PENN STATE ASTRONOMY LABORATORY è 11 THE STRUCTURE OF THE MILKY WAY GALAXY I. Objective In this lab, yu will learn that we live in the Milky Way Galay. Our slar system and all the stars yu can see with yur wn eyes are in this galay. Yu will learn where we are in the galay, what it lks like, and hw ld it is. II. Eercises Our Milky Way Galay is made up f a disk èwhich yu see in the sky as the band f the Milky Wayè, a bulge èa large clump f stars surrunding the galactic centerè, and a hal èa larger, spherical clud f stars that surrunds the entire galayè. The hal is much larger than the bulge. Our Milky Way Galay is made up f mstly stars, gas, and dust. The dust blcks ut light frm distant stars, and makes it hard t see a lt f the galay, especially the bulge and parts f the disk. Nw, we knw that we live in a galay that has a disk-like shape. But, where in the disk d we live? Are we at the center, r æ n ne side? In the early 1920's, an American astrnmer named Harlw Shapley studied this questin. He fund that there were clusters f stars, called glbular clusters, that were spread arund the center f the galay evenly in a spherical distributin in the hal. Harlw Shapley realized that depending n where yu are in the galay, the pattern f glbular clusters will lk diæerent. Let's try t perfrm Harlw Shapley's eperiment and ænd the center f the galay frm the glbular clusters. We must start, hwever, with a discussin f the crdinates invlved. Skip the net tet sectin if yu are already familiar with right ascensin and declinatin. SKY COORDINATES The celestial sphere is divided up in a system like the ëlngitude-latitude" system n the Earth's surface. The latitude f an bject in the sky is called the declinatin; an bject èlike Plarisè whse psitin is ver the Earth's nrth ple has a declinatin f +90 degrees; an bject ver the suth ple is at,90 degrees. Objects n the celestial equatr have a declinatin f zer. Lngitude n the sky is called right ascensin. On Earth, the line deæning zer degrees lngitude is fairly arbitrary it's the circle that ges frm the nrth ple t the suth ple which passes thrugh Greenwich, England. Right ascensin èraè n the sky als has an arbitrary zer pint it's a circle frm the nrth celestial ple èplarisè t the suth celestial ple that passes thrugh ne f the pints where the ecliptic crsses the celestial equatr èin the cnstellatin f Piscesè. The nly diæerence between lngitude n earth and right ascensin n the sky is that lngitude is usually measured in degrees èfrm 0 t 360 degreesè, while right ascensin is measured in HOURS èfrm 0 t 24 hursè. The pint t the abve discussin is that the crdinate system astrnmers have fr bjects in the sky is similar t the crdinate system that map makers have n earth. Just as every lcatin n earth has a lngitude and latitude, every bject in the sky has a right ascensin and declinatin. 69
The diagram belw represents a view f Earth frm utside the Milky Way, lking dwn at the Earth's Nrth Ple. Earth is at the center, the lines shw the varius directins f right ascensin, and each circle represents tw kilparsecs f distance frm the earth. èon this scale, the Earth wuld be a VERY tiny pint at the center.è The lcatins and distances fr a sample f glbular clusters are pltted. 6 c. With an uncertainty f æ 1 2 hur, what is the right ascensin f the galactic center? 70
Belw is a diagram which views the Earth frm abve its equatr. Again, Earth is a tiny dt at the center, and each circle represents tw kilparsecs in distance, but this time, directins are degrees f declinatin. The same sample f glbular clusters is pltted again with declinatin against their distance in this diagram. d. With an uncertainty f æ10 degrees, what is the declinatin f the galactic center? e. Frm bth diagrams, what is the apprimate distance t the galactic center? 71
On page 73 is a plt cmparing the distributins in the sky f tw classes f bjects: glbular clusters and grups f yung stars. This plt represents the whle sky, pltted in a diæerent ëlatitude-lngitude" crdinate system, where the rigin f the plt is the directin t the center f the Galay èwhich yu just fund the crdinates f in the RA and Dec. systemè. ëlngitude" is measured alng the plane f the Galay, in degrees ranging frm 0 æ èat the centerè t 180 æ èppsite frm the centerè t 360 æ èback t the centerè. ëlatitude" is measured abve and belw the plane f the Galay in degrees and ranges frm: 0 æ n the plane f the galay t +90 æ directly abve the plane, and,90 æ belw the plane. f. Cmpare the distributin f glbular clusters with that f the yung grups f stars. èare the tw grups spread evenly ver the sky r are they clustered in speciæc parts f the sky?è g. Which grup wuld yu study t learn abut the prperties f the disk f ur Galay and why? Which grup wuld yu study t learn abut the hal f ur Galay and why? h. In lab 10 yu fund the ages f a ëyung" grup f stars èthe Pleiadesè and a glbular cluster èm15è. Based n the ages yu fund in lab 10, and yur answer t questin g., what can yu cnclude abut the relative ages f the hal and disk f ur Galay? 72
Distributin f Glbular Clusters and Yung Star Grups n the Sky +90 Yung Star Grups Glbular Clusters +80 +60 +40 +20 73 180 150 120 90 60 30 0 330 300 270 240 210 20 40 180 60 80 90
Assuming that yung grups f stars trace the structure f ur Galay's disk, then by eamining their distributin, we we can study the structure f the disk f ur Galay. The fllwing plt shws the spatial distributin f yung star grups arund the Sun. At the center f the graph is the Sun. Distance away frm the Sun is pltted radially in kilparsecs èkpcè. Arund the circle are angles measured alng the plane f the Galay away frm the galactic center è0 æ represents the directin t the center f the Galay, 180 æ represents the directin eactly ppsite the center f the Galayè. Think f this plt as a ëpicture" lking dwn n a little piece f the disk f ur Galay frm far abve the Sun. i. Describe the distributin f yung star grups near the Sun. èare the yung grups f stars evenly distributed arund the Sun r are they clumped? Are there any trends in their arrangement?è 74
Our Galay is classiæed as a spiral galay. This means that the disk f the galay is in the shape f a ëspiral," ëvrte," r ëwhirlpl." S within the disk f ur Galay there are spiral arms, which are lng streams f denser cncentratins f stars, gas, and dust. j. In the diagram f yung star grups near the sun, n page 74, there are stars frm parts f three diæerent spiral arms near the sun. Mark the lcatins f the three spiral arms in that diagram. k. The circle belw represents the entire disk f ur Galay viewed frm abve. The actual diameter f this disk is abut 30 kpc. T scale, add and label the fllwing structures t cmplete this scale mdel f the Galay: 1. the central spherical bulge èdiameter = 2 kpcè, 2. the psitin f the Sun èyu fund this earlierè, 3. a circle representing the etent f the yung star grups near the Sun frm the ægure n page 74 ènote: all f the stars we can see in the sky are cntained within this circleè. l. Create and label an edge-n view f this mdel in the right-hand margin, net t the face-n view abve. Include: the thickness f the disk and bulge èthe disk is abut 0.6 kpc thick and the bulge is abut 0.8 kpc thickè, and the lcatin f the Sun. 75
m. Why can't we see stars n the ppsite side f the Galay frm the Sun in visible light? èremember that all the stars in ur galay's disk that we can see in the sky are cntained within the circle deæned by the yung star grups that yu drew n the mdel f the Galay.è n. Summarize the facts and ideas presented in this lab. Als include any additinal questins that yu may have. 76