M31 - Andromeda Galaxy M110 M32

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1 UNIT 4 - Galaxies XIV. The Milky Way galaxy - a huge collection of millions or billions of stars, gas, and dust, isolated in space and held together by its own gravity M110 M31 - Andromeda Galaxy A. Structure our sun and solar system belong to a galaxy called the Milky Way within the flattened, circular region called the galactic disk seen in the night sky as a band of light called the Milky Way by ancients - looked like a trail of milk spilled in the sky by a goddess nursing her baby light is the glow of billions of stars of our galaxy M32 the center of our galaxy contains older stars in a broader region called the galactic bulge spherical ball of faint stars surrounds our galaxy - called galactic halo William Herschel (18 th C) estimated the size and shape of our galaxy based on counting stars thought the galaxy was flattened and disk shaped with the sun at the center Herschel did not know of the intervening gas and dust that blocks our view of stars within the Milky Way hides the true shape of our galaxy - allows us to see only so far to know the true shape of the Milky Way, the distance to very distant stars must be found astronomers use variable stars to find the distance to distant parts of the Milky way and other galaxies variable stars change in luminosity over time some pulsate regularly with a characteristic light curve - intrinsic variables

2 intrinsic variable stars represent a post main sequence stage of evolution where a star undergoes a period of instability temperature and radius of the star vary in a regular way when radiation is unable to radiate away from the interior of the star Structure RR Lyrae variables - low mass post main sequence stars Cepheid variables - high mass post main sequence stars Cepheid's variables can be seen for millions of light years their period of variability is related to their luminosity comparing the star's luminosity with its apparent brightness will give an estimate of its distance by the inverse square law: apparent brightness = luminosity distance 2 Harlow Shapley measured the distance of RR Lyrae stars in globular clusters found these clusters to be many thousands of light years away when measuring the distance and direction of these clusters, the sun was found not to be at the center of all the globular clusters, but in the direction of Sagittarius

3 globular clusters are contained within the halo of the galaxy this maps out the true galactic center of the Milky Way and its size ten times larger than previously thought Structure the sun is not at the center of the Milky Way, but rather approximately 2/3 away from the center the Milky Way appears as a giant pinwheel (spiral) approximately 100,000 light years across spiral arms contain gas and dust and is the site for star formation within the galaxy the disk is 1,000-3,000 light years thick young stars (population I stars; have heavier elements) and gas are confined close to the galactic plane where they form older stars (population II stars; contain only hydrogen and helium) drift further out the galactic halo contains no gas and dust no new stars form there all stars formed there formed during the formation of the Milky Way (at least 10 billion years ago) central bulge is football shaped and about 10,000 light years thick outer part contains old stars (gas poor) inner part has both old and young stars (gas rich) Milky Way rotates (like a pinwheel) once in about 225 million years (sun's distance from center) rotation is differential - the Milky Way rotates faster closer to the center and slower further out also observed in other spiral galaxies halo stars and globular cluster orbit the center of the galaxy in randomly in all directions

4 Structure Interstellar gas and dust prevent observations of stars to great distances within the galaxy radio telescopes (long wavelengths) can "see" through the dust and allow astronomers to map the distribution of gas and dust in the galaxy - Milky Way has a spiral structure Spiral arms contain gas and dust and is the site for star formation within the galaxy appears bright and blue due to the blue color of high mass stars do not live long so they are concentrated within the spiral arms the galactic bulge is densely populated with stars approximately 1 million times more dense than the region about the sun at the galactic center (strong radio source called Sagittarius A) lies a supermassive black hole based on observations of a rotating ring of material over a few light years across B. Mass to measure the mass of the galaxy, the orbital motion of stars at the outermost regions must be measured objects (stars) furthest from the center of the galaxy should orbit around the center slower than objects closer to the center as you get further from the center, there should be less material (stars, gas) at a distance of approximately 50,000 light years from the center, the orbital speed of stars and gas increases slightly this implies that there is more mass outside the visible part of the galaxy than can be accounted for the visible part of the galaxy is only a small portion of the true extent of the galaxy the visible portion of the galaxy is surrounded by a dark halo which consists of dark matter cannot be seen only its gravitational effects can be observed may consist of black dwarfs, brown dwarfs, black holes (all know as MACHOs - MAssive Compact Halo Objects) exotic massive subatomic particles that do not interact with ordinary matter called WIMPs (Weakly Interacting Massive Particles) that were produced in the formation of the universe A map of the galaxy cluster CL : dark matter appears as a halo in blue, while visible matter is in red

