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University of British Columbia
ASTR 311
Jeremy Heyl


 17: The Stars: 
 Trigonometric Parallax: The distances to the nearest stars can be measured by trigonometric ▯ ▯ parallax.
 ▯ -a star with a parallax of 1 arc second is 1 parsec or 3.3 lightyears away from earth 
 -stars have real motion through space as well as apparent motion as the earth moves through its orbit 
 Proper motion: its true motion across the sky, and is a measure of the stars velocity perpendicular to our line of sight, 
 radial velocity: along the line of sight, measured by the doppler shift of the spectral lines emitted by the star 
 apparent brightness: the rate at which energy from the star reaches a detector. apparent brightness falls off as the inverse square of the distance
 -astronomers use magnitude scale to express and compare stellar brightness 
 -the greater the magnitude the fainter the star
 ▯ - a difference of 5 magnitudes corresponds to a factor of 100 in brightness
 apparent magnitude: is a measure of the apparent brightness from our view 
 absolute magnitude: is the apparent magnitude it would have if placed at a standard distance of 10 pc from the viewer. It is a measure of the luminosity of a star 
 Star Temperature: measured by measuring their brightnesses through two or more optical filters and then fitting a black-body curve to the results
 Photometry: the measurement of the amount of starlight received through each member of a set of filters 
 spectroscopic: observations provide an accurate way to tell stellar temperatures and stellar composition 
 -classify stars according to their absorption lines in their spectra
 -the standard stellar spectral classes are in order of decreasing temperature:
 ▯ O, B, A, F, G, K, M
 -only a few stars are large and close enough that their radii can be measured directly 
 -most star size is estimated through radius-luminosity-temperature relationship 
 Dwarfs: stars smaller than or comparable in size to the sun 
 Giants: stars up to 100 times larger than the sun 
 Supergiants: stars that are more than 100 times larger than the sun
 -the sun is considered a ‘normal star’ but their are two other classes
 Red Giants: are large, cool, and luminous
 Red Supergiants: larger, cool, and luminous
 White dwarfs: small, hot, faint 
 HR-Diagram: a plot of stellar luminosities versus stellar spectral classes (or temperatures), also known as a colour magnitude diagram
 -about 90% of all stars plotted on the diagram lie in the main sequence, which stretches from 
 hot, bright blue supergiants and blue giants through intermediate stars like the sun, to cool, faint red dwarfs 
 -most main sequence stars are red dwarfs, blue supergiants are rare 
 -about 9% of stars fall in the white dwarf region
 -about 1% of stars are red giants 
 Spectroscopic parallax: if a star is known to be on the main sequence, measuring its spectral type allows its luminosity to be measured and therefore its distance. This is value for stars up to several thousand parsecs from earth
 luminosity class: of a star allows astronomers to distinguish main sequence stars from giants Page 1 of 29 and super giants of the same spectral type, because the larger it is the more luminous it is 
 Binary Star Systems: most stars aren't isolated but orbit other stars. 
 ▯ - visual binary: when both stars can be seen and their orbit is charter 
 ▯ -spectroscopic binary: the stars cant be resolved but their orbital motion can be detected 
 ▯ spectroscopically. 
 -studying binary star systems allow stellar masses to be measured 
 eclipsing binary: the orbit is oriented in such a way that one star passes in front of the other and dims the light we receive periodically. 
 Stars Mass: determines the size, temperature, and brightness 
 -High mass stars burn their fuel rapidly and have shorter lives than the sun
 -low mass stars consume their fuel slowly and may live on the main sequence for trillions of years
 -hot blue giants are much more massive than the sun 
 -cool red dwarfs are less massive 
 18: The Interstellar Medium: 
 Interstellar medium: occupies the space among the stars, is made of cold gas and atomic or molecular hydrogen and helium, and dust grains 
 Interstellar dust: highly effective at blocking our view of distant stars, even though the density of the medium is low. thought to be composed of silicates, graphite, iron, and dirty ice
 -dust particles are apparently elongated or rod like
 -interstellar medium is distributed patchily 
 extinction: the general diminution of starlight by dust 
 -dust preferentially absorbs short wavelength radiation, leading to a distinct reddening of light passing through interstellar clouds 
 Polarization: of starlight provides a means of studying these particles 
 Nebula: is a fuzzy bright or dark batch on the sky, it is an interstellar of dust, hydrogen, helium, and other ionized gasses. Nebula is kind of a blanket term for a large cloud of matter in space 
 Emission nebulae: are extended clouds of hot, glowing interstellar gas. They are associated with star formation and result when hot O- B- type stars heat and ionize their surroundings 
 -studies of emission lines produced by excited nebular atoms allow us to measure the properties of nebulae 
 -they are often crossed by dark dust lanes, which are part of the larger cloud from which they formed 
 Dark dust clouds: are cold irregularly shaped regions in the interstellar medium whose constituent dust diminishes or completely obscures light from background stars 
 Molecular clouds: found a lot in the interstellar medium, are cold and dark, dense enough that much of the gas exists in molecular form. Dust in these colds probably protects these molecules and act as catalysts to help them form. they are usually found in groups, forming enormous molecular cloud complexes, millions of times more massive than the sun 
 -are observed through the radio radiation emitted by the molecules they contain
 12-centimeter radiation: the radio spectrum length at which you can observe cold, dark regions of interstellar space containing atomic hydrogen, produced when the electron in an atom of hydrogen reverses its spin, changing its energy slightly in the process 
 -radio waves are not appreciable absorbed by the interstellar medium, so astronomers making observations at these wavelengths can see further distances 
 Hydrogen: the most common constituent of molecular clouds, but molecular hydrogen is hard to observe. 
