Spiral GalaxyEdit

Spiral galaxies are one of the most striking and well-studied classes of galaxies in the cosmos. They are characterized by a flattened, rotating disk that hosts prominent spiral arms, a central bulge, and a surrounding halo. The disk contains gas, dust, and a mix of young and old stars, while the halo is dominated by dark matter and a population of ancient stars and globular clusters. Spiral galaxies span a range of sizes and star-formation rates, from relatively compact systems to grand-design spirals with orderly, well-defined arms.

Because spiral galaxies are common in the nearby universe, they serve as essential laboratories for probing the physics of star formation, galactic dynamics, and the interaction between baryonic matter and dark matter. The Milky Way, our home galaxy, is a spiral galaxy of this class, and nearby neighbors like the Andromeda Galaxy illustrate the diversity within the family. Spiral galaxies are observed across the electromagnetic spectrum, from radio measurements that trace cold gas to infrared data that reveal dust-enshrouded star-forming regions and optical light from mature stellar populations.

Morphology and structure

Components and layout: A typical spiral galaxy comprises a central bulge containing older stars, a thin or thick galactic disk housing gas and younger stars, and a dark matter halo enveloping the visible parts. Spiral arms wind outward from the central regions and often delineate bright regions of ongoing star formation along interstellar gas clouds.
Spiral arms and morphology: Arm patterns range from the regular, symmetric structures of grand-design spirals to the patchier, flocculent patterns seen in other systems. Dust lanes frequently trace the spiral arms, signaling regions where star formation is actively converting gas into new stars.
Classification: In the conventional scheme, galaxies are categorized by the presence and character of their arms and bars. Unbarred spirals are denoted SA-type, barred spirals SB-type, with intermediate SAB-types representing transitional cases. Within each class, subtypes (Sa, Sb, Sc, etc.) describe the prominence of the bulge and the openness of the arms. See also Hubble sequence for the broader context of galaxy morphology.
Star formation and stellar populations: The spiral arms are commonly sites of enhanced star formation, hosting H II regions and clusters of young, hot stars. The inner bulge is typically dominated by older stars, while the disk contains a mix of stellar populations and ongoing chemical evolution.
Kinematic structure: The stars and gas in the disk orbit the galactic center in a differential rotation pattern, with orbital speeds that depend on radius. This rotation underpins the observable velocity fields used to study mass distribution and dark matter.

Dynamics and kinematics

Rotation curves and dark matter: Measurements of how orbital velocity changes with radius—the rotation curve—show that speeds often remain high far from the center, implying substantial mass beyond the visible disk. This mismatch is a primary line of evidence for dark matter halos surrounding spiral galaxies.
Density waves and arm formation: The persistence of spiral patterns has driven substantial theoretical work. Density wave theory posits that spiral arms are long-lived wave patterns in the stellar disk that compress gas and trigger star formation as they rotate through the disk. Alternative views emphasize transient, recurrent arm features arising from gravitational instabilities and the complex dynamics of the disk.
Gas dynamics and star formation in the arms: The compressed gas within arms fosters molecular-cloud formation and subsequent star birth, shaping the observed luminosity distribution and chemical enrichment of the disk.

Formation and evolution

Early assembly and disk settling: Spiral galaxies emerge from hierarchical growth in the context of a cosmological framework in which dark matter halos assemble first, followed by the gradual accretion of gas that settles into rotating disks. The detailed history—timing, gas inflows, and internal processes—affects the ultimate morphology and star-formation history.
Secular evolution and bars: Bars can reorganize material within the disk, funneling gas toward the center, fueling star formation and feeding central black holes in some cases. This secular evolution modifies the structure of the galaxy over timescales longer than major mergers.
Environment and interactions: Encounters with neighbors, minor mergers, and tidal forces can perturb spiral structure and, in some situations, transform spirals into lenticular or more disrupted systems. The environment plays a significant role in shaping the observed diversity of spiral galaxies.
Notable cases and analogs: The Milky Way and Andromeda Andromeda Galaxy offer close-up laboratories for studying spiral structure, while more distant spirals reveal how common or rare certain morphologies are across cosmic time.

Notable spiral galaxies

The Milky Way Milky Way is a barred or weakly barred spiral with a central bulge and a grand, rotating disk.
The Andromeda Galaxy Andromeda Galaxy (M31) is a nearby spiral that provides a detailed glimpse of a large disk galaxy in our local neighborhood.
The Triangulum Galaxy Triangulum Galaxy (M33) is a smaller spiral that helps illuminate the lower end of the spiral population.
The Whirlpool Galaxy Whirlpool Galaxy (M51) is a classic example of a grand-design spiral with prominent, well-defined arms.

Observations and methods

Wavelengths and instruments: Optical telescopes reveal the distribution of stars, while radio observations (notably the 21 cm line from neutral hydrogen, 21 cm line) map gas reservoirs. Infrared instruments penetrate dust to view embedded star formation, and ultraviolet data highlight the youngest stellar populations.
Distance and mapping: Determining distances to spiral galaxies relies on standard candles and other distance indicators, enabling calibration of luminosities, sizes, and mass estimates. Kinematic data from spectroscopy yield rotation curves and insights into mass distribution.
Mass estimates and dark matter: By combining rotation curves with luminous mass measurements, researchers infer the presence and distribution of dark matter halos surrounding spiral galaxies, a central component of modern galaxy formation models.