A. The First Stars and the Milky Way
Matter generates forces. Atoms, as small as they are, all generate a gravitational force that attracts other atoms. Gravity is the only force that can act across astronomical distances, so it quickly became the dominant force in shaping the growing universe.
If the atoms in the early universe had been spaced apart perfectly evenly, then each atom would have been gravitationally pulled “this way” just as much as “that way.” These forces would have cancelled each other out, leading to a static situation. Atoms never would have coalesced together, and the universe would still be a featureless cloud of gas today! Luckily for us, atoms were randomly concentrated a little more heavily in some regions of space than other regions. This lumpiness is referred to as the heterogeneity of matter. Our best understanding is that the heterogeneity started as random quantum fluctuations during the big bang. 2
The heavily concentrated atoms eventually started to clump together into giant molecular clouds hundreds of light-years across. Because a molecular cloud is itself heterogeneous, when it contracts it breaks up into several smaller clouds. After millions of years of gravitational clumping, the universe got populated by ever-denser clumps of atoms, separated by ever-sparser regions of empty space.
When a critical mass of hydrogen atoms gets together, the force of gravity shapes the cluster into a coherent sphere: the beginning of a star. The first stars (much older than the sun) formed about 200,000,000 years after the big bang. 3 Around the same time, stars started grouping themselves into the first galaxies, including our own Milky Way. 4 A galaxy is a large collective of stars permeated by gas and dust, the interstellar medium. Most galaxies, including the Milky Way, become so dense at the center as to form a supermassive black hole, millions of times more massive than a star. 5 Ours is called Sagittarius A* because it is behind the constellation Sagittarius.
On an even larger level, galaxies gravitationally bind each other in small groups or large clusters. The Milky Way is part of a local group containing tens of galaxies, including three others that are barely visible to the naked eye. Galaxies within a group orbit each other and regularly collide and coalesce, influencing star formation. In fact, there is evidence of local group galaxies’ colliding with the Milky Way in the past 6 , the future, 7 and maybe even the present. 8
The universe is now so large that matter is negligible compared to space. On the largest scales, gravity is getting dominated by a mysterious “dark” energy, which acts like an outward pressure on space. 9 This will have profound consequences for the universe’s fate. Fortunately, it is merely interesting trivia in a study of our past and present.
B. How Stars Make Energy and Elements
Stars are the powerhouses of the universe. We know that the sun is our planet’s source of energy. More than that, the materials of planet Earth itself were forged in the bellies of stars long gone.
A star is gaseous, and we know from ordinary experience that gas can be compressed. After achieving a critical mass, a star is forevermore gravitationally bound. It is committed to attempting suicide by compressing itself to a tiny point, but it can’t do so as long as other forces intervene to hold it up. Those opposing forces are generated by the star’s own gravitational energy! This makes a star a perfect example of a negative feedback loop, a system whose effects counterbalance their causes and lead to equilibrium.
The immense gravitational pressure makes the star exceedingly hot and dense. Hot gas exerts pressure and tends to expand, like the air inside a popcorn kernel. This heat pressure is what pushes back against gravity to support the star. The heat tears atoms apart, so a star (like the early universe) is made of plasma, a big ball of protons and electrons zipping around freely. At the critical temperature of about 10,000,000 Kelvins, 1 the protons in a star’s core become hot enough to fuse together into helium nuclei. Thus begins stellar nucleosynthesis, the creation of new atomic nuclei inside stars. Each time two nuclei fuse together to create a larger one, a small fraction of their mass is converted to energy. The cumulative energy of all these nuclear reactions keeps the star hot and pressurized for billions of years. Stellar nucleosynthesis involves a force called the weak nuclear interaction, which is not part of our ordinary macroscopic lives.
Helium is denser than hydrogen, so it sinks to the star’s core. Eventually, the helium core becomes so dense and hot (over 100,000,000 K!) that the star enters a whole new sequence of reactions. Three heliums fuse together to create carbon. The carbon sinks into a smaller core, until it is hot and dense enough to fuse with helium to create oxygen. This pathway can continue for quite a few steps, until the star’s deepest core is synthesizing iron and nickel. Some stars are able to synthesize elements as heavy as bismuth, the heaviest stable element.
At the star’s surface, a steady stream of matter and energy emanates into the interstellar medium. Nuclear fusion in the core creates gamma-ray photons. By the time those photons reach the star’s surface, they have cooled. A star radiates electromagnetic energy from X-rays to radio waves, mostly in the middle of the spectrum where light is visible. 2 The stream of matter is called solar wind. It is made of thin plasma that is hot and fast enough to escape the star’s gravity. Since plasma particles are electrically charged, solar wind has electromagnetic effects. We see clear evidence of solar wind today in the Northern Lights, radio interference, and the tails of comets as they approach the sun.
