Cosmic Dawn

by George Rhee

  The key to understanding stellar collapse lies in the fact that the most stable atomic nucleus is iron. Nuclear energy can be released from lighter elements than iron by fusing them together. However, no nuclear energy can be released by splitting or adding to an iron nucleus. As stellar evolution progresses, massive stars find themselves with increasingly heavier elements in their centers. Finally when a star contains an iron core, no more energy release through fusion is possible, collapse is bound to follow. The collapse happens in less than a second and produces a neutron star. During the collapse, the neutron star bounces slightly. The bounce expels matter at tremendous speeds. During the formation of the neutron star huge numbers of neutrinos are produced, which contribute to the energy of the explosion.

  In 1934, two astronomers working at Caltech, Walter Baade and Fritz Zwicky, suggested that the transition of an ordinary star into a neutron star consisting mainly of neutrons could be accompanied by a tremendous explosion known as a supernova. Zwicky is a legendary figure among astronomers. Known for his eccentricity in a field of eccentrics, he came up with a remarkable number of original ideas, including the notion of dark matter in galaxy clusters. The formation of a neutron star should produce a huge flux of neutrinos. It was a triumph of astrophysics when the neutrinos from a supernova were first observed. In 1987, a supernova was seen to go off in the Large Magellanic cloud, a companion galaxy to the Milky Way. Luckily for astrophysicists, two neutrino detectors, one in Ohio and one in Japan, were operational at the time. About 18 h before the supernova was first observed, the detectors caught 19 neutrinos during a 12 s interval. This was an amazing confirmation of the concept of neutron star formation proposed by Zwicky and Baade 53 years earlier. The existence of pulsars, spinning neutron stars, was discovered back in 1967, but no neutrinos had ever been detected from a supernova explosion. There is a PBS NOVA special on the discovery of supernova 1987a as it is called which really conveys through interviews the sense of wonder and enthusiasm that scientists felt at that discovery.

  Fusion in the cores of stars converts hydrogen into helium, helium into carbon, and, for the most massive stars silicon and iron, but there the sequence ends. The elements such as copper, gold and tin that we see around us were created in supernovas. As James Dunlop puts it

  The past history of star-formation activity even affects today’s financial markets, with the seeming ever rising price of rare commodities such as gold being due, in large part, to the rarity and brevity of the violent supernova explosions in which all gold was originally forged.

  Supernovas: The Ultimate Cosmic Fireworks

  During the supernova phase, a star can become one billion times more luminous than the Sun, and for a short time outshine an entire galaxy. Supernovas are characterized by the way they brighten and fade and by their spectra. Supernovas as classified into several categories; Type Ia, Ib Ic and Type II. Of primary interest to cosmologists are Type Ia supernovas because they all reach a similar maximum luminosity. Type Ia supernovas occur by the thermonuclear explosion of a white dwarf star. These white dwarfs accrete mass from a companion star until they exceed their mass limit and a thermonuclear explosion occurs. Other supernovas are caused by iron core collapse of a massive star.

  The oldest records of supernovas come from China. In the year A.D. 1054, a supernova was observed in the constellation Taurus and recorded by Chinese astronomers. A star in that constellation became so bright that it was visible even in daylight. It could be seen with the naked eye for a few weeks following its peak in brightness. We see today at the location of this star an expanding cloud of gas known as the Crab Nebula (Fig. 3.5). The nebula consists of the outer layers of the star that were blasted into space at enormous speeds. Following a supernova event expanding clouds can be visible for thousands of years before fading from view. At the center of the Crab nebula lies a neutron star 15 miles in diameter that rotates over 30 times a second. This neutron star formed during the supernova event. The first supernova seen in a galaxy other than our own was observed in 1885 in the Andromeda nebula. Many supernovas have been observed in external galaxies since then. The recently discovered supernova in M101 also known as the Pinwheel Galaxy is shown in Fig. 3.6. The star was too faint to be detected in images taken with ground based telescopes and the Hubble Space Telescope prior to the explosion. The last supernova observed in our own galaxy was seen in 1604.