 -astronomers study these clouds by observing other tracer molecules that are less common, but Page 2 of 29 easier to detect 
 -many complex molecules have been identified in these regions. 
 19: Star Formation: 
 -stars form when an interstellar cloud collapses under its own gravity and breaks up into pieces comparable in mass to our sun 
 -a cold interstellar cloud may fragment into many smaller clumps of matter, from which stars eventually form 
 -the evolution of a contracting cloud can be represented as an evolutionary track on the HR diagram 
 -as a collapsing pre stellar fragment heats up and becomes dense, it eventually becomes a 
 protostar: a warm, very luminous object that emits infrared radiation. eventually a protostar’s central temperature becomes hot enough for hydrogen fusion to begin, and it becomes a star. 
 -For a sun like star, the whole formation process takes 50 million years 
 -massive stars pass through similar stages, but faster 
 -less massive stars take longer to form 
 Zero-age main sequence: is the region of the HR diagram where stars lie when the formation process is over. 
 -Mass is key in determining a stars characteristics and lifespan 
 Bigger stars: gave the shortest formation times and main sequence lifetimes. 
 smallest stars: Some fragments are so low mass they never reach the point of nuclear ignition and become brown dwarfs 
 -objects predicted by the theory of star formation have been observed 
 ▯ -dark interstellar regions near emission nebulae often show evidence of cloud ▯ ▯ ▯ fragmentation and protostars 
 -radio telescopes are used to study early phases of cloud contraction and fragmentation
 -infrared observations allow us to see later stages of the process 
 -many well known emission nebulae, lit by O and B type stars, are partially engulfed by molecular clouds, portions of which are fragmenting and contracting, with smaller sites forming protostars 
 protostellar winds: produced by protostars. These winds meet less resistance in the distance perpendicular to the stars protostellar disk, expel jets of matter in the directions of protostars poles in a bipolar flow 
 -protostellar winds gradually destroy the disk, and eventually the wind flows away from the star in all directions 
 Shock waves: are produced as young hot stars ionize their surrounding gas, forming emission nebulae. they can compress other clouds and trigger more star formation, possibly producing chain reactions of star formation in molecular cloud complexes 
 Star cluster: a single collapsing and fragmenting cloud can give rise to hundreds of thousands of stars, forming a cluster. 
 -the formation of the most massive stars may play a role in suppressing the further formation of stars from lower mass cluster members 
 open clusters: have a few hundred to a thousand stars, are found mostly in the plane of the milky way. contain bright blue stars, indicating they formed recently 
 Globular clusters: found mainly away from the milky way plane and may contain millions of stars. they have no main sequence stars that are larger than the sun, indicating that they formed long ago
 -infrared observations revealed young star clusters or associations in several emission nebulae
 -eventually clusters break up into individual stars, but the process may take hundreds of millions Page 3 of 29 of years 
 20: Stellar evolution: 
 Core Hydrogen Burning Phase: where stars spend most of their lives on the main sequence, stably fusing hydrogen into helium at their centres. ▯ -Stars leave the main sequence when the hydrogen in their cores is exhausted 
 -the sun is halfway through its main sequence life and will reach this stage in 5 billion years 
 Hydrogen-shell burning stage:when the central nuclear fires in the interior of a star like sun cease, the helium in the stars core is too cool to fuse into anything heavier. with no internal energy source, the helium core cannot support itself against its gravity and it begins to shrink. ▯ - it has a non burning helium core and is surrounded by a layer of burning ▯ ▯ n e g o r d y h ▯ 
 ▯ -the energy released by the contracting helium core heats the hydrogen burning shell, 
 ▯ increasing the nuclear reaction rates on the surface. the star becomes brighter, while 
 e h t ▯ envelope expands and cools 
 -a sun like star moves off the main sequence first along the sub giant branch , then vertically up the red giant branch 
 -eventually the contracting core reaches the point where helium starts to fuse into hydrogen
 electron degeneracy pressure: at this point of helium burning the electrons in the core are tiny hard spheres that, once brought into contact, resist being compressed further. this pressure makes the core unable to react to the new energy source, and helium begins burning violently in a Helium Flash:
 the flash expands the core and reduces he stars luminosity, sending the star into the horizontal branch: the star now has a core of burning helium surrounded by a shell of burning hydrogen
 -as the helium burns in the core, it forms an inner core of non burning carbon
 -the carbon core shrinks and heats the overlying burning layers, and the star once again becomes a red giant, even more luminous than before 
 -It reenters the red-giant region of the HR diagram along the asymptotic-giant branch
 -the core of a low mass star never becomes hot enough to fuse carbon, and continues to ascend the asymptotic giant branch until its envelope is ejected into space as a planetary nebula 
 ▯ -at this point the core is visible as a hot, faint and extremely dense white dwarf
 ▯ -the planetary nebulae diffuses into space, carrying helium and some carbon into the 
 ▯ interstellar medium 
 ▯ -when the white dwarf cools and fades, it becomes a cold black dwarf 
 High Mass Stars: evolve more rapidly because larger mass results in a higher central temperature. they never initiate a helium flash, and they attain central temperatures high enough to fuse carbon 
