Astronomy PDF: Studying Our Star

The Cosmic Perspective Ninth Edition Chapter 14 Lecture Our Star 14.1 A Closer Look at the Sun Our goals for learning: – Why does the Sun shine? – What is the Sun's structure? Why Does the Sun Shine? A Closer Look at the Sun Is it on Fire? A Closer Look a...

The Cosmic Perspective Ninth Edition Chapter 14 Lecture Our Star 14.1 A Closer Look at the Sun Our goals for learning: – Why does the Sun shine? – What is the Sun's structure? Why Does the Sun Shine? A Closer Look at the Sun Is it on Fire? A Closer Look at the Sun Is it on Fire? … No! Chemical energy content ~ 10,000 years Luminosity  A Closer Look at the Sun Is it Contracting? Ideal Gas Law: As pressure increases from the PV = nRT = NkT weight of upper layers the density becomes P = nkT, where n ≡ N/V. and temperature increase. R and k are constants, specific to the gas content A Closer Look at the Sun Is it Contracting? … No! Gravitational potential ~ 25 million years Luminosity  A Closer Look at the Sun It can be powered by Nuclear energy! (E = mc ) 2 Nuclear potential energy ( core ) Luminosity = Energy ~ 10 billion years time Luminosity The Stable Sun Gravitational equilibrium: Energy supplied by fusion maintains the pressure that balances the inward crush of gravity. The Stable Sun Energy Balance: The rate at which energy radiates from the surface of the Sun must be the same as the rate at which it is released by fusion in the core. The Stable Sun Gravitational contraction: Provided the energy that heated the core as the Sun was forming. Contraction stopped when fusion began creating enough outward pressure (i.e., gravitational equilibrium). What Is the Sun's Structure? The Sun's Atmosphere (1 of 7) Solar wind: A flow of charged particles from the surface of the Sun in all directions. These charged particles escape along magnetic field lines from the Sun until they become part of the solar wind. The Sun's Atmosphere (2 of 7) Corona: Outermost layer of solar atmosphere ~1 million K (106 K) Observed as X- rays. Very low-density and the density of the matter increases as we descend (i.e., weight of upper layers). The Sun's Atmosphere (3 of 7) Chromosphere: Middle layer of solar atmosphere 10,000-100,000 K (104-105 K). Observed in the ultraviolet (UV). The Sun's Atmosphere (4 of 7) Photosphere: Visible surface of Sun (visible light) ~5800K which emits as visible light to us Density less than Earth’s atmosphere Sunspots form here. Basics of Light and Observations of It As the temperature increases so does the energy of the photons released. We will revisit this in Chapter 15. Depending on the energy of the photons, this will determine the wavelength/frequency of the light. For the corona at ~106 K (1 million K), this is X-ray emission. The Sun's Atmosphere (5 of 7) Convection Zone: Energy transported upward by rising hot gas (plasma) Each layer of the Sun and all stars are basically spheres (balls) of plasma – a gas consisting of ions (i.e., atomic nuclei) and electrons The Sun's Atmosphere (6 of 7) Radiation Zone: Energy transported upward by photons The Sun's Atmosphere (7 of 7) Core: Energy generated by nuclear fusion at ~ 15 million K Density 100 times that of water Pressure 200 billion times > Earth’s surface What Have We Learned? Why does the Sun shine? – Chemical and gravitational energy sources could not explain how the Sun could sustain its luminosity for more than about 25 million years. – The Sun shines because gravitational equilibrium keeps its core hot and dense enough to release energy through nuclear fusion. What Have We Learned? What is the Sun's structure? – From inside out, the layers are: ▪ Core ▪ Radiation zone ▪ Convection zone ▪ Photosphere ▪ Chromosphere ▪ Corona Which part of the Sun’s structure has the hottest temperature? What Have We Learned? What is the Sun's structure? – From inside out, the layers are: ▪ Core ▪ Radiation zone ▪ Convection zone ▪ Photosphere ▪ Chromosphere ▪ Corona Which part of the Sun’s structure has the hottest temperature? Core 14.2 Nuclear Fusion in the Sun Our goals for learning: – How does nuclear fusion occur in the Sun? – How does the energy from fusion get out of the Sun? – How do we know what is happening inside the Sun? How Does Nuclear Fusion Occur in the Sun? Nuclear Fusion in the Sun (1 of 5) Fission Fusion Big nucleus splits into Small nuclei stick together to smaller pieces. make a bigger one. (Example: nuclear (Example: stars) power plants) Nuclear Fusion in the Sun (2 of 5) High temperatures enable nuclear fusion to happen in the core. Strong force at small distances ~ 10-15m (nuclei size) appears and is 100 times stronger than the EM force. Inverse Square Law: 1/r2 for EM and gravitational forces Nuclear Fusion in the Sun (3 of 5) The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus. Nuclear Fusion in the Sun (4 of 5) Hydrogen Fusion by the Proton-Proton Chain The proton–proton chain is how hydrogen fuses into helium in the Sun. Nuclear Fusion in the Sun (5 of 5) In 4 protons Out 4 He nucleus 2 gamma rays 2 positrons 2 neutrinos Total mass is 0.7% lower. Thought Question 2 (1 of 