Download EvoluGon of high mass stars Solar-‐type stars end their lives by

Evolu&on of high mass stars Solar-­‐type stars end their lives by going through a red giant phase, leading to the loss of the envelope and leaving behind a C and O white dwarf. The white dwarf maintains stability against gravita=onal contrac=on because of electron degeneracy pressure. There is a limit to the mass that a white dwarf can have before degeneracy pressure is unable to support the star against gravita=onal contrac=on. This mass is 1.4 solar masses, and is referred to as the Chandrasekhar limit. Stars with masses higher than 8 – 10 solar masses evolve and end their lives very differently than lower mass stars. Fundamentally this is because of their ability to sustain nuclear reac=ons in their cores involving heavier elements than H and He. The ability to burn heavier elements comes about because of the much higher temperatures in their cores, brought about by con=nued contrac=on when the core exceeds the Chandrasekhar limit. The early evolu=on of a high mass (e.g. 25 Msun) is similar to a solar-­‐type star: Burning hydrogen to helium -­‐> burning helium to carbon and oxygen. At this stage the core mass exceeds the Chandrasekhar mass. When helium burning ceases the core contracts and heats up un=l the temperature reaches 600 million K. Carbon burning switches on and produces 16O, 20Ne, 23Na, 23Mg and 24Mg. Cessa=on of carbon fusion leads to further contrac=on and a rise of temperature to 109 K when Ne fusion begins, producing O and Mg. When Ne fusion ends the core contracts un=l T=1.5 x 109 K, and O fuses to produce 28Si. Next comes Si fusion when T=2.7 x 109 K, producing 32S, 56Fe, 56Ni. Some reac=ons produce neutrons that are captured by a variety of nuclei, producing elemental isotopes. Each successive stage of burning lasts for a decreasing length of =me, as shown in the table. Each stage of core fusion generates a new shell of material around the core. As the core temperature increases these shells ignite, producing an onion-­‐like structure. The extremely large luminosity generated by the combined energy output from these burning shells causes the stellar envelope to expand to a radius of 5 AU, the semi-­‐major axis of Jupiter’s orbit. This envelope expansion leads to the surface temperature reducing down to ~3500 K, causing the star to develop a red-­‐ish colour. This star is now a red super-­‐giant. Examples in the night sky include Betelgeuse and Rigel in Orion and Antares in Scorpius. The process of adding fusion shells to the core does not con=nue forever because Fe does not burn – fusing Fe involves endothermic rather exothermic reac=ons (i.e. energy must be put in if Fe is to fuse). Forma=on of an Fe core by the fusion of Si is the last stage of core fusion. At this stage the core radius is approximately equal to that of the Earth. Core-­‐collapse supernovae The core of a massive star gets progressively hober as it contracts and ignites each of the nuclear fuels described in the previous slide. The photons emibed also become more energe=c (Wien’s law). Once the temperature reaches a few 100 million K, the photons are energe=c enough to ini=ate nuclear reac=ons that lead to the emission of neutrinos which escape the core and carry off energy, effec=vely causing it to cool. This energy loss can be compensated for by increasing the rate of fusion reac=ons, un=l an Fe core is formed. The only way in which internal energy can be generated in this core is for gravita=onal contrac=on to occur. Once the Fe core is formed, and due to the small size and extreme density in the core, collapse occurs very rapidly and the temperature shoots up to 5 x 109 K within 0.1 second. At these temperatures high energy gamma rays are generated (hν ≈ kT), and these are able to photodissociate the Fe nuclei into He nuclei, a process that takes a frac=on of a second. Within another 0.1 second, the core becomes so dense that the electrons combine with protons to form neutrons, a process that also produces a flood of neutrinos: e-­‐ + p n + ν 0.25 seconds aher the core collapse begins, the core radius reaches 10 km. The density of the neutron dominated core has reached nuclear densi=es – the density in the nucleus of an atom. At these densi=es the core becomes rigid and the contrac=on ends with a bounce that sends a shock wave outward into the outer core. During the neutrino-­‐induced cooling and collapse of the inner core, the pressure in the region surrounding the core decreases drama=cally, and material collapses toward the core, reaching speeds of 0.15 c. This material crashes down onto the rigid core just as the core bounces, and meets the outward travelling pressure wave. The inward moving material around the core reverses, and moves back towards