Stellar Populations in the Milky Way. Star clusters. Cluster HR diagrams B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope. Prof Andy Lawrence

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1 Stellar Populations in the Milky Way Prof Andy Lawrence Star clusters Globular clusters have red stars NGC 3293 Open clusters have blue stars Cluster HR diagrams B.J. Mochejska, J. Kaluzny (CAMK), 1m Swope Telescope NGC 3293 Raboud et al Colour-magnitude (HR) diagram for open cluster HR diagram for Globular cluster... the blue stars are missing.

2 Evolution of HR diagram recall t nuc / M 2.3 As cluster ages, smaller stars gradually leave main sequence Brooks / Cole Thomson Learning The Pleiades is ~60 million years old (although some recent work suggests 115 million...). Globular clusters ~12-13 billion yrs old : cf Earth ~4.5 billion yrs and age of Universe ~13-14 billion yrs (see later lectures) Distribution of globular clusters Brooks / Cole Thomson Learning Globulars do not follow the plane of the Milky Way. They are distributed in a roughly spherical system, but concentrated towards the centre. There are also many individual old stars in this "halo" component. This doesn't mean the disc doesn't have old stars too. Its just that it keeps making new stars whereas the halo has stopped making stars. Components of the Milky Way 2MASS project The IR star counts seem to show several components Thin disc Halo Thick disc Bulge Note there is no hard line between these components but they are clearly different

3 Stellar composition In astronomy, heavy elements are referred to as "metals". The relative abundance of these compared to Hydrogen and Helium is called the "metallicity" of the star compare the Calcium lines in these stars lower metallicity star Typical disc star, high metallicity "Population I" Spectra from SDSS project halo star, v.low metallicity "Population II" Diagram by Peter Woitke The cycle of matter Stars are made from gas clouds During main sequence lifetime, heavy elements are gradually made by nuclear fusion Star death recycles material back into the interstellar medium Further stars form Successive generations of stars have higher metallicity Those low metallicity halo stars and globular clusters (Pop II) really are an earlier generation of stars than we see in the disc of the Milky Way (Pop I) Stellar Motions Nick Strobel, Astronomy Notes Pop I and II stars have different motions And they have different colours

4 Thin vs thick disc Figure shows how number of stars falls off with height z above the disc. Note vertical axis is logarithmic so straight line is an exponential fall-off N = N 0 e z/h where h="scale height". Then : ln N =lnn 0 z/h So the youngest stars are in a disc which is three times thinner than older stars Summary of Milky Way structure Diagram by James Schombert, Oregon Star counts What dominates the Milky Way? Big stars or little stars? What you see in pictures is very misleading : O5 B5 A5 G5 MV % of nearby stars % of naked eye stars To a given apparent magnitude, most stars seem to be quite blue, AB stars. But luminous stars are heavily over-represented because they can be seen further away. Recall L=4πD 2 F ; so if you survey to a flux limit F a star of luminosity L can be seen to distance D=(L/4πF) 1/2. So the effective volume you survey is V = 1 6 1/2 3/2 L F O stars can be seen over a volume a million times larger than G stars

5 Stellar Luminosity / Mass Functions Correcting for the bias, surveys show that by number, most stars are very faint, low mass red dwarves. These stars represent most of the stellar mass/ However, recall L / M 3.3 so those high mass stars make most of the light even though they are a small fraction of the mass Initial Mass Function With time, the mass function N(m) of a region will change as the younger stars "peel off" the main sequence Correcting for this, we can calculate the Initial Mass Function (IMF) that a region started with This seems to be universal - why it should be so is an unsolved problem of star formation.. but it does seem to be an observed fact... Mass to light ratio It is easier to measure light than to measure mass (especially in distant external galaxies) In external galaxies we cannot measure individual stars; only their total light output; so we would like a conversion factor from total light to total mass. If we take the stellar mass function N(m), luminosity dependence L(m) and integrate, we can get total mass M and total luminosity L in solar units For the solar neighbourhood, we find γ=m/l~2 So if we measure a total luminosity Lgal solar luminosity units from an external galaxy, we estimate its stellar mass to be M=γ Lgal solar masses

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