  Fig. 3.5The Crab Nebula, the result of a supernova seen in 1054 A.D., is filled with filaments. This image was taken by the Hubble Space Telescope. The Crab Nebula spans about 10 light-years. In the nebula’s very center lies a pulsar: a neutron star as massive as the Sun but with only the size of a small town. The Crab Pulsar rotates about 30 times each second (Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University))

  Fig. 3.6These images show Type Ia supernova PTF 11kly, the youngest ever detected, over three nights in August 2011. The left image taken on August 22 shows the location before the supernova went off. The center image taken on August 23 shows the supernova at about 10,000 times fainter than the human eye can detect. The right image taken on August 24 shows that the supernova is six times brighter than the previous day (Credit: Peter Nugent and the Palomar Transient Factory)

  In a typical galaxy we expect to see a supernova every 50 years. With modern digital detectors we can survey large areas of the sky and check for supernovas automatically. In 2011 290 supernovas were detected in this way.

  Supernovas as Cosmological Tools

  What kinds of stars end their lives as supernovas? It is believed that any star born with a mass of more than 8 solar masses will end its life as a supernova. Such supernovas have a wide range of peak luminosities. Supernovas in binary star systems (Type Ia supernovas) have a small range of luminosities and can be used as distance indicators. Binary stars consist of two stars orbiting each other, bound together by their mutual gravity. As the stars evolve, mass can be transferred from one to the other (Fig. 3.7). A white dwarf accretes matter until it reaches the limit of 1. 4 solar masses, at which point it explodes. The exact cause of the explosion is not known. Possibly, the star collapses to a neutron star, or the hydrogen in its atmosphere becomes so hot that it ignites and starts to fuse, producing a huge nuclear detonation. The fact that Type Ia supernovas explode when their progenitors all have the same mass explains why their peak luminosities are very similar.

  Fig. 3.7The mechanism leading to a Type Ia supernova (Credit: NASA, ESA and A. Feild (STScI))

  The search for high luminosity distance indicators is a holy grail of astronomy. It was the dream of Hubble and Sandage to measure the curvature of space by measuring the apparent brightness of objects of known luminosity located at increasingly large distances from us. The problem was that objects detectable at large distances, such as quasars, have a wide range of luminosity. If we were studying light bulbs instead of stars and all light bulbs were 75 W light bulbs, we could tell the distance to any light bulb at night by seeing how bright it appeared to our eyes. If however light bulb luminosities (wattages) varied from say 60 to 80 W, a bright light bulb in the sky could be a 60 W light bulb nearby or an 80 W light bulb a bit further away. We want to know the scatter in wattages so to speak of a given class of astronomical object such as quasars or supernovas. The smaller the scatter, the more accurately we can calculate the distance to the object.

  In order to measure the curvature of space or density of the universe, one has to be able to measure the apparent brightness of object of known luminosity. Why not use Cepheids, the variable stars that Hubble detected in the Andromeda nebula? The most luminous Cepheids are indeed quite bright, the brightest are about 30,000 times as bright as the Sun. With the Hubble Space Telescope, we can use Cepheids to measure the distances to galaxies as far as 50 million light years from our galaxy. The problem is that to measure the curvature of space we need to measure the distances to objects hundreds of times further away than Cepheids. Supernovas reach peak luminosities
of 10 billion solar luminosities, much more luminous than Cepheids. Supernovas are thus a perfect tool for cosmology. We will discuss these findings that were central to the discovery of dark energy in Chap.​ 7.

  Galaxies

  We know that galaxies are immense aggregations of stars, gas, and dust and dark matter held together by gravity. A wide variety of galaxies appear in images taken with telescopes. Broadly speaking galaxies can be placed in one of three categories: spiral galaxies, elliptical galaxies and irregular galaxies. The visible universe contains over 50 billion galaxies.

  Spiral galaxies (Fig. 3.8) contain flat disks with bulges at their centers. The disks contain cool gas and dust interspersed with hotter gas and they exhibit spiral arms. The disks show evidence of ongoing star formation. The disks of spiral galaxies lie within a halo that can extend out to over 100,000 light years. Most galaxies (about 80 %) in the universe are spirals, and they usually have fewer close neighbors than elliptical galaxies.

