1 II-9 Normal Galaxies and Clusters of Galaxies (Main Ref.: Lecture notes; FK 19-5, 23-1 to 3, 5, 6; Box 23-1; CD photos shown in class) (i)Stellar Population (Main Ref.: Lecture notes; FK Sec ) Two kinds of populations: Population I: Stars in young clusters (e.g., associations, open clusters) Young stars - blue stars,etc. Metal rich; i.e., spectral lines of heavy elements strong Population II: Stars in old clusters (e.g., globular clusters) Old stars - red stars,etc. Metal poor; i.e., spectral lines of heavy elements absent or weak See, e.g., Fig.II-81 Population II stars created long time ago. Note: globular clusters are > ~ years old! All massive stars have finished life long ago, and are gone. Only small mass stars with long life are still around. Lec 8

2 Why population II stars are metal-poor? In the early Universe right after the big bang, only light elements H and He are created. All `metals’ (= elements heavier than He) are synthesized during the pre-supernova stage of massive stars. Population II stars were born early in the evolution of the Universe, soon after the big bang, before metals are formed during the pre-supernova evolution of massive stars. So, metal-poor! Population I stars are born relatively recently, and so still young. So, they are found in young clusters. Why metal-rich? Pop. I stars were born recently from the dense region of the interstellar medium (ISM), e.g. molecular clouds, dark nebula,etc., which are contaminated by ashes of nucleosynthesis which went on in earlier generation massive stars during the pre- supernova stages. Since the matter out of which Pop. I stars is born comes out of the material ejected by supernova explosions of earlier generation massive stars, they contain more metals, and hence, metal-rich!

3 See class notes for further details. Now we also have Pop III stars (see class notes). Will come back to more discussions of properties of Pop. I, II and III stars in the galaxy section. Fig. II-81: Spectra of Metal-Poor and Metal-Rich Stars

4 (ii) Normal Galaxies (Main Ref.: Lecture notes; FK Sec. 23-2,3; Reading FK Sec 23-1 optional – not covered in class) Reading assignment – FK Ch 22 Our Galaxy (not covered in class.) Galaxy Types: There are three types of galaxies – Spiral, Elliptical, and Irregular. Properties - study Table II-7. See class notes for further detailed explanation. Enjoy CD photos shown in class and various pictures in FK. Table II-7: Properties of different galaxies

5 Examples: Elliptical – NGC 4365; Spiral – Andromeda (M31); Barred Spiral – NGC 1365; Irregular - LMC Hubble’s Classification and Hubble’s Tunig Fork Diagram: Galaxies are classified according to the tuning fork diagram. The subclasses are: Left to right (see Fig. II-82): Ellipticals – E0 to E7; Spirals – S0, Sa to Sc; Barred Spirals – SB0, SBa to SBc See class notes for changing Shapes from left to right, and what that means. Note: among ellipticals, as we go from E0 to E7 the shape goes from circular to more flattened (i.e., pancake) shape. Spirals are normal spirals and barred spirals (spirals with a bar). Among spirals and barred spirals, as we go from a to c it goes from most tightly coiled to most loosely winding arms, and from the largest bulge to the smallest. Back to Pop I, II, and III stars : Their age, location in different galaxies, Velocities (Pop I low, Pop II high), etc., and why? See your class notes. Fig. II-82: Hubble’s Tuning Fork Diagram

6 (iii) Hubble Law (Main Ref.: Lecture notes; FK Sec. 23-5) What it says : Further away a galaxy is, faster it is receding from us! Discovery purely from observations of distance and redshift of many galaxies. Galaxies are made up of stars, gas and dust. Star light includes absorption lines. Gas in emission nebular emits emission lines. So, we can measure redshift of these lines. Distance is the hardest to measure, but could be done – come back later. Each galaxy has its own local motion, but if sufficiently far away, the receding velocity (hence redshift) overtakes. Importance: Proof of expanding Universe – one of the most important discoveries in cosmology! – come back later. Cosmological Redshift z = redshift due to cosmological expansion of the Universe after big bang  Receding radial velocity v c

7 z = ( – 0 ) / 0 =  / 0 = v c / c (if non-rel ) Eqn(II-28) Hubble Law expressed mathematically is: v c = H 0 d Eqn(II-29) Where v c is receding velocity due to cosmological redshift; d is distance, and H 0 is Hubble’s constant. From Eqns (28) and (29), we get: d = v c / H 0 = z c / H 0 Eqn(II-30) Note: Eqn(30) is a powerful method to find distance d of galaxies far away (when it cannot be known in other ways), if H 0 is known. By simply measuring redshift, can find d! So, it is important to find H 0 correctly. Hubble Constant : Find H 0 by measuring distance d and redshift z of galaxies whose distances can be found by other methods (see later section on distance measurement).

8 Latest from HST (Hubble space telescope): H 0 = 73 km/s-Mpc Current range of unertainty : 60 < H 0 < 90 km/s-Mpc Here we use: H 0 = 70 km/s-Mpc EX 52: Singly ionized Ca K line in NGC Measured = nm; 0 = nm. What is the distance of NGC 4889? Ans: d = 93 Mpc. See class notes for the details. Fig II-83: Hubble Law

9 (iv) Clusters and Superclusters of Galaxies (Main Ref.: Lecture notes; FK Sec. 23-6) FK gives good description of the subject. So, study your textbook carefully. Here I give summary only. Galaxies come in clusters. The smallest is our local group, consisting of our Milky Way Galaxy, Anromeda, LMC, SMC, etc. See FK Fig The closest member of the Local Group – the Canis Major Dwarf (see FK Fig ). The largest – Andromeda = M31. Large cluster of galaxies – e.g., The Coma Cluster, see FK Fig – many dense collection of galaxies, denser toward center. Have many elliptical galaxies. Superclusters of galaxies: Distribution of clusters of galaxies – see FK Fig They look like soap bubbles with big voids. Clusters of galaxies lie along the walls of the bubble-like voids – filament- looking in 2D(two dimensional) picture. See pictures in CD photos shown in class and those in FK.