the star’s surface at high speed. The extremely high densi=es in and around the core allow a frac=on of the escaping flood of neutrinos to be absorbed by the outward moving material, and this increases the energy of the escaping material and drives it outward. This general picture is derived from supercomputer simula=ons that model the processes described above. The absorp=on of neutrinos is crucial in accelera=ng the pressure wave out toward the surface of the star. Eventually the wave exceeds the speed of sound in the star’s envelope and it becomes an outward moving shock wave. A shock wave is very efficient at transferring its momentum to the surrounding gas, pushing it outward. Aher a few hours the shock wave reaches the surface of the star and the envelope of the star is driven off. As the envelope becomes op=cally thin the light from the hot interior escapes and the object brightens drama=cally. The star has now become a supernova. The image below is taken from a simula=on, and shows the highly turbulent nature of the outward flowing envelope. Depending on the mass of the progenitor star, the inner core leh behind either forms a neutron star or a black hole. Supernova that occur at the end of a star’s life are called core-­‐collapse supernovae. The basic picture described in the previous slides was confirmed by supernova 1987A which exploded in the Large Magellanic Cloud (a satellite galaxy to the Milky Way). This was the first =me that neutrinos were detected from a nearby supernova explosion, providing confirma=on of their important role in the driving mechanism. Supernovae are classified according to their light-­‐curves, and there are three classes of core-­‐collapse collapse supernovae: type II (the progenitor is a red super-­‐giant whose gaseous envelope is intact when the explosion occurs) type Ib (there are no hydrogen lines in the spectrum, indica=ng that the hydrogen envelope was lost before the explosion) type Ic (no hydrogen or helium lines, so all the hydrogen and helium has been lost before the explosion). The images on the right show the progenitor star and the supernova 1987A, and a supernova remnant (Cassiopeia A). Supernovae also occur when a white dwarf in a close binary system explodes (type Ia). Here the white dwarf accretes gas from a companion star through an accre=on disc. The white dwarf is supported by degeneracy pressure, but when its mass exceeds the Chandrasekhar limit it contracts and heats up, igni=ng carbon fusion at its centre. In the absence of degeneracy pressure the increasing temperature due to fusion would increase the pressure and the star would expand and cool, regula=ng the fusion reac=ons. Degeneracy pressure prevents this, and the white dwarf undergoes explosive fusion of carbon that blows the star apart. Igni=on occurs near the centre, where the temperature is highest, and propagates outward (in the form of a “flame front”) as the surrounding layers are heated to temperatures that allow carbon fusion to occur, leading to an explosion that destroys the star. See diagram for more details. The diagram shows the different types of supernova and their spectral features. The diagram below compares the light-­‐ curves of type Ia and type II supernovae. The decay in brightness of a supernova is due to the radio-­‐ ac=ve decay of radioisotopes generated during the explosion. Type Ia generate large quan==es of 56Ni, for example. Neutron stars These are stars composed almost en=rely of neutrons, with radii ≈ 10 km, leh over aher a core-­‐collapse supernova. They are supported against gravita=onal collapse by neutron degeneracy pressure. Their existence was predicted by Zwicky & Baade in the 1930’s, but was not confirmed un=l the discovery of pulsars in 1967 by Jocelyn Bell and Anthony Hewish. These are sources of radio emission that emit highly regular periodic pulses of radio emission (with typical periods of ≤ 1 second). At first it was not clear what the source of these radio pulses could be because of the extremely short period. The discovery of a pulsar in the Crab nebula, a well-­‐known supernova remnant (the supernova was recorded in Chinese records from 1054), finally demonstrated that the pulsars were in fact rapidly rota=ng neutron stars. The Crab nebula pulsar emits radio pulses with a period of 0.0333 seconds, indica=ng that it is rota=ng 30 =mes per second! The collapse of the core of a rota=ng star leads to the forma=on of a rapidly rota=ng neutron star because of conserva=on of angular momentum (if the Sun was compressed into a radius of 10 km it would rotate at 1000 =mes per second). The images to the right show the Crab nebula, which is ~ 13 lyr across, and the inner regions where the pulsar may be seen surrounded by a rota=ng disc of tenuous plasma and two