Cosmic Dawn

by George Rhee

  By looking over great distances we look back to ever earlier times. To understand our findings we make the assumption that, averaged over sufficiently large scales, the universe is essentially the same everywhere. We thus assume that the galaxies we see in the distant past resemble what galaxies in our local ‘neck of the woods’ would have looked like at comparably early times. The practical challenges are; the most distant galaxies are extremely faint, their light gets shifted to progressively longer wavelengths and, dust and gas can obscure the light from young stars.

  We shall describe this fascinating world of galaxies, the realm of the nebulae as Hubble called it, in Chap.​ 3.

  Further Reading

  The First Three Minutes. S. Weinberg. New York: Basic Books, 1993.

  Just Six Numbers. M. Rees. New York: Basic Books, 2000.

  The Magic Furnace: The Search for the Origins of Atoms. M. Chown. Oxford: Oxford university Press, 2001.

  The Origin of the Chemical Elements. R. J. Tayler and A. S. Everett. London: Wykeham, 1975.

  Seeing Cosmology Grow. P. J. E. Peebles. Annual Reviews of Astronomy and Astrophysics, 2012

  George RheeAstronomers' UniverseCosmic Dawn2013The Search for the First Stars and Galaxies10.1007/978-1-4614-7813-3_3© Springer Science+Business Media, LLC 2013

  3. The Visible Universe

  George Rhee1

  (1)Department of Physics & Astronomy, University of Nevada, Las Vegas, Nevada, USA

  Abstract

  For an astronomer, there are really two forms of matter; visible and invisible. Visible matter emits electromagnetic radiation, which travels through space and is detected with telescopes. Invisible matter does not produce detectable amounts of radiation. The presence of invisible, or dark matter, can only be inferred through its gravitational effect. The study of visible matter enables us to infer the distribution of the dark matter whose gravity was responsible for the formation of the first stars and galaxies. In this chapter we explore the visible matter as seen in stars, galaxies and gas.

  All humans are brothers. We came from the same supernova.

  Allan Sandage

  For an astronomer, there are really two forms of matter; visible and invisible. Visible matter emits electromagnetic radiation, which travels through space and is detected with telescopes. Invisible matter does not produce detectable amounts of radiation. The presence of invisible, or dark matter, can only be inferred through its gravitational effect. The study of visible matter enables us to infer the distribution of the dark matter whose gravity was responsible for the formation of the first stars and galaxies. In this chapter we explore the visible matter as seen in stars, galaxies and gas.

  The Lives of the Stars: Birth

  Galaxies such as the Milky Way are continuously forming stars. Stars form out of the very tenuous (by earth standards) gas clouds that exist between the stars. Known as molecular clouds, these have temperatures of 10–30 K, sufficiently low that hydrogen can exist in molecular form. Interestingly, molecular clouds contain other molecules, such as carbon monoxide, water, ethanol, ammonia, and even amino acids. The existence of organic molecules in the interstellar medium suggests the possibility that life may have originated in molecular clouds in interstellar space.

  The inward acting force of gravity is countered by the gas pressure in these clouds. Star formation can be triggered in molecular clouds by a sudden increase in density. This may be caused by the formation of stars nearby; star formation can spread like wildfire once it gets going. Figure 3.1 shows a nearby star-forming region with young stars and gas clouds.

  Fig. 3.1Undulating bright ridges and dusty clouds cross this close-up of the nearby star-forming region known as the Lagoon Nebula. A color composite of visible and near-infrared data from the 8-m Gemini South Telescope, the entire view spans about 20 light-years. The cosmic Lagoon is found some 5,000 light-years away toward constellation Sagittarius and the center of our Milky Way Galaxy (Credit: Julia I. Arias and Rodolfo H. Barbá, (Dept. Fisica, Univ. de La Serena), ICATE-CONICET, Gemini Observatory/AURA)

  Molecular clouds rotate, such that when clouds start to collapse, a disk forms. It is out of such a disk that our own solar system is believed to have formed. These disks have been observed, associated with jets of matter flowing along the rotation axis of protostars.

  In the initial stages of their lives, stars shine very bright. Since they are radiating away a lot of their energy, they can shrink in size, releasing more energy and heating up the core of the star. Eventually, the temperature rises to the point that nuclear fusion reactions begin in the star’s core. The central temperature of the star continues to rise, as do fusion rates, until a balance is achieved. That is to say, the energy generated in the star’s core becomes equal to the energy sent into space at the surface of the star.

  Brown Dwarfs and Planets Outside the Solar System

  The minimum mass required for a contracting gas cloud to turn into a star is about one tenth of the mass of the Sun. These stars are called red dwarfs, they burn their hydrogen so slowly that they can keep nuclear fusion going for 100 times longer than the Sun, about 1 trillion years. Clouds of gas having a mass less than one tenth of the Sun’s mass do not heat up sufficiently in their core for nuclear reactions to start. Such objects are known as brown dwarfs. Some examples of brown dwarfs have been found with the Hubble Space Telescope. Because brown dwarfs are so faint, it is difficult to estimate how many of them exist in our galaxy. We do know that less massive stars occur much more frequently than more massive stars. If we extrapolate this trend to brown dwarfs, the numbers of these objects could be quite high.

