Stellar Midlife. A. Main Sequence Lifetimes. (1b) Lifetime of Sun. Stellar Evolution Part II. A. Main Sequence Lifetimes. B. Giants and Supergiants

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1 Stellar Evolution Part II 1 Stellar Midlife 2 Stellar Midlife A. Main Sequence Lifetimes B. Giants and Supergiants C. Variables (Cepheids) Dr. Bill Pezzaglia Updated Oct 9, 2006 A. Main Sequence Lifetimes 3 (1a) Mass-Lifetime Relation 4 1. Mass-Lifetime Relation 2. Hertzsprung Plots of Clusters Lifetime= (Fuel)/(Rate) Fuel mass Rate Luminosity Luminosity (mass) Turn-off Point of Cluster Lifetime 1/(mass) 2.5 (1b) Lifetime of Sun Fusion of Hydrogen to Helium converts 0.71% mass to energy. If converts total mass, sun would last for 100 billion years But, can only fuse hydrogen in core, hence 10 billion years. 5 (1c) Lifetime of Stars Lifetime= (10 billion years)/(mass) 2.5 Mass in solar masses Spectral Class Mass (solar masses) Lifetime O million B million A million F billion G billion K billion M billion 6 1

2 7 (1e) H & Chi Perseus 8 (1d) Hodge 301 in Tarantula Nebula (2ai) Young Cluster 9 10 H and Chi Perseus Double Open Cluster Its very young (10 million years?), only the O stars have burnt out (become red giants in upper right), rest are on the main sequence 11 (2bi) M45 Pleiades Hertzsprung discovered the main sequence by plotting the Pleiades and Hyades. This cluster is slightly older, the O and are gone, and B stars are beginning to burn out 12 2

3 (2bii) TAURUS 13 (2c) Hyades in Taurus 14 Pleiades=tail of bull Hyades=face of bull (its very close) Hertzsprung discovered the main sequence by plotting the Hyades. This cluster is slightly older, the O and B stars are gone (become giants, and a few have had time to evolve to white dwarfs) M11 Wild Duck Cluster 15 M67 Cancer 16 One of the oldest open clusters, 3 to 4 billions years The very bright blue O and B stars will outshine all the other stars in a young cluster. M11 is perhaps 200 million years old, so the O and most of the B stars have evolved to the bright yellow and red giants seen here. All the OBA stars have evolved into giants. (2f) M55 Glob Cluster in Sagittarius 17 (2g) Blue Stragglers? 18 You d expect all the blue stars to be long since gone in an old globular cluster. All the OBAF stars have evolved into giants. All globs are about 10 billion years old. The blue stars are stragglers The theory is that these stars have recently formed from merging binary stars or stellar collisions. Its not well understood yet. 3

4 (3a) The Turn-Off Point 19 (3b) Put it all together 20 Overplot all the clusters. The youngest cluster has the most stars on the main sequence. The oldest (M67) turns off the main sequence at type F5, so its about 5 billion years old. Very Young Clusters: we see that the low mass stars take longer to form & reach the main sequence The turn off point of a cluster is the spectral class where stars are leaving the main sequence. It tells us the age of the cluster. Older clusters: show us main sequence stars evolve into giants (3c) Evolution into Giants Clusters tell us main sequence stars evolve to giants 21 B. Giants and Supergiants Evolution into a Giant 2. AGB Stars 3. Supergiants (1a). Shell Burning 23 (1b). Eventually reactions stop 24 Helium is building up in core of sun The sun started out with 25% helium uniformly distributed throughout its body Eventually core reactions will stop Reactions can only take place in the hot dense core Hydrogen fusion continues in shell around core Helium can t fuse, it requires higher temperature to ignite 4

