Galaxies

Key Features of Galaxies Galactic disk – an immense, circular, flattened region containing most of our Galaxy's luminous stars and interstellar matter Galactic bulge – Thick distribution of warm gas and stars around the center of a galaxy Galactic halo – region of a galaxy extending far above and below the galactic disk where globular clusters and other old stars reside

Spirals bulge and disk with a spiral structure; the tighter the arms the later the letter in the classification scheme; barred spirals have a bar of interstellar material across the center Stars are forming in the sprial arms (bright O and B type stars)

Elliptical smooth and regular; no disk or bulge; fuzzy football Contain little or no cool gas and dust; no evidence of young stars, made of mostly old-reddish, low mass stars

Lenticular look smooth and regular; has a disk and a bulge; spiral galaxies with NO arms

Irregular no well-defined shape, symmetry or structure

Spiral Structure Spiral density waves – coiled waves of gas Differential rotation makes it impossible for any large-scale structure “tied” to the disk material to survive Spiral density waves – coiled waves of gas compression that move through the galactic disk

Measure the Size of Our Galaxy Variable Stars – stars whose luminosity changes with time Examples: Eclipsing binary stars Pulsating variable stars – which have a consistent pattern RR Lyrae – luminosity changes in a characteristic way and all have about the same average luminosity Cepheid variables – stars whose luminosity varies in a characteristic way, rapid rise in brightness followed by a decline. Determine the luminosity, compare with its’ apparent brightness yields distance

Why pulsating? Opacity – how much light is let through Opacity increase – the internal pressure increases because the radiation is trapped Opacity decrease – the internal pressure decreases and the star shrinks Evolved past the main-sequence

Instability Strip

Period-Luminosity relationship long periods, high luminosity

Tully-Fisher Observation of how fast a galaxy rotates Looking at a particular emission line on the emission spectrum (in this case neutral hydrogen) The line becomes smeared because of the rotation Doppler Effect Red shift as going away Blue shift as coming towards

Visual Example

Measure Even Further Out Standard Candles – easily recognizable astronomical objects whose luminosities are confidently known Have a well defined luminosity Be bright enough to be seen at a large distances

To the outer reaches Hubble’s Law – the rate at which a galaxy recedes is directly proportional to its distance from us Cosmological redshift – as its moving away, the Doppler effect comes into play and it is redshifted Recessional velocity = Hubble’s constant x distance H0 = 70km/s/Mpc