Galaxies come in diverse shapes and sizes, from elegant spirals to smooth ellipticals. We classify them using Hubble's , which considers features like spiral arms and size. This helps us understand their structure and evolution.
Beyond appearance, galaxies differ in properties like stellar populations, gas content, and star formation rates. We use spectroscopy to analyze their composition and motion, while functions help us study galaxy populations and evolution across cosmic time.
Galaxy Classification and Properties
Galaxy classification scheme
Top images from around the web for Galaxy classification scheme
Hubble's tuning fork diagram organizes galaxies into ellipticals (E), spirals (S), barred spirals (SB), lenticulars (S0), and irregulars (Irr) based on visual appearance
Morphological features considered include presence of spiral arms, bulge-to- ratio, and presence of a bar
Elliptical galaxies classified E0 to E7 based on ellipticity (roundness to flattened shape)
Spiral galaxies classified Sa, Sb, Sc based on tightness of spiral arms and bulge size (tightly wound to loosely wound)
Limitations include not accounting for galaxy evolution and difficulty classifying distant or faint galaxies
Properties of galaxy types
Elliptical galaxies exhibit smooth, featureless appearance with little gas or dust, contain older stellar populations, and supported by random stellar motions
Spiral galaxies feature disk component with spiral arms, central bulge, younger stellar populations in disk, ongoing star formation in spiral arms (HII regions), and rotation-supported structure
Irregular galaxies lack definite shape or structure, often result from galaxy interactions (mergers), typically smaller and less massive, and display active star formation regions
Spectroscopy for galaxy analysis
Emission and absorption lines indicate presence of specific elements (hydrogen, helium, oxygen)
Doppler shift measures galaxy's radial velocity (approaching or receding)
Spectral energy distribution reveals galaxy's overall composition and age (young vs old stars)
measurements determine galaxy distances and calculate Hubble constant
Metallicity indicators trace chemical evolution of galaxies (iron, oxygen abundance)
indicators include H-alpha emission line strength
Active galactic nuclei identified by broad emission lines (quasars, Seyfert galaxies)
Galaxy luminosity functions
Number of galaxies per unit volume as a function of luminosity
Schechter function models luminosity function: ϕ(L)=ϕ∗(L/L∗)αe−L/L∗
Parameters include ϕ∗ (normalization factor), L∗ (characteristic luminosity), and α (faint-end slope)
Applications study galaxy evolution across cosmic time, constrain galaxy formation models, and estimate total luminosity density of the universe
Variations exist with galaxy type and environment (ellipticals vs spirals, clusters vs field)