Today s Topics & Events

Transcription

1 ASTR 1120: Stars & Galaxies Prof. Juri Toomre TA: Licia Ray Lecture 34 Wed 7 Apr 04 zeus.colorado.edu/astr1120-toomre toomre NGC 1232 Today s Topics & Events Today we look at the challenge of measuring big distances to figure out where things are in universe Class meets in Fiske Planetarium this Friday (April 7) go there directly: John Bally: Birth of Stars and Planets New Homework Set 7 on Cosmic Distances and Hubble Law still available due next Monday Homework 6 returned (graded+answers( graded+answers) ) today, along with more of the extra-credit projects 1

2 Clicker -- reading on galaxies How might you classify this galaxy? A. Sa B. SBb C. E D. SO B. Reminder Cepheid variable stars as distance indicators: standard candle Vital discovery by Henrietta Leavitt (1912) 2

3 Cepheid stars in H-R H R diagram Instability strip -- region in H-R H R diagram with large, bright stars (Chap 16, p 536) Outer regions of star are unstable and tend to pulsate Star expands and contracts, getting brighter and fainter Reminder (Fig 16.13) Cepheid variable stars Period - Luminosity relation brighter Cepheids have longer periods DISTANCE ESTIMATE 2 3

4 Andromeda found to be far outside Milky Way another island universe : galaxy! Edwin Hubble in 1924 identified Cepheids in Andromeda (M33) showed they were far outside of Milky Way! His first big discovery. Next was expansion of the universe wow! Hubble using new 100 Hooker telescope at Mt. Wilson (above LA) Andromeda M31 4

5 Hubble: showed universe appeared to be expanding! Vesto Slipher (1912) reported that most galaxies showed Doppler redshifts Edwin Hubble,, using new 100 telescope, started busily measuring galaxy redshifts Hubble (1929) announced that redshifts of galaxies appear to increase with distance from us This was startling: suggests an EXPANDING UNIVERSE! Hubble and recession of galaxies: Further away, greater redshift! Hubble guessed their distances by size and brightness -- underestimated by factor 10! 5

6 Hubble s Law Hubble s (1929) original Scatter here from random velocities of nearby galaxies, unreliable distance estimates Best current values for expansion velocity H o = 71 +/- 4 km/s/mpc HUBBLE CONSTANT distance Hubble (1929) plot extended only to 2 Mpc,, H o was ~500! 6

7 Universe expands like raisin bread! Mapping the universe: need distances to galaxies! Identify (and calibrate) properties of galaxies that could serve as STANDARD CANDLES -- beyond direct measure by trigonometric parallax 1. Make some measure of an object which identifies its luminosity (like period in Cepheid) 2. Use this luminosity and measure apparent brightness to infer distance to it 7

8 DISTANCE ESTIMATE 1 Main-Sequence Fitting Start with cluster A (upper) whose distance known via parallax A Compare with other cluster B (lower) B Use brightness difference to estimate B distance M-S S Fitting pinned to Hyades Cluster, 151 ly away 8

9 DISTANCE ESTIMATE 2 1. Measure period of variability Cepheids variables as standard candles 2. From period- luminosity relation, infer the luminosity 3. Compare with apparent brightness and thus determine distance Cepheid variable in M100 (HST) Clicker Cepheids and distance Two Cepheid stars, Fred and Barney, have the same apparent brightness.. Fred has a period of 5 days, and Barney of 10 days. Which is closer? A. Fred B. Barney A. 9

10 Why A. Fred? Fred has a shorter period and so must be less luminous Period-Luminosity Relation Less luminous but the same apparent brightness means that Fred is closer to us DISTANCE ESTIMATE 3 Even brighter: White dwarf supernovae Standard explosion = fusion of 1.4 solar masses of material Always the same amount of energy released 10

11 Bright enough to be seen halfway across observable universe Useful for mapping the universe to the largest distances Practical difficulty: White dwarf SN Need to catch them within a day or two of the explosion About 1 per galaxy per century Need to monitor thousands of galaxies to catch a few per year galaxy clusters are useful 11

12 DISTANCE ESTIMATE 4 Tully-Fisher Relation Fast rotation speeds in spiral galaxies Lots of mass High luminosities Measure rotation speeds to infer the luminosity Need bright edge-on spirals Distance ladder Overlapping standard candles 12

13 Distance ladder to measure universe Different standard candles are useful for different distances Reading for Next Lecture Complete your reading of 20.3 Measuring Cosmic Distances Read 20.4 Measuring Cosmic Ages Remember: go directly to Fiske Planetarium for this Friday lecture 13