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E1: Intro to the Universe Where are we??? • We are on planet Earth • In the Solar System with 8 planets revolving around a star called the Sun • We are just one star in billions in the Milky Way Galaxy • Our galaxy is one of a group of 40 called “The Local Group” • We are in a cluster of groups, called a Super cluster • We are just one of billions of Galaxies in the universe which is about 15 billion ly across (the age of the universe is 15 billion years) • Click here to get a better picture of where we are Galaxy: a Star city •A galaxy is a huge collection of stars held together by gravity •The largest galaxies contain thousands of billions of stars • Our own galaxy is called the Milky Way. • 50 billion galaxies are visible to modern telescopes •Most galaxies are thought to have a black hole in their centre •The nearest galaxies are 10^6 ly away and the furthest are 10^10 ly away Types of Galaxy •There are 2 types of galaxy: disc and elliptical (but also irregular) •It is thought that the spiral arms of galaxies arise as a result of collisions between galaxies. The Milky Way Galaxy • The Milky Way is a disc galaxy with spiral arms • It contains about 200 billion stars ( 2 x 109 stars) •At the centre of our galaxy is a black hole with a mass of approx 100 million suns •It is 2000 ly thick and 100,000 ly long •The closest star in our galaxy, Proxima Centauri, is 4 ly away M31 Andromeda Galaxy A disc galaxy Distance: about 700 kpc The most distant object which can be seen by the naked eye Diameter: 60 kpc (about twice the diameter of the Milky Way Contains 400 billion stars Whirlpool Galaxy A disc Galaxy with well defined spiral arms. Distance about 6 Mpc (notice the dwarf galaxy orbiting it) M104 Sombrero Galaxy M32 Elliptical Galaxy Triangulum Galaxy Whirlpool galaxy Blackeye Galaxy M82 Irregular Galaxy Clusters and Superclusters • Our Galaxy is just one of a group of galaxies called the Local Group. • The Local Group includes about 40 galaxies, including the Andromeda Galaxy, The Magellanic Clouds and many dwarf galaxies. • The Local Group, the Virgo Cluster, and Coma Cluster form a Supercluster about 300 million parsecs in radius. • The distribution of galaxies in the universe is not uniform (contrary to recent theories). Virgo Cluster A large cluster of about 2500 galaxies. Distance approx 17 Mpc Globular Cluster Nebulas( fuzzy regions) •The word ‘Nebula’ comes from the Latin word meaning ‘cloud’ •Many nebula are bright regions of gas where star formation is taking place • Dark nebulae are large dusty regions where light cannot pass Triffid Nebula The dark area (see below) is illuminated by bright stars hidden behind the dust Tarantula Nebula Cone Nebula Catseye Nebula Horsehead Nebula Coalsack dark Nebula Cone Nebula Structure of the solar system •Planets revolve around the sun in a counterclockwise (viewed from above) elliptical orbit. •There is an asteroid belt (leftover debris from the sun’s formation) between Mars and Jupiter. •MVEM (inner/terrestrial planets) are composed of rock an metal •JSUN (gas planets) are mostly hydrogen and helium. Relative sizes of the planets Relative distances of the planets http://www.youtube.com/watch?v=AUUkjWsNC9k&feature=related 2:20-2:40 Distances in terms of an Astronomical Unit (AU): M.4 V.7 E1 M1.5 J5.2 S9.5 U19.6 N30 P30-50 (sometimes Pluto is closer to the sun than Neptune…but Pluto is no longer a planet, it is a “dwarf planet”) • Stellar cluster: A group of stars so numerous they appear to be a cloud. Example: Globular cluster located in the constellation Hercules • Constellation: A group of starts that appear to form a pattern. Example: Orion • Light Year (ly): The distance light travels in one year = 10^13 km • Parsec (pc): The distance to a point when the parallax angle is 1” (one second of an arc (1/3600 degrees)). The distant stars appear stationary to the Earth observer. Closer stars appear to move throughout the year (parallax). The distance to the imaginary star is one pc if the angle it makes (parallax angle) is 1” (one arc second). 1pc = 3.26 ly Click on the stellar parralax animation for a good example of this. Pisces Orion North Pole Leo The time is mid-day. The observer O at the equator looks straight up and sees the sun overhead. O Sun Serpens Pisces Six hours later, the earth has turned through 90o. The observer has to look towards the horizon (i.e. horizontally) to see the sun. Direction of the sun Orion North Pole Leo Sun Serpens Pisces From the observer’s point of view, the sun has gone from overhead down to the western horizon. Direction of the sun Orion North Pole Leo Sun Serpens Pisces Direction of Orion Orion Now, if he looks overhead, he sees the constellation Pisces. Orion is just appearing at the eastern horizon. Direction of the sun North Pole Leo Sun Serpens Pisces Orion O North Pole Leo After another 6 hours it is midnight. The observer sees Orion straight overhead. Sun Serpens Pisces Pisces has reached the western horizon and Leo is just appearing at the eastern horizon. Direction of Pisces Orion O North Pole Direction of Leo Leo Sun Serpens Pisces Orion has moved up from