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The Astronomical Universe COMPLETE NOTES [4.0ed the final exam]

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Department
Astronomy
Course
CAS AS 102
Professor
All Professors
Semester
Winter

Description
Luminosity--> brightness and distance Temperature--> Wien's Law and measuring the spectra in different colors. Size--> Temperature and Luminosity COMPOSITION OF STARS Stars have atmosphere. -when we give electrons too much energy we ionize the atoms. -Hotter stars have hotter atmospheres and vice versa. -Most of the atmosphere is made of hydrogen and helium. The rest is heavy elements (anything beyond He) Spectral types: -Stars that are really hot are blue, cold are red. -There are more absorption lines in colder stars, because atmosphere colder, so the electrons are near the atmosphere (surface temperature will be colder). -by the number of absorption lines we can determine the temperature of stars. H-R Diagrams -Annie Jump Cannon came up with names and classification for stars. -she based it on the surface temperature of stars. -O, B, A, F, G, K, M -The hottest stars are called O Stars, they have a surface temperature of 30,000 K. -The coolest stars are called M Stars, they have a surface temperature of 2,800 K. -The Sun is a G star! The type is G2. • Numbers go from 0-5. The lowest number mean the hottest and vice versa. • Ex: B0 are the hottest, B5 are the coolest stars. -O stars are more blue (hot) -M stars are more red (cold) • Why do we have absorption? Hotter stars have much hotter atmospheres, so it gives lots of energy to the electrons, so we don’t have a lot of atoms( they escape)--> Less absorption lines. -Black holes absorbs because of gravity. -Temperature is flux, and luminosity related with radii. If I go to low temperature, means low luminosity and big size. Mass is related to gravity. P=length of a circle/circular speed Circular speed=sqrt(G Msun/r) P=2pi2r/sqrt2 3 Msun/r) Or P =4(pi) r /G Msun -if we know the orbit of the planet and the period we can measure the mass of a star. • When we have two objects that are really massive, they are going to orbit on the center of mass. Most of the stars have a binary, companion star. --> Binary Stars If the objects have the same mass, the center of mass will separate them exactly in the middle. If the 1st object is more massive, then the center of mass will be nearer to the massive object. -the stars are always facing each other. • A less massive star moves faster on a larger orbit. V1/V2=M2/M1 • When we measure the period of the star, we will be able to measure their masses. • How to measure the speed? By Doppler shift! • Spectroscopic binary: we can see their absorption lines shifting from right to left. • When we have doppler shift, we are talking about the speed of radial. In a speed that is perpendicular to us, we don’t see the doppler shift. • When we see the spectra of the stars overlapping, we see the mixed absorption lines coming from the atmospheres. If we measure the blackbody in a given time, the lines shift. In a given amount of time, the movement repeats itself (periodic)--> it has to do with the doppler shift (movement of stars towards or away from us)-->we can count a period--> it has to do with the motion of the stars. • Wavelength (obs)=(1+v/c)* wavelength(rest) • When observing, if we stay perpendicular to the sight of the central mass, we won't see any doppler shift. Because motion of the stars is perpendicular to the observer. • What happens in other cases? (if not perpendicular)?--> What will happen with absorption lines if Star 1 is approaching us and Star 2 leaving us?-->S1 will be blue shifted--shorter wavelength(it will move to the left in the I-wavelength diagram), and S2 will be red-shifted, longer wavelength(it will move to the right)-->The absorption line s will start separating. • The period of Star 1 and 2 is the same! It will take the same time for both of them to go around the center of mass. • From Doppler shift ,we can get the speed of each star! • If we get the speed of each star, we can get the ratio of the masses, and also the period! The period will be the same for each star. • Period is related with the total mass of the system. P=m1+m2. we can measure it by checking the spectra. • Speed(doppler shift)==> Ratio of masses==> Period (total mass of system)==> Individual masses! • H-R Diagrams is the life-story of stars • Most of the stars lifetime will be in the main sequence. 90% of the stars lie along a ribbon (neighborhood) called Main Sequence -Hipparcos Satellite--> measure the distance of nearby stars by using parallax. Unit of the sun's luminosity--> Solar luminosity Why is H-R Diagram handy? 1. By looking at the spectrum you can tell if the star is in the main sequence or not. 2. By knowing the temperature and by placing the star in the main sequence we can measure its luminosity. 