A small – medium star, about the size of the sun, ends its life in a whisper. Its outer layers gradually dissipate into space, blown away by a super-hot core. This allows a rich mixture of elements to be made available for the next generation of stars and planets. A more massive star dies a more spectacular death, in the form of a supernova explosion. Astronomers routinely observe supernovae in remote galaxies, and occasionally in our own. A supernova is the third major natural source of new elements. It produces enough energy to fuse heavy nuclei together, even if only for a few seconds. Supernova nucleosynthesis is the only source of very heavy elements such as uranium.
C. Atoms: from Physics to Chemistry
After synthesizing almost 100 new kinds of atomic nuclei, the first generation of stars exploded or dissipated, releasing their new nuclei into the molecular gas clouds of outer space. When these nuclei cooled down, they captured electrons to form complete atoms. Quantum physics is the science of fundamental particles and their assembly into atoms. Chemistry picks up from there, describing the interactions of atoms to create larger and more complex forms of matter.
The structure of an atom is a small, positively charged nucleus (protons and neutrons) surrounded by a cloud of negatively charged electrons. The nucleus is permanent, while electrons have some freedom to come and go. Electrons situate themselves in orbitals around the nucleus of the atom, somewhat (but only somewhat) analogous to planets orbiting the sun. Orbitals come in layers. The outermost orbital is the layer that chemists call the valence orbital, and it is especially important because this is where the atom interacts with the outside world. It is the coming and going of valence electrons that causes most electrical and chemical phenomena.
Atoms were a major step forward in the early universe. They make ideal building blocks for matter. Atoms are complex enough to provide an endless variety of combinations. They have a natural tendency to bond together to form larger units such as molecules and crystals. Atoms provide the tangible structure of matter, and their bonds influence intangible physical characteristics from color to hardness. Just as importantly, atomic bonds store energy in a manner that is easy to form, sustain, and access. 3 The energy that powers our homes, cars, and bodies all comes from making and breaking bonds between atoms. The energy stored within the nucleus of an atom is about 1,000,000 times greater than the energy in chemical bonds. However, that energy is notoriously difficult to store or access. It takes stars and specialized power plants to harness nuclear energy. You can access chemical energy just by eating a sandwich.
A substance made out of identical molecules is a compound. Water is a compound that is common in outer space as well as on Earth. 4 Water’s chemical notation H2O indicates that each molecule of water is formed from two hydrogen atoms bonded to one oxygen atom. Many compounds form in outer space, ranging from hydrogen’s stable form as a diatomic compound (H2) to massive balls of carbon atoms as large as fullerene (C70). Some chemicals are inherently unstable. Molecules like C5H6 simply fall apart, while atoms like H are highly attractive to other particles. The end result is a cosmic form of evolution: the stable substances become the common ones in the gases and dusts of outer space.
D. Stability, Complexity, and Chance
We have glimpsed the beginning of a theme that will persist throughout natural history – the progression toward stability and complexity. The early universe consisted of amorphous, ephemeral fundamental particles. After hundreds of millions of years, they had coalesced into long-lived molecules, stars, and galaxies. Molecules have structure and asymmetries. This allows them to diversify and to relate to each other in numerous ways, characteristics that we can think of as bearing “information.”
Do the developing stability and complexity of nature point to a “plan”? After all, the most complex structures in the known universe are we humans ourselves, and we owe our lives to stable, complex environmental conditions on Earth. From a religious perspective, it feels like the world is literally God’s gift to us. But of course, it’s easy to describe anything as pre-ordained in hindsight. Complexity by definition means that many options are possible. Some of them happen, and some don’t. Nature is full of chance moments. We have no authority to argue that any outcome “beat the odds” or “had to be” unless we predicted it beforehand.
As for our reliance on the environment, we have to remember the order of things. It’s not that humans had random needs, which were then met fortuitously by nature. The world was here first; humans evolved in response to it.
The anthropic principle is a scientific point of reference that looks backward from the human present to frame the past. If anything would have prevented our existence, then we know it didn’t happen. The anthropic principle does not address “why” things happened, but it does have some predictive power. Though scientists debate what the early Earth was like, they all have to agree that at some time it had the right kind of chemistry to support the origin of life, simply because we know that life originated here! Now, this is not the same as saying that Earth had to support life. Earth could have formed too close to the sun to support liquid water. In fact, that’s just what Venus did, and consequently we are not on Venus.