 -these stars become red supergiants forming heavier and heavier elements in their cores at an increasingly fast pace, and eventually die explosively 
 How to test: theory of stellar evolution can be tested by observing star clusters, in which stars form at the same time,
 ▯ -as time passes, the most massive ones leave the main sequence first, then middle 
 ▯ mass, then smallest 
 ▯ -at any instant, no stars with masses above the cluster’s main sequence turnoff mass 
 ▯ remain on the main sequence 
 ▯ -stars below this mass have not yet evolved into giants and still lie on the main sequence 
 -by comparing main sequence turnoff mass with theoretical predictions, we can measure the age of the cluster
 Page 4 of 29 -stars in binary systems can evolve differently from isolated stars because of their interactions 
 -each star is surrounded by a teardrop shaped Roche lobe, and all matter within this region belongs to the star 
 -as binary stars evolve into the giant phase, it may over flow its roche lobe, and gas flows from the giant onto its companion. Stellar evolution in binaries can produce states that are not achievable in single stars
 21: Stellar Explosions: 
 Nova: a star that suddenly increases greatly in brightness, then slowly fades back to its normal state over a period of months. it is the result of a white dwarf in a binary system drawing Hydrogen rich material from its companion
 -the gas spirals inward in an accretion disk and builds up on the surface of the white dwarf, eventually becoming hot and sense enough for Hydrogen to burn explosively, temporarily causing a large increase in the dwarfs luminosity 
 Core collapsing supernovae: stars larger than 8 solar masses form heavier and heavier elements in their cores, at a more and more rapid pace. As they do, their core forms a layered structure of burning shells of successively heavier elements 
 -the process stops at Iron, whose nuclei can neither be fused together or split to produce energy 
 -the iron core grows in mass, and eventually is unable to support itself against its gravity and starts to collapse 
 -at the high temperatures produced during the collapse, Iron nuclei are broken down into protons and neutrons, the protons combine with electrons to form neutrinos
 -eventually when the core becomes so dense that the neutrons are brought int contact with one another, the collapse stops and the core rebounds, sending a violent shock wave out through the rest of the star, and it explodes 
 Supernovae: are classified in two types 
 ▯ Type 1: are hydrogen poor and have a light curve similar in shape to a nova. Occurs 
 ▯ when a carbon-oxygen white dwarf in a binary system gains mass, collapses, and 
 ▯ explodes as its carbon ignites, this is called a carbon-detonation super novae 
 ▯ Type 2: are hydrogen rich and have a characteristic plateau in the light curve a 
 ▯ few months after maximum. This type 2 supernova is a core collapse supernova 
 -theory predicts that super nova visible from earth should occur within our galaxy once a century, but none have been observed in the last 400 years 
 supernova remnant: evidence of past super novae visible to us, a shell of exploded debris surrounding the site of an explosion and expanding into space at high speed 
 Stellar nucleosynthesis: all heavier elements than helium are formed this way. the production of new elements by nuclear reactions in the cores of evolved stars. 
 helium capture: elements heavier than carbon form this way rather than by the fusion of more massive nuclei
 -at high enough temperatures, photo disintegration breaks apart some heavy nuclei, providing HE4 nuclei for the synthesis of even more massive elements, up to iron 
 Neutron Capture: how elements beyond iron form, in the cores of evolved stars. During supernova, rapid neutron capture occurs producing the heaviest nuclei of all 
 -comparisons between theoretical predictions of element production and observations of element abundances in stars and supernovae provide strong support for the theory of nucleosynthesis 
 -the process of star formation, evolution, and expansion form a cycle that constantly enriches the interstellar medium with heavy elements and sows the seeds for new generations of stars
 Page 5 of 29 -without elements produced in supernovae life on earth would be impossible
 22: Neutron Stars and Black Holes: 
 Remnant: a core collapsed supernova may leave behind a remnant taking the form of an ultra compressed ball of material called a neutron star 
 Neutron Stars: created when giant stars die in supernovas and their cores collapse, with the protons and electrons essentially melting into each other to form neutrons. are extremely dense and very hot at formation, sternly magnetized and rapidly rotating. They cool down and lose much of their magnetism and they slow and age 
 Lighthouse Model: according to this model, neutron stars, because they are magnetized and rotating, send regular bursts of electromagnetic energy into space. The beams are produced by charged particles confined by the strong magnetic fields 
 Pulsar: what we call them when we can see the beams from earth. the pulse period is the rotation period of the neutron star. because the pulse energy is beamed into space and because neutron stars slow down as they radiate off their energy, not all neutron stars are seen as pulsars 
 Binary Neutron Star: a neutron star in a close binary can draw matter from its companion, forming an accretion disk 
 accretion disk: a structure, often a circumstellar disk, formed by diffused material in an orbital motion around a massive central body. Gravity causes the disk to spiral inward towards the central body. 
 ▯ -the material in the disk heats up before it reaches the neutron star, making the disk a 
 ▯ strong source of X-Rays. 
