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Galactic plane

The Galactic plane, also referred to as the galactic equator, is an imaginary plane that divides the Milky Way galaxy into northern and southern halves, coinciding with the midplane of its flattened disk where the majority of the galaxy's stars, interstellar gas, and dust are concentrated.^[1] This plane serves as the fundamental reference for the galactic coordinate system, with positions defined by galactic longitude (l) ranging from 0° to 360° and galactic latitude (b) measured from -90° to +90°, where b = 0° lies exactly on the plane.^[2] The plane's orientation is tilted at approximately 63° relative to the celestial equator and about 60° to the ecliptic, reflecting the Milky Way's independent structure from Earth's orbital geometry.^[3] In this coordinate system, the north galactic pole is positioned at right ascension 12h 51.4m and declination +27.13° (epoch J2000.0) in equatorial coordinates, with the plane itself intersecting the celestial sphere along the band of the Milky Way visible from Earth.^[4] The Sun lies near this plane, at a distance of roughly 20–35 parsecs (about 65–114 light-years) above it, depending on the precise definition of the midplane derived from stellar distributions or radio observations.^[5] This proximity allows our solar system to orbit within the galactic disk, approximately 8 kiloparsecs (26,000 light-years) from the galactic center, contributing to the observed concentration of material along the plane.^[6] The galactic plane demarcates the primary structure of the Milky Way, encompassing a thin disk component with an overall vertical scale height of around 300 parsecs (about 1,000 light-years), where gas and young stars are concentrated in a thinner layer of about 100 parsecs, embedded within a thicker disk extending to about 1–2 kiloparsecs, and surrounded by a spherical halo of older stars and globular clusters.^[7]^[8] Dense concentrations along the plane, such as spiral arms and the central bulge, host active star formation regions obscured by dust at optical wavelengths but prominent in infrared and radio surveys, making the plane a key focus for studying galactic dynamics, chemical evolution, and high-energy phenomena like gamma-ray emissions.^[9] Observations perpendicular to the plane reveal the galaxy's overall symmetry, while views along it highlight its complexity due to extinction effects.^[10]

Fundamentals

Definition

The galactic plane is an imaginary surface that passes through the center of the Milky Way galaxy, bisecting its flattened disk structure and serving as the midplane where the majority of stars, interstellar gas, and dust are concentrated.^[11] This plane represents the primary symmetry axis of the galaxy's disk component, with the distribution of material becoming sparser above and below it due to the gravitational potential well that confines most baryonic matter to this equatorial layer.^[12] The physical extent of the galactic plane spans approximately 100,000 light-years in diameter, reflecting the overall scale of the Milky Way's disk.^[13] Its thickness varies regionally, measuring about 1,000 light-years (300 parsecs) in the denser inner regions near the galactic center and about 1,000 light-years (300 parsecs) in the vicinity of the Sun, where the stellar and gaseous components exhibit lower vertical dispersion.^[14] The concept of the galactic plane was first conceptualized in the 18th century by astronomer William Herschel, who used systematic star counts—known as "star gages"—to infer the Milky Way's flattened, lens-shaped structure with the Sun positioned near its center.^[15] This early model laid the groundwork for understanding the galaxy's disk-like geometry, though it overestimated the Sun's centrality. The definition was significantly refined in the 20th century through radio astronomy, particularly with observations of neutral hydrogen (HI) emissions that allowed for a more precise determination of the plane's orientation and position, culminating in the International Astronomical Union's (IAU) adoption of a standardized system in 1958.^[16] Mathematically, the galactic plane is defined as the z=0 surface in cylindrical galactic coordinates, with the galactic center at the origin (R=0, z=0, φ arbitrary) and the Sun located at a distance of about 8 kiloparsecs along the positive x-axis.^[17] This coordinate framework, briefly related to the broader galactic system, provides a reference for positioning celestial objects relative to the plane's symmetry.^[18]