2) What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? A. The core would expand and heat up. B. The core would expand and cool. C. The Sun would blow up like a hydrogen bomb. Thought Question 2 (2 of 2) What would happen inside the Sun if a slight rise in core temperature led to a rapid rise in fusion energy? A. The core would expand and heat up. B. The core would expand and cool. C. The Sun would blow up like a hydrogen bomb. The solar thermostat keeps burning rate steady. Solar Thermostat Decline in core Rise in core temperature temperature causes fusion causes fusion rate to rise, rate to drop, so core so core expands and contracts and heats up. cools down. Energy Escape from the Sun (1 of 4) Random Walk Remember density is greater closer to the core and decreases outward changing the average step size from small to larger. Energy Escape from the Sun (2 of 4) Energy gradually leaks out of the radiation zone in the form of randomly bouncing photons, Radiative Diffusion Energy Escape from the Sun (3 of 4) a This diagram shows convection beneath the Sun's surface. Hot gas (light yellow arrows) rises while cooler gas (black arrows) descends around it. Convection (rising hot gas) takes energy to surface. Energy Escape from the Sun (4 of 4) b This image shows the mottled appearance of the Sun's photosphere. The bright spots, each about 1000 kilometres across, correspond to the rising plumes of hot gas in part a. Bright blobs on photosphere show where hot gas is reaching the surface. How We Know What Is Happening Inside the Sun? We Learn About the Inside of the Sun By … making mathematical models observing solar vibrations observing solar neutrinos We Learn About the Inside of the Sun By … making mathematical models – Use laws of Physics to predict internal conditions – Basic model of star’s composition and mass – Solves equations to describe: ▪ Gravitational equilibrium and Energy Balance – Computer models can be created to determine ▪ Pressure (P), Temperature (T), and density (ρ) at all depths – Good models will predict accurately properties of a star: ▪ E.g., Size, Luminosity, Age, Surface Temp Solar Vibrations (1 of 4) Patterns of vibration on the surface tell us about what the Sun is like inside (e.g., earthquakes or sound waves through air). Here, vibrations (i.e., ripples on surface) revealed by Doppler shifts are shown. Solar Vibrations (2 of 4) Data on solar vibrations agree very well with mathematical models of solar interior. This is helio- seismology. Solar Neutrinos (3 of 4) Neutrinos created during fusion fly directly through the Sun. Only interact with weak and gravitational forces. Observations of these solar neutrinos can tell us what's happening in the core (detectors placed deep underground). Solar Neutrinos (4 of 4) Solar neutrino problem: Early searches for solar neutrinos failed to find the predicted number. More recent observations find the right number of neutrinos, but some have changed form. Neutrinos can change passing through matter and early detectors weren’t designed for that change. What Have We Learned? (3 of 5) How does nuclear fusion occur in the Sun? – The core's extreme temperature and density are just right for nuclear fusion of hydrogen to helium through the proton–proton chain. – Gravitational equilibrium acts as a thermostat to regulate the core temperature because fusion rate is very sensitive to temperature. What Have We Learned? (4 of 5) How does the energy from fusion get out of the Sun? – Randomly bouncing photons carry energy through the radiation zone. – Rising of hot plasma carries energy through the convection zone to the photosphere. How do we know what is happening inside the Sun? – Mathematical models agree with observations of solar vibrations and solar neutrinos. 14.3 The Sun–Earth Connection Our goals for learning: – What causes solar activity? – How does solar activity vary with time? What Causes Solar Activity? (1 of 2) Solar Activity is like "Weather" Sunspots Solar flares Solar prominences All these phenomena are related to magnetic fields. Solar Activity (1 of 10) Sunspots Are cooler than other parts of the Sun's surface (4000 K ) elvin Are regions with strong magnetic fields a This close-up view of the Sun's surface shows two large sunspots and several smaller ones. Each of the big sunspots is roughly as large as Earth. Solar Activity (2 of 10) Zeeman Effect We can measure magnetic fields in sunspots by observing the splitting of spectral lines. b Very strong magnetic fields split the absorption lines in spectra of sunspot regions. The dark vertical bands are absorption lines in a spectrum of the Sun. Notice that these lines split where they cross the dark horizontal bands corresponding to sunspots. Solar Activity (3 of 10) Charged particles spiral along magnetic field lines. Solar Activity (4 of 10) b This x-ray photo (from NASA's TRACE a Pairs of sunspots are connected by mission) shows hot gas trapped within tightly wound magnetic field lines. looped magnetic field lines. Loops of bright gas often connect sunspot pairs. Solar Activity (5 of 10) Magnetic activity also causes solar prominences that erupt high above the Sun's surface. The loops of bright gas (plasma). Solar Activity (6 of 10) The corona appears bright in X-ray photos in places where magnetic fields trap hot gas. Coronal Holes The dark regions in X-ray photos near the poles is where magnetic field lines extend into space causing charged particles to escape becoming part of the solar wind.  