  It is not clear what fraction of stars have their own planetary systems. Discoveries of several hundred nearby planetary systems suggest that the formation of planets surrounding stars may be a fairly frequent phenomenon.

  The History of Star Formation in the Universe

  How can we detect the birth of stars in other galaxies? Newly born stars are still surrounded by clouds of gas, and dust, which absorbs the light from the young stars and re-emits it at infrared wavelengths. Our galaxy re-emits about a third of the light from stars as infrared radiation. Telescopes such as the European Space Agency’s Herschel telescope (Fig. 3.2) are providing a new view of star-forming regions in our and other galaxies.

  Fig. 3.2Front view of the Herschel Satellite. The first observatory to cover the entire range from far-infrared to sub-millimeter wavelengths and bridge the two. The satellite was launched 14 May 2009 on an Ariane rocket from Kourou French Guyana (Credit: ESA/AOES Medialab)

  We can estimate the amount of star formation taking place by measuring the infrared light from galaxies. Star-forming regions have hot young stars associated with them. These stars emit ultraviolet radiation. We can use the amount of ultraviolet light emitted per unit volume to estimate the star formation rate per unit volume. We must also take into account the fact that ultraviolet light is scattered by dust in galaxies.

  We can measure star-forming activity in nearby galaxies, but we can do the same with distant galaxies and compare the results. Are distant galaxies forming stars at a faster rate than nearby galaxies or has the rate of star formation been fairly constant over time? The answer is shown in Fig. 3.3 which depicts the star formation history of the universe. The rate of star formation increases steeply during the first billion years, peaks about 1 billion years after that, and then gradually decreases until the present day. It is remarkable that we can measure how the number of stars per year being born in a volume of the universe has changed since the big bang. We have come a long way in the 400 years since the telescope began to be used as an astronomical tool!

  Fig. 3.3The cosmic star formation history derived from the ultraviolet luminosities of galaxies. The rate at which a typical region of space formed stars increased rapidly during the first 2 billion years of the universe and then started a slow decline to the present day. It is during the first 2 billion years following the big bang that galax
ies such as our Milky Way came into existence. The units of the horizontal axis are Gyr, short for Gigayear which is a billion years

  The Lives of the Stars: Middle Age

  Stars are massive spheres of hot gas that glow because of their high temperatures. The nearest middle-aged star to us is the Sun, a rather average star in our galaxy. The radius of the Sun is about twice the Earth-Moon distance. The mass of the Sun is about 300,000 times the Earth’s mass. If we could capture 1 s worth of the total energy emitted by the Sun, we could fill the energy needs of the human race for the next million years. The average density of the Sun is about equal to that of water.

  The composition of the Sun is 28 % helium, 70 % hydrogen and 2 % heavier elements. The surface temperature of the Sun is about 6,000 K or about double the temperature of a blowtorch. The temperature near the center of the Sun is about 15 million degrees Kelvin. It was only in the 1930s that the true source of the Sun’s energy was discovered. We pointed out in Chap.​ 2, that the center of the Sun is hot enough that nuclear fusion reactions can take place. Keep in mind though that 90 % of the helium that is present in the Sun was produced in the early universe.

  The fusion of hydrogen into helium releases energy, and it is this energy that powers the Sun and makes it shine. The fusion process takes place in three steps. The net effect of these three steps is that four protons fuse together to produce a helium nucleus and some radiation. This fusion process does not take place at room temperature because of the repulsion force between protons. The strong force, which holds atomic nuclei together, has a very short range. Two protons must collide with very high energy in order to get close enough to each other to feel the strong force. This only happens at very high temperatures.

  Since a helium nucleus is less massive than four protons, the mass difference must be accounted for. The mass difference is, in fact, converted to energy, following Einstein’s famous E = mc2 equation. The mass difference is a very small fraction, 0.7 % of the original mass. When 1 kg of hydrogen fuses into helium, 993 g of helium are produced, while 7 g of mass turn into energy. Four million tons of mass per second are converted to energy in the Sun. The Sun is, in effect, a controlled hydrogen bomb. It is amazing that the laws of nature have given rise to nuclear fusion reactors capable of maintaining a stable energy output over billions of years.

  We know how much energy the Sun produces per second, and we know the mass of the Sun, so we can calculate how much longer we can expect the Sun to keep on shining. If the Sun remains as it is until it has used up all of its hydrogen supply, it will have enough fuel to keep shining for another 70 billion years. In fact the Sun cannot use up all its hydrogen because it is only in the core that it is hot enough for fusion to happen. Calculations show that the Sun will change in structure after using up about 13 % of its total store of hydrogen. This should take about 9 billion years. Since the Sun has already been around for 4.6 billion years, it is already in middle age.

  The fact that stars like the Sun shine by converting hydrogen into helium together with the fact that stars have a finite mass and thus a finite supply of fuel suggests that stars evolve. Stars are born and go through several stages before they reach the end of their energy producing “lives”. The Sun is not an extreme star. Its properties such as mass and size are near the middle of the properties that are measured for stars in our galaxy.