oppositely directed jets. Pulsars have extremely strong magne=c fields that are responsible for the radio emission. The strong field results from the fact that the magne=c field lines in a star are =ed to the gas when it is a plasma. As the stellar core collapsed it dragged the magne=c field with it, amplifying it to large values. The field strength at the Earth’s surface ~ 5 x 10-­‐5 Tesla, a fridge magne=c field is ~ 10-­‐2 Tesla, the Sun’s surface ~ 10-­‐4 Tesla, and a pulsar field has strength ~ 108 Tesla. The large field strength, combined with rapid rota=on, leads to the genera=on of strong electric fields near the neutron star surface, and the electric field causes pair produc=on –> spontaneous produc=on of electron-­‐positron pairs. These par=cles spiral along the magne=c field lines, and emit radio (and other radia=on) as they are accelerated. The radio emission occurs in beams that point toward the Earth once per spin period, genera=ng a light-­‐house effect, as illustrated by the diagram. Black holes Once the mass of a neutron star exceeds approximately 3.2 Solar masses, neutron degeneracy pressure is unable to support the star against gravita=onal collapse. There is no known force in nature that can prevent the collapse of such an object, so contrac=on must result in the forma=on of a black hole. A black hole is an object with all of its mass concentrated in a point of zero volume, such that it has infinite density – such an object is called a singularity. The gravita=onal accelera=on in the vicinity of this object is so strong that not even light can escape -­‐ hence the name “black hole”. Einstein’s rela&vity Einstein ini=ally developed the theory of special rela=vity (1905) that deals with objects moving with uniform velocity. It is worth recapping some of the predic=ons of special rela=vity before discussing the general theory. The major founda=on stone of special rela=vity is that the speed of light in vacuum is constant for all observers. Time dila=on: an observer measuring the passage of =me on a clock that is moving rela=ve to the observer will no=ce that the moving clock =cks more slowly than one at rest in the observer’s frame of reference. Length contrac=on: an observer measuring the length of a moving object will determine that the length of the object is smaller along the direc=on of mo=on than the length of an iden=cal object at rest in the observer’s frame of reference. In 1915 Einstein published his theory of general rela=vity, which provides a descrip=on of gravity -­‐> gravity is due to the curvature of space by an object with mass. Gravity influences the passage of =me: a clock located in a strong gravita=onal field is observed to =ck more slowly than one that is in a weak gravita=onal field by an observer located far from a gravita=ng object. General rela=vity describes the warping of both space and =me – space=me. The fact that space=me itself is curved leads to the predic=on that light rays will be bent as they pass by a gravita=ng object. Newton’s theory of gravity predicts that gravita=onal forces are only experienced between pairs of objects with mass (or more correctly with finite ‘rest-­‐mass’ given the equivalence of mass and energy in rela=vity), and therefore does not predict that light should be influenced by a gravita=ng object. Numerous experimental tests have been performed to confirm rela=vity, and these include the following: i. measuring the bending of light from distant stars as it passes the Sun during a total Solar eclipse ii. measuring the precession of Mercury’s orbit (rela=vity due to the Sun’s gravity induces a precession rate of 43” per century that can be measured) iii. Advanced LIGO detec=on of gravita=onal waves from two coalescing black holes iv. Measuring the passage of =me at different heights above the Earth using atomic clocks in aircrah: gravita=onal slowing of =me (see diagram below) v. Measuring gravita=onal redshih: the slower passage of =me in a strong gravity field causes the frequency of emibed electromagne=c radia=on to decrease. The frequency of gamma-­‐rays emibed at Earth’s surface and detected at significant heights above the Earth’s surface are observed to have reduced frequencies (see diagram below) vi. The components of a well-­‐known binary pulsar PSR B1913+16 are observed to be slowly spiraling in toward each other because gravita=onal wave emission leads to a loss of energy from their orbit. As a black hole forms from the collapse of a star, light rays from its surface become increasingly curved un=l they follow paths that curve back toward the body. At this point a black hole has been formed. Photons emibed in direc=ons perpendicular to the surface are gravita=onally red-­‐shihed un=l they have no energy and cannot be detected by an