Chapter 31: Galaxies and the Universe

Transcription

1 Chapter 31: Galaxies and the Universe Sectin 1: The Milky Way Galaxy Objectives 1. Determine the size and shape f the Milky Way, as well as Earth s lcatin within it. 2. Describe hw the Milky Way frmed. A. Discvering the Milky Way 1920s: Astrnmers mapped ut the lcatins f the glbular clusters, which are believed t be the ldest knwn bjects in the galaxy, and calculated the distances t them by identifying variable stars within them. : giants that pulsate in brightness because f the expansin and cntractin f their uter layers (1) : have pulsatin perids between 1.5 hurs and 1 day; have the same luminsity (n average); are used t calculate the distances t the glbular clusters (2) : have pulsatin perids between 1 and 100 days «Lnger pulsatin perids = Astrnmers used t determine that the glbular clusters are distributed and centered arund a pint in space and tward the cnstellatin ; they believe that this pint is the center f the Milky Way. B. The Shape f the Milky Way Was determined by mapping it with Is shaped like a Has a surrunding the galactic center, r, that sticks ut f the galactic disk Astrnmers believe that, the center f the galaxy, is a supermassive (~2.6 millin times the mass f the Sun but smaller than ur slar system) that prbably frmed when the galaxy s disk was frming. Our Sun is apprximately 28,000 ly frm the galactic center and travels, s its rbital perid is. Has a spherical surrunding the nuclear bulge and disk that cntains glbular clusters 31-1

2 Has majr spiral arms (Centaurus, Cygnus, Perseus, and Sagittarius) and numerus minr arms, including, which cntains ur slar system C. Stars in the Milky Way Our Sun is just ne f billins f stars that make up the Milky Way. Mst f the Milky Way s yung stars are lcated in its spiral arms, where the interstellar gas and dust are cncentrated, whereas the ldest-knwn bjects the glbular clusters are lcated in its hal. Astrnmers divide stars in the Milky Way int tw classes: (1) stars: are lcated in the disk and arms and have small amunts f heavy elements; are the yungest stars (EX: ) (2) stars: lcated in the hal and bulge and cntain nly traces f heavy elements; are the ldest stars (EX: ) D. Frmatin and Evlutin f the Milky Way 1. The Milky Way began as a spherical clud with the first stars frming in what is nw the hal The inner prtin f the clud became the nuclear bulge. (This is based n the fact that the hal and bulge are made exclusively f ld stars.) 2. The clud eventually cllapsed under the frce f its wn gravity and began rtating, which created its disk-like shape. 3. Stars that frmed after this time have rbits lying in the plane f the disk. Sectin 2: Other Galaxies in the Universe Objectives 1. Describe hw astrnmers classify galaxies. 2. Identify hw galaxies are rganized int clusters and superclusters. 3. Describe the expansin f the universe. A. Classificatin f Galaxies Galaxies are classified accrding t their shapes: Disk-like Galaxies (1) Spirals Are disk-like galaxies with spiral arms Are divided int tw subclasses: (S) and (SB), which are further subdivided based n: ( ) tightly wund arms and a large, bright nucleus; ( ) lsely wund arms and a small, dim nucleus; ( ) having characteristics between a and c 31-2

3 (2) Nn-spirals are disk-like galaxies that d nt have spiral arms and are dented. Elliptical Galaxies Are nt flattened int disks D nt have spiral arms Are dented Are divided int subclasses (0 7) based n the apparent rati f their majr and minr axes: runder ellipticals are classified as, while very elngated ellipticals are classified as Irregular Galaxies D nt have distinct shapes, and thus d nt fit int either the spiral r elliptical classificatin Are dented by B. Masses f Galaxies Smallest galactic masses : dwarf ellipticals Large galactic masses : large spirals, such as the Largest galactic masses : giant ellipticals C. Grups f Galaxies Mst galaxies are lcated in grups: 1. Small clusters Have diameters <5 millin ly Example: Is ~2 millin ly in diameter Has knwn members, including the and galaxies, which are the tw largest galaxies Cntains mstly dwarf elliptical galaxies 2. Large clusters Have diameters ranging frm ~5 t 30 millin ly May have 100s f members Cntain mstly ellipticals in their inner regins and a mix f ellipticals and spirals in their uter regins Example: Virg cluster 3. Superclusters Are enrmus clusters f galaxies that are in diameter Appear sheetlike and threadlike 31-3