Solar Activity (7 of 10) Coronal mass ejections send bursts of energetic charged particles out through the solar system. Caused by energetic releases of plasma from stressed regions in the Sun’s magnetic field corresponding to sunspot groups or prominences, where the expelled plasma quickly fans out as huge bubbles of charged particles. Solar Activity (8 of 10) Magnetic activity causes solar flares that send bursts of X-rays and charged particles into space, which are the most explosive form of coronal mass ejections. Effects of Solar Activity on Earth Charged particles streaming from the Sun can disrupt electrical power grids and can disable communication satellites.  How Does Solar Activity Vary with Time? a This graph shows how the number of sunspots on the Sun changes with time. The vertical axis shows the percentage of the Sun's surface covered by sunspots. The sunspot cycle has a period of approximately 11 years. Interesting result in the solar cycle where the magnetic poles flip every 11 years for a full cycle of 22 years to flip back. b This graph shows how the latitudes at which sunspot groups appear tend to shift during a single sunspot cycle. Solar Activity (9 of 10) There are additional variances over longer periods. Less direct evidence of long term cycles can come from trees or specifically, Carbon 14 and tree rings. Solar Activity (10 of 10) The sunspot cycle has to do with the winding and twisting of the Sun's magnetic field, which eventually causes the magnetic poles to flip resetting the cycle. The Sunspot Cycle and Earth's Climate Despite an 11-year cycle, the amount of sunlight has remained approximately constant while Earth has warmed. What Have We Learned? (5 of 5) What causes solar activity? – Stretching and twisting of magnetic field lines near the Sun's surface cause solar activity. How does solar activity vary with time? – Activity rises and falls with an 11-year period. The Cosmic Perspective Chapter 15 Surveying the Stars 15.1 Properties of Stars Our goals for learning: – How do we measure stellar luminosities? – How do we measure stellar temperatures? – How do we measure stellar masses? How Do We Measure Stellar Luminosities? Apparent Brightness The brightness of a star depends on both distance and luminosity. Apparent Brightness Versus Luminosity Luminosity: Amount of power a star radiates (energy per time = watts = joules/s) Apparent brightness: Amount of starlight that reaches Earth (energy per time per unit area = watts/m2 joules/s/m2) Thought Question 1 (1 of 2) Alpha Centauri and the Sun have about the same luminosity. Which one appears brighter? A. Alpha Centauri B. The Sun Thought Question 1 (2 of 2) Alpha Centauri and the Sun have about the same luminosity. Which one appears brighter? A. Alpha Centauri B. The Sun Apparent Brightness Versus Luminosity The amount of luminosity passing through each sphere is the same. Area of sphere: 4p ´ ( radius ) 2 Divide luminosity by area to get brightness. We are using the Inverse Square Law for light, 1/r2. Apparent Brightness Versus Luminosity The relationship between Apparent Brightness and luminosity depends on distance: Luminosity Apparent Brightness = 4p ´ ( distance ) 2 We can determine a star's luminosity if we can measure its distance and apparent brightness: Luminosity = 4p ( distance ) ´ (brightness ) 2  Apparent Brightness Versus Luminosity Detectors are not capable of measuring light across the complete EM spectrum. For a detector measuring visible light, the apparent brightness measured calculates the visible-light luminosity. We use terms of total luminosity (bolometric luminosity) and total apparent brightness to describe luminosity and brightness, if we could measure light from the entire EM spectrum. Thought Question 2 (1 of 2) How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/ 3 as bright B. It would be only 1/ 6 as bright. C. It would be only 1/ 9 as bright. D. It would be three times brighter. Thought Question 2 (2 of 2) How would the apparent brightness of Alpha Centauri change if it were three times farther away? A. It would be only 1/ 3 as bright B. It would be only 1/ 6 as bright. C. It would be only 1/ 9 as bright. D. It would be three times brighter. Stellar Distance So how far away are these stars? Stellar Parallax (1 of 2) Parallax is the apparent shift in position of a nearby object against a background of more distant objects. Apparent positions of nearest stars shift by about an arcsecond as Earth orbits the Sun. Parallax angle depends on distance. Parallax is measured by comparing snapshots taken at different times and measuring the shift in angle to the star. Stellar Parallax (2 of 2) Parallax and Distance p = parallax angle 1 ( d in parsecs =) ( p in arcseconds ) 1 d ( in light-years ) = 3.26 ´ p ( in arcseconds ) d(in meters) = 1 (AU to m) / p(in radians) We can write arcseconds as arcsecs or `` π (radians) = 180º and 1º = 60 arcmin = 3600 arcsec Angular Measurements Full circle = 360º = 2π radians 1º = 60ˊ (arcminutes) 1ˊ = 60˝ (arcseconds) Luminosity Range of Stars Most luminous stars: 106 LSun Least luminous stars: 10 -4 LSun (LSun is luminosity of Sun) LSun = LSol = L☉; ☉=Sun Sun’s luminosity is about the middle of stellar luminosity. There are far more dimmer stars than brighter ones. Luminosity = 4p ( distance ) ´ (brightness ) 2 The Magnitude Scale m = apparent magnitude, M = absolute magnitude Apparent brightness of Star 1 Apparent brightness of Star 2 ( = 1001/5 m2 - m1) Luminosity of Star 1 Luminosity of Star 2 ( = 1001/5 M2 - M1) Faintest stars observable with human eye is m = 6 designated originally by Hipparchus (190-120 BC).  Star color or How Do We Measure Stellar spectrum gives Temperatures? stellar temperature. Stellar Temperatures (1 of 4) JWST (Stellar Spectrum diagram) Every object emits thermal radiation with a spectrum that depends on its temperature. Stellar Temperatures (2 of 4) An object of fixed size grows more luminous as its temperature rises. Properties of Thermal Radiation 1. Hotter objects emit more light per unit area at all frequencies. 2. Hotter objects emit photons with a higher average energy (its peak). Thought Question 3 (1 of 2) The radiation from a star gives temperature information for what part of the star? A. The core B. The photosphere C. The radiation zone D. The convection zone Thought Question 3 (2 of 2) The radiation from a star gives temperature information for what part of the star? A. The core B. The photosphere C. The radiation zone D. The convection zone Stellar Temperatures (3 of 4) Hottest stars: 50,000 K Coolest stars: 3000 K (Sun's surface is 5800 K.) Stellar Temperatures (4 of 4) Level of ionization also reveals a star's temperature. Spectral Type (1 of 2) Absorption lines in star's spectrum tell us its ionization level. Types of Spectra Types of Spectra Continuous Spectrum: A continuous spectrum contains all wavelengths of light in a certain range. Hot, dense light sources like stars, for example, emit a nearly continuous spectrum of light, which travels out in all directions and interacts with other materials in space. The broad range of colors that a star emits depends on its temperature. Absorption Spectrum: When starlight passes through a cloud of gas, some of the light is absorbed and some is transmitted through the gas. The wavelengths of light that are absorbed depends on what elements and compounds it is made of. An absorption spectrum has dark lines or gaps in the spectrum corresponding to wavelengths that are absorbed by the gas. Emission Spectrum: Starlight can also heat up a cloud of gas, exciting the atoms and molecules within the gas, and causing it to emit light. The spectrum of light that a cloud of gas emits depends on its temperature, density, and composition. An emission spectrum consists of a series of colored lines that correspond to wavelengths emitted by the glowing gas. Spectral Type (2 of 2) Lines in a star's spectrum correspond to a spectral type that reveals its temperature. (Hottest) O B A F G K M (Coolest) Remembering Spectral Types (Hotter) O B A F G K M (Cooler) Oh, Be A Fine Guy/Girl, Kiss Me Or pick something else (an animal) that works with the letter G. Go crazy! Astronomers further sub classify spectral types into a number, e.g., B0 through B9. B0 is hotter than B9. The sun’s designation is G2. Thought Question 4a (1 of 2) Which kind of star is hottest? A. M star B. F star C. O star D. K star Thought Question 4a (2 of 2) Which kind of star is hottest? A. M star B. F star C. O star D. K star Thought Question 4b (1 of 2) Which kind of star is hottest? A. M1 star B. F2 star C. O8 star D. K9 star Thought Question 4b (2 of 2) Which kind of star is hottest? A. M1 star B. F2 star C. O8 star D. K9 star Which one is the cutest? (Bark, Bark, Woof, Woof!)  Pioneers of Stellar Classification Annie “Jump” Cannon and the "calculators" at Harvard laid the foundation of modern stellar classification. These “calculators” as Pickering referred to his assistants, mostly women, were some of the most prominent astronomers of the 19/20th century. The original “Hidden Figures”. How Do We Measure Stellar Masses? Measuring Stellar Masses and Radii (1 of 2) The orbit of a binary star system depends on center of mass. Types of Binary Star Systems Visual binary Spectroscopic binary Eclipsing binary About half of all stars are in binary systems. Kepler’s laws of motion Kepler’s third law: p2 = a3 p: period a: semi-major axis (average distance between objects) Visual Binary We can directly observe the orbital motions of these stars. 4p 2 p2 = a3 Newton’s version of G ( M1 + M2 ) Kepler’s third law p = period a = average separation Green circle is observer on Earth and red circle is the star. Spectroscopic Binary We determine the orbit by measuring Doppler shifts. Eclipsing Binary We can measure periodic eclipses. Only binary that allows the true measurement of orbital velocities and even stellar radii. Measuring Stellar Masses and Radii (2 of 2) We measure mass using gravity. Direct mass measurements are possible only for stars in binary star systems. 