the eastern horizon to the zenith (overhead). Direction of Pisces Orion O North Pole Direction of Leo Leo Sun Serpens Pisces Orion At dawn, the observer can see Orion at the western horizon and the sun is rising in the east. Leo is now overhead. North Pole Direction of Orion Direction of sunrise Leo Sun Serpens Pisces As seen from “above” the north pole, the earth rotates counterclockwise. To an observer on the earth, the sky appears to move clockwise – i.e. from east to west. Apparent rotation of sky Orion North Pole Rotation of earth Leo Sun Serpens During the course of a night, half of the entire sky can be seen. Which half depends on where the earth is in its orbit. The other half of the sky is still there, but it is not bright enough to see during the daytime. Something similar happens during the year. Pisces Position of Earth in September Orion Serpens Sun Position of Earth in December Position of Earth in June Position of Earth in March Leo Pisces In December, Orion is overhead at midnight. Pisces is setting on the western horizon and Leo is rising in the east. Orion Sun Serpens Position of Earth in December Serpens cannot be seen because of the daylight. Leo Pisces In March, Leo is overhead at midnight. Orion is setting on the western horizon and Serpens is rising in the east. Orion Pisces is concealed by the glare of the sun. Sun Position of Earth in March Leo Serpens Pisces In June, Serpens is overhead at midnight. Leo is setting on the western horizon and Pisces is rising in the east. Orion Serpens Sun Position of Earth in June Orion is out of sight. Leo Pisces In September, Pisces is overhead at midnight. Serpens is setting on the western horizon and Orion is rising in the east. Position of Earth in September Orion Serpens Sun Position of Earth in December Position of Earth in June Position of Earth in March Leo cannot be seen. Leo During the entire year, the Earth rotates once around the sun in a counterclockwise direction. The entire sky appears to rotate round the Earth once in that period of time. E2: Stellar radiation and stellar types E2: Stellar radiation and stellar types • • • • • • The sun produces energy by fusion. Squeezing two hydrogen atoms (at high T and high P) into a helium atom. This produces what is called “radiation pressure” which tries to expand the star (essentially blow it up). The star is massive enough that it has a large “gravitional pressure” which tries to compress and crush the star. In a stable star there is an equilibrium between the gravitational and radiation pressure. Luminosity (L): The total power radiated by a star in watts (J/s) Apparent brightness (b): The amount of radiation (in watts) that hits an area on Earth perpendicular to the incoming radiation. Measured in Watts per square meter. Note: A star could have a greater L but a smaller b as compared to another star if it is further away from Earth. » b=L/(4pid^2) note: inverse square law » If b and L are known, then the star’s distance can be determined Stefan-Boltzmann Law • Stefan-Boltmann Law: Power radiated per unit area from the surface of a body is proportional to the fourth power of it’s kelvin T. » L=eσAT^4 » L=Luminosity (in watts) » e=emissivity : a number (0 to 1) that is characteristic of the material. Very black surfaces have an e=1 which means they are very good absorbers and emitters of radiation (blackbodies) » σ=Stefan-Boltzmann consant 5.67E-8 W/m^2K^4 » A=Area of object (Sphere=4xpixradius^2) » T=Temperature in Kelvin Example: What is the Luminosity of the Sun? The surface T=5780K and the radius=6.96E5 km. Assume the sun is a perfect blackbody (e=1) L=(5.67E-8)(4x 3.14x6.96E8^2)(5780^4)=3.85E26 Watts A star has half the Sun’s surface T and 400 times its L. How many times bigger is it? L=eσAT^4 (e=1 since they are blackbodies, A=4piR^2) L AT 4 L 4 R 2T 4 L 4 R 2T 4 2 4 Ls 4 Rs Ts L R 2T 4 2 4 Ls Rs Ts 4 LTs R2 2 4 LsT Rs 4 400 LsTs R2 4 2 Ls Ts / 2 Rs R2 400 x16 6400 2 Rs R Rs 6400 80 The star has a radius that is 80 times that of the Sun. Wien’s Law • Wien’s Law: The surface T of a star depends on the peak wavelength (λp) emitted by the star. T=2.90x10^-3/λp – A blackbody (perfect emitter and absorber of radiation) emits a characteristic radiation spectrum The spectrum contains a continuous range of frequencies. As T increases, the peak wavelength becomes shorter (higher frequency) and the total radiation (area under curve) increases. Notice the Sun (6000K) peaks in the visible part of the spectrum. Lower T appear reddish, higher T appear blue. Example: What is the surface T of a star whose λp is 966 nm? T = 2.90E-3/966E-9 = 3000K Stellar Spectra • • • • • • Photons emitted from a star passes through its own atmosphere. The photons are of a continuous wavelength (see the spectrum on the previous slide). As the photons pass through the atmosphere of the star some of them are absorbed. Certain wavelengths are absorbed depending on the composition of the star’s atmosphere. Click here to see the absorption spectrum of different gases. If the star or galaxy is moving away from Earth, the spectrum will be shifted toward the longer wavelengths (same as the Doppler effect for sound). This