When simple units come together to form larger or more complex structures, these new structures often have completely new and unpredictable characteristics called emergent properties. For example, hydrogen and oxygen are two flammable gases. When chemically combined, they create water. The properties of clouds, oceans, waterfalls, and ice crystals could not be predicted by studying elemental hydrogen and oxygen. These properties emerge only after the water is formed. Likewise, chemicals combine to produce life. When people come together into cities, they are able to invent technology that seems downright miraculous – well beyond any one person’s skill set. Yet from the bottom to the top of the organizational pyramid, none of this requires magic potions or spells. Everything is made of the same fundamental particles. The amazing properties of living things do not come from any special life ether or divine forces, but from the ways they are organized.
Atoms make excellent building blocks for organized matter. They are not only stable but interchangeable, making them essentially eternal. Their extremely small size allows a lot of complexity to be jam-packed into a small amount of space. Even a small animal such as a mouse requires billions of trillions of atoms. The fact that atomic bonds can store energy is just another amazing property of these tiny building blocks.
Some skeptics believe that increasing complexity violates a principle called the 2nd Law of Thermodynamics, and therefore that complex phenomena can only be created by God performing miracles. 11 That argument is not based on the 2nd Law itself, but on oversimplified and inaccurate analogies of the law such as, “Order tends to chaos.” (If that were literally true, then the planet would just be a puddle of mud by today!) The 2nd Law correctly states that whenever energy is used to do work, some energy is lost to waste heat. For instance, whenever you eat, some of the calories are wasted away as body heat. Most of the energy is still used productively to turn simple food products into your complex body. The generation of complexity is less than 100% efficient, not less than 0% efficient.
Stable things survive. Stable simple things occasionally combine to create more complex things, and if they are stable then the whole process repeats itself. That’s just the natural order.
Back to Section 10.II: The Big Bang
Continue to Section 10.IV: Sun, Earth, and Moon
- Image NASA / ESA, public domain ↩
- Fernando Porcelli and Giancarlo Scibona, “Large-Scale Structure Formation via Quantum Fluctuations and Gravitational Instability”, Int’l Journal Geosciences 5:634-656 (May, 2014), https://www.scirp.org/journal/PaperInformation.aspx?PaperID=46224 (accessed and saved 6/16/19). ↩
- Judd D. Bowman et al., “An absorption profile centred at 78 megahertz in the sky-averaged spectrum”, Nature 555:67-70 (3/01/2018), https://www.nature.com/articles/nature25792 (accessed and saved 6/17/19). ↩
- Howard E. Bond et al., “HD 140283: A star in the solar neighborhood that formed shortly after the big bang”, The Astrophysical Journal Letters 765:L12, 1-5 (2/13/2013), https://iopscience.iop.org/article/10.1088/2041-8205/765/1/L12 (accessed and saved 6/17/19). ↩
- A.M. Ghez et al., “Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits”, The Astrophysical Journal 689(2):1044-1062 (8/21/2008), https://iopscience.iop.org/article/10.1086/592738 (accessed and saved 6/17/19). ↩
- Sukanya Chakrabarti et al., “Antlia2’s role in driving the ripples in the outer gas disk of the galaxy”, The Astrophysical Journal Letters, draft version accepted for publication (6/12/2019), https://arxiv.org/abs/1906.04203 (accessed and saved 6/22/19). ↩
- Roeland P. van der Marel et al., “First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33”, The Astrophysical Journal vol. 872, no. 1, Article 24 (2/10/2019), https://iopscience.iop.org/issue/0004-637X/872/1 (accessed and saved 6/22/19). ↩
- N. F. Martin et al., “A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way”, Monthly Notices of the Royal Astronomical Society, vol. 348, Issue 1 (2/11/2004), https://academic.oup.com/mnras/article/348/1/12/1411293 (accessed and saved 6/23/19). ↩
- Adam G. Riess et al., “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant”, Astronomical Journal 116:1009-1038 (5/15/1998), https://arxiv.org/abs/astro-ph/9805201 (accessed and saved 7/29/19). ↩
- Image by User:Rursus / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/), https://upload.wikimedia.org/wikipedia/commons/3/37/Evolved_star_fusion_shells.svg (accessed and archived 2/24/20). ↩
- See e.g. Jim Stephens, “# 31 The Second Law of Thermodynamics”, 101 Proofs for God (4/09/2013), http://101proofsforgod.blogspot.com/2013/04/31-second-law-of-thermodynamics.html (accessed and saved 6/30/19). ↩
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