Astronomy 141 -- Winter 2012 In-Class Quiz 4 Study Guide Properties of Stars Luminosity: total energy output from a star Colors of stars and relation to Temperature Spectral Classification Spectral Sequence is a Temperature Sequence Main Spectral Types: O B A F G K M L The Hertzsprung-Russell Diagram: Plot of Luminosity vs. Temperature for stars. Main Sequence Stars Giant Stars Supergiant Stars White Dwarfs The Lives of Stars Star shine because they are hot The primary source of energy in stars in nuclear fusion Main Sequence stars generate energy via Hydrogen fusion into Helium The Main Sequence is a mass sequence: Low-mass = cool and low-luminosity High-mass = hot and high-luminosity How does a star's main-sequence lifetime depend on its mass? Larger Mass = Shorter Life. Smaller Mass = Longer Life. What are typical lifetimes for O-stars, M-stars, & the Sun? How might a star's main-sequence lifetime affect the possibility of life arising on planets around it? What happens when a star runs out of Hydrogen for fusion in the core? Low Mass stars (M < 4 Msun) Star becomes a red giant Loses its envelope Remnant core becomes a White Dwarf Intermediate-Mass Stars with 4 < M < 8 Msun Star becomes a red supergiant Loses its envelope Remnant core becomes a White Dwarf High-Mass Stars with M > 8 Msun Star becomes a red supergiant Burns through a succession of nuclear fusion fuels up through Iron Catastrophic collapse of the Iron Core causes a supernova explosion Remnant core becomes either a neutron star or a black hole Supernovae Heavy elements beyond Iron are created in the supernova explosion Explosion debris seeds interstellar space with heavy elements Habitable Zones around Stars Likely place to look for life is on rocky planets in the habitable zones of low-mass stars Basic requirements for life in a planetary context Stable source of energy: long-lived low-mass star Raw materials for Complex chemistry Location for life to emerge (solid surface) Benign environmental conditions (low UV radiation) Where is the Sun's habitable zone today? HZs get wider and farther from the star as its luminosity increases Other factors affecting habitability Tidal Locking - important for red dwarfs where HZ is close to the star Dangerous superflares from red dwarfs UV radiation hazard from high-mass main-sequence stars Around which types of main-sequence stars are we most likely to find potentially life-bearing rocky planets? The Solar Neighborhood What are the nearest stars to the Sun? Local space is very empty and stars are very far apart Could the Alpha Centauri system harbor habitable planets? About half of nearby stars are in binaries or triples The average distance between stars locally is about 6 light years The distribution of stars is dominated by low-mass stars Stars like the Sun are relatively rare About 75% of nearby stars are red dwarfs (M-type stars) The Milky Way is our Galaxy It is a flattened disk of stars with a central bulge It is nearly 100,000 light years across and 1000ly thick There are nearly 200 billion stars in the Milky Way Extrasolar Planetary Systems Searches for planets around other stars Astrometric Wobble Method Doppler Wobble (RV) Method Planetary Transit Method Gravitational Microlensing Method Which techniques are the most successful? What types of planet are each technique most sensitive to? Properties of Extrasolar Planetary Systems We find many Jupiter-sized planets close to their parent stars What are the prospects for finding Earth-like planets? We find many planets on eccentric orbits We tend to find planets around metal-rich stars Finding Earths (The Pale Blue Dot) What are the challenges of finding very small planets like Earth? What does the spectrum of the Earth look like? What do we learn about a planet from its thermal infrared spectrum? What are the features of the reflection spectrum of the Earth? What are the important Spectroscopic Biomarkers of life? What can we learn from variability in the brightness of an exoEarth? Intelligent Life in the Universe What are the criteria we think define "intelligent" beings? What is the encephalization quotient (EQ)? How long did it take for intelligence to emerge on Earth? How do we define "advanced technology" The Drake Equation Roughly how many stars are there in the visible Universe? What is the Drake Equation? What factors in the Drake Equation are known from observations? What factors in the Drake Equation are conjectural? What does the Drake Equation imply about the number of civilizations in our Galaxy at any given time? What does the Drake Equation imply about the average distance between civilizations in our Galaxy at any given time? What is the main limitation of the Drake Equation?