5 XV. Normal Galaxies Galaxies do not all look alike they are not all like spiral galaxies as the Milky Way Edwin Hubble in 1926 categorized galaxies based on their appearance called Hubble Classification Scheme still used today there are four basic types of galaxies: Spirals Barred Spirals Ellipticals Irregular Barred Spiral Spiral Elliptical Irregular A. Spiral Galaxies the Milky Way and its neighbor the Andromeda Galaxy are examples of Spiral Galaxies all contain a galactic disk (with spiral arms), a central galactic bulge, and a faint halo of old stars spiral galaxies are denoted by the letter S and subdivided by their central bulge size: a, b, or c Sa galaxies have the largest central bulges and tightly wrapped spiral arms Sc galaxies have the smallest central bulges with loose, poorly defined spiral arms

6 Barred Spiral Galaxies have an elongated bar shaped concentration of matter through the center of the galaxy spiral arms project from the end of the bars rather than the central bulge barred spirals are denoted by the letters SB and subdivided into categories a, b, and c (just like spirals - based on size of the central bulge) B. Barred Spiral Galaxies Elliptical Galaxies have no spiral arms and no flattened galactic disk many are egg shaped stellar density increases towards the center elliptical galaxies are denoted by the letter E and subdivided by how elliptical (egg shaped) they are the most circular are E0, the most elongated are E7 there is a large range of sizes of elliptical galaxies from giant ellipticals (contain trillions of stars) to dwarf ellipticals (contain a few million stars) dwarf ellipticals are far more numerous than giant ellipticals elliptical galaxies lack spiral arms contain little gas and thus have old, red, low-mass stars intermediate between ellipticals and spirals are galaxies with a flat disk, but no gas and no spiral arms denoted S0 or SB0 (if bar present) C. Elliptical Galaxies

7 D. Irregular Galaxies Irregular Galaxies have no set geometric shape they usually have much gas and dust and young blue stars smallest irregular galaxies are called Dwarf Irregulars - most common irregular type of galaxy Small Megellanic Cloud Large Megellanic Cloud E. Hubble Classification Scheme F. Distribution of Galaxies in Space to know where galaxies are in the universe, the distance to them must first be known Standard Candles - easily seen objects with luminosities that are know comparing an objects apparent brightness to its luminosity gives the objects distance (and the galaxy it resides in) include Type 1 supernova, Cepheid Variables, and by observing the rotation speed of galaxies (rotation speed gives the mass of the galaxy)

8 1. Galaxy Clusters galaxies tend to congregate in galaxy clusters the Milky Way is part of a cluster called the Local Group includes: 45 galaxies (Milky Way, Megellanic Clouds, Andromeda Galaxy)- all gravitationally bound together Local Group's diameter is approximately 6.5 million light years across Virgo Cluster - next close galaxy cluster approximately 60 million light years from the Milky Way contains 2,500 galaxies all gravitation bound together in a space 10 million light years wide galaxy clusters tend to congregate into clusters themselves forming huge superclusters the Local Group, the Virgo Cluster and several other clusters form a huge supercluster of galaxies centered near the Virgo Cluster called the Local Supercluster the Local Supercluster contains several tens of thousands of galaxies 150 million light years across beyond the Local Supercluster are countless other superclusters of galaxies 2. Superclusters G. Masses of Galaxies Newton's laws of gravity allow astronomers to measure the mass of a galaxy masses of spiral galaxies are determined by the rotation speed of the spiral arms measure the doppler shift of spectral lines for the galaxy when measuring the mass of superclusters of galaxies, there appears to be much more mass within the cluster than can be accounted for by the light emitted by the galaxies this is dark matter 90% of the universe is composed of dark matter

9 H. Formation and Evolution of Galaxies Astronomers do not know for certain how galaxies evolve into the categories within the Hubble classification scheme galaxies grow by the merging of smaller galaxies galaxy mergers are supported by computer simulations of the early universe and by observations of galaxies at great distances that show these galaxies are smaller in size, blueish in color (due to star formation from mergers) and less regular in appearance ex.: Hubble Deep Field galaxy collisions are common - galactic cannibalism some collisions between galaxies can cause distortions leading to the formation of spiral arms where none existed before takes several hundred million years Evolution can account for the Hubble classification sequence: Spiral galaxies grow from the merger of small galaxies with a larger spiral the larger spiral retains its shape but grows larger ex. Milky Way - shows evidence of past mergers Elliptical galaxies result from the merger of galaxies of similar sizes these mergers destroy the spiral structure of the galaxies the result after star formation has stopped is an elliptical galaxy observations of superclusters support this giant ellipticals appear towards the center of superclusters