external observer. The distance from a black hole where not even light can escape is called the Schwarzschild radius. This radius can be calculated as being the distance from the black hole where the escape velocity is equal to that of light, c: RS = 2 G M / c2 An object that ventures closer to the black hole than the Schwarzschild radius cannot escape. An imaginary sphere that surrounds the black hole with radius RS is called the event horizon, because we cannot see any “events” that are occurring within it. Black holes in astrophysics Direct evidence for the existence of black holes is difficult to obtain given that they do not emit electromagne=c radia=on. Recent direct evidence has come from the detec=on of gravita=onal waves by the Advanced LIGO experiment. Strong circumstan=al evidence for black holes is obtained from the study of maber in orbit. We have evidence for the existence of stellar mass black holes (with masses in the range ~ 3 – 10 Msun) in close binary star systems, and supermassive black holes (with masses ~ 106 – 109 Msun) at the centres of galaxies, including our own Milky Way. Cygnus X-­‐1 is stellar mass black hole candidate. It is the strong X-­‐ray emiber, and is a binary system consis=ng of a B0 supergiant with surface temperature T ~ 31,000 K (and mass ~ 30 Msun), and a compact companion object. (Note: in astrophysics objects with stellar masses but much smaller than main-­‐sequence stars are referred to as compact objects. White dwarfs, neutron stars and black holes are compact objects.) B0 stars are not strong emibers of X-­‐rays, so the source of these must be the compact companion. Furthermore, the X-­‐ray flux is observed to flicker over =me scales of 0.01 second. Given that nothing can travel faster than light, this flickering sets a limit on the maximum size of the emiyng area assuming that there is a coherent physical change occurring that explains the changing flux. This tells us that the X-­‐ray emiyng region can only be about 3000 km in diameter. Spectral lines of B0 are seen to Doppler shih due to its orbital mo=on, and the orbital period suggests a mass for the compact object of 7 Msun. Cygnus X-­‐1 is likely to be a black hole as its mass is too large for a neutron star. The diagram to the right describes a model for Cygnus X-­‐1: a binary system consis=ng of a B0 super-­‐giant and a 7 Msun black hole, where the =dal forces due to the black hole are able to pull material from the surface of the star. Because the star and black hole orbit their common centre of mass, this material has angular momentum, and so cannot fall onto the black hole directly. Instead it forms a gaseous accre=on disc in orbit around the hole. This accre=on disc is in Keplerian rota=on around the black hole, so the disc material nearest the hole orbits faster than material further out. This differen=al rota=on creates shear between neighbouring regions in radius, and fric=onal forces ac=ng between these disc components cause material to lose angular momentum and spiral in toward the black hole, eventually falling into the event horizon. The fric=on in a gas is called viscosity, and this also causes hea=ng of the disc. The inner regions of the disc are believed to achieve temperatures of T ~ 2 x 106 K, hot enough to emit X-­‐rays. Supermassive black holes appear to orbit at the centres of many galaxies. The image to the right shows radio emission from a jet (with length ~ 15,000 pc) being launched from the centre of a galaxy. A close-­‐in image obtained by the Hubble space telescope shows the inner regions of the galaxy, with a gaseous and dusty torus of radius 250 pc orbi=ng around a bright central object. The jets are launched in a direc=on perpendicular to the disc. Measurement of the Doppler shih from material on either side of the disc shows orbital speeds of 100s km/s. Given the size of the region and this velocity, the disc must be orbi=ng a central object with a mass ~4 x 108 Msun. The disc is observed to have an inner radius ~ the size of the solar system, so the object appears to be a black hole. Supermassive black holes are believed to be the central engines of quasars – ac=ve galaxies that have high red-­‐shihs and are extremely luminous across a range of wavelengths from the radio to X-­‐rays. The energy is emibed from a =ny region at the centres of these galaxies, and the most likely explana=on is a supermassive black hole being fed by a gaseous accre=on disc. The Milky Way Galaxy Our solar system orbits within a rota=ng and flabened distribu=on of stars, gas and dust – the Milky Way Galaxy. The Milky Way is believed to contain approximately 200 x 109 stars. Even with a large op=cal telescope most of these stars are not visible because light in the visible spectrum is absorbed by dust grains