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Measuring the Stars How big are stars? How far away are they? How bright are they? How hot? How old, and how long do they live? What is their chemical composition? How are they moving? Are they isolated or in clusters? By answering these questions, we not only learn about stars, but about the structure and evolution of galaxies they live in, and the Universe. Energy mosquito lands on your arm = 1 erg 1 stick of dynamite = 2 x 1013 ergs 1 ton of TNT = 4 x 1016 ergs 1 atomic bomb = 1 x 1021 ergs Magnitude 8 earthquake = 1 x 1026 ergs Earth’s daily solar input = 1 x 1029 ergs Planet cracker = 1 x 1032 ergs Luminosity of the sun = 4 x 1033 ergs/sec Review Burners and Stars Size, Temperature, and Luminosity Building the Hertzsprung-Russell (H-R) Diagram Which is larger, X or Y? => A.) X B.) Y C.) same size D.) can’t tell Building the Hertzsprung-Russell (H-R) Diagram Which is larger, Y or T? => A.) Y B.) T C.) same size D.) can’t tell The Hertzsprung–Russell Diagram Once many stars are plotted on an H–R diagram, a pattern begins to form: These are the 80 closest stars to us; note the dashed lines of constant radius. The darkened curve is called the main sequence, as this is where most stars are. Also indicated is the white dwarf region; these stars are hot but not very luminous, as they are quite small. The Hertzsprung–Russell Diagram An H–R diagram of the 100 brightest stars looks quite different. These stars are all more luminous than the Sun. Two new categories appear here – the red giants and the blue giants. Clearly, the brightest stars in the sky appear bright because of their enormous luminosities, not their proximity. The Hertzsprung-Russell (H-R) Diagram Red Supergiants Red Giants Increasing Mass, Radius on Main Sequence Sun Main Sequence White Dwarfs The Hertzsprung–Russell Diagram This is an H–R plot of about 20,000 stars. The main sequence is clear, as is the red giant region. About 90 percent of stars lie on the main sequence; 9 percent are red giants and 1 percent are white dwarfs. Extending the Cosmic Distance Scale Spectroscopic parallax: Has nothing to do with parallax, but does use spectroscopy in finding the distance to a star. 1. Measure the star’s apparent magnitude and spectral class. 2. Use spectral class to estimate luminosity. 3. Apply inverse-square law to find distance. Extending the Cosmic Distance Scale Spectroscopic parallax can extend the cosmic distance scale to several thousand parsecs. Extending the Cosmic Distance Scale The spectroscopic parallax calculation can be misleading if the star is not on the main sequence. The width of spectral lines can be used to define luminosity classes. Star Classifications Super Giant => From 100 to 1000 times larger than the Sun Giant => From 10 to 100 times larger than the sun Dwarf => any star of size comparable or smaller than the Sun White Dwarf => about the size of the Earth Figure 10.13: H-R diagram for nearest stars Stadium Analogy © 2013 Pearson Education, Inc. Fig. 10.14: H-R diagram for brightest stars Building the Hertzsprung-Russell (H-R) Diagram Which stars have the same temperature? How can you tell? Are stars of the same temperature always of the same type? Building the Hertzsprung-Russell (H-R) Diagram Which star(s) on this diagram (A – G) match this description? This star is very bright (high luminosity) and very hot (high temperature) Building the Hertzsprung-Russell (H-R) Diagram Which star(s) on this diagram (A – G) match this description? This star is very dim and very cool Building the Hertzsprung-Russell (H-R) Diagram Which star(s) on this diagram (A – G) match this description? This star is very dim and very hot Building the Hertzsprung-Russell (H-R) Diagram Which star(s) on this diagram (A – G) match this description? This star is very bright and very cool Stellar Masses Many stars are in binary pairs; measurement of their orbital motion allows determination of the masses of the stars. Orbits of visual binaries can be observed directly; Doppler shifts in spectroscopic binaries allow measurement of motion; and the