 ▯ -as gas builds up on the stars surface, the star eventually becomes hot enough to fuse 
 ▯ hydrogen, when the hydrogen burning stars, it does so explosively, and an X-ray 
 ▯ burster results 
 -the rapid rotation of the inner accretion disk causes the neutron star to spin faster as 
 new gas arrives at its surface 
 -the eventual result is a very rapidly rotating neutron star, called a millisecond pulsar 
 -many millisecond pulsars are found in the hearts of old globular clusters, haven't formed recently so they must have been spun up by interactions with other stars 
 -some pulsars are orbited by planet sized objects 
 Gamma Ray Bursts: very energetic flashes of gamma rays observed about once per day, distributed uniformly over the entire sky, ones we have measured are far away from us and must therefore be extremely luminous 
 -the leading theoretical model for these explosions think the violent merger of neutron stars in a distant binary system or the re-collapse and violent explosion following a failed supernovae in a very massive star 
 Theory of Relativity: deals with the behaviour of particles moving at speeds comparable to the speed of light. at low velocities it agrees with newton, but different predictions for high speed motion
 General Theory of Relativity: replacement for newtonian gravity, describes gravity in terms of the warping or bending of the space-time by the presence of mass, more mass means a greater warping. 
 -all particles, including photons, respond to that warping by moving along curved paths 
 Black Holes: formed from collapsing neutron stars. the upper limit on the mass of a neutron star is 3 solar masses, beyond this, the star cannot support itself and collapses under its own gravity, forming a black hole, a region of space from which nothing can escape 
 -very massive stars, after exploding into supernovae, form black holes rather than neutron stars 
 Page 6 of 29 -conditions in and near black holes can only be described using general relativity
 Schwarzchild Radius: the radius at which the escape speed from a collapsing star equals the speed of light 
 Event horizon: the surface of an imaginary sphere of radius equal to the schwarzchild radius, surrounding a black hole is called the event horizon. Point at which the gravitational pull becomes so great that escape is impossible even for light 
 Gravitational redshift: what light that is falling into a black hole would be subject to redshift, as the light climbed out of the holes intense gravitational field 
 -at the same time a clock on a spaceship would show time dilation- the clock would slow down as it approached the event horizon, the observer would never see the ship reach the surface of the hole 
 -once within the event horizon, no known force can prevent a collapsing star from contracting all the way to a point like singularity: ▯ ▯ ▯ - the point at which both the density and the gravitational field of the star becomes infinite 
 ▯ -singularities are where the known laws of physics breaks down 
 -once matter falls into a black hole, it can no longer communicate with the outside, but on its way in it can form an accretion disk and emit x-rays, just as the case of a neutron star
 -the best candidates for black holes are binary systems in which one component is a compact x- ray source.
 Cygnus X-1, a well studied X-ray source in the constellation Cygnus is a long standing black hole candidate 
 -studies of orbital motion imply that some binaries contain compact objects too massive to be neutron stars, leaving black holes as the only alternative 
 Supermassive black holes: in the centres of most galaxies, which we have strong evidence for 
 23: The Milky Way: 
 -one of 100 billion galaxies in the observable universe, made up of gas, stars, dust, neutron stars, black holes, held together by its own gravity 
 -our sun lies in the Galactic disk, the large flat region that has most of the stars and interstellar matter 
 -to try and map out the milky way we have to study other more easily observed systems 
 -Andromeda is one of these, 2.5 million light years away 
 -structure of a galaxy: Galactic disk, Galactic bulge in the middle, The disk and bulge are embedded in a spherical ball of faint old stars known as the Galactic Halo , dozens of kpc across
 ▯ - Galactic Disk: thin sheet of young stars, gas and dust that cuts through the centre of 
 ▯ the halo
 ▯ - Halo: contains no gas or dust, the opposite of the disk and bulge. 
 -there is difference in composition and appearance of stars in each: 
 ▯ - bulge and Halo stars are redder than those on the disk 
 ▯ -all bright blue stars in our sky, and young open star clusters and star forming regions ▯ ▯ are part of the disk, 
 ▯ -cooler, older, redder stars are uniformly distributed through all three 
 ▯ - disk looks blue because of O- and B- supergiants ▯
 ▯ -the disk is where ongoing star formation occurs 
 ▯ - no new stars form in the halo, hence lack of dust 
 ▯ - galactic bulge has intense gas density and is vigorously creating new stars 
 -Hershel tried to measure the shape of Galaxy by counting how many stars he saw in each direction, assuming they were of the same brightness. Thought it was flat, disk shaped and the Page 7 of 29 sun was near its centre, others estimated the dimensions of his model at 10 KPC in diameter by 2. This was wrong, were only observing at visible wavelengths and not considering the absorption of light by interstellar gas
 Population I stars: young disk stars 
 Population II stars: old halo stars 
 -globular clusters: where studies on the structure of the galaxy looked. They are tightly bound swarms of old reddish stars, 150 known in our galaxy. Most are between 10-12 billion years old 
 -Spiral galaxies: also looked at when mapping out galactic structure, they are comparable in size to our own. But they didn't know how far away either of these were so they couldn't fully understand them 
 Variable stars: a by product of the cataloging of stars in the 20th cent led to the study of these stars whose luminosity changes with time, some erratic others regularly. Only a small amount of stars are variable stars. 