in the inter-­‐ stellar medium – this is known as interstellar ex=nc=on. Observing at longer wavelengths, (i.e. in the IR) allows distant stars to be observed. Longer wavelength radia=on is not absorbed by small interstellar grains – as shown by the figures. Measuring distances to stars using the parallax method is limited to rela=vely nearby stars. In the early 20th century it was realised that two classes of variable stars could be used as “standard candles” to measure distances across the Galaxy and beyond: Cepheids and RR Lyrae stars. Large scale visible view of the Milky Way, and a close-­‐in IR view showing many stars and dust clouds not visible at shorter wavelengths Cepheid variables are stars which pulsate. The pulsa=on period varies in a predictable way with their intrinsic average luminosity. RR Lyrae stars also pulsate, but they all have the same brightness, independent of pulsa=on period. By iden=fying Cepheids and RR Lyrae stars in distant parts of the Galaxy, or in globular clusters (dense star clusters containing ~ 106 stars that orbit the Milky Way galaxy in a spherical halo), astronomers were able to measure the size of the Galaxy and the distances to globular clusters. It is now accepted that the distance to the centre of the Milky Way is 8 kpc (kiloparsecs). Infrared observa=ons demonstrate that stars and dust form a flabened disc in the Milky Way, which has a central bulge containing a high density of stars. The diagram on the right shows a schema=c picture of the structure of the Galaxy: -­‐ A disc containing gas, dust and metal-­‐rich (Popula=on I) stars -­‐ A halo containing metal-­‐poor (Popula=on II) stars and globular clusters (also composed of Pop II stars) -­‐ A bulge containing a mixture of Popula=on I and II stars. Note that Popula=on II stars are old stars that formed before the products of fusion reac=ons in stars enriched the gas clouds from which they formed with heavy elements (“metals”). Popula=on I stars formed more recently from metal-­‐enriched gas clouds, and hence contain heavy elements in their atmospheres that can be detected from their spectra. Popula=on II stars probably formed at the same =me as the Milky Way Galaxy formed, and their orbits are highly =lted rela=ve to the galac=c disc. Popula=on I stars formed rela=vely recently from the gas clouds contained in the galac=c disc, so they orbit within the plane of the disc. We observe galaxies outside of the Milky Way and no=ce that many have flabened disc-­‐like structures and display spiral arms. Mapping the structure of our galaxy is difficult because of interstellar ex=nc=on, but is made possible by the fact that atomic hydrogen emits radia=on at a wavelength of 21 cm. This arises when the spin of the electron transi=ons from a state that is aligned with the spin of the nucleus (a higher energy state) to one in which their spins are an=-­‐aligned. The distribu=on and kinema=cs (mo=on) of atomic hydrogen in the Galaxy can be mapped using the Doppler technique described in the diagram to the right. This analysis reveals that the Milky Way galaxy has spiral arms similar to the above image. Rota&on of the Milky Way Doppler shih measurements of 21 cm H emission indicate that stars and gas orbit in the same direc=on around the centre of the Galaxy. These measurements indicate that the velocity of gas and stars about the galac=c centre is fairly uniform throughout much of the Galaxy’s disc. The mo=on of the Sun around the centre of the Galaxy is es=mated by determining the Sun’s mo=on rela=ve to background galaxies and globular clusters by measuring the average Doppler shih. These galaxies and globular clusters act as a fixed frame of reference against which the Sun’s mo=on can be measured. The Sun’s velocity around the galac=c centre is measured to be ~ 220 km/s. Given that the distance of the Sun from the galac=c centre is ~ 8 kpc, the =me required to orbit the Galaxy once is 220 million years. Using Kepler’s 3rd law, we can determine the Galaxy mass interior to the Sun. This turns out to be ~ 9 x 1010 Msun. The mass of the Galaxy is even larger than this given that it extends beyond the loca=on of the Sun from the Galac=c centre. Measuring the rota=on of the Galaxy out beyond the orbital radius of the Sun using 21 cm H emission shows that the amount of maber in the Galaxy does not drop off as we move from the Galac=c centre. The orbital speed of gas around the galac=c centre should decrease if the amount of mass present at large radii decreases (just as the orbital veloci=es of planets around a star decrease with distance from the star). The diagram shows that the rota=on curve of the Galaxy (orbital velocity versus distance from galac=c centre) is fairly flat out to large distances, instead of falling off as expected. This