  Fig. 3.8Similar in size to the milky way, the spiral galaxy NGC 3370 lies about 100 million light-years away. The Hubble Space Telescope’s Advanced Camera for Surveys has imaged Cepheids stars in this galaxy to accurately determine NGC 3370’s distance. NGC 3370 was chosen for this study because in 1994 the spiral galaxy was also home to a well studied stellar explosion – a Type Ia supernova. Combining the known distance to this standard candle supernova, based on the Cepheid measurements, with observations of supernovas at even greater distances, has helped to reveal the expansion rate of the entire Universe itself (Credit: NASA, ESA, Hubble Heritage STScI/AURA)

  Elliptical galaxies (Fig. 3.9) lack a disk component and, indeed, look like footballs in shape. They contain very little gas and dust, although they do contain some very hot gas which emits X-rays. Elliptical galaxies are more common in clusters of galaxies, and they come in a wide range of masses, sizes, and luminosities. The largest known galaxies are elliptical galaxies and are about 50 times the size of our Milky Way.

  Fig. 3.9Elliptical galaxies lack gas and dust to form new stars. Their randomly swarming older stars, give them an ellipsoidal (egg-like) shape. M87 is the dominant galaxy at the center of the Virgo Galaxy Cluster. This elliptical galaxy is over 120,000 light-years in diameter (Credit: Robert Gendler www.​robgendlerastrop​ics.​com)

  Our third class of galaxies, irregular galaxies (Fig. 3.10) have a white to bluish color and are less organized than ellipticals or spirals. The nearby Magellanic clouds are an example of irregular galaxies. As we look into the past the fraction of galaxies classified as irregular rises dramatically. Dwarf irregular galaxies contain relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the universe. Some irregular galaxies are small spiral galaxies that are being distorted by the gravity of a larger neighbor.

  Fig. 3.10NGC 4449 is an irregular galaxy. It is less than 20,000 light-years across, similar in size to one of our Milky Way’s satellite galaxies, the Large Magellanic Cloud. This Hubble Space Telescope image highlights the reddish glow of hydrogen gas which traces star forming regions. Features include interstellar arcs and bubbles blown by short-lived, massive stars (Credit: Data – Hubble Legacy Archive, ESA, NASA; Processing – Robert Gendler, www.​robgendlerastrop​ics.​com)

  Cosmic Mayhem: Galactic Collisions

  The nearest large spiral galaxy to our own is Andromeda, about 2 million light years away, it is visible to the naked eye on dark nights as a faint, fuzzy patch of light. As the universe expands galaxies are drawn away from each other. In some instances two galaxies can be close enough that their gravity overcomes the expansion and the galaxies start to move towards each other. The Andromeda galaxy feels the pull of gravity from our own galaxy and is, in fact, moving toward us at about 300,000 miles per hour. The speed at which the two galaxies are approaching each other is increasing as they get closer to each other. Eventually (in about 2 billion years) the two galaxies will sideswipe each other and eventually merge to form a single elliptical galaxy. It is unlikely that individual stars will actually collide during this galaxy collision, why?

  If we were to shrink the Sun to the size of a basketball, the nearest star would be about 3,000 miles away. In contrast, if our galaxy were shrunk to the size of a basketball, Andromeda, the nearest large spiral, would only be about 4 m away. One can infer from these rough estimates that collisions between stars are very, very unlikely. It is equally clear, that since galaxies are separated by only a few times their diameters, galaxy collisions will occur fairly frequently in the universe. The collisions take hundreds of millions of years to occur, so we can, at best, see mere snapshots of galaxy collisions occurring now.

  Because galaxies were closer together in the past mergers (collisions) occurred more frequently. Images of galaxies at high redshifts show more distorted and irregular shapes, suggestive of merging events. It is absolutely remarkable that we can take pictures of galaxies as they appeared when the universe was less than a billion years old and compare these with galaxies that we see around us today.