period of eclipsing binaries can be measured using intensity variations. Stellar Masses Mass is the main determinant of where a star will be on the main sequence. How does a star's Luminosity depend on its Mass? L M 3 (Main Sequence stars only!) Stellar Masses Stellar mass distributions – there are many more small stars than large ones! How Long do Stars Live (as Main Sequence Stars)? A star on Main Sequence has fusion of H to He in its core. How fast depends on mass of H available and rate of fusion. Mass of H in core depends on mass of star. Fusion rate is related to luminosity (fusion reactions make the radiation energy). So, lifetime α Because luminosity lifetime mass of core fusion rate mass of star luminosity (mass) 3, mass or 3 (mass) 1 (mass) 2 So if the Sun's lifetime is 10 billion years, a 30 MSun star's lifetime is only 10 million years. Such massive stars live only "briefly". Clicker Question: The HR diagram is a plot of stellar A: mass vs diameter. B: luminosity vs temperature C: mass vs luminosity D: temperature vs diameter Clicker Question: What would be the lifetime of a star one tenth as massive as our sun? A: 1 billion years = 109 years B: 10 billion years = 1010 years C: 100 billion years = 1011 years D: 1 trillion years = 1012 years Summary of Chapter 10 • Distance to nearest stars can be measured by parallax. • Apparent brightness is as observed from Earth; depends on distance and absolute luminosity. • Spectral classes correspond to different surface temperatures. • Stellar size is related to luminosity and temperature. Summary of Chapter 10, cont. • H–R diagram is plot of luminosity vs. temperature; most stars lie on main sequence. • Distance ladder can be extended using spectroscopic parallax. • Masses of stars in binary systems can be measured. • Mass determines where star lies on main sequence. The Interstellar Medium (ISM) of the Milky Way Galaxy Or: The Stuff (gas and dust) Between the Stars Why study it? Stars form out of it. Stars end their lives by returning gas to it. The ISM has: a wide range of structures a wide range of densities (10-3 - 107 atoms / cm3) a wide range of temperatures (10 K - 107 K) Compare density of ISM with Sun or planets: Sun and Planets: 1-5 g / cm3 ISM average: 1 atom / cm3 Mass of one H atom is 10-24 g! So ISM is about 1024 times as tenuous as a star or planet! ISM consists of gas (mostly H, He) and dust. 98% of mass is in gas, but dust, only 2%, is also observable. Effects of dust on light: 1) "Extinction" Blocks out light 2) "Reddening" Blocks out short wavelength light better than long wavelength light => makes objects appear redder. Grain sizes typically 10-5 cm. Composition uncertain, but probably silicates, graphite and iron. Gas Structures in the ISM Emission Nebulae or H II Regions Regions of gas and dust near stars just formed. The Hydrogen is essentially fully ionized. Temperatures near 10,000 K Sizes about 1-20 pc. Hot tenuous gas => emission lines (Kirchhoff's Laws) Rosette Nebula Lagoon Nebula Tarantula Nebula Red color comes from one emission line of H (tiny fraction of H is atoms, not ionized). Clicker Question: What does does ionized Helium, He II, contain? A: He nucleus only B: He nucleus and one electron C: He nucleus and two electrons D: He nucleus and three electrons Why red? From one bright emission line of H. But that requires H atoms, and isn't all the H ionized? Not quite. Sea of protons and electrons Once in a while, a proton and electron will rejoin to form H atom. Can rejoin to any energy level. Then electron moves to lower levels. Emits photon when it moves downwards. One transition produces red photon. This dominates emission from nebula. Why is the gas ionized? Remember, takes energetic UV photons to ionize H. Hot, massive stars produce huge amounts of these. Such short-lived stars spend all their lives in the stellar nursery of their birth, so emission nebulae mark sites of ongoing star formation. Many stars of lower mass are forming too, but make few UV photons. Why "H II Region? H I: Hydrogen atom H II: Ionized Hydrogen ... O III: Oxygen missing two electrons etc.