Lecture 19: Clusters of stars; Review Astronomy 111 Wednesday November 1, 2017
Reminders Exam #2: Monday, 6 November 2017 No homework this week Star party tonight! Also next Tuesday:
Testing stellar evolution The Problem: Stellar Evolution happens on billion-year time scales. Astronomers only live for a few 10 s of years. The Solution: Make H-R Diagrams of star clusters with a wide range of ages.
Star clusters Groups of 100 s to 1000 s of stars. All stars in a cluster... are at the same distance easy to measure relative Luminosity don t need distances to individual stars have the same age. (generally) have the same chemical composition.
The main sequence revisited The Main Sequence is a Mass Sequence: High-mass stars are hotter and brighter. Low-mass stars are cooler and fainter. Main Sequence Lifetime depends on Mass: High-mass stars have short M-S lifetimes Low-mass stars have long M-S lifetimes. Low-Mass stars take longer to form.
Progressive evolution As a cluster ages: High-mass stars reach the M-S first, with the low-mass stars still approaching. High-mass stars run out of hydrogen in their cores first, evolving into supergiants. As successively lower mass stars run out of hydrogen in their cores, they too evolve off. Peel off the Main-Sequence from the top.
Luminosity (L sun ) ASTR111 Lecture 19 Age: ~1 Myr 10 6 10 4 10 2 1 10-2 10-4 Zero Age Main Sequence 40,000 20,000 10,000 5,000 2,500 Temperature (K)
Luminosity (L sun ) ASTR111 Lecture 19 Age: ~10 Myr 10 6 10 4 10 2 B Stars 1 10-2 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K)
Luminosity (L sun ) ASTR111 Lecture 19 Age: ~100 Myr 10 6 10 4 10 2 A Stars 1 10-2 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K)
Luminosity (L sun ) ASTR111 Lecture 19 Age: ~1 Gyr 10 6 10 4 10 2 1 F Stars 10-2 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K)
Luminosity (L sun ) ASTR111 Lecture 19 Age: ~10 Gyr 10 6 10 4 10 2 1 10-2 G Stars 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K)
Main-Sequence Turn-off Point where the Main-Sequence turns off towards giant stars. As cluster ages, the stars at the turn-off are lower mass Low mass stars have redder colors. Indicator of the cluster age: Older Clusters have redder turn-off points.
Age: ~10 Myr Age: ~1 Gyr B Stars F Stars Blue T Red T
Types of clusters Open Clusters: Sparse clusters (few 100-1000 stars) few parsecs in diameter Globular Clusters: Rich spherical clusters (10 5-10 6 stars) 10-30 parsecs in diameter
Open Cluster 100 s of stars Many blue M-S stars Few giants Young Ages (100 s of Myr)
Globular Clusters 100,000 s of stars Many giants No blue M-S stars Old ages (~13 Gyr)
Open Clusters H-R Diagrams of Open Clusters show: They are young to middle-aged Have blue Main-Sequence stars Few supergiants or giants Older Open Clusters have more red giants Don t see a horizontal branch Youngest still have gas clouds associated
Globular Clusters H-R Diagrams of Globular Clusters show: Very old: 10-15 Billion Years Red turnoffs and no blue Main-Sequence stars Lots of red giants A prominent Horizontal Branch Slightly bluer and fainter Main Sequence due to having less metals than nearby stars
Luminosity (L sun ) ASTR111 Lecture 19 Typical Globular Cluster H-R Diagram 10 6 10 4 10 2 1 Zero-Age Main Sequence Horizontal Branch 10-2 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K)
Open vs. Globular Clusters Open cluster: 1000 s of stars of a wide range of temperatures (young stellar population) Globular cluster: 100,000s of stars, only cool red stars present (old stellar population)
Conclusions of the tests Cluster H-R Diagrams give us a snapshot of stellar evolution. Observations of clusters with ages from a few Million to 15 Billion years confirms much of our picture of stellar evolution. Remaining challenges are in small details, but the big picture is secure.
Exam #2 Review: Lecture 9: Formation of the Solar System How did our Solar System form? Characteristics of inner and outer Solar System How did angular momentum play a role? Define: planetesimals, radioactive age dating, terrestrial, jovian, nebular theory
Clues to how Solar System formed: What things are made of Sun: Mostly Hydrogen (H) and Helium (He). Jovian planets: Rich in H and He, low density. Terrestrial planets: Mostly rock and metal, high density.