3. By measuring luminosity we can measure distance. (this is the way we determine distances beyond parallax) 4. Stars at the main sequence are distributed according to the masses. High mass stars are in the top left, low mass stars are in the bottom right. Temperature-->Main Sequence-->Luminosity-->Distance-->Mass-->Location where is located in the main sequence-->Mass Determines the fate of a star -NOT ALL STARS ARE IN THE MAIN SEQUENCE! • The other stars that are not in the sequence live in the top right (very luminous, cold giant stars) (Remember: Luminosity is related to temperature by the Radii, L/4pi r2=cT4;;Radius huge-temperature cold) or in the bottom left (Tiny stars the size of earth and are called white dwarf, they are very hot) • Baby stars are below the main sequence. The trajectory of a star after leaving the main sequence will depend on its mass. • The stars stay in the main sequence because of something keeping them stable, otherwise they become detached and become giants and die. • Stars tend to be in certain areas: Main Sequence, Supergiants, Giants, Subgiants , White dwarfs--> Classes 1 to 5 • Main Sequence are Class 5 stars. CHAPTER 14: THE MYSTERIOUS SUN -the features of the Sun's surface have to do with the magnetic field of the sun. Mass: 300.000x mass of the earth Radius: 100X of Earth Luminosity: More energy per second than all electric power plants on Earth would generate in half-million years -Sun has an Atmosphere: • The best way to see the atmosphere is by solar eclipse, because it is easy to see the extended light of atmosphere. Atmosphere of sun extends really far away. Its atmosphere is not uniform (features called streamers) • If we look through Sun with X-ray, we are sensitive to the temperature more than 5000K. • Gases in the sun that are above the surface in the sun that have a temperature of a million kelvins. They are not distributed parallel, they are distributed in a belt . • We call these regions "active regions"- AR. • Sun ejects materials called flares. Part of these materials ejected that can hit earth (Space weather) • Another material leaving the sun is called Proeminence (seen by satellites sensitive to colder materials) • If we look at the surface of the Sun: are Magnetic Structures, it is a gas mixed with magnetic field and plasma --> intense magnetic field coming out of the Sun. What is a star?- All stars are "cooking" in their center Gravity vs. Energy Released by Nuclear Reactions 90% of the stars lie along a ribbon (neighborhood) called Main Sequence -Hipparcos Satellite--> measure the distance of nearby stars by using parallax. Unit of the sun's luminosity--> Solar luminosity Why is H-R Diagram handy? 1. By looking at the spectrum you can tell if the star is in the main sequence or not. 2. By knowing the temperature and by placing the star in the main sequence we can measure its luminosity. 3. By measuring luminosity we can measure distance. (this is the way we determine distances beyond parallax) 4. Stars at the main sequence are distributed according to the masses. High mass stars are in the top left, low mass stars are in the bottom right. Temperature-->Main Sequence-->Luminosity-->Distance-->Mass-->Location where is located in the main sequence-->Mass Determines the fate of a star -NOT ALL STARS ARE IN THE MAIN SEQUENCE! • The other stars that are not in the sequence live in the top right (very luminous, cold giant stars) (Remember: Luminosity is related to temperature by the Radii, L/4pi r2=cT4;;Radius huge-temperature cold) or in the bottom left (Tiny stars the size of earth and are called white dwarf, they are very hot) • Baby stars are below the main sequence. The trajectory of a star after leaving the main sequence will depend on its mass. • The stars stay in the main sequence because of something keeping them stable, otherwise they become detached and become giants and die. • Stars tend to be in certain areas: Main Sequence, Supergiants, Giants, Subgiants , White dwarfs--> Classes 1 to 5 • Main Sequence are Class 5 stars. CHAPTER 14: THE MYSTERIOUS SUN -the features of the Sun's surface have to do with the magnetic field of the sun. Mass: 300.000x mass of the earth Radius: 100X of Earth Luminosity: More energy per second than all electric power plants on Earth would generate in half-million years -Sun has an Atmosphere: • The best way to see the atmosphere is by solar eclipse, because it is easy to see the extended light of atmosphere. Atmosphere of sun extends really far away. Its atmosphere is not uniform (features called streamers) • If we look through Sun with X-ray, we are sensitive to the temperature more than 5000K. • Gases in the sun that are above the surface in the sun that have a temperature of a million kelvins. They are not distributed parallel, they are distributed in a belt . • We call these regions "active regions"- AR. • Sun ejects materials called flares. Part of these materials ejected that can hit earth (Space weather) • Another material leaving the sun is called Proeminence (seen by satellites sensitive to colder materials) • If we look at the surface of the Sun: are Magnetic Structures, it is a gas mixed with magnetic field and plasma --> intense magnetic field coming out of the Sun. What is a star?