is called “red-shift” as the light is shifted toward the red wavelengths. If the star or galaxy is moving towards Earth, the spectrum is shifted toward the shorter wavelengths called “blue-shift”. Spectral classification Spectral Classes: O, B, A, F, G, K, M. You can remember this by using “Oh Be A Fine Girl/Guy Kiss Me”. The systems is based on the surface temperature of the star. O ≥ 30,000 K blue (most massive) B 10,000–30,000 K blue to blue white A 7,500–10,000 K white F 6,000–7,500 K yellowish white G 5,200–6,000 K yellow K 3,700–5,200 K orange M ≤ 3,700 K red (least massive) Our star, The Sun, is a class G star and is yellowish. Types of Stars Single star: our Sun Binary star: two stars that orbit each other around its common center of mass. If you can visually see two stars they are called “visual binaries”, if not they are “spectroscopic binary” (more on that later) and they can also be “eclipsing binary” if one eclipses the other during its orbit. Cepheids: stars which brighten and dim periodically (more later) Red Giants: A star in a late stage of evolution. The outter atmosphere has inflated and its surface T is low. It is very luminous due to its large size. Supergiant: A more massive and larger Red Giant. More luminous than Red Giants due to their size. White Dwarfs: A star with a small mass (such as our Sun) in the final stage of evolution. It is the size of Earth but with the mass of the Sun. Its surface T is very high but its luminosity is low due to its size. Hertzsprung-Russell (HR) Diagram Most stars are in the region called “The Main Sequence”. Supergiants and Giants are above The Main Sequence stars and White Dwarfs are below The Main Sequence stars on the diagram. Notice that Supergiants are at a relatively low T but are very luminous due to their large size. White Dwarfs are very hot but not luminous due to their small size. This is not a linear scale. Mass of Main Sequence star depends on the position in the diagram. O is most massive, M is least massive. Spectroscopic Binary When two stars can not be seen but can be inferred due to the shift in their spectral lines. As one star (B) is moving away from Earth, its spectral lines (or absorption lines) will be red-shifted. As the other star is moving towards Earth (A), its lines will be blue-shifted. As they two stars are moving horizontally with respect to the Earth, the spectral lines are normal. Then B moves towards Earth and A moves away. The spectral lines move apart then come together twice per revolution. See the simplified animation below. Eclipsing Binary Stars • • When in a binary star system, one of the stars passes in front of the other, during their orbit, relative to Earth. The intensity of the system decreases relative to Earth when an eclipse occurs. If the stars are of different intensity the dip in the intensity during the eclipse depends on which star is being seen. E3: Stellar Distances How to determine stellar distances • • • • Stellar Parallax Method Absolute and Apparent Magnitude Method Spectroscopic Parallax Method Cepheid Variables as “Standard Candles” Stellar Parallax Method • Only can be used for stars within 100 pc because as the star is further away the parallax angle becomes too small to be able to measure accurately. • Distance 1-2 is 2 AU. • Θ = parallax angle • D= distance of the star • Click here for animation of this method Example: How far is the closest star to us, Alpha Centauri, if the largest parallax angle measured is 0.76” (0.76 arc seconds)? Use your formula booklet. Θ=1AU/D (small angle approx only works with radians) .76” = 2.1E-4 degrees = 3.7E-6 radians D=1AU/3.7E-6 = 271,400 AU = 4.1E16 m =4.3 ly Simplified formula… d(parsec)=1/p(arc-secpond) Apparent magnitude (m) scale Greece 300 B.C.: Astronomers classified all the stars in the sky according to how bright they appeared. 1 was the brightest and 6 was just visible by eye. Magnitude 1 is 100x brighter than a magnitude 6 star. A difference in one magnitude is 2.51 times difference in brightness. (The fifth root of 100 = 2.51) Problem: What magnitude is the Sun or the full Moon? What about stars that we can only see with telescopes? Unfortunately we have kept this system. m Sun = -26.73 m full Moon = -12.6 m faintest objects = 31.5 Absolute magnitude (M) scale An apparent magnitude of 1 appears brighter than the that of an apparent magnitude of 2 for two reasons: It is actually a brighter star (more luminous) It is much closer than the m=2 star (but may actually be less luminous) Therefore if the apparent magnitude is less (appears brighter) it may not actually be more luminous. To correct this problem we use absolute magnitude. Absolute magnitude of a star is the apparent magnitude of the star if it was 10 pc from us. So it is as if we have put all the stars the same distance away from us so we can accurately compare them to each other. The M scale is the same as the m scale (1 step is a change of 2.51 times the brightness) m-M=5lg(d/10) (d is in parsecs) Examples on next slide m vs. M apparent magnitude m Absolute magnitude M Distance Modulus m-M Sirius -1.44 1.41 -2.85 Betelgeuse 0.45 -5.14 5.59 GJ 75 5.63 5.63 0.00 Star