Coordinate system

The galactic coordinate system is a spherical coordinate framework centered on the Sun, used to specify positions on the celestial sphere relative to the Milky Way's structure. Galactic longitude l measures the angle eastward along the galactic plane from the direction of the galactic center, ranging from 0° to 360°. Galactic latitude b measures the angular distance north (+) or south (-) of the galactic plane, ranging from -90° to +90°, with b = 0^\circ defining the plane itself.^[19] This system was formally defined by the International Astronomical Union (IAU) in 1958 to standardize mappings of galactic features based on radio observations of neutral hydrogen and updated to the J2000.0 epoch in 1984. The north galactic pole, at b = +90^\circ, is positioned at right ascension \alpha = 12^\mathrm{h} 51.4^\mathrm{m} and declination \delta = +27.13^\circ in the J2000.0 epoch. The galactic plane is inclined at an angle of approximately \phi = 62.9^\circ to the celestial equator, with the ascending node of the galactic equator on the celestial equator at a position angle of \theta \approx 122.93^\circ from the vernal equinox.^[19] Transformations between equatorial (right ascension \alpha, declination \delta) and galactic coordinates involve rotation matrices that align the celestial equator with the galactic plane using the defined angles. The standard rotation first applies an angle \theta around the polar axis to position the ascending node, followed by a rotation of \phi to tilt the plane. The resulting 3×3 rotation matrix R for converting from equatorial to galactic Cartesian coordinates (x_\mathrm{eq}, y_\mathrm{eq}, z_\mathrm{eq}) to (x_\mathrm{gal}, y_\mathrm{gal}, z_\mathrm{gal}) is given by:

\begin{pmatrix} x_\mathrm{gal} \\ y_\mathrm{gal} \\ z_\mathrm{gal} \end{pmatrix} = R \begin{pmatrix} x_\mathrm{eq} \\ y_\mathrm{eq} \\ z_\mathrm{eq} \end{pmatrix},

where R incorporates \cos\theta, \sin\theta, \cos\phi, and \sin\phi in its elements, such as the (3,3) element being \cos\phi. Detailed matrix elements and inverse transformations are derived from the IAU parameters for precise numerical conversions.^[19]^[17] In astronomical research, the galactic coordinate system serves as the primary reference for investigating the Milky Way's structure, dynamics, and distribution of stars, gas, and dust. By definition, the direction to the galactic center is at l = 0^\circ, b = 0^\circ, though the Sun's actual position is offset from the true center by approximately 8 kpc along this line of sight. This heliocentric origin facilitates targeted surveys and modeling of galactic phenomena.^[19]^[20]

Structure and components

Thin disk

The thin disk of the Milky Way represents the primary flattened component aligned with the galactic plane, characterized by a relatively small vertical scale height of approximately 300 parsecs near the solar neighborhood. This structure dominates the visible appearance of the galaxy's plane, embedding spiral arms, H II regions, and molecular clouds that trace regions of active star formation and dense interstellar material. The disk's thin profile arises from the concentration of younger stellar populations and gas, contrasting briefly with the more extended distribution of older stars in the thick disk.^[21] In terms of composition, the thin disk accounts for the majority of the galaxy's stellar mass, comprising roughly 90% of the total visible stellar content, with a total mass estimated at approximately 3–4 × 10^{10} solar masses (as of 2025).^[22] It features high metallicity due to enrichment from multiple generations of star formation, alongside ongoing processes that sustain the interstellar medium (ISM) with an average density of around 0.1 atoms per cubic centimeter in the plane. The ISM within the thin disk includes a mix of atomic and molecular hydrogen, with central densities for H I reaching about 0.32 cm^{-3} and H_2 around 4 cm^{-3}, distributed exponentially across the disk.^[21]^[23] The formation of the thin disk is attributed to the collapse of a rotating protogalactic cloud approximately 10 billion years ago, during which conservation of angular momentum led to the development of a flattened, rotating structure. This process followed the initial formation of older components, with the thin disk emerging as subsequent gas settled into the plane, enabling continued star formation. Seminal models, such as those based on hierarchical merging and gas accretion, support this timeline, indicating the disk's youth relative to the galaxy's overall age of about 13.6 billion years.^[24] Key features of the thin disk include prominent spiral arms, such as the Perseus Arm and the Scutum-Centaurus Arm, which are embedded within its structure and extend across multiple galactic quadrants. These arms are traced by young O and B-type stars illuminating H II regions, as well as dust lanes associated with giant molecular clouds, revealing concentrations of gas and dust that enhance the disk's spiral morphology. Observations of over 4,500 H II regions and 1,300 giant molecular clouds confirm the continuity of these arms, with the Perseus Arm prominent in the outer disk and the Scutum-Centaurus Arm connecting inner regions near the galactic bar.^[25]