indicates that the total mass of the Galaxy is ~ 1012 Msun. Dark maDer Coun=ng up all of the “visible” mass of the Galaxy in the form of stars, gas clouds and dust leads to an es=mate of the mass of the Galaxy ~ 1011 Msun, only 10% of the mass inferred from the rota=on curve. The nature and origin of the missing mass remains a mystery and the subject of intense research. It has been given the name dark maber by virtue of our inability to detect it directly. It appears that this maber may have an exo=c non-­‐baryonic nature, because abempts to detect the missing mass in the form of low luminosity stars and brown dwarfs have failed. Rota&on curves in galaxies and dark maDer: Genzel et al. (2017), Nature, 543, 397 Rota=on curve of high redshih galaxies decrease with radius -­‐> they were baryon-­‐dominated, dark maber was less concentrated Spiral density waves Early explana=ons for the origin of spiral waves in disc galaxies suggested that the pabern was established during the forma=on of the Milky Way and maintained because of rigid rota=on. We know, however, that the Milky Way is differen=ally rota=ng, with stars nearer the galac=c centre orbi=ng in a shorter =me than stars near the outer edge. This differen=al rota=on makes it impossible for a rigid spiral structure to exist, as demonstrated by the diagram below, or for a par=cular group of stars with different orbital radii to define a spiral pabern over long =me scales. Differen=al rota=on causes a spiral to wind up over =me, and aher many rota=ons any appearance of a spiral pabern defined by a par=cular group of stars will disappear. This is known as the winding dilemma. Spiral density waves are regions of a galaxy where material slows down and becomes concentrated for a period of =me, before moving away from the spiral arm. A well-­‐known analogy is a traffic jam, where cars slow down as they enter the region of conges=on, but then move off again at increased speed as they leave the area of the jam. Stars and gas clouds do the same thing in the spiral arm of a galaxy. Consequently, the spiral waves in a galaxy tend to move more slowly around the galaxy than the maber itself, and the stars and clouds that make up a spiral arm are constantly changing as they move around the galaxy. The compression of gas clouds as they enter a spiral arm triggers forma=on of stars. This explains why images of spiral galaxies show luminous spiral features -­‐ these are the young O and B stars that heat up the surrounding gas clouds (so called HII regions because they contain ionised hydrogen due to the hot stars). The short lives of the O & B stars means they don’t travel far out of the spiral arm before they die. The inter-­‐arm regions s=ll contain lower luminosity stars that are not so apparent in the images. It remains unclear why density waves form in the first place. Central black hole Observa=ons indicate that the centre of the Galaxy hosts a massive black hole. An energe=c radio source is located in the direc=on of the constella=on Sagibarius – the source is known as Sagibarius A* and has a size ~ 0.3 AU. A dense cluster of high mass stars is observed near Sagibarius A*, and the trajectories of some of these display orbits around the black hole allowing the mass to be measured as ~ 4 million Msun. Galaxies 100 years ago astronomers believed the universe was a few thousand light years across, and that nothing lay beyond our Milky Way Galaxy. We now know that the Milky Way is just one of billions of galaxies contained in a universe that is billions of light years across. In the 1920’s a debate raged between Harlow Shapley, who believed that the “spiral nebulae” observed through telescopes were rela=vely small objects scabered within our Galaxy, and Heber Cur=s who believed that they were “island universes” located outside of Milky Way. Confirma=on that these objects were galaxies in their own right, located great distances outside of the Milky May, was provided by Edwin Hubble who used photographs of Cepheid variables in Andromeda (M31) to es=mate its distance. The distance to the Andromeda galaxy is ~ 750 kpc (2.5 million light-­‐years). Its diameter is ~ 75 kpc, considerably larger than the Milky Way. Distances to galaxies are usually expressed in megaparsecs (Mpc) -­‐ 106 parsecs. M31 and M51 Hubble classified galaxies into 4 categories: spirals, barred spirals, ellip=cals and irregular galaxies. The diagram (called the “tuning-­‐fork” diagram) shows how the classifica=on scheme corresponds to galaxy shapes. Note that the irregulars do not appear in this diagram. Spirals – galaxies with substan=al gas and dust frac=ons, that display prominent spiral arms where on-­‐going star forma=on occurs. Spiral galaxies contain both old Popula=on II stars, and stars with high