4 D. The Expanding Universe In 1929, Edwin Hubble, by measuring the and distances f many galaxies, fund that the farther away frm Earth a galaxy is, the faster it is mving away It s expanding! Hubble determined that the universe is expanding by graphing the speed at which a galaxy is mving versus its distance, which resulted in a straight line that can be expressed as a simple equatin,, where v = d = H = Is measured in km/sec/mpc currently Is the slpe f the graphed line f measured speeds (v) and distances (d) f many galaxies Can be used t find distances t far away galaxies; t calculate the rate at which the universe is expanding; and E. Radi Galaxies Are ften giant elliptical galaxies that emit as much as r mre energy in radi wavelengths than in visible light wavelengths Emit radi wavelengths frm the tw huge lbes f very ht (and likely inized) gas n ppsite sides f the visible prtin f the galaxy and frm the jets f very ht (and likely inized) gas that link the lbes t the galaxies May cntain a highly energetic bject r activity in its cre an that emits as much r mre energy than the rest f the galaxy F. Quasars ( ) Were discvered in the 1960s Are star-like bjects Are strng radi emitters with highly redshifted lines in their spectra The large redshifts indicate that they are very far away (the mst remte nes are several billin ly away) and are believed t be a result f the expansin f the universe. May be extra-bright f very dim galaxies, which Indicates that they existed billins f years ag, and Suggests that many galaxies went thrugh a quasar stage when they were yung and that tday s AGN might be frmer quasars that are nt quite as energetic as they were lng ag OR Bth AGNs and quasars may be, which wuld explain 31-4

5 Why AGNs and quasars emit far mre energy than rdinary galaxies but are as small as slar systems, and The jets that frm when beams f charged particles stream ut f the cres f radi galaxies Sectin 3: Csmlgy Objectives 1. Explain the different theries abut the frmatin f the universe. 2. Describe the pssible utcmes f universal expansin. A. Mdels f the Universe The fact that the universe is expanding implies that it had a beginning. Origin theries (1) : prpses that the universe des nt change with time; prpses that new matter is created and added t the universe as it expands, and thus, the verall density f the universe des nt change (2) : prpses that the universe began as a pint and has been expanding ever since Current evidence supprts the Big Bang thery. If the universe began in a highly cmpressed state, as the Big Bang thery suggests, it wuld be filled with radiatin In 1965, astrnmers discvered, which Is backgrund nise caused by weak radiatin that cmes frm all directins in space Matches the prperties f the Big Bang thery s predicted leftver radiatin Was mapped in detail in 1989 by NASA s rbiting bservatry, Csmic Backgrund Explrer (COBE) Cannt be explained by prpnents f the steady-state universe thery B. The Big Bang Thery In the Big Bang mdel, there is cmpetitin between the utward mmentum f expansin and the inward frce f gravity as the matter in the universe acts t slw its expansin What ultimately happens t the universe will depend n which frce is strnger. Assuming that the universe s rate f expansin has slwed dwn since its beginning, there are three pssible utcmes fr the universe: (1) Open: (2) Clsed: (3) Flat: The Critical Density Average density: the ttal amunt f matter in the universe 31-5

6 : kg/m 3 and is the dividing pint between a clsed r pen utcme f the universe By cmparing the average density and critical density, astrnmers can predict the utcme f the universe: (1) Average density < critical density: utcme* (2) Average density > critical density: utcme (3) Average density = critical density: utcme *Current data suggests that the average density is less than the critical density, which supprts an utcme. Expansin Rate Astrnmers can cmpare the expansin rate lng ag t the expansin rate tday t predict the fate f the universe. The expansin rate lng ag can be determined by measuring the redshifts f and the distances t the mst remte galaxies. Astrnmers have fund that the rate f expansin slwed fr a while but is currently, which they believe is due t an unknwn frce is pushing the galaxies apart. When the expansin rate f the universe is knwn, it is pssible t calculate the time since the expansin began, r the age f the universe. Based n the best value fr H that has been calculated, as well as the apprpriate crrectins (t allw fr the fact that universal expansin was mre rapid at the beginning and is currently accelerating), the age f the universe is hypthesized t be. Inflatinary Universe Mdel Is a versin f the Big Bang that states that the universe began as a fluctuatin in a vacuum and expanded very rapidly fr a fractin f a secnd befre settling int a mre rderly expansin Predicts a utcme, which in turn, explains the bserved density f the universe (with an allwance fr dark matter) and the expansin s apparent acceleratin 31-6