- All stars are "cooking" in their center Gravity vs. Energy Released by Nuclear Reactions NEUTRINOS -they are the messengers that tells us what happens in the interior of the Sun. • In the process of hydrogen fusion, there are particles being emitted--> Neutrinos. • Neutrinos don’t interact with almost anything. They pass through matter. Neutrino Problem • Maybe the nr of neutrinos coming from hydrogen fusion is wrong. • Maybe we do understand Neutrinos, but maybe on their path to earth they change. Neutrinos might have more than 1 type. They found three types of neutrinos. ENERGY TRANSPORTATION FROM INTERIOR OF THE SUN • How does it get transported from the interior to the surface? Most of the energy in our lives is transferred by thermal conduction: in solids. ( you need a lot of atoms to transport heat). Sun doesn’t have atoms, because it is too hot in the interior. The atoms are broken down into their particles. =Radiative Transfer= The energy will be transferred by photons! Hot region==>more photons • Depending on the region of the sun , in the way how energy is transferred, it will be very different. • In the outer surface of the sun, we use the process of the convective zone. • The energy hits the convective zone and something different happens, energy will go around. • Energy gets accumulated at the top of the radiative zone and then another process called convection happens: The whole material will go up and down, it will go round. It has material rising and falling. • If standing on the surface of the sun, we would be constantly moving. The surface is in motion. • The trajectory of a photon from the interior takes 100,000 years. The light we see now from the sun, was produced in the interior of the sun 100,000 years ago. It has a lot of interaction with matter. Photosphere: Layer where light is emitted. The most dense part of the sun. Average temperature of 5700 K. Sunspots: little dark spots, most of them are located near the equator of the sun. they rotate and change, the convection cells stir them. Areas of strong magnetic field. Surrounded by hotter material and that's why they look dark. They are 4000 K. The size of sunspots are like the size of Earth. -from the photosphere,if we go above it, the temperature will go down -But that doesn’t happen with the Sun! • Temperature declines a little bit, but then goes up again to the corona. It goes up to a temperature like inside of the core. --> Coronal heating problem. • Photosphere->chromosphere->corona • Solar wind-->not vacuum • Temperature declines outward in the photosphere • Corona-photosphere-chromosphere --> from hottest to coolest • Chromosphere: • -Above the photosphere, lower temperature than the photosphere • -gives off a red emission • Corona: above the chromosphere • Very hot, emits X -rays • Not visible with the eye unless there is a solar eclipse. • • SOLAR WIND • The idea before was that it was vacuum. • The sun starts ejecting material, in a cold medium. There is not a lot of density. • Wind speed 400 km/sec. • The wind clashes against the rest of the galaxy. The wind extends everywhere until the edge of the solar system. • The magnetic field is responsible for the coronal heating problem. Something beside the wind is making the corona hotter. • If you shake magnetic fields they create waves. The wind + waves==> the energy created (high temperature) • The magnetic field+ convection makes this material be ejected from the interior of the sun. • Speed of the wind can change. The sun has two types of winds : slow and fast (800 km/s) • The sun has a cycle--> Minima(fast wind near the pole and slow wind in the equator) and maxima (high speed everywhere) • In the corona we see bright explosions. Those spots continuously expel in little explosions. They come from regions where sun has strong magnetic field. • Sunspots: when you go to solar maxima, there are a lot of sunspots--> it has to do with the magnetic field of the sun. • Sunspot cycle: Nr. of sunspots vs time-there are variations. • Active regions in the corona are the sunspots. • When the sun is in solar maxima, Solar maxima-->lots of sunspots-->lots of active regions-->Sun is very active. • Every 11 years the sun goes quiet and then active. • Maunder Minimum-a huge period when there were no sunspots. • Active regions--> explosions when the sun goes from the active region and spits a big amount of material that has magnetic field in it --> Coronal mass ejections and sun flares! • Solar flares are more concentrated in space. • Solar prominence--> gas that is standing above the chromosphere. • • White light-extended corona. • Why are explosions important?