 -eclipsing binary stars are variable stars because of their change in brightness from ▯▯ ▯ rotation, called Cataclysmic variables because of their more violent consequences in ▯ ▯ novae 
 -other stars have variability as a basic trait, called. these include 
 -pulsating variable stars: vary cyclically in luminosity, ex. RR Lyrae and Cepheid Variables.
 ▯ -both these variable stars are easily recognized by the shapes of their light curves. 
 ▯ - RR Lyrae pulse with only small differences in period, looks wavy periods are usually 
 ▯ 0.5-1 days . Many are found in globular clusters 
 ▯ - Cepheid variables have a sawtooth pattern, but they can have different pulsation 
 ▯ periods 
 -they pulsate because they are in an unbalanced state causing the opacity (thickness of stars interior) to expand, causing the radiation to swell up the star and diminish in luminosity, then fall 
 then repeat. These conditions don't happen in the main sequence stars but on the instability strip 
 ▯ - High mass stars on the instability strip become Cepheid 
 ▯ - Low mass stars on the instability strip become RR Lyrae, form the horizontal branch 
 ▯ lying across the 
 -these variable stars are passing through a brief million year phase of instability as a natural part of stellar evolution 
 Cosmic Distance: Once we spot a RR Lyrae or Cepheid, we can infer its luminosity, and we can measure distance from it using inverse square law: Apparent brightness = Luminosity/ distance squared 
 ▯ -infer its luminosity by looking at the horizontal branch of the HR Diagram, all same L
 ▯ -for Cepheid measure using the period luminosity relationship, long period - large ▯ ▯ luminosity, Cepheids help us see farther out to near galaxies, 
 the Galactic globular cluster system: most clusters lie at great distances from the sun, and by measuring the direction and distance of each cluster you can determine the 3D distribution of the clusters in space. They map out the galactic halo for us. This maps out for us a large spherical volume of space 30 kpc across, centre of distribution being nowhere near the sun. 
 -optical vision can only see a small portion of galactic disk, most knowledge comes from radio observations, particularly 21 cm radio emission line produced by atomic hydrogen 
 -centre of gas distribution is the same centre of the globular cluster centre 
 -the thickness of the Galactic disk depends on the kinds of objects measured 
 ▯ - Young stars are more tightly confined to the plane than Sun like stars, 
 ▯ - sun like stars are more tightly confined than older K- and M- type dwarfs 
 ▯ -this is because stars form in clouds close to the disk and drift out as they age
 Page 8 of 29 ▯ - as stars age, their abundance above and below the plane of the disk slowly increases 
 -the thick disk part, about 2-3 KPC in thickness, are the medium between ancient stars in the galactic halo and the younger disk 
 -the central bulge is about 6 kpc across and 4 kpc thick. actually football shaped, believe the middle of our glaxy may have an elongated ‘bar-like’ appearance and that our galaxy would then be a barred spiral type. 
 -Motion of galaxies members is ordered on a larger scale 
 ▯ -Blueshifted: motion of radiation in the disk from stars and gas clouds in the upper ▯ ▯ right and lower left quadrants, meaning they are approaching the sun 
 ▯ -red shifted: Motion of the radiation from stars and gas in the upper left/ lower right, ▯ ▯ receding from the sun 
 -tells us the disk is rotating around the galactic centre, orbits determined by galaxies gravitational pull. the disk rotates differentially, not as a solid object 
 -Galactic year: how long it takes for the sun to complete one interval around the galactic centre, 225 million years at 220 km/s 
 -this motion only applies to the disk, stars in the bilge and halo aren't so orderly 
 ▯ - globular clusters and faint red stars in the halo and bulge orbit at random, filling a 3d ▯ ▯ volume not a 2 d disk 
 ▯ -the motion of disk and red stars don't collide because of the extreme distances between 
 ▯ stars in the disk, the scale is incomprehensible 
 Spiral Arms: 
 -spiral arms consisted of interstellar gas, dust, young stellar and pre stellar objects like emission nebulae, O- & B- stars, and recently formed open clusters. 
 -conclusion is spiral arms are part of the galactic disk where star formation occurs, objects are so bright so the spiral arms are visible 
 -how do spiral arms keep their form with differential rotation? 
 -spiral density waves : one explanation, that they are coiled waves of gas compression moving through the disk squeezing gas clouds and triggering formation 
 -this wave pattern wouldn't be tied to a part of the disk and avoids the differential rotation problem, it is a pattern moving through the disk not great masses of matter moving together, like sound waves 
 the spiral wave pattern moves more slowly than stars and gas 
 Dust lanes mark regions of high density gas. 
 -Bright stars like O and B blue giants only live for short times so they are never found outside of the arms because they die close to where they are born 
 Self propagating star formation: Formation of stars drives the waves, not vice versa. another theory 
 ▯ -the formation of massive stars creates emission nebulae and supernovae that send 
 ▯ shock waves through surrounding gas, triggering new star formation
 ▯ -opposing theory to spiral density waves 
 ▯ -the problem with this is they can only make parts of spirals and not galaxy wide spiral ▯ ▯ arms
 Mass of Galaxy: measured by studying motions of gas clouds and stars n the disk. 