Thick disk

The thick disk of the Milky Way represents an older, vertically extended stellar component that intersects the galactic plane, distinguished from the thinner, younger disk by its greater scale height and distinct kinematic and chemical properties.^[26] It has a scale height of approximately 0.75–1.1 kpc (roughly 2,450–3,600 light-years), allowing it to extend to galactic latitudes of |b| ≈ 20° where its stellar density remains detectable. Recent Gaia data (as of 2025) reveal a young thick disk population aged ~6.6 billion years with a scale height of ~0.64 kpc, suggesting additional complexity in disk evolution.^[27] ^[26] ^[28] Locally near the Sun, the thick disk's stellar density is about 5–10% that of the thin disk, reflecting its lower overall mass contribution while spanning a comparable radial range.^[26] Its stars exhibit lower metallicity, with a mean [Fe/H] ≈ -0.6, and enhanced α-element abundances relative to the thin disk, indicating formation from less enriched gas.^[26] The thick disk is primarily composed of old stars with ages ranging from 8 to 12 billion years, including a significant population of red giants, and is associated with a small number of globular clusters that share its kinematic properties.^[26] Kinematically, it features higher velocity dispersions, particularly in the vertical direction (σ_z ≈ 35 km/s), compared to the thin disk's σ_z ≈ 20 km/s, resulting from dynamical processes that puffed up an earlier disk layer.^[26] Radially, the thick disk extends similarly to the thin disk out to about 12 kpc from the galactic center but shows flaring at larger radii beyond 12 kpc, where its surface density profile becomes flatter due to reduced differential rotation effects.^[28] This component was discovered in the 1980s through star counts in the galactic plane's vicinity, which revealed an excess of stars at heights inconsistent with a single thin disk model, leading to the identification of a distinct thicker layer with a scale height of about 1.35 kpc.^[29] Formation scenarios emphasize dynamical heating of an ancient thin disk precursor, either through scattering by molecular clouds or via minor mergers with satellite galaxies that deposited stars and increased vertical dispersion without major disruption.^[26] These processes likely occurred early in the Milky Way's history, preserving the thick disk's coherent rotation while distinguishing it from the more dynamically cold thin disk.^[26]

Observations and visibility

Challenges from Earth

The Solar System is embedded within the galactic plane, approximately 25,000 light-years (8 kpc) from the Galactic center and situated in a minor spur known as the Orion Arm. This location results in highly overlapping lines of sight when viewing along the plane, where distant structures project onto nearer ones, creating significant confusion in resolving individual components of the disk.^[30]^[31] Interstellar dust concentrated in the galactic plane absorbs and scatters much of the optical radiation, with typical extinction rates ranging from 1 to 5 magnitudes per kiloparsec. This effect severely limits direct views of the disk's stellar content in visible light; toward the Galactic center, the visual extinction A_V approaches 20 magnitudes over the 8 kpc path.^[32]^[33] These obscuration effects define the zone of avoidance, a wedge-shaped region spanning galactic latitudes |b| < 20^\circ where dust hides the majority of extragalactic sources, exacerbating challenges in measuring distances through crowded, confused fields along the plane. Early efforts to map the plane were hampered by these barriers, with comprehensive star counts only becoming feasible in the 1920s through the work of Jacobus Kapteyn, whose analyses revealed asymmetries in stellar densities, later attributed to local structures like the low-density local bubble surrounding the Sun.^[34]^[35]

