Stars & Gas: Building Blocks of the Galaxy

Stars & Gas: Building Blocks of the Galaxy Stars as Black Body Radiators Hertzsprung-Russell diagram: Luminosity vs. Temperature Radiation over a `continuous' or broad range of frequencies (continuum radiation) Gas Nebulae Radiation at specific

frequencies (line radiation) Emission lines: from energetic atoms in gas cloud Absorption lines: a gas cloud seen against the backdrop of a continuum radiation source Line radiation from nebulae allows us to probe their physical conditions (composition, temperature,

density) Doppler shift of line radiation due to motion along the line of sight: If the speed (v) is much smaller than the speed of light (c), / = - / = v / c where: and are the

changes in wavelength () and frequency (), respectively Introduction to Galaxies Basic Structure How densely packed are stars in a galaxy? Size (diameter) of a typical star: 106 km Distance between stars: 1 pc = 3 x 1013 km Analogy: 1 cm sized marbles separated by 300 km!

What fills in the space between stars? Interstellar medium: gas, dust Galaxy Types Spirals & irregulars (disk galaxies); ellipticals Morphological (structural) features: Disk, bulge, bulge+disk, presence/absence of central bar Nature of kinematics (internal motion of

stars and gas): Coherent rotation of stars and gas in a disk; differential rotation Random motion of stars in the bulge of a spiral galaxy or elliptical Hubble Sequence of Galaxies Tuning fork diagram: E0-E7, S0 Sa-Sd / SBa-SBd, Irr

Morphological trends along the sequence: Shape (flattening) Bulge-to-disk ratio Spiral arms Kinematical trends along the sequence: Ellipticals: mostly random motion, hardly any rotation Spirals: mostly rotation, hardly any random motion Trends in the stellar mix: Ellipticals: mostly cool (old) stars

Spirals: dominated by hot (young) stars The Local Group of Galaxies Two large spiral galaxies Milky Way & Andromeda (Messier 31 or M31) Distance between them: D = 700 kpc = 2.3 x 106 light yrs Each large spiral galaxy has several smaller satellite galaxies in orbit around it

Milky Way: Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), Sagittarius dwarf, etc Andromeda: Messier 32 (M32), NGC 205, NGC 147, etc You are here

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