 ▯ total solar mass = orbital size cubed/ orbital period squared 
 ▯ - suns distance to centre is 8 kpc and period is 225 million years, making the mass 90 ▯ ▯ billion times the mass of the sun, but what does this mass indicate
 ▯ -the galaxies matter is not concentrated in the centre, but all over a large volume of ▯ ▯ space, some outside the orbit of the sun, so therefore the equation only gives us the ▯ Page 9 of 29 ▯ mass within 8 kpc of the galactic bulge / the suns orbit 
 to get the full mass we have to measure the orbital motion of stars at a greater distance through radio observations of gas, we have determined our galaxy’s rotation rate at various distances from the centre
 Galactic Rotation curve: the plot of rotation Galaxy Formation. plots the orbital speeds of visible stars or gas in that galaxy versus their radial distance from centre 
 ▯ -the true rotation curve looks different than expected, instead of falling off at larger ▯ ▯ distances it rises slightly, implying the amount of mass continues to grow beyond the ▯ ▯ orbit of the sun, out to a distance of 40-50 kpc 
 -this means at least twice the amount of mass lies outside of the luminous part of our galaxy 
 -astronomers refer to the luminous portion- everything in the globular clusters and spiral arms, as the tip of the galactic iceberg
 -a dark halo surrounds it, which extends further than the 15 kpc radius originally assigned to our galaxy, assuming it is mostly invisible dark matter 
 -even in the visible part there is substantial dark matter, roughly 2/3rds 
 -dark matter escapes detection at all wavelengths, only known through its gravitational pull 
 ▯ -a small amount could be stellar sized black holes 
 ▯ -other ‘stellar’ contenders could be brown dwarfs: low mass pre stellar objects, white ▯ ▯ dwarfs and faint, low mass red dwarfs. called Massive Compact Halo Object (MACHOS), ▯ could exist in great number but hard to see cause they are so faint 
 ▯ -however hubble images argue against the last three of Macho, and found that stars with ▯ a very low mass are unexpectedly rare 
 -other suggestion is dark matter is made of exotic subatomic particles that pervade the entire universe 
 -these would have to have mass and also not interact with normal matter 
 -dubbed Weakly interacting Massive Particles WIMPS 
 -gravitational lensing: how we could detect unseen matter by using Einsteins observations of light bending due to gravity, if a faint object passes in front of a star the light should deflect and make the star seem brighter than usual. Very rare occurrence but we are observing enough to be able to see this happen and estimate the amount of stellar dark matter in the galactic halo 
 -most likely that more than one type of dark matter exists 
 Galactic Centre: 
 -theory that the centre is densely packed with billions of stars, higher density closer to centre 
 -have to use other wavelengths to peer in, find a place so dense stellar collision is most likely frequent 
 -radiation detected huge dust clouds, radio shows a ring of molecular gas 400 pc across with 100,00’s solar masses of material rotating around the cent 
 -region called Sagittarius A we think is at the centre of the galaxy, extended filaments suggest strong magnetic fields operate in the vicinity of the centre 
 -chandra picked up region of X-ray omitting gas, associated with supernovae remnant, within it is a rotating disk of molecular gas with streams of matter spiralling inward 
 -what is causing all this activity? in order to keep the gas moving as fast as it is the object in the centre has to be something more than a million solar masses and extremely small in size, most likely a supermassive black hole 
 ▯ -black hole isn't the energy source but the cast accretion disk of matter drawn towards ▯ ▯ the hole due to its gravity emits the energy as it falls in 
 -astronomers think that Sgr A* at the heart of Sagittarius A might be the black hole, radio observations suggest it is a violent place, total energy output estimated to be 10 to the power of Page 10 of 2 9 33 watts, more than a million times the sun 
 ▯ Focus Questions: ▯ Why do we see the Milky Way as a band of light across the sky?▯ Why it is difficult to map out the Milky Way from Earth?▯ How can variable stars be used to map out the structure of the Galaxy?▯ Why do astronomers regard the disk and halo of the Galaxy as two separate ▯ ▯ ▯ components?▯ Why are there no young halo stars?▯ Why can the spiral arms simply be gas and dust orbiting the centre of the Galaxy?▯ How do astronomers measure the mass of the Galaxy?▯ What is gravitational lensing?▯ How do astronomers use gravitational lensing to probe the properties of dark matter?▯ What are some possible candidates for dark matter?
 24: Normal and Active Galaxies 
 -have to study galaxies based solely on what they look like, Edwin hubble was the first to categorize them into four types called the Hubble classification scheme: 
 Spirals: milky way, andromeda. all have flattened disk with spiral arms, a bulge and dense nucleus, halo of faint old stars. Stellar density is highest in nucleus. The bulges and Halos hold old reddish wars and globular clusters. Light comes from A-G type stars in the disk. However many spiral galaxies have a big variety of shapes. denoted by S, sub sectioned a) b) c) according to the size of the bulge. Sc has the smallest bulge, Sa the largest. The larger the bulge the more tightly wrapped spiral arms. Sc galaxies have the most interstellar matter, Sa the least. Arms look blue because of bright blue O and B stars. 
 Barred spirals: a variation of spiral galaxies. They're main difference is the elongated bar of stellar and interstellar matter passing through the centre and extending beyond the bulge. the spiral arms project from the ends of the bar and not the bulge. designated letters SBa , b, c, depending on size of bulge, which again is related to tightness of spiral as before. 
