Week 8 Galaxies Reading: Chapter 15, Sections 1, 3 (9 pages) Galaxy/Not a Galaxy? 1 2 5 4 3 6 7 11 8 9 10 Types of Galaxies The Hubble Classification System: • Spirals • Barred Spirals • Ellipticals • Irregulars Spiral Galaxies Have nuclear bulges and spiral arms. Spiral arms have clouds that are forming new stars. Three Sub-types: Spiral galaxies are classified according to the size of their central bulge and the tightness of their arms. Nuclear Bulge Spiral Arms Sa Big Tightly wound Sb Moderate Sc Small Moderately wound Loosely wound Spiral Galaxies (Face-on) Sa Tight arms, big bulge Sb Sc Loose arms, small bulge Spiral Galaxies (Edge-on) Sa Tight arms, big bulge Sb Sc Loose arms, small bulge Face-on Edge-on Barred Spiral Galaxies • Like normal spirals, but have central bars • Bars are extended, linear bulges • Half as common as “normal” spirals SBa SBb SBc Spiral Density Waves Spirals arms like traffic jams – jam always there, but different cars. Stars and gas clouds rotate around galaxy faster than the spiral density wave. Clouds get compressed when passing through wave; new star formation is triggered. Spiral Density Waves Elliptical Galaxies Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of less than a million stars. Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation. Elliptical Galaxies Elliptical in shape Mostly older stars (note yellowish color) Seven Sub-types: E0, E1, E2, E3, E4, E5, E6, and E7 E0 – almost perfectly round E3 – somewhat elongated E7 – very elongated Elliptical Galaxies E0 E3 E6 Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated). Which class depends on angle; E0 can look like E7 if seen “edge-on”. Giant Elliptical Galaxies ~20x as big as the Milky Way Dwarf Elliptical Galaxies So few stars that you can see right through them. S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas: Irregular Galaxies Irregular galaxies have a wide variety of shapes. Both these galaxies appear to be undergoing interactions with other galaxies. Irregular Galaxies SMC LMC = Large Magellenic Cloud Galaxy classification tutorial www.galaxyzoo.org Galaxy Zoo How We Know that Galaxies are Far Away Cepheid variables 1912 – Henrietta Leavitt’s Cepheid PeriodLuminosity Relation Cepheid Variables in Andromeda 1923 – Edwin Hubble discovers Cepheids in the Andromeda “Nebula”, M31. It is 2.2 million light-years beyond the Milky Way The Local Group The Local Group is the cluster of galaxies to which the Milky Way belongs. • LG is relatively poor: ~40 galaxies • Andromeda (M31): largest and most massive • More than 1/3 are dwarf ellipticals • 3 are spirals (MW, M31, M33) • New dwarf ellipticals being discovered The Local Group Here is the distribution of galaxies within about 1 Mpc of the Milky Way. A nearby galaxy cluster is the Virgo cluster; it is much larger than the Local Group, containing about 3500 galaxies. Very Distant Galaxies Most galaxies are too far away to pick out their Cepheid variables. How do we measure distances to them? Using atomic spectra, Doppler shifts and the Hubble law. Cosmic Fingerprints Spectral lines are like fingerprints – they identify the element that produces them. We use these fingerprints to study the chemical composition and distances of objects in space. Doppler Shifts A moving source of light appears to produce different lines (fingerprints) than a stationary source of light. That is, a moving object’s fingerprints will be shifted with respect to the fingerprints from a stationary object. This effect is called a Doppler Shift. The Doppler Effect Depends only on the relative motion of source and observer. If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer. Light moving towards you is blueshifted. Light moving away from you is redshifted. Very Distant Galaxies Most galaxies are too far away to pick out their Cepheid variables. Their distances are instead found through their Doppler shifts. All galaxies seem to be moving away from us, with the redshift of their motion correlated with their distance: Hubble’s Law These plots show the relation between distance and recessional velocity The relationship (slope of the line) is characterized by Hubble’s constant H0: The value of Hubble’s constant is currently uncertain, with most estimates ranging from 50 to 80 km/s/Mpc. Note that the Universe is expanding. More on this in the last week of class.