Galactic recycling Stars produce elements heavier than Hydrogen & Helium These metals eventually find their way to clouds of gas from which new stars and planets are formed.
Nebular theory The solar system formed from the gravitational collapse of a large cloud of gas Postulated by Kant (1755) and Laplace (1796)!
Summary of Solar System formation: ASTR111 Lecture 19
Forming the Jovian planets Moons of Jovian planets form in miniature disks:
Inner vs. outer Solar System Inner solar system: where rocks & metals could condense into solids Frost Line: point where Hydrogen compounds can form ices (solids)
Radioactive age-dating Age of oldest Earth rocks = 4 billion years Age of oldest Moon rocks = 4.5 billion years Age of oldest meteorites (meteoroids that survive the plunge to Earth) = 4.56 billion years Radioactive age-dating indicates that the Solar System is 4.56 billion years old.
Lecture 10: Terrestrial planets Characteristics of Earth, the Moon, and terrestrial planets How do we know the age of the Earth? Where did the Moon come from?
Terrestrial Planets ASTR111 Lecture 19
Where did the Moon come from? Current favorite theory: COLLISIONAL EJECTION THEORY A protoplanet the size of Mars struck the young Earth an oblique blow, just over 4.5 billion years ago.
Synchronous rotation (WRONG!) 3-to-2 spin-orbit coupling (RIGHT!)
On the Earth, the cold rigid crust is broken into plates: On Venus, the hot plastic crust does not break: No plate tectonics on Venus.
Topographical map of Mars ASTR111 Lecture 19
Lecture 11: Jovian planets and smaller bodies Characteristics of Jovian planets, dwarf planets, and other solar system objects Define dwarf planet, asteroid, Kuiper belt, comet
Jovian Planets ASTR111 Lecture 19
The Solar System: List of Ingredients Ingredient Percent of total mass Sun Jupiter other planets everything else 99.8% 0.1% 0.05% 0.05%
Jupiter and Saturn Jupiter and Saturn consist mainly of hydrogen and helium. Jupiter: Escape speed = 60 km/sec Air temperature = 165 K (-160 o F) Saturn: Escape speed = 35 km/sec Air temperature = 93 K (-290 o F) Earth: Escape speed = 11 km/sec Air temperature = 290 K (60 o F)
Galilean Moons: A miniature analog to the Solar System Revolution counterclockwise, on nearly circular orbits, in nearly the same plane. Io and Europa: mostly rock Ganymede and Callisto: rock and ice
IAU: definition of planet The International Astronomical Union defines planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape; and (c) has cleared the neighborhood Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
IAU: dwarf planet The International Astronomical Union defines a "dwarf planet" as a celestial body that, within the Solar System, (a) is in orbit around the Sun; (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape; (c) has NOT cleared the neighborhood around its orbit; and (d) is not a satellite Dwarf Planets (so far): Eris, Pluto, Haumea, Makemake, Ceres
Many smaller bodies Many rocky asteroids & icy comets populate the solar system.
Comets are big, dusty snowballs. If a comet comes close to the Sun, the ice is vaporized, and the dust is freed. Thus, comets in the inner Solar System are surrounded by dust clouds. If the Earth passes through the dust, a meteor shower results. ASTR111 Lecture 19
Lecture 12: Extrasolar planets Name the four techniques astronomers use to find extrasolar planets Define: Hot Jupiter, Earth Analog Why do we search for extrasolar planets?
Detecting extrasolar planets Direct Methods: Pictures or spectra of the planets Indirect Methods: Precision measurements of stellar properties (position, brightness, spectra) that reveal the effects of orbiting planets Doppler shift Microlensing method Planetary transits
Direct imaging Most detected planets are 1 to 10 times more massive than Jupiter; many are Hot Jupiters. Astronomers have directly imaged only a few exoplanets to date. Large future telescopes will be able to image more exoplanets. ASTR111 Lecture 19
Doppler technique Measures motion of star along our line-of-sight seen as shifts in the star s spectrum due to gravitational force of planet on the star.
Planetary transits A transit occurs when a planet passes in front of its star, slightly dimming the light from the star.
