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General Astronomy Stars and Galaxies

by: Stephan Kuvalis

General Astronomy Stars and Galaxies ASTR 1120

Marketplace > University of Colorado at Boulder > Astronomy > ASTR 1120 > General Astronomy Stars and Galaxies
Stephan Kuvalis

GPA 3.89

Philip Armitage

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Philip Armitage
Class Notes
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This 27 page Class Notes was uploaded by Stephan Kuvalis on Thursday October 29, 2015. The Class Notes belongs to ASTR 1120 at University of Colorado at Boulder taught by Philip Armitage in Fall. Since its upload, it has received 11 views. For similar materials see /class/231953/astr-1120-university-of-colorado-at-boulder in Astronomy at University of Colorado at Boulder.


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Date Created: 10/29/15
ASTR 1120 Section 1 3 credit hours Spring 2006 SUMMARY OF KEY CONCEPTS WEEK 1 This is the first of weekly sulnmaries of what I consider to be the key concepts introduced in class These sumlnaries are not intended to replace your own notes and are by no means inclusive ie anything we ve covered in class or in readings from the textbook is fair game for an exaIn question but they should help when it comes to reviewing the class for the tests Lecture 1 What do we mean by science and what is involved with studying a subject using the scientific method Science seeks the simplest nataral explanation for observed phenomena note natural rather than supernatural and advances via a process of first formulating hypotheses and then testing them against experiments or new observations A theory that makes no testable predictions is not normally regarded as scientific Chapter 3 of the textbook either edition discusses this in detail We then did a superquick overview of the Universe 1 The Sun is a star Stars have a range of masses and mass is the most ilnportant factor controlling how they evolve Stars don t live forever though the lifetjlne of the Sun about 10 billion years is very long and when they die they normally eject gas back into space and leave behind a stellar remnant If the star is massive enough this can be a black hole 2 A galaxy is an island of stars held together by gravity Our own galaxy called the Milky Way has about 100 billion stars On the largest scale of the whole observable Universe the galaxy is the basic building block Galaxies often collide with each other very very slowly over billions of years and like stars they come in different types 3 We observe that ahnost all galaxies are moving away from us Hubble s Law We infer from this that they must have been closer together in the past and that the Universe had a definite beginning This is part of the evidence for the Big Bang We believe based on strong evidence to be discussed later that at early times the distribution of matter in the Universe was much smoother than it is today gravity has subsequently caused the mass to clulnp together forming galaxies stars and planets Lecture 2 textbook Chapter 14 43911 edition or 15 3 d edition Our star The visible surface where the light comes from of the Sun is called the photosphere beyond that is a region of very hot diffuse gas called the corona and beyond that the solar wind The Sun is mostly made of hydrogen gas Sunspots appear on the photosphere and rotate around with the Sun their nu1nber varies on an 11year cycle called the Solar Cycle We know that sunspots are regions where the magnetic field of the Sun is escaping into space but the details of why the Solar Cycle behaves as it does remain unclear magnetism and differential rotation of the Sun are almost certainly involved There are suggestions but no proof that changes in the Solar Cycle in the past may have in uenced clilnate on Earth As well as sunspots we can observe a boiling pattern on the photosphere Technically this is called granulation and it s the surface manifestation of a process called convection in which hot gas rises to the surface loses energy and cools before dropping back down Convection is how the energy generated in the deep interior of the Sun makes its way to the surface In the corona magnetic fields power various phenomena solar ares prominences and coronal mass ejections These can be studied most easily by looking at the Sun in Xrays or UV light that trace the very hot coronal gas 1 million K The Earth is partially protected against coronal mass ejections or Solar storms by our own magnetic field observable effects include the aurora and occasional damage to power grids and satellites in places exposed to the high energy radiation released during Solar storms Astronauts away from low Earth orbit e g en route to Mars would have to be especially careful ASTR 1120 Section 1 3 credit hours Spring 2006 SUMMARY OF KEY CONCEPTS WEEK 5 Lecture 9 textbook Chapter 52 Space and Time We began discussing the Theory of Relativity which was developed in two parts The Special Theory of Relativity describes how space and time which we normally perceive to be completely different concepts are actually closely linked The Special theory does not include gravity The General Theory of Relativity goes much further and explains how mass curves spacetime Special relativity is based on two concepts 1 The laws of physics are the same in all inertial frames an inertial frame is one in which the experimenter is not subject to any external forces such as gravity In particular this means that there is no preferred frame as far as velocity is concerned observers moving at different velocities relative to each other are in equally valid positions 2 The speed of light is the same constant when measured in any inertial frame regardless of how that frame is moving relative to the source of the light This is very weird