Figure 69.01a. Formation of Stars

Transcription

1 1. One cloud many clumps 2. Up to 1000 cores can form within 1 clump 3. Core: begins to build a star by attracting material from the cloud 4. Protostar, MS star 5. Up to 1000 stars 6. Stars usually form in groups out of huge clouds of interstellar matter 7. Star clusters Formation of Stars Figure 69.01a Radio-map of a star-forming molecular cloud (cores) Figure 69.01b Figure 69.01c 1

2 Figure 69.01d Stellar associations Don t present obvious clustering ~ 300 stars still associated by common kinematics Large, loose, unbound and unstable OB associations (high mass O-B2 stars) R associations (medium mass stars) T associations (low-mass stars) Slide 6 The star-forming field of Norma Individual young stars Deep Photographic Image Clusters and Associations of recently born stars Clusters of young stars Associations of young stars Interstellar matter Dust HII regions Atomic H GMC (H 2, CO, ) Young stars are often found in groups: clusters and associations Young Slide 8 field stars: 50 % (?!) 2

3 The Ursa Major Moving Cluster Which cluster is younger? Cluster nature discovered by R.A. Proctor 1869 and Huggins light years from us spread over a volume of 30 light years length and 18 light years width currently approaching us at 10 km/sec NGC 2264 Credit & Copyright: Michael Bessell (RSAA, ANU), MSO Messier 41 H-R Diagram for Cluster M41 Which cluster is younger? H-R diagram for Cluster NGC 2264 Galactic (open) stellar clusters Slide 11 Fig 21-12, p.473 Slide 12 3

4 Structure of our galaxy Globular clusters Disk young stars, gas and dust Bulge, containing the galactic center Halo oldest stars and globular clusters, no interstellar matter, no star formation there A 12 bill. yrs. old cluster 4

5 Globular clusters Which cluster is younger? Slide 18 The Hertzsprung Russell Diagram Figure Henry Norris Russell (US) and Ejnar Hertzsprung (Denmark) M related to L Spectral type related to T Almost all stars fall in fairly well structured bands and zones. 5

6 Figure Figure HR diagrams of clusters 10 million years 7 billion years Figure Figure HR diagrams of clusters: true population of stars of each LC Stars visible to the unaided eye Selection effect we see only giants and supergiants 10 million years 7 billion years 6

7 Initial Mass Function: how many stars of different mass are born within the cluster HR diagram of the known stars within 10 pc of the Sun Figure The Milky Way Overview: Wide-angle photo of the Milky Way Number of Stars Mass Shape Size Age Sun s location First ideas about MW structure 7

8 Figure The shape and size of MW Star Counts: William Herschel: 1785 The further the Milky Way extends in some direction, the more stars we should see. Herchel s Star Count Method Stellar distances not available A given star is about as bright as any other star The dimmer star is farther away Herschel s Map Hershel could not determine the actual size of the Galaxy, but he calculated that it was 5 times wider than it was thick. In this model, the Sun was located very close to the center of the galaxy Light Years 6400 Light Years 8

9 Globular Clusters Dust in the plane of the Milky Way: Limits view in visible light to a few 1000 light years at best s: Harlow Shapley M3: Inconstant Star Cluster Credit & Copyright: J. Hartman & K. Stanek (Harvard CfA) 9

10 RR Lyrae Stars 1 solar mass 100 x larger than Sun 1000 x more luminous Pulsation periods: 1.5 h to 1 day Absolute magnitude well known Distances to globular clusters M = 0.75 mag m depends on distance d = (m-M+5) Estimation of distances: 1. RR Lyra stars in clusters 2. Angular size of clusters Shapely (~1900): Shapely s Map of 93 Globular Clusters The system of globular clusters is centered on a point some 26,000 light years away. If these clusters are evenly distributed, that should be where the center of the Milky Way is located. 10

11 Fig The shape of the Galaxy Disk: structure (thin and thick) Bulge + bar Halo Revolution of all matter around the Galactic center A NASA artist's conception of what the Milky Way would look like if seen from its axis. 11