Observational techniques

Observing the galactic plane requires techniques that circumvent the heavy obscuration by interstellar dust in visible wavelengths, which limits direct optical views to within a few kiloparsecs.^[36] Radio astronomy plays a central role in mapping the galactic plane, utilizing the 21-cm emission line of neutral hydrogen (HI) to trace atomic gas distribution and derive rotation curves. The Leiden-Argentine-Bonn (LAB) survey, combining data from multiple radio telescopes, provides the most sensitive all-sky map of Galactic HI emission, enabling detailed studies of gas kinematics across the plane.^[37] Observations of the 21-cm line reveal a nearly flat rotation curve, with the orbital velocity at the Sun's position measured at approximately 220 km/s, indicating a massive dark matter halo or modified dynamics. Additionally, carbon monoxide (CO) millimeter-wave emissions serve as a primary tracer for molecular gas, highlighting dense clouds where star formation occurs, with surveys like those from the CfA telescope delineating spiral arm structures. Infrared observations penetrate dust more effectively, revealing embedded stars and gas structures. The Spitzer Space Telescope's GLIMPSE survey mapped the inner galactic plane at 3.6, 4.5, 5.8, and 8 µm, detecting polycyclic aromatic hydrocarbon (PAH) emissions at 7-8 µm that trace photoexcited regions in the interstellar medium. Complementing this, the Wide-field Infrared Survey Explorer (WISE) mission conducted an all-sky survey in the mid-infrared, identifying over 30 million sources in the galactic plane when combined with GLIMPSE data, including young stellar objects and dust lanes otherwise hidden. High-energy observations in X-rays and gamma-rays probe the hot, diffuse interstellar medium (ISM) and energetic events. The Chandra X-ray Observatory has imaged supernova remnants and hot ISM plasma along the plane, revealing temperatures exceeding 10^7 K from shock-heated gas. Similarly, the Fermi Gamma-ray Space Telescope detects diffuse gamma-ray emission from cosmic-ray interactions, including the Fermi bubbles—large, bipolar structures extending perpendicular to the plane, spanning about 50,000 light-years and likely originating from past activity at the galactic center. Recent advances as of 2025 enhance resolution in the crowded plane. The James Webb Space Telescope's Near-Infrared Camera (NIRCam) has produced high-resolution images of star-forming regions, such as Sagittarius B2, resolving individual massive stars and protostellar disks within dense molecular clouds.^[38] Gaia's Data Release 3 (DR3) catalogs astrometric data for 1.8 billion stars, providing parallaxes with uncertainties below 0.1 mas for sources brighter than G=15, allowing precise 3D mapping of stellar populations even in obscured plane regions.^[39]