but it s an experimental fact We showed that these concepts imply some strange behavior moving clocks appear to run slow we actually derived this one and moving objects appear to be length contracted The effects are only significant once we are moving close to the speed of light hence we don t notice them in everyday life We outlined the twin paradox suppose one twin ies in a superadvanced spaceship to a distant star at close to the speed of light and then returns to Earth Because of time dilation the twin on the spaceship can reach the distant star even if it s say 100 light years away since the clock on the spaceship runs slow a journey that takes at least 100 years as seen from Earth requires less tjine as seen by the astronaut But when the twin returns to Earth there s a big mismatch in the amount of tjine that has elapsed on Earth as compared to on the spaceship and the twin on Earth is probably long dead Lecture 10 textbook Chapter S3 Spacetime and Gravity The central physical idea there s also a lot of mathematics behind the General Theory of Relativity is the equivalence principle the effects of gravity and acceleration are exactly equivalent indistinguishable locally locally here means that we restrict ourselves to experiments that could be done in a small box on much larger scales we can measure effects such as tides which are characteristic of gravity not acceleration The equivalence principle is in some ways obvious in an elevator accelerating upward you certainly feel heavier just as if gravity had become stronger but it s also profound since gravity and acceleration appear to be completely different concepts One of the most immediate consequences is that gravity de ects light by an amount that turns out to be exactly twice what Newtonian theory predicts This light bending was detected shortly after the publication of the General Theory of Relativity and has subsequently been measured very accurately lt agrees with the prediction very well More recently exaInples of gravitational lensing have been observed which are another manifestation of the same effect ASTR 1120 Section 1 3 credit hours Spring 2006 SUMMARY OF KEY CONCEPTS WEEK 3 Lecture 5 textbook Chapter 14 43911 edition or 15 3 d edition Our star Concept of gravitational equilibrium how does the Sun and other stars support itself stably against the force of gravity which otherwise would cause a rapid collapse We noted two things first that at each radius in the Sun there is a balance between the inward force of gravity and an outward pressure gradient high pressure in the center low pressure at the surface The high pressure comes about because an ordinary gas pressure increases with temperature and density and the center of the Sun is very hot Second the core of a star acts as a kind of thermostat to prevent a runaway nuclear reaction if the rate of nuclear reactions increases the core expands and counterintuitively cools off restoring the balance Photons produced in the Solar core leak out of the Sun very slowly over thousands of years or longer because of multiple collisions along the way Nuclear reactions in the core also produce subatomic particles called neutrinos which have the property of interacting extremely weakly with ordinary matter They can easily escape the Sun without being scattered or absorbed This allows us a window into the Solar core ip side is that it s very hard to capture the neutrinos in experiments on Earth and detect them The Solar neutrino problem was an apparently large discrepancy between the predicted number of Solar neutrinos from models of the Sun and the number actually measured in experiments mostly situated deep underground The eventual resolution of the problem was that electron neutrinos the type produced in the core can change into other avors muon and tau neutrinos on their way to Earth Early experiments could only detect the electron neutrino leading to the apparent shortfall Lecture 6 textbook Chapter 5 43911 edition or Chapter 6 3 d edition on Light We surmnarized the different sources of information we can glean about the Universe light images and spectra a spectrum divides light into its constituent colors neutrinos dust and rocks from the Solar System and in the future gravitational waves Light is much the most important Gammarays Xrays ultraviolet visible light infrared and radio waves are all the same basic phenomenon called electromagnetic radiation Electromagnetic radiation has simultaneously and weirdly this is quantum mechanics both particle and wave properties Viewed as a particle the basic unit is the photon a single particle of light Photons have specific energies and in a vacuum all travel at the same speed the speed of light c 300000 km s A bright source just emits more photons than a dim source of the same color Viewed as a wave electromagnetic radiation has 4 properties the speed c the same as for photons the wavelength denoted by the greek letter A lambda this is the distance between successive peaks in the wave the freaaeneyf how many peaks pass by each second measured in units of cycles per second or Hertz and the amplitude which is a measure of the brightness of the wave An ilnportant relation exists between the wavelength frequency and speed of a wave WAVELENGTH x FREQUENCY SPEED or M c You need to understand how this works short wavelength electromagnetic radiation has a high frequency whereas long wavelengths mean low frequency this is simple because all electromagnetic radiation has the same speed ASTR 1120 Section 1 3 credit hours Spring 2006 SUMMARY OF KEY CONCEPTS WEEK 2 Lecture 3 textbook 4353 43911 edition or 4243 3 d edition We discussed the concept of forces a force being something that changes the velocity of an object The four fundamental forces are gravity electromagnetism the weak nuclear force and the strong