51 Gravitational microlensing
Lecture 13: Energy transport in the Sun Define: hydrostatic equilibrium, P-P chain, CNO cycle, neutrinos Know what the Perfect Gas Law means Describe how the P-P chain makes energy in the Sun through fusion List the three modes of energy transport
Hydrostatic Equilibrium Gravity Gas Pressure
Core-envelope structure Compact Core Extended Envelope
Energy Generation Stars shine because they are hot. To stay hot stars must make up for the energy lost by shining. Two Energy sources available: Gravitational Contraction (Kelvin- Helmholtz) Only available if star is NOT in equilibrium Nuclear Fusion in the hot core.
Main-Sequence stars Generate energy by fusion of 4 1 H into 1 4 He. Proton-Proton Chain: Relies on proton-proton reactions Efficient at low core Temperature (T C <18M K) Low-mass stars like the Sun CNO Cycle: Carbon acts as a catalyst Efficient at high core Temperature (T C >18M K) Massive stars
Radiation Energy is carried by photons. Photons leave the core Hit an atom or electron within ~1cm and get scattered. Slowly stagger to the surface ( random walk ) Break into many low-energy photons. Takes ~1 Million years to reach the surface.
Convection Energy carried by bulk motions of the gas. Analogy is water boiling: Hot blob rises cooler water sinks
Conduction Heat is passed from atom-to-atom in a dense material from hot to cool regions. Example: Holding a spoon in a candle flame, the handle eventually gets hot.
The Sun ASTR111 Lecture 19
Lecture 14: Studying the stars Calculate distance to a star given its parallax Define: apparent brightness, luminosity, true binary, visual binary, spectroscopic binary, eclipsing binary What quantities can we measure using binary stars? Why are stars different colors? Why do stars have absorption lines in their spectra? Give the order of the spectral sequence Be able to calculate luminosity and know its units (energy/sec) Be able to use the Stefan-Boltzman law and know its units (energy/sec/area) Know the main components and axes of an H-R diagram
Method of trigonometric parallaxes June p December Foreground Star Distant Stars
Luminosity Luminosity is the total energy output from an object. Measured in power units: Energy/second emitted by the object (e.g., Watts) Independent of distance Important for understanding the energy production of a star.
Inverse Square Law of Brightness The apparent brightness of a source is inversely proportional to the square of its distance: B 1 d 2 2-times Closer = 4-times Brighter 2-times Farther = 4-times Fainter ASTR111 Lecture 19 d=1 B=1 d=2 B=1/4 d=3 B=1/9
Flux-luminosity relationship Relates apparent brightness (Flux) and intrinsic brightness (Luminosity) through the Inverse Square Law of Brightness: Flux = Luminosity 4 d 2
Types of binaries Visual Binary: Can see both stars & follow their orbits over time. Spectroscopic Binary: Cannot resolve the two stars, but can see their orbit motions as Doppler shifts in their spectra. Eclipsing Binary: Cannot resolve the two stars, but can see the total brightness drop when they periodically eclipse each other.
The spectral sequence O B A F G K M L T Hotter Cooler 50,000K 2000K Bluer Redder Spectral sequence is a Temperature sequence
Stellar spectra in order from the hottest (top) to coolest (bottom).
Luminosity-Radius-Temperature Relation Stars are approximately black bodies. Stefan-Boltzmann Law: energy/sec/area = st 4 The area of a spherical star: area = 4 R 2 Predicted Stellar Luminosity (energy/sec): L = 4 R 2 st 4
Luminosity (L sun ) H-R Diagram 10 6 Supergiants 10 4 10 2 Giants 1 10-2 10-4 White Dwarfs 40,000 20,000 10,000 5,000 2,500 Temperature (K) ASTR111 Lecture 19
Mass-Luminosity Relationship For Main-Sequence stars: L L M sun M sun In words: More massive M-S stars are more luminous. Not true of Giants, Supergiants, or White Dwarfs. 3.5
Lecture 15: Star formation and the ISM Interstellar medium contains gas and dust Cool clouds within the ISM can gravitationally collapse to form stars A protostar will heat up as it collapses This heat will cause the protostar to begin fusing hydrogen into helium, which is when it becomes a main-sequence star
Interstellar medium ISM is the stuff between stars Composition (by mass) 75% Hydrogen 23% Helium 2% Heavy metals Also has some dust (about 1%)
Stars form in the gravitational collapse of a molecular cloud ASTR111 Lecture 19
A star is born ASTR111 Lecture 19
Star formation does not occur with the same frequency for all stellar masses Massive stars are rare Many more low-mass stars (M<Msun)
Lecture 16: Low mass stars Main Sequence stars burn H into He in their cores. The Main Sequence is a Mass Sequence. Lower M-S: p-p chain, radiative cores & convective envelopes Upper M-S: CNO cycle, convective cores & radiative envelopes Larger Mass = Shorter Lifetime Stage: Main Sequence Red Giant Horizontal Branch Asymptotic Giant White Dwarf ASTR111 Lecture 19 Energy Source: H Burning Core H Burning Shell He Core + H Shell He Shell + H Shell None!