abundances of heavy elements indica=ve of younger Popula=on I stars. The Hubble classifica=on corresponds to large central bulge regions with smooth spiral arms for Sa galaxies, and small central bulge regions with narrow highly defined spiral arms for Sc galaxies. Hubble defined galaxies with prominent bulge components, and a disc component that does not show evidence of spiral structure as S0/SB0 “len=cular galaxies”. Barred spirals – here the spiral arms originate at the end of a bar-­‐feature that runs through the galaxy’s nucleus. A SBa galaxy has a large central bulge and thin, =ghtly wound spiral arms. A SBc galaxy has a small central bulge and loosely wound spiral arms. Ellip&cals – these galaxies have no spiral arms, but dis=nctly ellip=cal shapes. These galaxies are devoid of interstellar gas and dust, and show no evidence of on-­‐going star forma=on. The stars are old metal-­‐poor Popula=on II stars that appear to have formed in one large burst of star forma=on when the galaxies formed. Irregulars – irregular galaxies have deformed shapes and are rich in interstellar gas and dust. They contain old and young stars. The Large Magellanic Cloud is an Irregular galaxy that orbits the Milky Way at a distance of 55 kpc. Distance determina&on and the distance ladder Determining the distances to astronomical objects presents a significant challenge. The more distant the object the greater the difficulty. Parallax is useful up to a distance of ~ 500 pc, and spectroscopic parallax (which uses the H-­‐R diagram and luminosity classes of stars) is accurate out to ~ 10 kpc – more distant stars are too dim. To determine the distances to galaxies, astronomers look for standard candles – objects with known luminosi=es that can be used to determine distance using the inverse-­‐square law for the observed flux.
Cepheids can be observed out to distances ~ 30 Mpc (i.e. about 40 =mes further than the Andromeda galaxy) using the HST, and are used to measure galaxy distances. Even the brightest Cepheids (L = 2 x 104 Lsun) cannot be seen beyond this distance. RR Lyrae stars are variable Popula=on II stars ohen found in globular clusters, but can only be used out to distances ~100 kpc. Type Ia supernovae are used for distances out to more than 1000 Mpc. But note that most galaxies do not contain type Ia supernovae when being observed, so this cannot be used for all distant galaxies. The speed of rota=on of a spiral galaxy is related to the mass of the galaxy (Kepler’s third law), and therefore its luminosity. The rota=on rates of distant galaxies can be measured from the Doppler effect using 21cm line emission of hydrogen which is broadened because of the combined redshih and blueshih induced by the rota=on of the galaxy. This leads to the Tully-­‐Fischer rela=on between the width of the 21 cm emission line and the luminosity of the galaxy. This can be used to measure distances out to just over 100 Mpc. These distance measuring techniques can be used to calibrate one another: -­‐ the brightness and distance of Cepheids and RR Lyrae stars can be calibrated using parallax or spectroscopic parallax measurements. -­‐ the Tully-­‐Fischer and type Ia supernova methods can be calibrated using Cepheids. Given that one method of distance determina=on leads to the next one, astronomers refer to the different distance determina=on techniques as the distance ladder. The Hubble law: distance vs red-­‐shiX rela&on for galaxies The first abempt to measure spectral lines from galaxies was undertaken by Vesto Slipher in 1914. Of the 15 galaxies measured, 11 showed evidence of significant red-­‐shih, indica=ng that they were moving away from the Earth. During the 1920s, Edwin Hubble and Milton Humason photographed the spectra of many galaxies using the 100 inch (2.5 metre) Mount Wilson telescope in California. By observing the apparent brightness and pulsa=on periods of Cepheid variables in these galaxies, they were also able to measure their distances. They found the following correla=on: The more distant a galaxy, the greater its red-­‐shih and the more rapidly it is receding from us. The recession of galaxies from us is called the Hubble flow. Redshih is denoted by z, and is defined by: z = (λ – λ0) / λ0 = Δλ / λ0 where λ is the observed wavelength of the shihed spectral line, and λ0 is the wavelength of the spectral line measured in the laboratory. Hubble used the Doppler formula to relate the observed red-­‐shih, z, to the recession velocity of the galaxies (see on-­‐line supplementary lecture notes) and plobed the data for velocity versus distance. He found that the points lie on a straight line, so that velocity and distance are directly propor=onal to each other. This is clear evidence that the universe is expanding. In 1929 Hubble published this