AST flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Hubble constant
- The present-day expansion rate of the universe, about 70 km/s per megaparsec; recession velocity divided by distance in the local universe.
- Distance modulus
- The relation m − M = 5 log10(d/10 pc), connecting apparent magnitude, absolute magnitude, and distance for unextincted starlight.
- Critical density
- The density for which the universe is spatially flat, equal to 3H0²/(8πG); about 9×10⁻²⁷ kg per cubic meter.
- Wien displacement law
- Peak wavelength of a blackbody times temperature equals 2.898×10⁻³ meter-kelvin, so hotter bodies peak at shorter wavelengths.
- Hydrostatic equilibrium
- The balance in a star where the outward pressure gradient exactly supports the weight of overlying layers at every radius.
- Virial theorem
- For a gravitationally bound system in equilibrium, twice the kinetic energy plus the potential energy equals zero.
- Main sequence turnoff
- The point on a cluster's Hertzsprung-Russell diagram where stars are leaving the main sequence; its mass indicates cluster age.
- Chandrasekhar limit
- The maximum mass, about 1.4 solar masses, that electron degeneracy pressure can support against gravity in a white dwarf.
- Mass-luminosity relation
- Main-sequence luminosity scales roughly as stellar mass to the 3.5 power, so massive stars are far brighter and shorter-lived.
- Density wave theory
- Spiral arms are interpreted as slow-moving patterns in the disk that compress gas and trigger star formation as material passes through.
- Tully-Fisher relation
- An empirical correlation between a spiral galaxy's luminosity and its maximum rotation velocity, used as a distance indicator.
- Rotation curve
- A plot of orbital speed versus radius in a galaxy; flat outer curves imply substantial unseen mass.
- Faber-Jackson relation
- An empirical correlation between the luminosity of an elliptical galaxy and its central stellar velocity dispersion.
- Roche limit
- The distance inside which a small body is torn apart by tidal forces from a more massive primary, forming rings or debris.
- Equilibrium temperature
- The temperature at which a planet radiates absorbed starlight, depending on stellar luminosity, orbital distance, and albedo.
- Jeans escape
- Atmospheric loss in which fast-moving light molecules exceed escape velocity and leak away from the top of the atmosphere.
- Tidal locking
- A state where a body's rotation period equals its orbital period, so the same hemisphere always faces its companion.
- Angular resolution
- The smallest angular separation a telescope can distinguish, approximately 1.22 λ/D radians for a circular aperture.
- Resolving power
- The ratio R = λ/Δλ of wavelength to the smallest resolvable wavelength difference in a spectrum.
- Quantum efficiency
- The fraction of incident photons that a detector converts into measurable signal; high values improve sensitivity.
- Adaptive optics
- A technique that deforms a mirror in real time to correct atmospheric turbulence and sharpen ground-based images.
- Vis-viva equation
- v² = μ(2/r − 1/a), giving orbital speed at distance r for a body with semimajor axis a and gravitational parameter μ.
- Escape velocity
- The minimum speed needed to leave a gravitational field without further propulsion, equal to √(2μ/r) at distance r.
- Kepler's third law
- The square of an orbital period is proportional to the cube of the semimajor axis, with the constant set by total mass.
- Tidal force
- The differential gravitational pull across a body, scaling as 1/r³, which stretches the body along the primary's direction.
- Jeans mass
- The minimum cloud mass for which gravity overcomes thermal pressure, scaling as T^(3/2) divided by the square root of density.
- HII region
- A cloud of ionized hydrogen around hot young stars, glowing in recombination lines and tracing recent star formation.
- Interstellar extinction
- The absorption and scattering of starlight by dust grains, stronger at shorter wavelengths and causing reddening.
- Initial mass function
- The distribution of masses for newly formed stars in a population, falling steeply toward higher masses.
- Ideal gas law
- The relation P = nkT linking pressure, number density, and temperature for a dilute gas of particles.
- Doppler shift
- The change in observed wavelength caused by relative motion along the line of sight, with approach producing blueshift and recession redshift.
- Gravitational lensing
- The bending of light by mass along the line of sight, producing magnified, distorted, or multiple images of background sources.