THE FIRST STARS
ABOUT 5 JANUARY; ABOUT 14,600 METERS UP THE LINE
Compared to our knowledge of other periods in the Universe’s development, there is still relatively little known about the cosmic dark ages, the period in our Universe’s history when no visible light was detectable. After the Big Bang, the cosmic microwave background radiation was for many centuries the only residue left from this momentous event. And for millions of years the Universe was basically opaque. But tremendous changes were about to occur, ones that would establish the foundations of the physical Universe as we know it, and create the conditions which were to prove indispensable to the formation of elements other than those created by primordial nucleosynthesis.
Current thinking in cosmology holds that after recombination, the dark ages began to wane, as we noted in the previous chapter, after about 200 million years. This date, based on the most recent data, surprised some observers, since it had been previously believed that the first stars had not materialized until 400 million years after the Big Bang. (Some now argue that stars before 100 million years after the Big Bang are likely.1) Evidence has been discovered of the remnants of these earliest of all stars. What do we know of the early Universe’s history?
Ionization and Reionization in the Early Universe
The period of recombination that occurred in the early Universe cleared up the cloud of ionized gas that had existed earlier and made the Universe’s hydrogen electrically neutral, temporarily producing a transparent Universe, albeit one in which there were no significant light sources. But this neutral hydrogen did absorb certain wavelengths of light, and as it spread out during the Universe’s expansion, it blocked much of the electromagnetic spectrum. This has made the study of the very earliest light-producing bodies in the Universe problematic. But it was some of these same early light sources, the very earliest stars, that probably contributed to the Universe’s reionization. The earliest stars may have been quite massive. According to cosmologist Abraham Loeb of Harvard, the very powerful ultraviolet radiation emissions from these ancient, enormous (and comparatively short-lived) stars heated and ionized the areas of gas that surrounded them. The formation of the first galaxies several hundred million years later seems to have contributed to this process in an even more significant way. The ionization of hydrogen radically altered its structure. As Loeb has summarized reionization,
Observations of the spectra of early galaxies, quasars, and gamma ray bursts indicate that less than a billion years [after recombination] later the same [hydrogen] gas underwent a wrenching transition from atoms back to their constituent protons and electrons…Indeed, the bulk of the Universe’s ordinary matter today is in the form of free electrons and protons, located deep in intergalactic space.2
So the reionization process seems to have been completed by around 1 billion years after the Big Bang. By this time the expansion of the Universe was such that the fundamental particles that had formerly been the constituents of hydrogen were not densely enough concentrated in most regions to block our view of the early Universe’s structures. What is called the intergalactic medium, or IGM, was now much more spread out than it was in the period before recombination. The Universe was again transparent.3
So how did the stars which were part of this process begin to form, the stars which would eventually become part of galactic structures of immense size?
The Formation of Stars
There are aspects of star formation that are poorly understood. But in general, star creation begins when energy-matter begins to aggregate in particular regions of space-time marked by energy fluctuations and gravitational instability. Most stars begin their lives as diffuse balls of hydrogen, what are known as protostars. Gravitation causes this aggregation of hydrogen gas to contract, along with dusty material. (Dust-like material was, however, probably absent when the first stars were forming.) The gravity must be strong enough to overcome the resistance of magnetic fields, and the turbulence of the gas and dust themselves. Dark matter appears to be a factor in the condensation of this material, but recent research has shown that it may not play a critical role.4 As the cloud of dust and gas contracts, it appears that part of this mass collapses faster than it can cool down, forming what is called a minimum fragment mass. This minimum mass is thought to typically be one one-hundredth of a solar mass [our Sun=1 solar mass] in size. During contraction the mass of material out of which the star will be formed assumes a disk-like shape. It also begins to spin rapidly as it contracts. As energy is expended in this process, spin (angular momentum) slows. The central part of the disk gathers material while the outer part of the disk often produces planets through the accretion of solid materials. As it loses angular momentum, the mass of material at the center of the disk gradually begins to form into a sphere. Because of immense pressure at its core, the center of the emerging star rises in temperature, and upon hitting 10 million degrees Kelvin, or about 18 million degrees Fahrenheit, the fusion process of hydrogen to helium begins.5 The length of a star’s life will be determined by how much mass it has to begin with. The majority of stars in our galaxy are called main sequence stars. They are neither immense in size (by stellar standards), and hence prone to instability, nor are they so small that their centers never ignite. They are relatively stable and they can burn for a very long time. Our Sun, as we will see shortly, is a notable example of such a star.
The Life Cycle of a Star
After protostars form from nebulae of dust and gases, if they are main sequence stars, they settle into the billions of years of their “life span”. The vast majority of the stars we see, about 90%, are using hydrogen as their chief fuel. Hydrogen burning releases about ten times as much energy per gram as helium burning.6 A star exists in a constant state of “conflict”, so to speak. Astrophysicist David Arnett puts it this way:
Stars last so long because of a delicate state of balance. The crush of gravity which pulls them in upon themselves is balanced by the increasingly high pressures in their interiors. Their prodigious radiation of energy, as light and neutrinos, is balanced by a prodigious generation of energy, by nuclear burning and gravitational contraction.7
Stars are therefore immense nuclear furnaces in which gravity and pressure are waging constant war. Stars on the main sequence can be engaged in this struggle for more than 10,000,000,000 years. It is not yet known what fraction of stars have planetary systems, but apparently a significant number of them do in light of the discovery of various exoplanets (worlds orbiting stars other than our Sun).