 Ellipticals: have no spiral arms and no obvious galactic disk. Made of dense central nucleus, have little internal structure. Have little to no cool gas or dust. Mostly no evidence of ongoing star formation. Made up mostly of old reddish low mass stars, with disordered rotation. Lots of Hot interstellar gas in their interiors E, divided up by how elliptical they appear in the sky E0 (circular)- E7 most elongated. Large range in size and number of stars. giant ellipticals can be hundreds of kpc across with trillions of stars, dwarf ellipticals being as small as 1 kpc with less than a million stars. 
 S0 Galaxies: intermediate between spiral and E7, they have a thin disk, flattened bulge, no gas or spiral arms. Also known as lenticular galaxies because they look like lenses. 
 irregulars: rich in interstellar matter and young blue stars, lack structure. Two sub classes, Irr I look like misshapen spirals, Irr 2 are rarer, have an explosive appearance and are probably the result of a collision of previously normal systems. they are smaller than spirals but larger than dwarf ellipticals. Smallest category are dwarf irregulars, this type is the most common just like dwarf ellipticals. 
 Magellanic Clouds: are a famous pair of Irr I galaxies orbiting our galaxy, are 50 kpc from our centre and have a lot of gas, dust, blue stars. 
 the Hubble Sequence: the variation in types across the tuning fork diagram of galaxies, ellipticals- spirals -irregulars 
 -main goal in this diagram as to point out similarities in appearances, even suggesting evolutionary process but there is no evidence of direct evolutionary connection along the Page 11 of 2 9 sequence 
 -there is evidence galaxies collide and interact that these encounters drive the evolution of galaxies. 
 Distances of Galaxies: 
 -galaxies are distributed uniformly but clump into larger agglomerations of matter 
 -estimated around 40 billion galaxies as bright or brighter than ours exist in the observable universe, some close enough that cepheid variables can tell us distance 
 -if galaxies have no cepheid stars or are too far away we have to rely on different methods:
 Standard Candles: bright, easily recognizable objects who’s luminosities are already known 
 ▯ -identified by its appearance or shape of its light curve, then the apparent brightness is 
 ▯ compared with known luminosity we can get the objects distance and its residing galaxy 
 ▯ -it has to have a well defined luminosity, be bright enough to see at large distances, 
 ▯ -some examples of standard candles are Novae, emission nebulae, planetary nebulae, 
 ▯ globular clusters, even entire galaxies, some have larger intrinsic spreads of luminosity
 -Planetary nebulae and type 1 supernovae are the most reliable 
 Tully- Fisher Relation: use correlation between rotational speeds and luminosity of spiral galaxies, then comparing the true luminosity with observed brightness and figure out the distance. because of doppler effect radiation moving towards us is blue shifted and away is redshifted, this results in the line radiation from the galaxy being broadened by the rotation, the faster it rotates the greater it broadens, measure the amount of broadening and determine its rotation speed, the relation tells us its luminosity. usually observe it in 21 cm radio line. This method can measure galaxies about 200 mpc away, after that the broadening is too difficult to measure accurately. 
 Our local group: our neighbourhood of Galaxies, Us, Andromeda and M33 are the only spirals and the rest are dwarf irregulars and dwarf ellipticals, diameter is about 1 mpc.
 - most of the smaller galaxies are gravitationally bound to the the two largest members, kind of a large scale star cluster. 
 -galaxy cluster is a collection of galaxies held together by mutual gravitational attraction 
 ▯ -in a cluster galaxies move randomly, but on the largest scale galaxy clusters move 
 ▯ around in ordered way 
 -Virgo Cluster: is the next large concentration of galaxies outside our local group 
 Hubbles Law: 
 -individual galaxies not part of clusters are steadily receding 
 -galaxy clusters too have overall recessional motion, but their galaxy members move randomly inside the cluster 
 -there is a connection between doppler shift and distance, the further away they are the more redshifted they are 
 -hubble diagrams plot the recessional velocity against distance for galaxies, indicates the rate they recede is directly proportional to its distance from us, this is called hubble’s Law 
 -this proves the universe is expanding ie. the vast distances separating the galactic clusters are expanding 
 -redshift resulting from hubble flow (the universal recession) is called cosmological redshift, not just redshift due to motion within the object. 
 -does this mean that they all started their journey at a single point? 
 Hubble’s Constant: The proportionality between recessional velocity and distance in Hubble’s law , denoted H0
 ▯ Recessional velocity = H0 x distance 
 -the value of Hubbles constant is the slope of the straight line, it specifies the rate of expansion of the entire cosmos 
 Page 12 of 2 9 -H0 accepted value is about 70 km/s/mpc 
 -using Hubbles law we can derive distance of a far off object by measuring its recessional velocity and dividing it by Hubbles constant. 
 Active Galactic Nuclei:
 -galaxies following into Hubble classes are normal galaxies, with luminosities from a million to a trillion solar luminosities 
 -bright galaxies are those with luminosities more than 10 to the 10 times the solar value, 
 -around 40 percent of bright galaxies don't fit into the normal category 
 active galaxies: galaxies who's spectra is different from normal galaxies and have extremely large luminosities. the brightest are the most energetic objects in the universe, represent an important phase of galactic evolution. A system who's abnormal activity is related to violent events occurring in or near the nucleus. there is still a variation within the properties of galaxies 
 though they may look like normal galaxies at optical wavelengths they look unusual under others 
 -the radiation from active galaxies does not peak in the visible wavelength on a blackbody curve like normal galaxies do, their radiation is said to be non-stellar 
 Starburst galaxies: an example of non-stellar emission galaxy that was normal but now has widespread episodes of star formation probably because of interactions with a neighbour 
 -Seyfert Galaxies : look like spiral galaxies, most of its energy emitted from the galactic nucleus. 10,000 times brighter than our nucleus. the majority emit their energy in the infrared. think that most of the high energy radiation is absorbed by dust in or near the nucleus then reemitted as infrared. rapid time variability and large radio + infrared luminosities imply violent non-stellar activity in the Nuclei, most likely similar in nature to processes in our own galaxy but on a bigger scale. 