  The Lives of the Stars: Mid-Life Crisis!

  In a few billion years the hydrogen in the Sun’s core will be converted to helium. The core of the Sun will then cease to generate energy, and start to shrink and heat up. Surprisingly, perhaps, while the core of the Sun undergoes this, the outer layers do just the opposite. They expand and cool down. The result is that, during this phase, a star like the Sun appears to get redder and larger, becoming what is known as a red giant. During the red giant stage, the Sun will engulf the inner planets and vaporize any life remaining on earth. If any humans are still around, they had better plan on finding a home on another planet or moon farther out from the Sun. In the red giant stage, the Sun will start fusing hydrogen into helium in a shell surrounding its core. Meanwhile, the core shrinks and gets hotter. When the core temperature reaches 100 million degrees, it becomes hot enough for helium fusion to take place, producing a heavier element, carbon. We are born of stars in that all the carbon in our bodies was manufactured inside them. During the red giant stage, stars can lose substantial amounts of mass and become surrounded by clouds of their own gas. The Hubble Space Telescope has taken some stunning images of these clouds.

  Figure 3.4 is a plot of surface temperature against energy output (or luminosity as we call it) for several stars. We can see that the range of star luminosities found in nature is much larger than the range of surface temperatures. When the stars are in the hydrogen burning phase of their lives, they occupy a diagonal line in this diagram. We refer to this line as the main sequence. The main sequence is ordered by mass such that the most massive stars are hot, blue and of high luminosity while the least massive stars are cool, red and have low luminosity. When a star becomes a red giant it leaves the main sequence and becomes more luminous and redder in color. Massive stars are known to have shorter lifetimes, so for a cluster of stars all born with different masses, but at the same time, the main sequence will appear to peel off. The point at which stars are leaving the main sequence is known as the turn-off point. The turn-off point is a measure of the age of the star cluster. Some of these clusters are very old, between 11 and 13 billion years. These ages give us an estimate of the age of the galaxy since it must be older than the oldest objects in it. The oldest stars in our galaxy have very little iron in them since they were formed from gas that had not been polluted so to speak by supernova explosions. We have found a 13 billion year old star in our galaxy that has 100,000 times less iron than is seen in the Sun. Since there actually is some iron in this star it cannot be one of the first stars to form after the big bang. We are still searching for the first stars.

  Fig. 3.4The Hertzrpung-Russell diagram. Each point represents a star. The star’s surface temperature is plotted against its luminosity. An L value of 1 in these units is the luminosity of the Sun. Our Sun is shown as the larger yellow circle. The points are color coded to make the point that stars with low surface temperatures appear red whereas hot stars appear blue in the night sky

  Certain stars during the red giant phase can become unstable and start pulsating. These are known as variable stars since the pulsation results in periodic variations in brightness. The pulsation time can vary from hours to years, depending on the kind of star and its mass. We can use these variable stars to estimate their distances from us. This is why Hubble was so excited to discover a variable star in the Andromeda nebula (see Chap.​ 1). It meant he could calculate the distance to that nebula and establish that it was a galaxy like our milky way. We are fortunate that these stars are so bright. A star like our Sun would not be detectable at the distance of the Andromeda galaxy.

  Rest in Peace: The Death of Stars

  The life of a star is a battle between the forces that generate heat in the stellar interior and the force of gravity, which wants to crush matter ever inward. After stars run out of fuel, they reach the final stages of their lives. The masses of stars determine their ultimate fate. Stars less massive than 1. 4 solar masses end their lives as white dwarfs. After the red giant phase, the Sun will shrink to the size of the Earth and stop generating energy. It will shine with a bluish glow but will be no brighter in the Earth’s sky than the full moon today. The only reason white dwarfs shine is that they have a high temperature, but eventually they will cool because the energy they radiate is not being replaced. The gravity on the surface of a white dwarf is very strong. If you dived off a 30 ft platform into a swimming pool on a white dwarf, it would take you about 1/100 of a second to hit the water compared to about 1.4 s on earth. The matter in a white dwarf has a crystalline structure, but what stops the white dwarf from collapsi
ng?

  An effect called the exclusion principle provides an effective counter to the crushing force of gravity. Electrons do not “like” to occupy the same position when they have the same velocities. A consequence of this is that it costs energy to shrink a star like the Sun beyond the size of the Earth, and a balance point is reached. A sugar cube’s worth of a white dwarf weighs about a ton. A lump the size of beach ball weighs as much as an ocean liner.

  Combining quantum physics and the theory of relativity an Indian physicist Subrahmanyan Chandrasekhar predicted in 1930, that a white dwarf having a mass larger than 1. 4 solar masses, will collapse to form a neutron star. The calculation was carried out on a ship sailing from India to England where the 20 year old Chandrasekhar was to begin graduate study in physics at Cambridge University. A neutron star has a radius of a few miles, and a teaspoonful of such a star weighs as much as a battleship. Neutron stars have their own upper mass limit. Stars more massive than five solar masses or so will end their lives as black holes–objects from which nothing, not even light itself, can escape. White dwarfs, neutron stars, and black holes are the three possible end states of stellar evolution.