Sirius has a smaller apparent magnitude than Betelgeuse but a larger absolute magnitude. What does that mean? It appears brighter from Earth but it is actually less luminous. Sirius has a smaller apparent magnitude than and absolute magnitude. What does that mean? That it is closer than 10 pc. As Sirius is moved to 10 pc it becomes dimmer, therefore it must be closer than 10 pc. Betelgeuse has as smaller absolute magnitude, what does this mean? That it is further than 10 pc, as it it moved to 10 pc it become brighter, therefore it must have moved closer. GJ 75 has equal magnitudes, why? It is already at 10 pc from Earth Find the distance of Sirius m-M=5lg(d/10) (d is in parsecs) -1.44-1.41=5lg(d/10) -2.85/5=lg(d/10)=-0.57 (antilog) 10^-.57=d/10=.27 d=2.7 pc Find the distance of Betelgeuse. Answer= 131 pc Spectroscopic parallax • • • • • • • • • Actually does not have anything to do with parallax For distances too far for stellar parallax (trigonometry) Can measure distances up to about 10Mpc The star’s apparent magnitude (m) is measured From its spectrum, the star’s class and type can be determined From the HR diagram, its absolute magnitude (M) can be determined Using the equation m-M=5lg(d/10), the d (in parsecs) can be calculated Gives an estimation of the d You can also use the formula b=L/(4pid^2) Apparent brightness and luminosity • Apparent brightness (b): The amount of radiation (in watts) that hits an area on Earth perpendicular to the incoming radiation. Measured in Watts per square meter. Note: A star could have a greater L but a smaller b as compared to another star if it is further away from Earth • Luminosity can also be determined from the HR diagram » b=L/(4pid^2) note: inverse square law » If b and L are known, then the star’s distance can be determined Example: A main sequence star has a peak wavelength at 2.4E-7 m. It’s apparent brightness is measured to be 4.3E-9 Wm^-2. How far away is the star? (Hint: you have to use Wien’s law to find the T and the HR diagram to find the Luminosity, Ls = 3.9E26). T=2.90x10^-3/λp T=12,000K L=10^2Ls=100x3.9E26 =3.9E28 b=L/(4pid^2) d= L 4b d=8.5E17m = 90ly = 28 pc Cepheid variables • • • • The outer surface of the star undergoes contraction and expansion the gravitation and radiation pressure not in equilibrium They are very bright and massive stars This produces a periodic variation in its luminosity The period (usually somewhere between one and 50 days) can easily be determined Cepheid variables • Once the period is known, its luminosity (average L) can be determined by a known relationship (notice it is a log scale on the y-axis) • Once you know the luminosity, you can measure its apparent brightness and use b=L/(4pid^2) to determine the distance • Cepheid variables are called “standard candles” because their luminosity is known (like a candle’s would be) so if you can spot a Cepheid variable in a galaxy you can determine its distance (can be used for objects very far away) Example 1: LMC has a period of 4.76 days days and mean apparent magnitude of 15.56. Find its absolute magnitude and distance. M = -3.57 M-M=5logd-5 antilog(15.56- -3.57+5)/5=d d = 67,000 parsecs Example 2: Zeta Gem has a period of 10 days and an apparent magnitude of 4.0. What is the absolute magnitude and the distance? M = -4.3 d = 460 pc E4: Cosmology (the study of the universe: its origin and future) (includes parts of E6) Olber’s Paradox Why is the night sky dark? Night Sky If you are in a big forest and keep walking you will bump into a tree • If you are in a big (infinite) universe and keep going in a straight line you will bump into a star. Newton thought the Universe was infinitely old and static Centre of mass • Gravity makes the stars collapse towards their centre of mass • There must be more stars pulling out to stop this happening • And more outside these to stop them collapsing • So the universe must be infinitely big or it would have collapsed Remember Inverse Square Law d 2d • As light travels it spreads out • At a distance of 2d the light will have spread out over 4 times the area • So the object will have quarter the brightness • Apparent brightness is proportional to 1/d2 Look in any direction Imagine spherical shells surrounding Earth d 2d Shell at 2d has 4 times the area, hence 4 times the volume. It contains 4 times as many starts so emits 4 times as much light 3d 9 times as many stars Each shell has same brightness • Shell at 2d has 4 time as many stars • Remember inverse square law • They have ¼ brightness • So apparent brightness of each shell is the same • • • • If the universe is infinite Then there are an infinite number of shells. Each shell has the same brightness If you add the light from each you get an infinitely bright sky Here’s the Paradox • So if the universe is infinitely big then the sky should be bright • But the sky is dark • So the universe is not infinitely big • So it should have collapsed (if it was infinitely old) What assumptions did Newton make? • 1. The universe is infinitely old. • 2. The universe is static Note that clouds of dust blocking light does not solve the problem because the clouds would heat up and re-radiate the