- Space weather. The sun exploding can hit us, the reason why it doesn’t clash us, is due to our magnetic field. • The energetic particles that are clashed from sun to the magnetic field of earth go to the poles and form the aurora in the north. The location of spots vary with the latitude, where they are located in the surface of the sun. --> Butterfly Diagram. It is only for the Sun , not other stars. Bow shocks- indications that other stars have winds and that they are moving. CHAPTER 15- Interstellar medium Interstellar Medium- the place where stars are formed. What is the matter between stars? Not vacuum! Interstellar medium (ISM) is very very tenuous. In ISM: 0.1 atom/cm3--> there is almost no air. 1% of the ISM material is interstellar dust. This is material between the stars. This material is made of iron, silicon, carbon. They stick together in cold dense regions--> if it's hot it will start bouncing, if cold gravity will hold them. These elements are made by stars. So dust comes from stars. There is also gas (most of it) -even though ISM is tenuous, it has a huge effect on how we see light. Dust distorts the light that comes to us. Dust blocks the light from stars, galaxies, etc. especially in the visible light --> Interstellar Extinction. • If there is a very long wavelength, it will pass the dust without interaction. • A short wavelength, it will interact. • Short wavelength will suffer from interstellar extinction. • Shorter wavelength are going to be blocked from the dust--> the light would be distorted. --> the stars will seem redder. We call it interstellar reddening. • Dust has its own blackbody emission. They are cool, 10-300 K. Dust glows in the infrared. The properties of ISM are not the same everywhere! Because lots of stars die with different temperatures and densities. It relates to the life story of stars. Is a very dynamic place • Is interstellar medium homogenous? No • Hot gas • Cold gas • Warm gas -Most gas and dust are concentrated in the interstellar clouds • Some regions can be very hot (as hot as solar corona)--> X - rays emitted. Extremely hot intercloud gas is thought to occupy half of the volume of interstellar space. How is this gas heated? • By supernovae explosions. If we will be standing in that gas, would we be warm? • Not really, it would be almost like vacuum. Too rarified, not a lot of density. Warm Gas: H-alpha : Recombination line-> combination of proton and electron. H atom's left in an excited state; the atom drop down emitted H-alpha. H2 regions: The bright spots in ISM (hydrogen doubly ionized) Ionized by UV light from hot, luminous stars. These regions tell where the stars are formed. Why O stars? = because they live very few million years, they evolve quickly and do not move very far away from where they are form. Neutral Gas: Related to the spin of the particles--> like the sides of the magnet. Particles tend to be aligned in the same direction, to rotate in the same way. Neutral gas will emit radio waves of 21 cm wavelength. . This light is long wavelength so it will penetrate dust. Here we are talking about cold temperatures. Cold Gas: Temperatures are 10 K. Density is high. It emits radio waves. Most common molecule is H-2. some clouds can have a mass as large as 10 million times as of the Sun-->called Giant molecular clouds. In these clouds, stars are formed. STAR FORMATION By molecular clouds! (because they are cold and super dense). Gravitational force is going to attracted to the center of mass) Angular Momentum: when the clouds start collapsing the shrinking is going to go faster. We are assuming that the cloud is spinning. As the cloud collapses, the gravitational force decreases! Suppose a radii decreases by a factor of 2, by how much will G.f will increase? By 4! As cloud collapses, it will start forming very dense regions. It's like a "snowball" process. It collapses into molecular-cloud cores. During the process of spining causes the creation of a star: Protostar (it will be spinning). The rotation of the baby star will be related to the rotation of cloud. The material around the baby star is still rotating and collapsing and it will form a disc of dense material. The protostar is not yet part of the main sequence. ISM: Warm Gas: from hot stars Hot Gas: Winds from explosions of supernovas Cold Gas: molecular clouds Formation of a Protostar: • In cold molecular clouds between gravity and temperature • They will start collapsing , gravity will act stronger. • The cloud forms dense molecular cores. • It collapses in a disk because in the axis it can conserve (r=0) (angular momentum=m*v*r) • At the center, there is a strong concentration of mass that is collapsing more • The cloud will keep falling, but in the center there will be a big blob of mass. • In the formed disk, in the middle the protostar will be formed. • The disk is called Accretion disk (falling towards the disk) • Protostar is a hot ball of gas, but it's not a star yet. It is formed in a collapsing cloud of gas and dust. It's coming from interstellar medium. This cold gas came from previous stars--> heavy molecules that are partly dust. • The rest of the mass is in the protoplanetary disk. • Planets are formed by it. Planets are called "failed stars" - they don’t have enough mass to become stars. • They came up with a rotating disk because of the planets rotating around the sun (stars) and because they rotate in te same way. Disks are full of dust . It is good to study disks in infrared wavelength. Protostar is hidden by the disk. Some materials falling from the disk to protostar are expelled in jets! (because of magnetic fields) • Protostars are huge (all gravitational eenergy converted to thermal energy)\ • They are very luminous, but they are cool. • How can we see them? • In infrared wavelength (because they are surrounded by dust) • Where will be a baby star located in HR diagram? -> Upper Right! • How will a baby star evolve? a. Which forces are fighting? Gravity vs Thermal energy (temperature) b. It will continue to accrete more material and will keep collapsing. Thermal energy will try to stop the collapse as much as possible, but since it is cold material gravity will work to pull material together. The star is radiating energy, so it balances pressure and gravity. The interior temperature and pressure rise. c.It will become a star! Gravity is winning, pressure is increasing, until center of the star will reach temperatures of milions of kelvin. In this point, hydrogen will start fusing, energy will be produce. This is the difference between failed stars--> when reaching high temperature, the star itself will start producing energy. Gravity will pull the layers, there is radiation, pressure increase, and ignition starts. Once you start burning hydrogen, you will have more energy in the center to stop the contraction from gravity. In this way it starts stabilizing. d. The protostar's energy source is gravitational energy, for a star is hydrogen ignition! e. In this way, the star will enter to the Main Sequence. f. Very low mass stars never start hydrogen fusion. These are called "brown dwarfs". A brown Dwarf is not a star, nor a planet, it is in between. Classified as L, T, Y Cooler than M stars. Glow in infrared due to internal heat LOW MASS STARS • MOVING ALONG THE HAYASHI TRACK -Protostars get less luminous and hotter. High luminosity and as they age they get less luminous. -The interior of the star gets hotter, not the surface. -For lower masses, protostars get less luminous, smaller in radius and hotter and they follow the Hayashi track Chushiro Hayashi explained why? • Atmospheres of stars and protostars contain a naturall thermostat: the H- ion. (H with an extra electron). • The amount of H- in the atmospheres will depend on the temperature of the protostar. If protostar is colder, the atmosphere is cooler too, we have more H- ions. If I have more H-, what these ions do, is they trap energy and don’t let the temeperature escape. • If a protostar has a certain T temperature, it is radiating, and has H- in atmosphere, H- work as blanket, they trap energy (temperature coming out of star) and keep it, not let it go out of space. This is going to heat the star. • If temperature goes higher it will be less H- ions, more energy will go out. • this H- will keep surface temperature in the range 3000-5000 K--> in the top right of HR diagram. • Low mass stars are in the top right of the HR diagram. The bottom axis of HR diagram is surface temperature. -As a protostar evolves (contracting), the surface temperature will be between 3000-5000 K blocked there. -If the T is the same what does it mean about energy that the star is giving out? How much is the total flux ? If the surface is the same, the flux is the same. -but star is contracting? What happens to luminosity? L=Area x Flux --> Flux not changing, Area going down, luminosity is going down! -So the evolutionary track goes down as the luminosity goes down! -The star moves down on the Hayashi Track and arrives on the main sequence. -Just before it enters the main sequence, the hydrogen start to burn. -as protostar goes down, it gets fainter and fainter. -As the star starts burning hydrogen, luminosity goes up and it enters the main sequence. • Only a few percent of the material in the molecular cloud ends up in stars. • Bipolar outflow of jets: material leaving the protostar (maybe it has to do with magnetic field) • We don’t know when the protostar starts having its own wind. When the wind begins it will start moving away the material on the disk, it will clear out the disk. • Once the material is blown away, we see them "revealed": T - Tauri Star (protostar) • Powerful jets can collide with the IM to make "Herbig Haro HH Objects"--> these can eject much of the mass that would otherwise land on the star. • Maybe the jets or wind of star eject part of the cloud. • Stars are usually found in groups: Star clusters. (bound by gravity) • These are regions where stars are forms. One gigantic cloud formed all these stars. • More massive stars evolve quicker. What defines a star in the main sequence? =Burning Hydrogen Where the burning happening? =In the core!!! All stars in main sequence burn hydrogen in their cores. What is the most important characteristic that distinguishes the evolution of stars? Mass! Why mass?- High mass stars evolve very different from low mass stars. Low mass stars--> M<3Msun Life span of Main-sequence Star Once
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