4p 2 Newton’s version of p2 = a3 G ( M1 + M2 ) Kepler’s third law p = period a = average separation Eclipsing binaries allow for the masses of each star to be determined accurately by studying the transits, which shows how much light comes from each star. Need Two Out of Three Observables to Measure Mass: 1. Orbital period (p) 2. Orbital separation (a or r = radius) 3. Orbital velocity (v) For circular orbits, v = 2p r / p. For elliptical orbits, the velocity can be determined but requires more complex spherical geometry. (*Ugh!*) Kepler’s first two laws (observations) and Newton’s laws of motion help with this. Stellar Mass Most massive stars: 100MSun Least massive stars: 0.08MSun MSun is the mass of the Sun. Also represented as: MSun = MSol = M☉ What Have We Learned? (1 of 5) How do we measure stellar luminosities? – If we measure a star's apparent brightness and distance, we can compute its luminosity with the inverse square law for light. – Parallax tells us distances to the nearest stars. How do we measure stellar temperatures? – A star's color and spectral type both reflect its temperature. What Have We Learned? (2 of 5) How do we measure stellar masses? – Newton's version of Kepler's third law tells us the total mass (and individual masses) of a binary system, if we can measure the orbital period (p) and average orbital separation of the system (a). – Studying the transits of eclipsing binary stars allows the individual masses to be determined the most accurately. 15.2 Patterns Among Stars Our goals for learning: – What is a Hertzsprung-Russell diagram? – What is the significance of the main sequence? – What are giants, supergiants, and white dwarfs? – Why do the properties of some stars vary? How would you classify these stars? Constructing an HR Diagram What is a Hertzsprung-Russell diagram? Hertzsprung-Russell Diagram (1 of 4) An H-R diagram plots the luminosity and temperature of stars. Hertzsprung-Russell Diagram (2 of 4) Most stars fall somewhere on the main sequence of the H-R diagram. Hertzsprung-Russell Diagram (3 of 4) Stars with lower Temperature (T) and higher Luminosity (L) than main-sequence stars must have larger radii. These stars are called giants and supergiants. (Ex. Betelgeuse) Hertzsprung-Russell Diagram (4 of 4) Stars with higher T and lower L than main- sequence stars must have smaller radii. These stars are called white dwarfs. Stellar Luminosity Classes A star's full classification includes spectral type (line identities) and luminosity class (line shapes, related to the size of the star): I - supergiant II - bright giant III - giant IV - subgiant V - main sequence Examples: Sun - G2 V Sirius - A1 V Proxima Centauri - M5.5 V Betelgeuse - M2 I Hertzsprung-Russell Diagram H-R diagram depicts: Temperature, Color, Spectral type, Luminosity, and Radius Thought Question 5 (1 of 2) Which star is the hottest? Thought Question 5 (2 of 2) Which star is the hottest? Thought Question 6 (1 of 2) Which star is the most luminous? Thought Question 6 (2 of 2) Which star is the most luminous? Thought Question 7 (1 of 2) Which star is a main-sequence star? Thought Question 7 (2 of 2) Which star is a main-sequence star? Thought Question 8 (1 of 2) Which star has the largest radius? Thought Question 8 (2 of 2) Which star has the largest radius? What is the Significance of the Main Sequence? Lifetimes Along the Main Sequence (1 of 3) Main-sequence stars are fusing hydrogen into helium in their cores like the Sun. Luminous main- sequence stars are hot (blue). Less luminous ones are cooler (yellow or red). Lifetimes Along the Main Sequence (2 of 3) Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones. Mass could then be used instead of temperature for the HR Diagram for the main-sequence, but only the main-sequence. Lifetimes Along the Main Sequence (3 of 3) The mass of a normal, hydrogen-fusing star determines its luminosity and spectral type. Mass is the most fundamental property that governs stars. Mass is the Most Fundamental Property Core pressure and temperature of a higher-mass star need to be larger in order to balance gravity. Higher core temperature boosts fusion rate, leading to increased luminosity. From binaries, these more massive stars are only ~10x larger radii, the huge increase in luminosity comes from the combination of hotter surface temperatures and larger radii. L = 4πr2 σT4 Stellar Properties Review (1 of 2) Luminosity: from brightness and distance 10 -4 Lsun - 106 Lsun Temperature: from color and spectral type 3000K – 50,000K Mass: from period (p) and average separation (a) of binary star orbit 0.08MSun – ~ 100MSun Stellar Properties Review (2 of 2) Luminosity: from brightness and distance ( 0.08Msun ) 10-4 Lsun - 106 Lsun (100Msun ) Temperature: from color and spectral type (0.08MSun ) 3000 K - 50,000 K (100MSun ) Mass: from period (p) and average separation (a) of binary star orbit 0.08MSun - 