energy • 1. Hubble constant estimates age of universe about 15 billion years. Light from stars beyond 15 billion light years hasn’t reached us yet • 2. The universe is expanding. Gravity slows the expansion and may eventually reverse it. (This depends on how much matter is in the universe.) Confused with the IDEA? Isaac Newton • discovery of the law of gravity • realized that gravity is always attractive. • Every object in the sky attracts each other If the universe were finite, the attractive forces of all the objects in the universe should have caused the entire universe to collapse on itself. Albert Einstein • theory of gravity in the general theory of relativity • encountered same as Newton’s problem • His equations said the universe was either expanding or collapsing. Created the cosmological constant : BAD! • “the greatest blunder of my life” My name is Albert! But why didn’t the universe collapse like Newton and Einstein’s equation suggested ? •Because the universe had been expanding from the moment of its creation and it is in a constant state of change. Heinrich Olbers • In the early 1800s, German astronomer, argued that our universe were finite. • He said, “If the universe was infinite and contained starts throughout, then if you looked in any particular direction, you line of sight would eventually fall on the surface of a star. Although the apparent size of a star in the sky becomes smaller as the distance to the star increases, the brightness of the smaller surface remains a constant.” • Therefore, if the universe was infinite, then the whole surface of the night sky should be as bright as a star. However, as we know, the sky has dark areas, proving that it is finite. Expanding Universe And the Big Bang ` Doppler Effect – ReD ShIfTs • Doppler effect is the result of sound increasing or decreasing in pitch as an object moves towards or away form you. (APPLIES TO ANY WAVES) • If a light-emitting object is moving away from a person, each wave of light leaves the object from a point slightly farther away from the person than the previous wave did. Therefore, the distance between waves, or wavelength, that the person sees is longer than it would be if the object were motionless. • In visible light, the longest wavelength belongs to red light, and shortest to the violet light • most often used by astronomers to measure the velocity of galaxies. Vesto Slipher • • First to encounter redshifting galaxies • Discovered a new cosmic riddle for astronomers of his time. In 1916, he observed about fifty nearby galaxies in Lowell Observatory in Arizona, spreading his light out using a prism, and recorded the results onto film • Almost every object he observed had its light stretched to redder colors, indicating essentially everything in the universe was moving away from earth. Hi, I’m Vesto Edwin Hubble • • • • • discovered the existent of other distant galaxies In 1929, while working at the Carnegie Observatories in Pasadena, California, he measured the red shifts of a number of distant galaxies and their relative distances When he plotted the numbers, he discovered, that the red shift of distant galaxies increased as a linear function of their distance = universe expands Also discovered that the galaxies were receding from us at a velocity proportional to their distance. The more distant the galaxy, the greater its red shift, and therefore the higher the velocity; this relationship was known as Hubble’s law. Big Bang model • • • • • • Light from distant galaxies is red shifted (more later) This means that the universe is expanding (the space between the galaxies is expanding, not matter itself) This means that the universe must have emerged from a tiny dense dot of matter (Big Bang) 10-20 billion years ago Time and space originated at the Big Bang Time and space is created as the universe expands You can not ask these questions… “What existed before the Big Bang?” “What is the universe expanding into?” The answer is……………………….NOTHING “But what is nothing?” ………………………………………..NOTHING Any questions? The Cosmic Background Radiation 1. 2. Proof for the Big Bang The light from distant galaxies is red shifted which means they are all getting further away from us which means the universe is expanding The Cosmic Microwave Background Radiation (CMB): • If the universe was a tiny dense speck it would have been very hot • High energy radiation would have been produced in the early stages of the bang • This radiation should still be “out there” • As the universe expands, the radiation would be red shifted (stretched out) or “cooled” and would now be long wave radiation • Penzias and Wilson discovered this background radiation emanating from all points in the sky • It was “cooled” to 3 K (in the microwave region of the EM spectrum) • We are constantly being bombarded by microwaves left over from the big bang, we are in a Universal Microwave Oven! Watch this video about the discovery (chapters 1 and 2) Models of our universe • Closed (left) – [high mass] a universe that would eventually stop expanding and re-collapses on itself, possibly leading to another big bang. • Flat (center) – [critical mass] a universe which