The Nature of Stars (Chapter 17 – Universe) The Nature of Stars (Chapter 17 – Universe)

Astronomy 12 – Final Project: Visit the following website – Pick one of the animations/simulations from the list. –Who came up with the original observations or ideas? Give a little history to why we would want to simulate this today. –How did they do this in real life? –Explain how the simulation works? –What does it mean? –Explain why it is important.

Requirements: PowerPoint Presentation for class about the sim, the history behind why the investigation was done in the first place, and why it is important today minutes Demonstration to the class of how to use the sim. 5 – 10 minutes

Spectral Types of Stars Spectral types are defined by the: Spectral types are defined by the: existence of spectral lines belonging to various elements, ions, & molecules in a star’s spectrumexistence of spectral lines belonging to various elements, ions, & molecules in a star’s spectrum the relative strengths of these linesthe relative strengths of these lines However, spectral type is not determined by a star’s composition However, spectral type is not determined by a star’s composition all stars are made primarily of Hydrogen & Heliumall stars are made primarily of Hydrogen & Helium

Spectral Type Classification System O B A F G K M Oh Be A Fine Girl/Guy, Kiss Me! 50,000 K 3,000 K Temperature

How can we classify stars 1)Collect information on a large sample of stars. 2)Measure their luminosities 3)Measure their surface temperatures (need their spectra)

HR – Diagrams: FgrKg4 (Overview) FgrKg4 Xtm1hs (Well explained) Xtm1hs

The Hertzsprung-Russell Diagram

HOTCOOL BRIGHT FAINT

The Hertzsprung-Russell Diagram The Main Sequence ~90% of all stars are in the main sequence (MS) ~90% of all MS stars are cooler spectral types than the Sun (i.e., at the lower MS) All MS stars fuse H into He in their cores.

The Hertzsprung-Russell Diagram Mass-Luminosity Relation: L  M 3.5 For example, if the mass of a star is doubled, its luminosity increases by a factor ~ 11. Mass of MS Star L  M 3.5 The relation is for main sequence stars only!

The Hertzsprung-Russell Diagram Red Giants - Red Giant stars are very large, cool and quite bright. e.g., Betelgeuse is 100,000 times more luminous than the Sun but is only 3,500K on the surface. It’s radius is 1,000 times that of the Sun.

The Hertzsprung-Russell Diagram Super Giants Very bright Very hot Size of Star:

The Hertzsprung-Russell Diagram White Dwarfs - White Dwarfs are hot, but since they are so small, they are not very luminous.

Main Sequence Lifetime All M-S stars have temperatures sufficient to fuse H into He in their cores.All M-S stars have temperatures sufficient to fuse H into He in their cores. Luminosity depends directly on mass:Luminosity depends directly on mass: –more mass = more pressure from upper layers –nuclear fusion rates must be high maintain equilibrium Higher mass stars have shorter lives!Higher mass stars have shorter lives!

The Hertzsprung-Russell Diagram Lifetime of Star Shorter Longer More mass, more fuel, very fast burning. Less mass, less fuel, slow, steady burning

HR – Diagrams: FgrKg4 (Overview) FgrKg4 Xtm1hs (Well explained) Xtm1hs

To do: Check out the following site: ray.org/Labs/StarLife/hr_interactive.htmlhttp://aspire.cosmic- ray.org/Labs/StarLife/hr_interactive.html xplorer.html (excellent resource) xplorer.html

Mass (M Sun ) Luminosity (L Sun ) Surface Temperature (K) Radius ( R Sun ) Main sequence lifespan (yrs) × , × , × , × , × , × , × , × ,00022, × ,00028, × ,00035, × ,00044, ×10 6 As mass of a M-S star increases, luminosity, surface temperature and radius (size) increaseAs mass of a M-S star increases, luminosity, surface temperature and radius (size) increase As mass of a M-S star increases, life span on M-S decreasesAs mass of a M-S star increases, life span on M-S decreases

Review Questions: The H-R Diagram 1.Where are most stars? 2.What is the common characteristics of MS stars? 3.What determines the location of a star in the MS? 4.Where do you find the largest stars? 5.The smallest? 6.The most massive one? 7.The coolest stars? 8.How do we know the age of a star?

Review Questions: The H-R Diagram 1.Where are most stars? 2.What is the common characteristics of MS stars? 3.What determines the location of a star in the MS? 4.Where do you find the largest stars? 5.The smallest? 6.The most massive one? 7.The coolest stars? 8.How do we know the age of a star? 1. MS, 2. H  He, 3. M, 4. upperright, 5. lowerleft, 6. upperleft, 7. lowerright, 8. normally we don’t

Key Ideas Measuring Distances to Nearby Stars: Distances to the nearer stars (within approx. 650 l.y.) can be determined by parallax, the apparent shift of a star against the background stars observed as the Earth moves along its orbit. The Population of Stars: Stars of relatively low luminosity are more common than more luminous stars. Our own Sun is a rather average star of intermediate luminosity.The Population of Stars: Stars of relatively low luminosity are more common than more luminous stars. Our own Sun is a rather average star of intermediate luminosity.

Key Ideas Spectral Types: Stars are classified into spectral types (subdivisions of the spectral classes O, B, A, F, G, K, and M), based on the major patterns of spectral lines in their spectra. The spectral class and type of a star is directly related to its surface temperature: O stars are the hottest and M stars are the coolest.Spectral Types: Stars are classified into spectral types (subdivisions of the spectral classes O, B, A, F, G, K, and M), based on the major patterns of spectral lines in their spectra. The spectral class and type of a star is directly related to its surface temperature: O stars are the hottest and M stars are the coolest. Most brown dwarfs are in even cooler spectral classes called L and T. Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusion.Most brown dwarfs are in even cooler spectral classes called L and T. Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusion.

Key Ideas Hertzsprung-Russell Diagram: The Hertzsprung- Russell (H-R) diagram is a graph plotting the luminosities of stars against their surface temperatures.Hertzsprung-Russell Diagram: The Hertzsprung- Russell (H-R) diagram is a graph plotting the luminosities of stars against their surface temperatures. The positions on the H-R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperature.The positions on the H-R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperature.

Key Ideas On the H-R diagram, giant and supergiant stars lie above the main sequence, while white dwarfs are below the main sequence.On the H-R diagram, giant and supergiant stars lie above the main sequence, while white dwarfs are below the main sequence. By carefully examining a star’s spectral lines, astronomers can determine whether that star is a main- sequence star, giant, supergiant, or white dwarf. Using the H-R diagram and the spectral lines, the star’s luminosity and distance can be found without measuring its stellar parallax.By carefully examining a star’s spectral lines, astronomers can determine whether that star is a main- sequence star, giant, supergiant, or white dwarf. Using the H-R diagram and the spectral lines, the star’s luminosity and distance can be found without measuring its stellar parallax.

Key Ideas Mass-Luminosity Relation for Main-Sequence Stars: Main-sequence stars are stars like the Sun but with different masses. The mass-luminosity relation expresses a direct correlation between mass and luminosity for main- sequence stars. The greater the mass of a main- sequence star, the greater its luminosity (and also the greater its radius and surface temperature).