Lecture 32: Galaxies We see that scattered through space out to infinite distances, there exist similar systems of stars, and that all of creation, in the whole extent of its infinite grandeur, is everywhere organized into systems whose members are in relation with one another… -- Immanuel Kant, 1755

Topics: Different types of galaxies Finding the distances to far away galaxies Hubble’s Law and the expansion of the Universe

Dwarf Irregular Galaxies Large Magellanic Cloud Small Magellanic Cloud

The Sagittarius Dwarf galaxy: our nearest companion

dwarf ellipticals Antlia Leo I

M31 – the Andromeda Galaxy

The Local Group

Spiral Galaxies

A Grand Design Spiral

Edge-on Spirals

Edge on spirals “Sombrero” Galaxy

A Lenticular galaxy

Elliptical Galaxies

Galaxies like to live in groups

The Virgo Cluster

The Virgo Cluster M87 giant elliptical galaxy

Hubble Sequence

Trends along the Hubble Sequence Lenticular Spiral Elliptical Irregular red blue old stars young stars gas poor gas rich no star formation lots of star formation

Measuring distances to far away galaxies many techniques for measuring distances in astronomy rely on finding standard candles these are objects that always have the same luminosity if we know the luminosity, we can measure the flux and find the distance

Example: main sequence fitting an example of a standard candle is a star of a particular spectral type – for example, a G-type star like the Sun we plot the main sequence for a nearby cluster of stars, for which we can find the distance by parallax then we assume that all main sequence stars of the same spectral type have the same luminosity.

main sequence fitting

other ways to measure distances Cepheid variable stars (period-luminosity relation) white-dwarf supernovae used as standard candles Tully-Fisher relation

Tully-Fisher Relation for spiral galaxies relationship between rotation velocity and luminosity

The Cosmic Distance Ladder

measuring recession velocity

reminder: Doppler formula redshift = z = (lobserved-lrest)/lrest

Hubble’s Law recession velocity distance

Hubble’s Law (modern version)

v = H0 r Hubble’s Law Formula recession velocity = constant times distance units of H0: km/s/Mpc

The expanding Universe Hubble’s Law implies that all galaxies are moving away from us the farther away they are the faster they are moving away from us this makes sense if the whole fabric of space-time is expanding

Please finish reading Ch. 19 over break!