The ultimate fate of a star depends on its mass. Main sequence stars will, when they exhaust their fuel, enter into a red giant phase. A red giant looks red because of its lower temperature, and the exhaustion and collapse of its center are accompanied by a vast expansion of its outer layers. A low mass star (like our Sun) will go through a red giant phase and then collapse, first into a white dwarf, and then ultimately to a black dwarf once it has cooled off completely. If a star is very large compared to our Sun (anywhere from about five solar masses on upward), it will have sufficient mass to not only become a red giant when it has burned through all its fuel, but to go supernova when its nuclear fuel is exhausted. In a supernova event, the star’s core goes through a catastrophic implosion, driving its heat up once more and unleashing an enormous wave of neutrinos. This shock wave of neutrinos will blast through the star’s outer layers, triggering an explosion of unimaginable violence.8 If what remains after the supernova event is 1.4 to about 3 times as massive as our Sun, it will become a neutron star.9 Neutron stars are the densest objects we know of, and even the smallest amounts of matter have enormous weight in them. If a star is of very high mass its remainder will, after its supernova event, collapse into a black hole, where the intense gravitational field created will even preclude the emission of light.
Types of Star Populations, Star Abundances
The earliest stars are known as Population III stars. They are usually thought to have been massively huge and of relatively short duration, although recent research indicates that the first generation of them may not have been as massive as previously believed, and that many of them were part of binary star systems. Subsequent generations of Population III stars are still considered primordial, however, and the second generation of Population III stars was probably much more numerous. There are thought to be no Population III stars remaining.10 Astrophysicists and astronomers call such early stars metal-poor or of low metallicity, metal in this context meaning any element heavier than hydrogen or helium. But many researchers are convinced that because of the large masses of many of the earliest stars (perhaps of the second generation) there were numerous supernova events among them. Population II stars are the oldest observable stars and generally have greater metallicity than Population III stars could have had, since they are so often formed out of the remnants of the oldest stars. In our galaxy Population II stars make up the great bulk of the galactic center. The stars with the greatest metallicity are the Population I stars, the newest ones, a group which includes our Sun.
Estimating the number of stars has always been problematic. In 2005 astrophysicist Alan Heavens estimated that over the course of the Universe’s 13.7 billion year existence 9 x 1021—9 sextillion—stars have been formed. (To put this number into perspective, the number of seconds that have passed since the Big Bang is approximately 4.32 x 1017. This means that the number of stars that have existed, according to this estimate, is more than 20,000 times larger.)11 But in 2010 a new estimate was made, based on a reassessment of the number of dwarf stars in elliptical galaxies (see next chapter). The new estimate is even more overwhelming: from 100 sextillion to perhaps 300 sextillion stars are thought to exist. Undoubtedly, these estimates will vary in the years to come.12
So why do stars have any degree of metallicity at all, and why do Population I stars have the greatest amounts of it? The answer is to be found in the processes by which the elements beyond hydrogen and helium were created.
All elements are the descendants of the nuclei that formed after the first three minutes of the Universe’s existence had gone by. All of the elements (with some rare exceptions; see below) were created either in the primordial nucleosynthesis that followed the Big Bang or in the processes of stellar nucleosynthesis and the phenomena associated with them. The elements that make up the bulk of the visible matter in the Universe are of primordial origin. But the largest number of different elements are of stellar/supernova origin.
At first, the problem of how stellar nucleosynthesis occurred was a daunting one, inasmuch as the temperatures by which hydrogen and helium could be converted into the heavier elements seemed unlikely to occur naturally. The first serious breakthrough in this research came in 1953 through the work of the astronomer Fred Hoyle and a team led by physicist Willy Fowler. These scientists demonstrated the theoretical means by which helium could be converted first to beryllium and then to carbon, the basis of living things. Further, their research indicated that when most stars die, they go through a long process of alternating implosion and stabilization which creates, over time, a wide variety of temperatures and pressures within them. A single star can be the birthplace of numerous elements during this very long process.13 Temperatures in this process can rise to phenomenal levels. The conversion of helium into carbon requires interior temperatures of more than 100 million degrees K, which are produced in the bigger red giants.14 The result of the process of alternating implosion and stabilization is that a red giant is structured in layers. The outer layer of a truly huge red giant consists of hydrogen and helium. Other layers contain successively heavier elements, such as carbon and nitrogen, then layers that contain heavier elements still, such as sodium and potassium. And at the core can be found heavy elements such as iron.15
In a landmark 1957 paper, Fowler, Hoyle, and two other scientists, E. Margaret Burbidge and G. R. Burbidge laid out the basic processes of nucleosynthesis in such a convincing manner that their work is still a major source of our knowledge. B2FH, as they are often referred to, established the following:
- Elements can be transmuted, i.e., converted to other elements.