discovery, which is known as the Hubble law: v = H0 d where v is the recession velocity (measured in km/s), d is the distance to the galaxy (measured in Mpc) and H0 is the Hubble constant. The Hubble constant has natural units of 1/=me, but because of the way in which v and d are expressed, H0 is normally expressed in units of km per second per megaparsec. In other words, if a galaxy located at a distance d=1 Mpc is receding from Earth because of the Hubble flow, it will have a velocity of H0 kilometres per second The value of H0 tells us how fast the universe is expanding, and H0 can be obtained by simply measuring the slope of the straight line on the above Hubble diagram. The currently agreed figure based on the most recent measurements gives a value H0 = 73 km/s/Mpc -­‐> a galaxy located a distance of 1 Mpc recedes from Earth with a velocity v=73 km/s. A galaxy located at a distance of 100 Mpc recedes with a velocity v=7300 km/s. Determining the value of H0 is not easy -­‐> uncertain=es in distance measurements to remote galaxies. It is only recently that a consensus has arisen around the value H0 = 73 km/s/Mpc. This value was derived from HST observa=ons of Cepheid variables out to distances ~ 30 Mpc. Earlier values based on type Ia supernova produced values in the range H0 = 40 – 65 km/s/Mpc -­‐> measurements of distances to galaxies using type Ia supernovae tended to produce larger values of d (note that H0 = v /d). Distance measurements based on the Tully-­‐Fischer rela=on tended toward smaller es=mates -­‐> values of H0 = 80 – 100 km/s/Mpc. Re-­‐analysis of these data has brought their es=mates of H0 closer to the H0 = 73 km/s/Mpc value. The spa&al distribu&on of galaxies Galaxies are not scabered uniformly throughout the universe, but are found in clusters. Galaxies within a cluster are in con=nual mo=on because of their mutual gravita=onal interac=on. This mo=on can be measured from the Doppler shih displayed by each galaxy, subtrac=ng the effects of the Hubble flow. The Hercules cluster located at distance 200 Mpc We expect that clusters of galaxies are held together by gravity, and so we can es=mate the total mass in the cluster by measuring the veloci=es of the galaxies and rela=ng the gravita=onal energy to the kine=c energy. The mass of galaxy clusters measured in this way is larger by a factor of 10 than mass es=mates based on coun=ng the numbers of galaxies and es=ma=ng the numbers of stars and gas clouds in each galaxy. We conclude that galaxy clusters are dominated by dark maber, in agreement with measuring the masses contained in individual galaxies through the rota=on curve of the Milky Way and other spiral galaxies. Both galaxies and galaxy clusters have too lible luminous (baryonic maber) to explain their dynamics -­‐ > hence the conclusion that their masses are dominated by dark maber. Our Milky Way galaxy is associated with a low density cluster with rela=vely few galaxies (called a “poor cluster”) which is known as the Local Group. This contains ~ 40 galaxies, including the largest galaxy in the group -­‐ Andromeda. A large number of dwarf ellip=cal galaxies are found in the Local Group, including the recently discovered Canis Major Dwarf which is being =dally disrupted by the Milky Way. It is also known that the Andromeda galaxy is approaching the Milky Way because of their mutual gravita=onal abrac=on. Current predic=ons indicate that these galaxies may collide in about 4 billion years =me (see movie Andromeda-­‐MilkyWay.mov) (hbps://www.youtube.com/watch?v=fMNlt2FnHDg) We can conclude that clusters of galaxies are highly dynamic environments in which galaxies evolve through mergers, =dal disrup=on and giant collisions. In addi=on to poor clusters, “rich clusters” also exist such as the Virgo (distance ~ 17 Mpc) and Coma (d ~ 90 Mpc) clusters. These contain 1000 – 2000 galaxies. Observing the distribu=on of galaxies on large scales, we see that clusters of galaxies are also grouped together into even larger clusters (clusters of clusters of galaxies). These associa=ons are referred to as superclusters. A typical supercluster contains dozens of individual clusters spread over a region ~ 45 Mpc. The Local Group is a member of the Local Supercluster that also contains the Virgo cluster. On large scales we see that the distribu=on of galaxies shows filamentary structure with empty voids. The diagram below taken from the 2MASS survey shows the spa=al distribu=on of 1.6 million galaxies. This filamentary structure is a natural outcome of supercomputer simula=ons that models the forma=on of large-­‐scale structure in the universe – the pabern is due to gravity and the random ini=al condi=ons. (show movie from Millenium simula=on -­‐ use VLC player) (hbps://www.youtube.com/watch?v=UC5pDPY5Nz4)