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 ergs 1 ton of TNT = 4 x ergs 1 atomic bomb = 1 x ergs Magnitude 8 earthquake = 1 x ergs Earth’s daily solar input = 1 x ergs Planet cracker = 1 x ergs Luminosity of the sun = 4 x 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.

Main Sequence White Dwarfs Red Giants Red Supergiants Increasing Mass, Radius on Main Sequence The Hertzsprung-Russell (H-R) Diagram Sun

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 © 2013 Pearson Education, Inc. Fig : H-R diagram for brightest stars Stadium Analogy

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.

L  M 3 How does a star's Luminosity depend on its Mass? (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). lifetime α mass (mass) 3 Because luminosity  (mass) 3, lifetime  or 1 (mass) 2 So if the Sun's lifetime is 10 billion years, a 30 M Sun star's lifetime is only 10 million years. Such massive stars live only "briefly". mass of core fusion rate mass of star luminosity  So,

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 = 10 9 years B: 10 billion years = years C: 100 billion years = years D: 1 trillion years = 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 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 ( atoms / cm 3 ) a wide range of temperatures (10 K K) Why study it?

Compare density of ISM with Sun or planets: Sun and Planets: 1-5 g / cm 3 ISM average: 1 atom / cm 3 Mass of one H atom is g! So ISM is about 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 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 Tarantula Nebula Lagoon 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. 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. Sea of protons and electrons

Why is the gas ionized? Remember, takes energetic UV photons to ionize H. Hot, massive stars produce huge amounts of these. Why "H II Region? H I: Hydrogen atom H II: Ionized Hydrogen... O III: Oxygen missing two electrons etc. 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.