100MSun Mass and Lifetime (1 of 4) Sun's life expectancy: 10 billion years Mass and Lifetime (2 of 4) Sun's life expectancy: 10 billion years Until core hydrogen (10% of total) is used up Mass and Lifetime (3 of 4) Sun's life expectancy: 10 billion years Until core hydrogen (10% of total) is used up Life expectancy of 10MSun star : 10 times as much fuel, uses it 104 times as fast as 1MSun 10 million years ~ 10 billion years ×10 / 104 Or simply divide by 103 Lifetime of a star depends on how mass scales to luminosity for fuel consumption. Mass and Lifetime (4 of 4) Sun's life expectancy: 10 billion years Until core hydrogen (10% of total) is used up Life expectancy of 10MSun star : 10 times as much fuel, uses it 104 times as fast 10 million years ~ 10 billion years ´ 10 / 10 4 Life expectancy of 0.1MSun star : 0.1 times as much fuel, uses it 0.01 times as fast 100 billion years ~ 10 billion years ´ 0.1/ 0.01 Or simply multiply by 10 Main-Sequence Star Summary High-Mass Star: High luminosity Short-lived Larger radius Blue Hotter Low-Mass Star: Low luminosity Long-lived Small radius Red Cooler What are Giants, Supergiants, and White Dwarfs? Sizes of Giants and Supergiants L = 4πr2 σT4 (r=900RSun)2 = 810,000 (T=0.63TSun)4 = 0.16 Off the Main Sequence Stellar properties depend on both mass and age: Those that have finished fusing H to He in their cores are no longer on the main sequence. All stars become larger and redder after exhausting their core hydrogen: giants and supergiants. Most stars end up small and white after fusion has ceased: white dwarfs. Thought Question 9 (1 of 2) Which star is most like our Sun? Thought Question 9 (2 of 2) Which star is most like our Sun? Alpha Centauri A Tau Ceti Thought Question 10 (1 of 2) Which of these stars will have changed the least 10 billion years from now? Thought Question 10 (2 of 2) Which of these stars will have changed the least 10 billion years from now? Proxima Centauri DX Cancri Thought Question 11 (1 of 2) Which of these stars can be no more than 10 million years old? Thought Question 11 (2 of 2) Which of these stars can be no more than 10 million years old? Any O stars on the MS (shown here only Beta Centauri). Why Do the Properties of Some Stars Vary? Variable Stars Any star that varies significantly in brightness with time is called a variable star. Some stars vary in brightness because they cannot achieve proper balance between power welling up from the core and power radiated from the surface. Such a star alternately expands and contracts, varying in brightness as it tries to find a balance. – Aren’t we all… Pulsating Variable Stars The light curve of this pulsating variable star shows that its brightness alternately rises and falls over a 50-day period. Cepheid Variable Stars Most pulsating variable stars inhabit an instability strip on the H-R diagram. The most luminous ones are known as Cepheid variables. Cepheid Variable Stars  Unique Variable Stars Artist's rendition of a polar Cataclysmic Variable. Dr. Mark A. Garlick  Unique Variable Stars Artist's rendition of a polar Cataclysmic Variable. Dr. Mark A. Garlick  Variable Star Names To avoid confusion with letter spectral types or the (now rarely used) Latin-letter Bayer designations. The letter R was chosen as a starting point. Start with the letter R and goes through Z Continue with RR...RZ, then use SS...SZ, TT...TZ and so on until ZZ Use AA...AZ, BB...BZ, CC...CZ and so on until reaching QZ After 334 combinations of letters naming continues with V335, V336, and so on. What Have We Learned? (3 of 5) What is a Hertzsprung-Russell diagram? – An H-R diagram plots stellar luminosity of stars versus surface temperature (or color or spectral type). What is the significance of the main sequence? – Normal stars that fuse H to He in their cores fall on the main sequence of an H-R diagram. – A star's mass determines its position along the main sequence (high-mass: luminous and blue; low-mass: faint and red). What Have We Learned? (4 of 5) What are giants, supergiants, and white dwarfs? – All stars become larger and redder after core hydrogen burning is exhausted: giants and supergiants. – Most stars end up as tiny white dwarfs after fusion has ceased. Why do the properties of some stars vary? – Some stars fail to achieve balance between power generated in the core and power radiated from the surface. (This is not the same as the solar thermostat!) 15.3 Star Clusters Our goals for learning: – What are the two types of star clusters? – How do we measure the age of a star cluster? What are the Two Types of Star Clusters? Open Cluster (Pleiades) Globular Cluster (M80) Stellar Clusters (1 of 5) Open cluster: A few thousand loosely packed stars Stellar Clusters (2 of 5) Globular cluster: Hundreds of thousands to about a million stars in a dense ball bound together by gravity How Do We Measure the Age of a Star Cluster? Stellar Clusters (3 of 5) Massive blue stars die first, followed by white, yellow, orange, and red stars. Assumptions made for star clusters: all stars in a cluster formed around the same time and have the same distance to us. Open clusters form in the disk of a galaxy and globular clusters form in the halo. We will discuss this in detail in Chapter 19. Star Cluster Age (1 of 2) The Pleiades cluster now has no stars with life expectancy less than ~100 million years. Star Cluster Age (2 of 2) The main-sequence turnoff point of a cluster tells us its age. For any star on the main- sequence, there has not been enough time to evolve off the main sequence. This is directly related to the age of the cluster, when it formed.  