5 25 (1d). Evolution to Red Giant Core reactions stop, no outward radiation pressure Core gravitationally contracts, and heats up 8. Subgiant Branch: Causes increased shell burning, Causes outer star to expand to giant size. Surface will be cooler as heat is spread over greater area 9. Helium Flash: the helium in core ignites in triple alpha (2a). Horizontal Branch Helium Flash: the helium in core ignites in triple alpha process Causes core expand and cool a bit, shell burning decreases (2b) Horizontal Branch 28 In the HR plot of an old cluster, you can easily see the Red Giant (sub giant) branch and the Horizontal Branch 10. Horizontal branch Outer star hence contracts in size, but gets hotter, stabilizes (2c) Triple Alpha Reaction 29 (2d). AGB: Asymptotic Giant Branch 30 The net reaction is 3 Heliums combine to make Carbon But this reaction takes place in 2 steps Additional hydrogen capture results in Nitrogen Additional helium capture results in Oxygen Carbon is building up in core of giant Eventually core reactions will stop Hydrogen and Helium fusion continues in shell around core 5

6 (2e). AGB: Asymptotic Giant Branch Core reactions stop, no outward radiation pressure. Core gravitationally contracts, and heats up. (2f) AGB Stars 32 A lightweight star (like the sun) will not be able to ignite the carbon core, instead it will blow up, forming a Planetary Nebula 11. Asymptotic Branch: Causes increased shell burning, Causes outer star to expand to giant size. Surface will be cooler as heat is spread over greater area Carbon Flash: More massive stars will ignite in carbon fusion. Lighter stars probably won t. (2g) AGB Stars 33 In the HR plot of an old cluster, you can easily see the Red Giant (sub giant) branch the Horizontal Branch The Asymptotic Giant Branch (AGB) (2h) Summary 34 (3a) Massive Stars 35 More massive stars will evolve faster, and travel a different evolutionary track on the HR diagram They will also go through more stages. After AGB, they will have a carbon flash. (3b) Supergiants 36 The ignition of the carbon core will again cause the star to move into a horizontal branch, building up a magnesium core. Then the magnesium will ignite, and so on. 6

7 (3c) Nucleosynthesis 37 (3d) 38 Many types of reactions are possible in a supergiant. There can be Helium capture making Oxygen and Neon. The stars zig zag up through luminosity classes of bright giants, supergiants and bright supergiants (3e). Iron Core Builds up in Supergiant The supergiant will eventually have a series of shells of fusion reactions An iron core will build up in the center 39 (3f) Supergiant Lifetime Each reaction takes less time to complete and gives off less energy. 40 C. Variables 41 (1) Instability Strip Instability Strip 2. RR Lyrae Variables There is a region of the HR diagram that whenever a star s evolutionary track crosses there, it will be a regular period variable. 3. Cepheid Variables 7

8 (2a) RR Lyrae Variables 43 Short Period Variables (less than one day) All are Population type II (metal poor, first generation stars), easy to identify (2b) Variable Properties 44 The brightness varies by less than 1 magnitude Corresponds to both change in size and temperature All about same brightness (50 Solar Luminosities, or M=0), so can use them as a standard candle to measure distances to objects that contain them. Can be seen out to about 100,000 Parsecs, so useful in our galaxy (determine distances to globular clusters) (2c) Theory 45 Giant stars have more Helium in the convection zone If star is hot enough, Helium is ionized, which acts like a dam, slowing down flow of heat Star swells up in size, starts to expand (bigger, brighter) Expansion cools outer star, Helium ions recombine with electrons, become more transparent, and releases trapped heat. Star s surface falls inward, compressing Helium, heating it, again causing it to ionize. (3a) Cepheid Variables 46 Periods can be anywhere from 3 to 100 days Very bright (up to 10,000 Solar Luminosities), so easily seen out to 50 Million Parsecs, so useful for the closer galaxies. Absolute Magnitude is proportional to their periods! So, measure period and apparent magnitude, can calculate distance to the variable! (3b) Period Luminosity Relation 47 Example: A 5-day Cepheid with m=+10 is how far away? From below, M=-2.5 (1000 Solar Luminosities) (m-m)=+12.5, this implies 3100 parsecs away (3c) Two Types of Cepheids 48 At first they didn t know that there were 2 types, so all the measurements of distances to galaxies were wrong! Type I: Population I (2 nd Generation stars, Metal rich) are brighter Type II: Population II (first generation stars, metal poor) 8

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