation (NSF).

Star formation and the interstellar medium

The interstellar medium in the disk fuels ongoing star formation at a modest present‑day rate of about 1–2 solar masses per year, a figure consistent with multiple diagnostics and summarized in both technical reviews and outreach materials (journal://Annual Review of Astronomy and Astrophysics|The Galaxy in Context|2016; Hubble/NASA). These rates imply only a few to several new stars form annually, with most mass locked into long‑lived, low‑mass stars, as explained in research summaries (e.g., Astronomy Magazine). The oldest stellar populations in the halo and globular clusters reach 12–13 billion years, marking star formation not long after the Big Bang (Hubble/NASA).

Satellites and environment

The Milky Way resides in the Local Group, a small association of more than 50 galaxies that includes the Andromeda Galaxy and prominent satellites like the Large Magellanic Cloud, a nearby dwarf companion now known to influence the Milky Way’s dynamics (NASA Science). The Local Group itself is part of larger structures such as the Virgo Supercluster and the Laniakea supercluster mapped by flow reconstructions (journal://Nature|The Laniakea supercluster of galaxies|2014; Britannica). Statistical microlensing surveys and exoplanet demographics indicate at least one planet per star on average, implying a minimum of ~100 billion planets galaxy‑wide (JPL/NASA; NASA Science).

Dynamics and future interactions

Earlier models using Hubble proper motions of Andromeda and related data pointed to a likely Milky Way–Andromeda merger in ~4–5 billion years, but an updated 2025 analysis combining Hubble and Gaia constraints and accounting for the LMC and M33 finds the probability of a direct merger within the next 10 billion years is roughly 50–50, with only a small (<2%) chance within 5 billion years (ESA/Hubble news release; AP/PBS; ADS/Nature Astronomy abstract). The revised outlook reflects improved galaxy mass and motion constraints and the perturbing effect of the Large Magellanic Cloud on the Milky Way’s trajectory (ESA/Hubble).

Observation and mapping

Because the Milky Way encloses the Solar System, mapping its structure requires multi‑wavelength observations to penetrate dust and infer spiral geometry, exemplified by Spitzer’s mid‑infrared surveys that clarified the two major arm morphology (JPL/Spitzer). The European Space Agency’s Gaia mission has produced the largest, most precise 3D map of the Galaxy—over two billion sources observed between 2014 and January 15, 2025, with further releases planned—transforming knowledge of stellar motions, Galactic structure, and accretion history (ESA Gaia mission page).

Historical study

Galileo’s early telescopic observations first resolved the milky band into stars, prompting recognition of the Galaxy’s stellar nature, and twentieth‑century debates about the scale of the universe culminated when Edwin Hubble identified Cepheid variables in Andromeda (1923), establishing external galaxies beyond the Milky Way and redefining the cosmic context, as documented by ESA/Hubble’s archival program on "Hubble’s Cepheid" V1 (ESA/Hubble).

Physical parameters (representative values)

• –Morphological type: barred spiral with two major stellar arms, Scutum–Centaurus and Perseus (JPL/Spitzer).
• –Stellar disk diameter: ≈100,000 light‑years; Sun at ≈26,000 light‑years from the center (Britannica; Hubble/NASA).
• –Stellar content: of order 100–400 billion stars (Britannica).
• –Total mass (to ~129 kly): ~1.5 × 10^12 solar masses (dark matter dominated) (ESA/Hubble).
• –Central black hole: Sagittarius A* ≈4 × 10^6 solar masses; first image released in 2022 (Event Horizon Telescope; NSF).
• –Globular clusters: ~150 known (NASA Science).
• –Current star‑formation rate: ≈1–2 solar masses per year (Hubble/NASA; journal://Annual Review of Astronomy and Astrophysics|The Galaxy in Context|2016).