nuclear force We noted that gravity is intrinsically the weakest it dominates over large scales in the Universe because a electric charges come in positive and negative flavors which tend to cancel out and b because the nuclear forces are very short range We then reviewed the structure of matter which is made up of atoms themselves composed of a nucleus containing protons and neutrons and electrons The nucleus is small 10 15 m and contains almost all the mass while the electrons orbit further out 1010 m and occupy most of the volume Chemical reactions involve changes in the distribution of electrons whereas nuclear reactions involve the nucleus We noted that a chemical element is defined by the number of protons in the nucleus for a neutral atom this equals the number of electrons Different isotopes of an element differ in having different numbers of neutrons in their nuclei eg most carbon nuclei have 6 protons and 6 neutrons but the carbon 13 nucleus has 6 protons and 7 neutrons Nuclear fission involves splitting heavy nuclei e g uranium into lighter pieces Nuclear fusion involves joining together light nuclei eg hydrogen to make heavier ones Fusion is hard because electric forces between the protons tend to repel nuclei we need high temperatures large average kinetic energy or velocity to create collisions violent enough for fusion to occur Lecture 4 textbook Chapter 14 43911 edition or 15 3 d edition Our star We defined the units of energy joules and power watts Energy is conserved while power is the rate of energy ow We showed that neither chemical reactions eg stuff burning nor gravitational contraction could yield an energy reservoir large enough to keep the Sun shining for the billions of years implied by the ages of rocks and meteorites though gravitational contraction is an important process that explains how gas collapsing to form stars gets hot in the first place Einstein s special theory of relativity revealed that mass was a form of energy with the conversion between the two expressed via the equation E mcz Because the speed of light is so large mass is a very concentrated form of energy but under normal circumstances there is no way to liberate that energy When light nuclei fuse together however the product nucleus has less mass than the starting nuclei with the balance being released as energy Although fusing hydrogen into helium releases only 07 of the rest mass of the hydrogen as energy this is still more than enough to power the Sun for billions of years The actual fusion process going on in the Sun is called the protonproton chain a moderately involved sequence of reactions that result in 4 protons eventually forming a single helium nucleus with two protons and two neutrons The energy liberated takes a long time thousands of years or more to leak out to the surface as the photons get scattered multiple times along the way BUT also nonluminous matter which we can t see directly Example that we know exists black holes that formed from stellar collapse velocity v For a circular orbit 2 GM v I where G is called the gravitational constant Numerically G 667 gtlt1011 m3 kg x s2 radius of orbit r gives the mass M interior to the orbit of the star r26x102 m v220 kms 2 26x102O m x 22 x105 ms 667 x10 11 m3 kg 32 2 gtlt1041 kg Rotation curve for gravity I Mercury Venus Earth Mars Jupiter Satu Ur Nepmne Pluto 1O 20 3O 40 5 mean distance from Sun AU Each star allows us to calculate the mass interior r to the orbit of that particular star Measures the mass distribution of the Milky Way l39llIEIIEI E1 IEIFIISIIJD39FILJ39Et ral ia39n IIIEEI39atnr39lg F39hcltlj EILJ ElEl39u39id l laLin SplmICalavaCC 4414 Inner galaxy enclosed mass rises so does the velocity Expected rotation curve of a galaxy if all the mass is due to observed stars and gas ASTR 1120 Spring 2006 Rotation curve of the Milk Wa ROTATION CURVE OF MILKY WAY TO ZRquot Instead observe a flat rotation curve velocity stays the same as we move further away from the center ASTR 1 120 Spring 2006 What does a flat rotation curve imply Go back to the formula More and more mass as we go to larger and larger distances from the center ASTR 1 120 Spring 2006 Flat rotation curve shows there is more matter out beyond the apparent edge of spiral galaxies Extra mass is called DARK MATTER infer existence from gravitational effect on ordinary stars and gas cannot be luminous or we would see it Use the Doppler shift of spectral lines emitted by hydrogen gas to infer velocity of rotation same idea as for measuring redshifts blueshifted A redshifted a J L C lt buer wavelength redder vi 2006 Pearson Educatian Inc publishing as Addison Wesley ASTR 1120 Spring 2006 dark matter Dark matter is more extended than the luminous matter Picture of the Galaxy very large massive m of dark matter Relatively insignificant sprinkling of ordinary stars and gas in the middle 2006 Pearson Education Inc publishing as Addison Wesley AS TR 1120 Spring 2006 existence of dark matter perhaps gravity does not behave in the same way as in the Solar System on very large scales 1 Very low mass stars Very low mass stars or brown dwarfs emit very little light for their mass Dim red stars and brown dwarfs failed stars 3 White dwarfs or neutron stars Perhaps the first stars in the Galaxy were all massive they died long ago leaving only stellar remnants which are now cool and hard to see 4 Black holes either formed from the collapse of very massive stars or primordial formed in the early Universe Called NONBARYONIC DARK MATTER V l39 3 27 V g l H H v all In the Solar System the density of ordinary matter vastly exceeds that of dark matter Very rarely the dark matter might collide with ordinary nuclei this might be detectable hoping to make the first direct detection of the dark matter


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