Star Cluster Ages Which cluster is older? A. M67 B. NGC188 C. Same age.  Star Cluster Ages Which cluster is older? A. M67 B. NGC188 C. Same age.  Star Cluster Ages M67 has no stars on the main-sequence with masses greater than 10MSun. What is the approximate age of the cluster? A. 10 billion years B. 5 billion years C. 5 million years D. 10 million years E. Unknown  Star Cluster Ages M67 has no stars on the main-sequence with masses greater than 10MSun. What is the approximate age of the cluster? A. 10 billion years B. 5 billion years C. 5 million years D. 10 million years E. Unknown Stellar Clusters (4 of 5) To determine accurate ages, we compare models of stellar evolution to the cluster data. We can even apply this to small galaxies, e.g., LMC and SMC (the Magellanic Clouds) If a star cluster only has F stars as the most luminous stars on the main-sequence and F stars have an expected lifetime similar to the Sun’s age, what is the age of this star cluster? A. 10 billion years B. 5 billion years C. 5 million years D. 10 million years Stellar Clusters (4 of 5) To determine accurate ages, we compare models of stellar evolution to the cluster data. We can even apply this to small galaxies, e.g., LMC and SMC (the Magellanic Clouds) If a star cluster only has F stars as the most luminous stars on the main-sequence and F stars have an expected lifetime similar to the Sun’s age, what is the age of this star cluster? A. 10 billion years B. 5 billion years C. 5 million years D. 10 million years Stellar Clusters (5 of 5) Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old or the approximate age of the Universe. What Have We learned? (5 of 5) What are the two types of star clusters? – Open clusters are loosely packed and contain up to a few thousand stars. – Globular clusters are densely packed and contain hundreds of thousands of stars. How do we measure the age of a star cluster? – A star cluster's age roughly equals the life expectancy of its most massive stars still on the main sequence. The Cosmic Perspective Ninth Edition Chapter 16 Lecture Star Birth 16.1 Stellar Nurseries Our goals for learning: – Where do stars form? – Why do stars form? Where Do Stars Form? Star-Forming Clouds Stars form in dark clouds of gas and dust in interstellar space. This is the raw material for stars to form. The gas and dust between the stars is called the interstellar medium. Composition of Clouds We can determine the composition of interstellar gas from its absorption lines in the spectra of stars. 70% H, 28% He, 2% heavier elements in our region of Milky Way Composition is similar for different clouds but fluctuate drastically in environment (i.e., temperature and density) Molecular Clouds (1 of 2) a A visible-light image of the nebula. The b A radio-wave image of the nebula, showing dark (horsehead-shaped) region is a emission from carbon monoxide (CO) molecules. molecular cloud. Most of the matter in star-forming clouds is in the form of molecules (H2 ,CO, etc.). These molecular clouds have a temperature of 10–30 K and a density of about 300 molecules per cubic centimeter (i.e., cold and dense). Molecular Clouds (2 of 2) a A visible-light image of the nebula. The b A radio-wave image of the nebula, showing dark (horsehead-shaped) region is a emission from carbon monoxide (CO) molecules. molecular cloud. Most of what we know about molecular clouds comes from observing the emission lines of carbon monoxide (CO) even though molecular hydrogen (H2) is the most abundant. Interstellar Dust Tiny solid particles of interstellar dust block our view of stars in these gas clouds. Particles are < 1 micrometer (micron) or 10-6 meters in size and made of elements like C, O, Si, and Fe. These small particles are more like smoke than sand grains. Interstellar Dust Allamandola et al. 1985 As with the gas molecules, the dust particles have the chemical makeup of molecules found in everyday things on Earth. Auto-soot and emission from Orion Bar gas cloud. Hubble Space Telescope Orion Treasury Project Team Orion Auto Soot and Star Formation … Red Alert?! Interstellar Reddening (1 of 2) Stars viewed through the edges of the cloud look redder because dust blocks (shorter- wavelength) blue light more effectively than (longer-wavelength) red light. phenomenon known as a A visible-light image of the dark molecular interstellar reddening cloud Barnard 68. Interstellar Reddening (2 of 2) Longer wavelength infrared light passes through a cloud more easily than visible light. Observations of infrared light reveal stars on the other side of the cloud.  