The Main Sequence ASTR111 Lecture 19
The main sequence is a mass sequence The location of a star along the M-S is determined by its Mass. Low-Mass Stars: Cooler & Fainter High-Mass Stars: Hotter & Brighter Follows from the Mass-Luminosity Relation: Luminosity ~ Mass 3.5
Main sequence lifetime Therefore: Lifetime ~ 1 / M 2.5 The higher the mass, the shorter its life. Examples: Sun: ~ 10 Billion Years 30 M sun O-star: ~ 2 Million years 0.1 M sun M-star: ~ 3 Trillion years
Consequences If you see an O or B dwarf star, it must be young since they only live for a few Million years. You can t tell how old an M dwarf is because their lives can be so long. The Sun is ~ 5 Billion years old, so it will last only for ~ 5 Billion years longer.
Luminosity (L sun ) ASTR111 Lecture 19 Planetary Nebula Phase C-O Core Envelope Ejection 10 6 10 4 10 2 1 10-2 10-4 White Dwarf 40,000 20,000 10,000 5,000 2,500 Temperature (K)
Core collapse to White Dwarf Contracting C-O core becomes so dense that a new gas law takes over. Degenerate Electron Gas: Pressure becomes independent of Temperature P grows rapidly & soon counteracts Gravity Collapse halts when R ~ 0.01 R sun (~ R earth ) White Dwarf Star
Summary Main Sequence stars burn H into He in their cores. The Main Sequence is a Mass Sequence. Lower M-S: p-p chain, radiative cores & convective envelopes Upper M-S: CNO cycle, convective cores & radiative envelopes Larger Mass = Shorter Lifetime
Lecture 17: High mass stars End of the Life of a Massive Star: Burn H through Si in successive cores Finally build a massive Iron core Iron core collapse & core bounce Supernova explosion: Explosive envelope ejection Main sources of heavy elements White Dwarf: Remnant of a star <8 M sun Held up by Electron Degeneracy Pressure Maximum Mass ~1.4 M sun Neutron Star: Remnant of a star < 18 M sun Held up by Neutron Degeneracy Pressure Pulsar = rapidly spinning neutron star
Summary End of the Life of a Massive Star: Burn H through Si in successive cores Finally build a massive Iron core Iron core collapse & core bounce Supernova explosion: Explosive envelope ejection Main sources of heavy elements
Summary White Dwarf: Remnant of a star <8 M sun Held up by Electron Degeneracy Pressure Maximum Mass ~1.4 M sun Neutron Star: Remnant of a star < 18 M sun Held up by Neutron Degeneracy Pressure Pulsar = rapidly spinning neutron star
SN 1987a For one second, more energy than in an entire galaxy!
Stardust Metal-enriched gas mixes with interstellar gas Next generation of stars includes these metals. Successive generations are more metal rich. Sun & planets (& us): Contain many metals (iron, silicon, etc.) Only ~5 Gyr old The Solar System formed from gas enriched by a previous generation of massive stars.
Galactic recycling Stars produce elements heavier than Hydrogen & Helium These metals eventually find their way to clouds of gas from which new stars and planets are formed.
Lecture 18: Black holes Black Holes are totally collapsed objects gravity so strong not even light can escape predicted by General Relativity Schwarzschild Radius & Event Horizon Unobservable objects are observable through their effect on surroundings X-ray binaries Jets Orbital mechanics Black Hole Evaporation Emit "Hawking Radiation"
Curvature of spacetime Matter tells space how to curve and space tells matter how to move.
Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape. ASTR111 Lecture 19
Lecture 19: Stellar clusters H-R Diagrams of Star Clusters Ages from the Main-Sequence Turn-off Open Clusters Young clusters of few 1000 stars Blue Main-Sequence stars & few giants Globular Clusters Old clusters of a few 100,000 stars No blue Main-Sequence stars & many giants
Open vs. Globular Clusters Open cluster: 1000 s of stars of a wide range of temperatures (young stellar population) Globular cluster: 100,000s of stars, only cool red stars present (old stellar population)