  Figure 3.11 shows the results of a galaxy collision in a computer simulation. Depending on the relative masses, separations, and orientations of the merging spirals, objects with rings and tails can form. Galaxies like these are occasionally seen in nature (Fig. 3.12). Computer models also suggests that two spirals in collision can give rise to an elliptical galaxy. This hypothesis may explain why so many ellipticals are found in dense environments like galaxy clusters.

  Fig. 3.11A computer simulation of the future collision of the Milky Way and Andromeda. The simulation used a total of 310 million particles. The simulation shows the mixing of the old (red) bulge and young (blue) disk stars as the galaxies merge. Simulations can be used to study the details of the merging process and its consequences for the structure in elliptical galaxies. An actual example of such a merger is shown in Fig. 3.12 (Credit: John Dubinski, University of Toronto)

  Fig. 3.12A strongly interacting pair of spiral galaxies. The image displays tidal arms. The bridge between the galaxies is created by tidal forces. Galaxy mergers can trigger high rates of star formation (Credit: NASA, ESA, the Hubble Heritage Collaboration, and A. Evans, University of Virginia, Charlottesville/NRAO/Stony Brook University)

  Mergers are believed to be associated with star formation. If turning spiral galaxies into elliptical galaxies necessitates large bursts of star formation, have these been observed? Possibly. Some galaxies (starburst galaxies) are very bright at infrared wavelengths and show disturbed morphologies. They have extended tails and ring-like structures, which can be explained by merging. Stars form much more frequently in these galaxies than in the Milky Way.

  When we think of human development we discuss the role of nature versus nurture. Was a person born with a certain talent, or was that talent nurtured by the environment that person was in? As with human beings both effects play a role in the lives of galaxies. We shall see in Chap.​ 10 that we can use galaxies as fossils to learn about the distant past, but galaxy collisions make us realize the limitation of this approach.

  We have come a long way from the Greek vision of the cosmos. The modern cosmos is a very turbulent place, where stars are constantly being born and are exploding, and where galaxies collide in dramatic events that change their appearance forever.

  Supermassive Black Holes: The Monsters at the Center

  Galaxies called active galaxies have extremely bright central regions. The emitting region is so small it can look like a bright star in images of the galaxy. Such galaxies usually look like normal elliptical, or spirals, but they radiate enormous amounts of energy at radio wavelengths. These galaxies are known as active galaxies. The central region of these spiral galaxies varies in brightness over a timescale of a few weeks. We can use this fact to set a limit on the size of the emitting region. Since the fastest speed at which information can be conveyed is the speed of light, the emitting region must be smaller than a few light months in size. To und
erstand this let us imagine that you and a bunch of friends are spread out in space and you want to give the signal to everyone to turn on their flashlights. The fastest way to do this is to tell them to turn their flashlights on when they see yours go on. The signal from your flashlight travels at a finite speed, the speed of light. Assuming your friends react immediately if they are located a light year away they will turn their light on 1 year after you. The ones located a light month away will take 1 month to react to your signal. Thus a region of space filling 1 light year will take about 1 year to havet all the flashlights go on. This is in essence the argument on the limits of the size a region emitting light based on the time it takes for the light to vary in brightness.

  The small sizes derived from the light variability arguments suggest that some novel mechanism must be at work for producing the energy we see. The central light year of an active galaxy produces more light than all the 100 billion stars in the whole galaxy.

  Some elliptical galaxies also show strange activity at their centers. Associated with this activity are enormous blobs of radio emission connected to the centers by jets of radio emission. The jets can be up to several million light years in length. These galaxies known as radio galaxies are so powerful that one can see them out to enormously large distances. Indeed, for a while, the most distant objects known were radio galaxies. It is interesting that radio astronomy started when the developers of radar during World War II noticed radar echoes coming from the upper atmosphere which they associated with meteor trails. When the war ended, radio astronomy came into existence as the scientists followed up on these observations. It is a spectacular example of serendipity in research, that a study of radio emissions in the upper atmosphere led to the discovery of the most distant and most luminous known objects in the universe.