Thought Question 1 (1 of 2) Why does the moon look redder as it gets closer to the horizon? A. The gas and dust particles in the atmosphere emit red light better. B. The gas and dust particles in the atmosphere block and scatter shorter wavelength light better. C. The curvature of the Earth causes the moon to be redder. D. The angle of the sunlight reflecting off the moon causes the color to change.  Thought Question 1 (2 of 2) Why does the moon look redder as it gets closer to the horizon? A. The gas and dust particles in the atmosphere emit red light better. B. The gas and dust particles in the atmosphere block and scatter shorter wavelength light better. C. The curvature of the Earth causes the moon to be redder. D. The angle of the sunlight reflecting off the moon causes the color to change. Observing Newborn Stars (1 of 2) Visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed. JWST (Stellar Spectrum diagram) Observing Newborn Stars (2 of 2) Observing the infrared light from a cloud can reveal the newborn star embedded inside it. JWST (Stellar Spectrum diagram) Pillars of Creation Glowing Dust Grains (1 of 2) Dust grains that absorb visible light heat up and emit infrared light of even- longer wavelength.  Glowing Dust Grains (2 of 2) Long-wavelength infrared light is brightest from regions where many stars are ET AL. currently forming. Vol. 692 Rieke et al. 2009 of galaxies but similar to gas clouds withing a galaxy Figure 6. Family of average templates for full radio, far-infrared, and mid- infrared spectral range. The templates are keyed to the log(L(TIR)) of the Why Do Stars Form? Gravity Versus Pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud. – Remember the ideal gas law P = nkT Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons. Mass of a Star-Forming Cloud A typical molecular cloud (T ~ 30 K, n ~ 300 particles / cm3 ) must contain at least a few hundred solar masses for gravity to overcome pressure. Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud. – Remember the CO image of the Horsehead nebula, which shows the radio photons escaping; and the heated dust cloud emitting long-wavelength IR light. Resistance to Gravity Molecular clouds tend to have thousands of solar masses but temperatures and densities that gravity could collapse for only a few hundred solar masses. A cloud must have even more mass to begin contracting if there are additional forces opposing gravity. Both magnetic fields and turbulent gas motions increase resistance to gravity. Fragmentation of a Cloud (1 of 4) Gravity within a contracting gas cloud becomes stronger as the gas becomes denser. Gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments, each of which may go on to form a star. Also recall the Inverse Square Law for gravity 1/r2. – It keeps showing up! Fragmentation of a Cloud (2 of 4) A turbulent cloud containing 50 MSun of gas a The simulation begins with a turbulent gas cloud 1.2 light-years across and containing 50MSun of gas. Fragmentation of a Cloud (3 of 4) The random motions of different sections of the cloud cause it to become lumpy. Also note how some patches are brighter (hotter) than others. b Random motions in the cloud cause it to become lumpy, with some regions denser than others. If gravity can overcome thermal pressure in these dense regions, they can collapse to form even denser lumps of matter. Fragmentation of a Cloud (4 of 4) Each lump of the cloud in which gravity can overcome pressure can go on to become a star. A large cloud can make a whole cluster of stars. c The large cloud therefore fragments into many smaller lumps of matter Its easier to collapse a corresponding to the bright yellow regions in this image. Each lump can go on to form smaller cloud than one or more new stars. larger one as the clumps break off. Fragmentation of a Cloud https://www.youtube.com/watch?v=YbdwTwB8jtc Isolated Star Formation Gravity can overcome pressure in a relatively small cloud if the cloud is unusually dense and cold. Such a cloud may make only a single star. Thousands of particles per cubic centimeter and ~10K. We do observe these clouds but the mechanism in not completely clear. Thought Question 2 (1 of 2) What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change. B. Its mass would increase. C. Its internal pressure would increase. Thought Question 2 (2 of 2) What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change. B. Its mass would increase. C. Its internal pressure would increase. The First Stars Elements like carbon and oxygen had not yet been made when the first stars formed, only H and He existed. Without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably warmer than today's molecular clouds. – Remember the emission lines we see from molecular clouds to explain the cooling. The first stars must therefore have been more massive than most of today's stars for gravity to overcome pressure. Simulation of the First Star Simulations of early star formation suggest the first molecular clouds never cooled below 100 K, making stars of ~200MSun. Some studies have suggested masses up to 500MSun. Newer simulations suggest lower masses due to stellar feedback (