would also expand forever, but the expansion rate would slow to close zero after an infinite amount of time . • Open (right) – [low mass] a universe that will continue its expansion forever C L O S E D O P E n The fate of the universe Notice that the age of universe depends on which type it is…. The fate of the universe depends on its density Critical density (pc) = the density of the universe for it to be flat. If p> pc than the universe is open (current evidence suggests the universe is open) If p= pc than the universe is flat If p< pc than the universe is closed The problem with finding the density is that there is a lot of “dark matter” out there, matter we can not see and measure Dark matter can be brown dwarfs, WIMPS (weakly interacting massive particles), neutrinos, and MACHOS (massive compact halo objects). Basically we have no idea what it is! E5: Stellar Processes and Stellar Evolution The evolution of a star… The evolution of a star… Click here for a step by step guide, then follow the summary below. The life cycle depends on the mass of the star NEBULA • Gas cloud (nebula) exists with at least 8% the mass of our Sun. • The gas cloud collapses under it own gravitational pressure MAIN SEQUENCE • Fusion start to occur in the core if there is enough mass to produce high enough pressure and temperature (two hydrogen atoms are squeezed together to produce a helium atom– this is commonly referred to “hydrogen burning” although it is not burning as in combustion) • The star is in equilibrium (gravitational P=radiation P) • Fusion can continue for up to 50 billion years, this is referred to as the “Main Sequence”, this is what our star, the Sun, is doing right now • More massive stars burn hydrogen quicker and live shorter but hotter lives • L mass 3 4 The exact value is not known GIANT PHASE • Hydrogen runs out in the core and the core contracts • Core starts to fuse Helium • T in core increases, surface expands due to increased T, surface can also burn left over hydrogen heating it up further • Surface expands and cools • This creates a Red or Super Giant, they are very luminous but are red due to a low surface T Death of star How a star dies depends on its mass • • • • • • Small Mass Outer shell rips off, called “Planetary Nebula” (it has nothing to do with planets!) Leaves hot core behind made of carbon Core glows white Core can not contract anymore due to “electron degeneracy pressure” Becomes a white dwarf Ejected shell becomes another nebula • • • • • • • • • Medium Mass There is enough gravitational pressure to fuse carbon Fusion continues until iron is produced Surface collapses onto the dense core and bounces off it and is ejected - called a supernova – heavier elements are formed The core collapses and electrons are squeezed into the protons which produces neutrons Core can not contract anymore due to “neutron degeneracy pressure”. Becomes a neutron star • • Large mass If at the point the neutron star is massive enough the gravitational pressure overcomes the neutron degeneracy pressure and it continues Forms a black hole Chandrasekhar limit – the upper limit of a white dwarf. If the star has a mass of over 1.4 solar masses than it will continue to collapse into a neutron star and will not become a white dwarf. Remember this is the mass AFTER ejecting its surface in the supernova Oppenheimer-Volkoff limit – the upper limit of a neutron star. If the star is greater than 2-3 solar masses (after the supernova) it will continue to contract into a black hole Pulsar – a rapidly spinning neutron star that emits pulses of radiation on the order of seconds Nucleosynthesis Elements above iron will not be formed (binding energy per nucleon peaks with iron…for elements above iron more energy is used than produced when they are formed by fusion) Stellar evolution on an HR diagram Draw the path of a small star’s evolution once it leaves the main sequence (hydrogen burning stops) The star leaves the main sequence and become a giant, then a white dwarf. E6: Galaxies and the Expanding Universe (and parts of E4) Remember Edwin Hubble? • • • • • discovered the existent of other distant galaxies In 1929, while working at the Carnegie Observatories in Pasadena, California, he measured the red shifts of a number of distant galaxies and their relative distances When he plotted the numbers, he discovered, that the red shift of distant galaxies increased as a linear function of their distance = universe expands Also discovered that the galaxies were receding from us at a velocity proportional to their distance. The more distant the galaxy, the greater its red shift, and therefore the higher the velocity; this relationship was known as Hubble’s law. Hubble’s Law • is a statement of a direct link between the distance to a galaxy and its recessional velocity as determined by the red shift. It states that the velocity (v) of a galaxy moving away from Earth is proportional to the galaxy's distance (d) from Earth. As distance increases, velocity also increases. The constant value that relates velocity and distance is called Hubble's constant and is usually written as H0. Or mathematically like this: • H0 is the