Astrophysical significance

Star formation and dynamics

Star formation in the galactic plane is primarily driven by density waves propagating through the spiral arms, where gravitational instabilities compress interstellar gas clouds, triggering gravitational collapse and the formation of new stars. These density waves create regions of enhanced gas density, facilitating the transition from molecular clouds to protostellar cores, with much of the activity concentrated in the inner disk where gas densities are highest. The Milky Way's overall star formation rate is estimated at 1.65–1.90 solar masses per year, reflecting the integrated efficiency of these processes across the plane.^[40] The relationship between gas density and star formation is quantified by the Schmidt-Kennicutt law, which states that the surface density of the star formation rate, \Sigma_\text{SFR}, scales with the total gas surface density, \Sigma_\text{gas}, as \Sigma_\text{SFR} \propto \Sigma_\text{gas}^{1.4}. This empirical power-law relation, derived from observations of nearby galaxies including the Milky Way, highlights the nonlinear dependence of star formation on available gas reserves, with molecular hydrogen dominating the process in dense spiral arm environments. Orbital dynamics in the galactic plane are governed by the axisymmetric gravitational potential, which supports nearly circular orbits for stars and gas in the midplane. Close to z=0, the potential takes the harmonic form \Phi(R,z) \approx \Phi_0(R) + \frac{1}{2} \nu^2 z^2, where \Phi_0(R) is the radial potential and \nu \approx 70 km s^{-1} kpc^{-1} is the vertical oscillation frequency at the solar radius, leading to simple harmonic motion perpendicular to the plane. This structure confines most material to thin layers, with vertical excursions limited by the restoring force from the disk's self-gravity.^[41] Supernovae explosions from massive stars provide crucial feedback that regulates the interstellar medium, injecting kinetic energy and momentum to stir turbulence, disperse clouds, and maintain a multiphase structure that modulates further collapse. These events heat and ionize gas, creating hot bubbles that limit the star formation efficiency to a few percent of the available gas mass. Locally, the Gould Belt serves as a prominent feature of recent star formation, an expanding ring of young stars and gas tilted by approximately 20° relative to the plane, representing a local ring-like distortion spanning several hundred parsecs vertically. Recent Gaia and Spitzer observations have identified the Radcliffe Wave, a large-scale sinusoidal structure of molecular clouds and young stars waving above and below the plane over approximately 9 kpc, which encompasses the Gould Belt and illustrates coherent wave-like dynamics in star formation along the disk. As of 2024, evidence suggests the Radcliffe Wave is oscillating through the plane while drifting radially outward from the galactic center.^[42]^[43]^[44]^[45] Contemporary numerical models, such as those from the IllustrisTNG simulations as of 2025, illustrate how dark matter subhalos and satellite interactions perturb the plane, generating warps through tidal torques that misalign the disk with the halo's symmetry axis. These simulations reveal that such dynamical perturbations sustain long-lived distortions, with warp amplitudes reaching several kiloparsecs in the outer disk, consistent with observed asymmetries in stellar distributions.^[46]

Role in galactic evolution

The galactic plane of the Milky Way is a key record of the galaxy's formation history, having settled from the monolithic collapse of a massive gas cloud approximately 10–13 billion years ago, as originally proposed in the seminal Eggen-Lynden Bell-Sandage model. This rapid collapse, lasting on the order of a few hundred million years, formed the initial disk structure from turbulent, well-mixed gas, leading to an early starburst that contributed to the thick disk population.^[47] Subsequent gas-rich mergers with satellite galaxies, such as those inferred from chemical and kinematic signatures, thickened the disk by injecting metal-poor gas and stirring vertical structure, with the thick disk acting as a relic of these early accretion events.^[47] Chemical evolution proceeded through inside-out growth, where star formation propagated outward from the inner regions, diluting metallicity gradients and establishing parallel abundance sequences in the thin disk as observed in age-metallicity relations.^[47] Over billions of years, secular evolution has reshaped the plane through internal dynamical processes, with radial migration—driven by resonances with transient spiral arms—scattering stars across radial distances of several kiloparsecs without significantly heating the disk or altering its thickness.^[48] This churning homogenizes metallicity gradients, as inner high-metallicity stars migrate outward, flattening observed abundance profiles in the solar neighborhood and beyond.^[48] The central bar further influences evolution by driving azimuthal-averaged gas inflows at rates up to 0.5 km/s inward of its corotation radius (around 3–4 kpc), funneling material toward the nucleus and fueling central star formation while mixing gas reservoirs.^[49] Looking to the future, the ongoing merger with the Sagittarius dwarf spheroidal galaxy is expected to disrupt the plane's structure, inducing vertical perturbations and potentially enhancing disk flaring through tidal torques over the next few billion years.^[50] The observed warping of the outer disk, extending to radii beyond 15 kpc, may intensify due to this interaction or misalignment with a tilted dark matter halo, with simulations showing halo tilts of up to 50 degrees propagating to stellar warps via gravitational coupling. A possible dark matter disk component could contribute to vertical oscillations, amplifying these effects as the halo responds to satellite infall. In the broader context of the ΛCDM cosmological model, the galactic plane serves as a tracer of hierarchical structure formation, with its flaring extending to at least 20 kpc as evidenced by rotation curve analyses of young disk stars, where non-baryonic contributions dominate the dynamics.^[51] Recent Gaia-based dynamical models, incorporating self-consistent potentials fitted to stellar kinematics, reveal an evolving disk with distinct thin and thick components, supporting scenarios of prolonged inside-out assembly and merger-driven thickening over 10 Gyr.^[52]

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