A galaxy is a gravitationally bound system of stars, interstellar gas and dust, stellar remnants, and dark matter.
The interstellar medium is the gas, dust, magnetic field, and energetic-particle population between stars in a galaxy.
A molecular cloud is a cold dense region of the interstellar medium in which hydrogen is predominantly molecular and stars can form.
Rotational carbon-monoxide line emission traces cold molecular clouds because abundant molecular hydrogen is difficult to observe directly. A conversion factor relates integrated CO intensity to molecular-hydrogen column density.
Interstellar dust absorbs and scatters short-wavelength starlight and reradiates the absorbed energy thermally in the infrared.
The neutral-hydrogen 21-centimeter line is the ground-state hyperfine transition of atomic hydrogen. Its brightness and Doppler shift map atomic-gas column density and line-of-sight velocity.
A cosmic ray is a high-energy charged particle propagating through space. Galactic cosmic-ray electrons emit synchrotron and inverse-Compton radiation, while cosmic-ray nuclei produce gamma rays through collisions with gas.
Galactic multiwavelength astronomy combines spectral bands that respond differently to stars, gas, dust, magnetic fields, temperature, and energetic particles.
Galactic synchrotron emission is nonthermal radio radiation from relativistic electrons spiralling in the Galactic magnetic field. Its broad latitude distribution reflects cosmic-ray transport and the magnetic halo.
Diffuse Galactic gamma rays arise mainly from neutral-pion decay after cosmic-ray nuclear collisions, electron bremsstrahlung in gas, and inverse-Compton scattering from radiation fields.
The Galactic scale height measures the vertical thickness of a tracer. Gravity, velocity dispersion, thermal pressure, magnetic and cosmic-ray support, particle transport, and source lifetime determine its value.
The Galactic bar is the elongated central stellar structure of the Milky Way. Extinction-corrected near-infrared standard candles and gas position--velocity maps constrain its distance, orientation, and three-dimensional shape.
The star formation rate is the mass converted into stars per unit time. Infrared luminosity traces dust-reprocessed light from young stars, while molecular-gas tracers locate their cold fuel.
A galaxy rotation curve gives circular speed as a function of radius. Its decomposition into stellar, gaseous, and dark-matter contributions is generally nonunique.
The maximum disk hypothesis assigns the largest stellar mass-to-light ratio compatible with a measured rotation curve, conventionally making the disk supply about of the circular speed near .
The disk--halo degeneracy is the ability of different stellar mass-to-light ratios and dark-halo profiles to reproduce nearly the same total galaxy rotation curve.
Galaxy dynamics studies the gravitational motions and evolution of stars, gas, dark matter, and satellite systems in galaxies.
Chandrasekhar dynamical friction is the drag on a massive body produced by its gravitational wake in a background of lighter particles. Its magnitude is proportional to the body's mass and the background density.
The tidal radius bounds material that remains gravitationally attached to a satellite in the differential gravitational field of its host.
Tidal stripping removes satellite material outside the tidal radius. The resulting mass loss weakens dynamical friction and can greatly delay or prevent a merger.
Dynamical-friction stalling by tidal stripping occurs when inward motion shrinks a satellite's tidal radius so rapidly that its remaining mass and frictional drag tend to zero before it reaches the host center.
Stellar dynamics studies the collective gravitational motion of stars in galaxies and star clusters.
Two-body relaxation is the cumulative randomization of stellar velocities by many weak gravitational encounters. Its timescale in a system of comparable stars is of order crossing times.
The Coulomb logarithm collects the logarithmically equal contributions of gravitational encounters over a wide range of impact parameters. For a self-gravitating system of comparable stars, is of order .
A stellar system is collisional on a timescale of interest when its two-body relaxation time is shorter than that timescale, so discrete encounters materially alter its distribution function.
A singular isothermal sphere with one-dimensional velocity dispersion has , enclosed mass , and constant circular speed .
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The term "Galaxy" can refer to several different concepts, depending on the context: 1. **Astronomy**: In astronomy, a galaxy is a massive system that consists of stars, stellar remnants, interstellar gas and dust, and dark matter, all bound together by gravity. Our own galaxy is called the Milky Way, which is just one of billions of galaxies in the observable universe.