 -Radio Galaxies : active galaxies emitting large amounts of radio energy. almost none of its radio emission comes from a compact nucleus but two extended regions called radio lobes - roundish clouds of gas extending beyond the visible galaxy. enormous, 10 times the length of the milky way. Centaurus A is an example of one, probably created by a collision of a spiral galaxy and E2. Bright radio galaxies are among the most energetic in the universe, luminosities up to a thousand times of that of the milky way. 
 -Quasars: have spectral lines of hydrogen redshifted to an extreme amount, meaning they are receding from earth super fast. these are the brightest known objects in the universe. the are called quasi-stellar radio sources (meaning star-like) more than 200,000 are known today. the distance to the closest is 250 mpc away. these objects represent the universe as it was in the distant past. They share properties with Seyferts and Radio galaxies. Radiation is non stellar and vary irregularly in brightness. some quasars have radio lobes or jets of matter. They omit most of their energy in the optical and infrared. We think today that they are really just intensely bright nuclei of distant galaxies 
 Engine of Active Galaxies: 
 -believe quasars, radio, seyferts and normal galaxies share the same energy-generation mechanism 
 -active nuclei have some or all of the following properties:
 ▯ - High luminosities 
 ▯ - energy emission is mostly non stellar (cant be explained as the combination of trillions ▯ ▯ of stars radiation) 
 ▯ - energy output can be variable, implying energy is emitted from small nucleus 
 ▯ - may have jets or signs of explosive activity 
 Page 13 of 2 9 ▯ -optical spectra may show broad emission lines indicating rapid internal motion within 
 ▯ the energy producing region 
 ▯ -actiivity associated with interactions between galaxies 
 energy production: 
 -accretion of gas onto a supermassive black hole, releasing huge amounts of energy as the matter sinks into the central object 
 -must be billions of times more massive than the sun to power active galaxies 
 -infalling gas forms an accretion disk, heating up within and emitting large amounts of radiation as a result
 -the origin of the accreted gas in active galaxies is entire stars and clouds of interstellar gas that got too close to the hole 
 -jets appear to be a common feature of accretion flows 
 Energy Emission: 
 -radiation emitted by the hot accretion disk spans broad range of wavelength because of the range of temperatures in the disk as the gas heats up 
 -in some cases high energy radiation emitted is reprocessed - absorbed and reemitted at longer wavelengths - by material beyond the nucleus before it reaches us 
 -think this might be done by a donut shaped ring of gas and dust surrounding the accretion disk 
 -another mechanism of reprocessing involves magnetic fields possibly produced in accretion disk and transported by jets and radio lobes into space, they make particles emit electromagnetic radiation producing synchrotron radiation, which is non thermal and there is no link to temperature, meaning it wouldn't be described by a black body curve, in fact its intensityy decrease with increasing frequency. 
 Focus Questions: 
 -what are the features based on which spiral galaxies are classified? In what ways are large spirals like the Milky Way and Andromeda not representative of galaxies as a whole?
 -Why is it difficult to classify elliptical galaxies from their appearance? What are the similarities and differences between elliptical galaxies and the halo of our own Galaxy?
 -Why are Type Ia supernovae good standard candles? How is using the Tully-Fisher relation an alternative to standard candles?
 -What is the most likely range of values for Hubble's constant? What are the uncertainties
 -How does the use of Hubble's law differ from the other extragalactic distance-measurement techniques we have encountered so far? 
 -Why is the radiation from active galaxies said to be nonstellar? 
 -How did the determination of quasar distances change astronomers' understanding of these objects?
 -How do we know that the energy-emitting regions of many active galaxies must be very small?
 -How does accretion onto a supermassive black hole power the energy emission from the extended radio lobes of a radio galaxy?
 25: Galaxy Evolution and Dark Matter: 
 To figure out if other galaxies have this same dark halos we need to calculate their masses and compare it to the amount of luminous matter observed. 
 Rotation curves: can calculate mass by determining their rotation curves, i.e.. speed vs. distance from the galactic centre, then the mass within any given radius in the galaxy follows newtons laws 
 Page 14 of 2 9 -far away galaxies too far for detailed curves to be drawn, but by using the tully fisher relation to observe the broadening of spectral lines we can measure the rotation speed 
 -estimate of size leads to estimate of mass. 
 -this approach is useful for measuring mass within 50 kpc of a galaxies centre. 
 Binary Galaxy Systems: 
 -used to measure mass further from the galaxies centre, because binary systems components are 100’s of kpc apart
 -orbit is usually too long to be measured, but by estimating the period of the semi major axis using the line of spite velocities and angular separation can get u an accurate mass
 -normal spirals and large ellipticals typically contain 10 to the 11-12 solar masses of material. 
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