10 I. Hubble's Law Edwin Hubble's observations of the spectra of galaxies show that all galaxies (except a few nearby galaxy systems) are moving away from the Milky Way in all directions galaxies furthest from us are moving faster away from us than galaxies that are closer they are redshifted more the greater the distance, the greater the red shift Hubble's Law - the rate at which a galaxy recedes is directly proportional to its distance from us: the entire universe is expanding galaxies are getting further away from us and each other the redshift caused by this expansion is called the cosmological redshift used as a distance measuring tool has implications for the past and future evolution of the universe Hubble's Constant (H 0 ) - the value of the slope of the line which gives the relation between recessional velocity and distance: recessional velocity = H 0 x distance the Hubble Constant specifies the rate of expansion of the universe distance to an object is found by measuring the recessional velocity and dividing by the Hubble constant furthest objects are 13 billion years old

11 Hubble's Law using Hubble's Law, the distribution of galaxies and galaxy clusters in the universe is not random clusters of galaxies lie within filaments (surface of "bubbles") which surround empty regions (voids) in space - much like suds on soapy water largest superclusters lie in regions where several bubbles meet - ex. Virgo Supercluster Great Wall - large scale filament structure - one of the largest known structures in the universe galaxies that are much brighter than normal are known active galaxies emit most of their radiation at long wavelengths (radio waves) as opposed to normal galaxies (visible wavelengths) XVI. Active Galaxies and Quasars Seyfert Galaxies have properties between normal galaxies and the most violent active galaxies represent an evolutionary link between these extremes there are three types of active galaxies: Seyfert Galaxies, Radio Galaxies, and Quasars active galaxies look like normal galaxies but emit enormous amounts of radio/infrared radiation A. Seyfert Galaxies have large redshifts - very distant in visible light, they look like normal spirals NGC 4261 radio

12 Seyfert Galaxies spectral analysis of Seyfert galaxies indicates very rapid rotation near the galactic nucleus energy emitted by Seyferts varies over time (can double or halve within a fraction of a year) does not happen in normal galaxies this rapid change in brightness means the source of energy is very small in size change in brightness occurs in less than a year so the size of the object must be less than one light year across - very small for the amount of energy released B. Radio Galaxies have many of the same characteristics as Seyfert galaxies emit radiation from much larger areas (than central nucleus as in Seyfert galaxies) often the radio energy is emitted in two large extended regions called radio lobes - rounded clouds of gas lobes are aligned with the center of the galaxy as material is ejected most radio galaxies are associated with elliptical galaxies radio sources with star-like visible objects - quasi-stellar radio sources (quasars) unusual spectrum - lines are redshifted cannot be stars - redshift indicates that these objects are very distant (from the early universe) their great distance implies these are the brightest objects known in the universe a quasars energy output varies irregularly the region of energy production must be very small recent observations suggest that most, if not all, Quasars reside in a host galaxy C. Quasars

13 D. Central Engine of Active Galaxies all active galaxies have: - high luminosities - energy emission is non-stellar - cannot be accounted for by just stars - energy output is variable - often have jets of material (explosive activity) - their spectra show rapid internal rotation within the energy producing region the huge energy produced must come from a supermassive compact object (supermassive black holes) at the center of the nucleus where gas or other material is accreted onto it (billions of times more massive than the sun) 1 billion solar mass black hole would have a diameter of only 20 AU gas within the accretion disk would be heated to billions of degrees observations from the Hubble Space Telescope support this theory - supermassive black hole with an accretion disk some quasars exhibit gravitational lensing - the gravitational field of a foreground galaxy (galaxy cluster) bends the light of a of a distant galaxy or quasar into multiple images can determine the masses of the galaxy (galaxy clusters) including dark matter

14 Central Engine of Active Galaxies large luminosity of quasars is the result of there being more gas (fuel) available in the early universe quasars spend a relatively short period of time in this highly luminous phase represents an early phase in galaxy evolution early formed galaxies may have merged and conditions existed (plentiful gas) to form supermassive black holes and thus highly luminous quasars the result as the fuel diminished, the quasars dimmed and the galaxy is visible as Seyfert and radio galaxies as fuel supply diminished further, central core became inactive and Seyfert spirals became normal spirals and radio galaxies normal ellipticals