proportionality between recession velocity and distance of a galaxy = the universes’ rate of expansion • H0 identified in 1929, but its numeric value is still uncertain. • Astronomers know that it falls between 50 and 80 kilometers per second-megaparsec. A megaparsec is a unit of length equal to 1 million parsecs. A parsec is 30.86 trillion km (19.18 trillion mi). • These units are used to make redshift calculations easier. • Could be represented as 1.5 × 10-15 to 3.1 × 10-15 1/s or • How do you find the the velocity of moving galaxies? Astronomers know the speed of which the galaxies are moving by their color. As I stated when something moves toward you very, very fast, it looks a little bit bluer, and when it moves away it looks redder. So astronomers can measure speeds by how red a galaxy looks. • How do you find the distance of the moving galaxy? You tell me because I already told you the answer. So what’s my point… • Hubble’s law could determine the age of our universe, about 12-15 billion years old because Ho offers the needed information. • When we know the definite value of the Hubble constant, the future of the universe could be determined. • Hubble’s law has an apparent linearity our universe is uniformly expanding no matter which galaxy we are in, all of the other galaxies are moving away from us earth is not the center of the universe and everything else is receding from us. • Suggests that the galaxies are not moving away through space, they are moving away with space. And again……. Edwin Hubble 1889-1953 HUBBLE’S LAW Hubble’s Law “the farther away a galaxy is from its observer, the faster it appears to be moving away from the observer” v = Hod V = redshift Ho = Hubble’s parameter at observer d = current distance of galaxy from observer By knowing Hubble’s constant, physicists can determine the rate of expansion of the universe and suggest scenarios for its ultimate fate If v=d/t…then t=d/v….so v/d=Ho…so d/v=Ho^-1…..so t=Ho^-1. t is the age of the universe!!! A value for the Hubble constant is 100 km s–1 Mpc–1. Use this value to estimate the age of the Universe in years. (1 Mpc ≈ 3 E19 km, 1 year ≈ 3 E7 s) v = Hod t= Ho^-1 Ho= 100 km s–1 Mpc–1=100000ms1/(3E22m) = 3.33E-17 t=3E16s=1E10years or 10 billion years old Expansion of the universe?? : The evidence • Edwin Hubble observed that the wavelengths of light from distant galaxies change and are longer than they were at their source i.e. that the spectra of most galaxies are red shifted (display a longer wavelength of light) Redshift : a change in the wavelength of light where the wavelength observed is greater than the wavelength emitted. possible causes of redshift: The Movement of the Source; the Doppler effect Stationary galaxy y x Galaxy moving y>x The Expansion of Space • Light will have a longer wavelength, (i.e. will experience red shift) when it passes through space which is expanding: expanding space will “stretch” the light to longer wavelengths • NB: The stretching of space is different to the movement of the source Space-time is stretching therefore wavelength of light increases Gravitationally bound galaxy (not stretching) Gravity • Gravitational effects of large masses cause the redshift or blueshift of light • This is usually a very small effect and therefore is unlikely to be the main reason for the redshift of light in the universe • BLACK HOLES are an exception: as light approaches the event horizon, redshift becomes infinite. Measuring redshift • Measured by comparing the absorption and emission spectra of atoms to those light from of the same atoms from galaxies. • Light shifts from higher to lower frequencies when its wavelength increases and this is apparent when the spectra are compared. The Cause of Redshift in the Universe is accepted as…. X The Movement of the Source; the Doppler effect (insufficient movement) X Gravity (insufficient strength) The Expansion of Space So: The Universe is Expanding! • Extrapolating back in time suggests that the universe was at “the beginning”, a gravitational singularity. present This led to the formulation of the UNIVERSE Singularity=when universe has no size Big Bang Theory The further the way the galaxy, The more space has expanded since the time the light left the galaxy, The more ‘stretched’ the light is, The greater the redshift, The faster the light appears to be moving away from us. The Expansion of the Universe A galaxies motion through space time is ignored when measuring red shift. Distant galaxies motions are insignificant when measuring the expansion of the universe. (rephrased, one component of a galaxies red shift may be due to the galaxies motion in space-time but for the most part the red shift is caused by the expansion of space-time itself) The universe is expanding equally in all directions as far as we can tell. The Expansion of the Universe IMPORTANT! The expansion of the universe is the expansion of space-time - matter itself is not ‘moving’ Planets and galaxies are gravitationally bound. Things which are gravitationally bound do not expand, however the space between these things does. There for the space between galaxies is increasing and this is what causes the