-The abundance distribution of elements gives us important clues as to their origins.
-All elements evolved from hydrogen, the most abundant element in the Universe
-The hypothesis that stars are the birthplace of most elements has the greatest observational support.
-As each nuclear fuel within a star is exhausted, the temperature of the star rises.
-It is this succession of higher temperatures at different stages in a star’s life that gives rise to heavier and more stable elements.
-Nuclear evolution is most advanced at the center of a star.
-Stars of different ages have different chemical compositions.
-Stellar material is spread by explosions, supernova explosions being the most efficient means by which stars scatter material.
-There is a wide variety of different processes by which stars create particular elements, and these processes occur over widely varying timeframes.16
The findings and hypotheses discussed in the B2FH paper have held up well over the years, although major research continues on stellar nucleosynthesis, and many of the observations the authors made have been explored in much greater detail. But the major points remain: the heavier elements are created in massive stars or in the supernova explosions that follow the implosions of those stars’ interiors. These elements are distributed by these same explosions.
Of all the stellar processes, the supernovae appear to be of the greatest significance in nucleosynthesis. The enormous amounts of matter that are emitted in supernova implosion/explosions spread elements that were contained in the exploded stars throughout the surrounding space, although elements at the very core of such stars are often captured in black holes. Since the Earth itself contains so much iron, almost 35% of its mass, the synthesis of this element was particularly significant to us. Recent research indicates that iron was first synthesized early in the history of the galaxy. The synthesis took place in the aftermath of the explosions of super-massive Population III stars, explosions which seem to have been of an extremely violent nature.17 Iron was also synthesized at the core of Population II and Population I stars that reached red giant status, and it is the last fuel at the core of the star that the star attempts to burn, without success. This is a sign that the red giant, if of sufficient mass (one much larger than our Sun), is nearing supernova status.
It is these implosions/explosions of massive red giants, called by astronomers Type II supernovae, that synthesize all the heaviest elements in the periodic table. The tremendous shock waves created by Type II supernova events blast any atoms within their range with huge numbers of neutrons contained within the supernova. The nuclei so affected can form all the heavy elements right up through uranium by “fattening up” on such masses of neutrons. When nuclei absorb too many neutrons, a neutron is converted into a proton through the process of negative beta decay (in other words, when an electron is emitted from an atom and a neutron undergoes a change), thus changing the atom from one type of element to another. The heaviest elements are neutron-rich, with very high isotopic numbers, such as 238uranium. Astronomers call this the r process, and among its products are gold, platinum, and all the highly radioactive elements.18 Type II supernovae tend to be found in spiral galaxies—like ours.19
Stellar winds can also spread star material outward, but such processes appear to be less efficient than the scattering produced by supernovae. Stellar winds are hot, ionized particles of dust and gas driven from the surface of stars by the pressure of the radiation that accumulates in their outer layers. They are a common feature of red giants. The material blasted out in these winds can be very important in the formation of new stars and planets.20
There are a handful of exceptions to the usual processes. As two cosmologists have put it,
The stars are the source of all the heavier elements of the periodic table, from carbon on up. The common isotopes of the elements between helium and carbon (beryllium, lithium, and boron) cannot be generated by ordinary stellar nucleosynthesis but are produced mainly by reactions involving cosmic rays. Cosmic rays are high-energy, relativistic particles, mostly protons, which are ejected from pulsars, supernovae, and other energetic sources. When these particles traverse interstellar gas, some collisions with the gas particles are inevitable. If the cloud has been enriched with carbon and oxygen by earlier generations of stars, a proton will occasionally strike a nucleus of one of these atoms; with so much energy, the proton literally knocks the nucleus apart, creating the light elements. The rarity of these formation processes accounts for the scarcity of these isotopes; they are by far the least abundant.21
Therefore, the elements out of which humans are comprised have in fact existed from the time of the Earth’s origins. The hydrogen in their bodies was made less than 400,000 years after the Big Bang. The other elements that make up humans are the products of solar furnaces and inconceivably violent explosions, which broke down and rearranged the elegantly simple hydrogen and helium atoms and produced an entire menagerie of unexpected specimens. Since the Sun is the source of all the elements out of which we are composed (probably bequeathed to it by supernovae) then it must be said that nothing out of which a human being is composed is less than 4.6 billion years old.
Some people scoff at the phrase, “Humans are made out of star stuff”, but it is, in fact, simply the bare expression of the truth. The formation of elements heavier than hydrogen, helium, and lithium took the rise of life in the cosmos from a remote possibility to a distinct one. Much of what we are made of indeed originated in the stars. If stars had not come into existence, human life wouldn’t have either. So when we are looking for our ultimate origins, we need do nothing more than venture outside on a clear, moonless night—and look up.