redshift. (Balloon does not account for this) Red shift and blue shift • So: spectra of most galaxies are red shifted • The universe is expanding equally in all directions • Some galaxies are actually blue shifted, these are nearby galaxies - their motion in space-time is towards us (they are close enough to feel the gravitational effects of our galaxy the Milky Way), and this overrides the expansion of space-time itself Using Hubble’s Law • Becomes more complex for very distant galaxies. • This is because the light we receive from the galaxies is from the distant past (as light has a finite speed of 3x108 m/s) The centre of the universe • There is no centre of the universe. Anyone at any point in space-time will think they are at the centre of the universe according to their observations. • This is because – and don’t forget! -motion is relative • Each observer regards their galaxy as stationary. Good explanation: • “If we see a galaxy B moving away from us at 10,000 km/s, an alien in galaxy B will see our galaxy A moving away from it at 10,000 km/s in the opposite direction. If there is another galaxy C twice us far away in the same direction as B we will see it moving at 20,000 km/s and the alien will see it moving at 10,000 km/s. • A B C from A 0km/s 10,000km/s 20,000km/s from B 10,000km/s 0km/s 10,000km/s So, from the point of view of the alien at B everything is expanding away from it, which ever direction it looks in, just the same as it does for us” - Usenet Physics FAQ Distance • For very distant galaxies, it is very difficult to measure distance. A method called the Distance ladder is used. • This involves using a series of methods in a stepwise manner to deduce the distance of the galaxy from the Earth, “narrowing down” on the real distance value with each step. Is it or not? NB. There some scientists who believe that it is not the expansion of space-time which is the cause of redshift, but rather something else. As a result, some of these scientists do not accept the expansion of the universe. Although it is widely accepted: maybe the universe isn’t expanding!?? Hubble’s constant • Hubble’s constant is not actually a constant at all, as it was recently discovered that its value varies with the age of the universe • It is now known as ‘Hubble’s Parameter’ • Used to determine the age of the universe 10-20 billion years old EINSTEIN • Einstein’s Theory of General Relativity, 10 years before Hubble discovered Hubble’s Law, predicted that the universe was either expanding or contracting. • Einstein didn’t like this idea EINSTEIN • So he added the cosmological constant so the universe was static in his theory. • After Hubble’s observations that galaxies appear to be receding away from us, Einstein described his inclusion of the cosmological constant as his “Biggest Blunder!” The cosmological constant remains a controversial matter. Now there is increasing talk about the existence of some cosmological constant so maybe Einstein was right after all! This is not yet well understood. There is also increasing support that the universes expansion is not slowing down but rather, accelerating. The Fate of the Universe Acceleration : the continual expansion of the universe – possible if gravitational pull of matter can be overcome Deceleration: if gravity is greater than expansion the expansion of the universe will eventually come to a halt and a reverse big bang will occur - a big crunch The early universe… Temperature was 10^32 K at the Big Bang, no nuclei could exist at this T. As the universe expanded and cooled, light nuclei stabilized 10^-43 s: Forces are unified 10^-35 s: Strong nuclear force separates, 10^27 K Inflation Epoch Begins, it lasts 10^-32 s, the size of the universe increases by a factor of 10^50 10^-12 s: Forces separate, 10^16 K 10^-2 s: Nucleon form, 10^10 K 3 minutes: Nuclei of light atoms form (nucleosynthesis), 10^9 K 3x10^5 years: Atoms form 5x10^5 years: First stars and galaxies form Matter vs Antimatter Click here for film on antimatter, then read the summary below Matter: The stuff we can see Antimatter: We can not see it…so how do we know it is there? In classic mechanics, there is not enough stuff (gravity) to hold galaxies together in clusters, so there has to be matter out there we can’t see holding them together In the early universe there was a slight asymmetry to particles and antiparticles There were slightly more particles, one in 10^9 particle-antiparticle pair (otherwise we wouldn’t be here) When the universe was very hot (over 10^13K) pair annihilation and pair production occurred at the same rate (see particle physics option) As the T dropped below 10^13 K, pair production stopped but annihilation continued What is left is the one extra particle per 10^9 pairs This is the matter we see today Resources • www.wikipedia.com • www.hubblesite.org • http://math.ucr.edu/home/baez/physics/index.ht ml (Usenet Physics FAQ) • http://cif.rochester.edu/~shera/expansion.htm • www.physicsforums.com • www.hyperspace.phyastr.gsu.edu/hbase/astro/hubble.html • Giancoli – Fifth Edition • http://highered.mcgrawhill.com/sites/0072482621/student_view0/i nteractives.html# animations