Cosmos by Carl Sagan

Good introductory book to science. Isn't a difficult read and Carl Sagan is a great science writer so this book can be read in less than a week. I had a large gap in what I highlighted between pages 30-201 so I didn't find it all that inspiring but Carl Sagan is one of the most influential science writers so could be helpful in learning about astrophysics for beginners who are curious. His most interesting part that I remember reading was about Einstein's theory of relativity and how traveling close to the speed of light you wouldn't age much at all but everyone else back home on Earth would. 


The time will come when diligent research over long periods will bring to light things which now lie hidden.  A single lifetime, even though entirely devoted to the sky, would not be enough for the investigation of so vast a subject…And so this knowledge will be unfolded only through long successive ages.  There will come a time when our decedents will be amazed that we did not know things that are so plain to them…Many discoveries are reserved for ages still to come, when memory of us will have been effaced.  Our universe is a sorry little affair unless it has in it something for every age to investigate…Nature does not reveal her mysteries once and for all. – Seneca, Natural Questions, Book 7, first century

The dimensions of the cosmos are so large that using familiar units of distance, such as meters or miles, chosen for their utility on earth, would make little sense.  Instead, we measure distance with the speed of light.  In one second a beam of light travels 186,000 miles, nearly 300,000 kilometers or seven times around the earth.  In eight minutes it will travel from the Sun to the earth.  We can say the sun is eight light minutes away.  In a year, it crosses nearly ten trillion kilometers, about six trilling miles of intervening space.  That unite of length, the distance light goes in a year, is called a light year.  It measures not time but distances – enormous distances.

All my life I have wondered about the possibility of lie elsewhere.  What would it be like?  Of what would it be made?  All living things on our planet are constructed of organic molecules – complex microscopic architectures in which the carbon atom plays a central role.  There was once a time before life, when the earth was barren and utterly desolate.  Our world is now overflowing with life.

The secrets of evolution are death and time – the deaths of enormous numbers of life forms that were imperfectly adapted to the environment; and time for a long succession of small mutations that were by accident adaptive, time for the slow accumulation of patterns of favorable mutations. 

What happened here on earth may be more or less typical of the evolution of life on many worlds’ but in such details as the chemistry of proteins or the neurology of brains, the story of life on earth may be unique in all the Milky Way Galaxy.  The earth condensed out of interstellar gas and dust some 4.6 billion years ago.  We know from fossil record that origin of life happened soon after, perhaps around 4 billion years ago, in the ponds and oceans of the primitive earth.  The first living things were not anything as complex as a once celled organism, already a highly sophisticated form of life.  The first stirrings were much more humble.  In those early days, lightning and ultraviolet light from the sun were breaking part the simple hydrogen rich molecules of the primitive atmosphere, the fragments spontaneously recombining into more and more complex molecules.  The products of this early chemistry were dissolved in the oceans, forming a kind of organic soup of gradually increasing complexity until one day, quite by accident, a molecule arose that was able to make crude copies of itself, using as building blocks other molecules in the soup.

Four billion years ago there were no predators.  Some molecules reproduced themselves inefficiently, competed for building blocks and left crude copies of themselves.  With reproduction, mutation and the selective elimination of the least efficient varieties, evolution was well under way, even at the molecular level.  As time went on, they got better at reproducing. Molecules with specialized functions eventually joined together, making a kind of molecular collective – the first cell.  Plant cells today have tiny molecular factories, called chloroplasts, which are in charge of photosynthesis – the conversion of sunlight, water and carbon dioxide in carbohydrates and oxygen.  The cells in a drop of blood contain a different sort of molecular factory, the mitochondrion which combines food with oxygen to extract useful energy.  These factories exist in plant and animal cells today but may once themselves have been free living organisms.

With the invention of sex, two organisms could exchange whole paragraphs, pages and books of their DNA code, producing new varieties ready for the sieve of selection.  And this is true not only of the microbes of two billion years ago.  We humans also have a palpable devotion to exchanging segments of DNA today.

For most of the four billion years since the origin of life, the dominant organisms were microscopic blue-green algae, which covered and filled the oceans.  Then some 600 million years ago, the monopolizing grip of the algae was broken and enormous proliferation of new life forms emerged, an event called the Cambrian explosion.  Life had arisen almost immediately after the ori9gin of the earth, which suggest that life may be an inevitable chemical process on an earth like planet.

Between the times of Aristarchus and Huygens, humans answered the question that had so excited me as a boy growing up in Brooklyn:  What are the stars? The answer is that the stars are mighty suns, light years away in the vastness of interstellar space.

But the number of stars we can see is only the tiniest fraction of the number of stars that are.  What we see at night is the merest smattering of the nearest stars.  Meanwhile the cosmos is rich beyond measure: the total number of stars in the universe is greater than all the grains of sand on all the beaches of the planet Earth.

The problems of simultaneity do not apply to sound as they do to light because sound is propagated through some material medium, usually air.  The sound wave that reaches you when a friend is talking is the motion of molecules in the air. Light, however, travels in a vacuum.  There are restrictions on how molecules of air can move which do not apply to a vacuum.  Light from the sun reaches us across the intervening empty space, but no matter how carefully we listen, we do not hear the crackle of sunspots or the thunder of the solar flares.  It was once thought, in the days before relativity, that light did propagate through a special medium that permeated all of space, called “the luminous aether”.  But the famous Michelson-Morley experiment demonstrated that such an aether does not exist.

Traveling close to the speed of light you would hardly age at all, but your friends and your relatives back home would be aging at the usual rate.  When you returned from your relativistic journey, what a difference there would be between your friends and you, they having aged decades, say, and you having aged hardly at all!  Traveling close to the speed of light is a kind of elixir of life.  Because time slows down close to the speed of light, special relativity provides us with a means of going to the stars.  But is it possible, in term of practical engineering, to travel close to the speed of light?  Is a starship feasible?

To make an apple pie, you need wheat, apples, a pinch of this and that, and the heat of the oven.  The ingredients are made of molecules – sugar, say, water.  The molecules, in turn, are made of atoms – carbon, oxygen, hydrogen and a few others.  Where do these atoms come from?  Except for hydrogen, they are all made in stars.  A star is a kind of cosmic kitchen inside which atoms of hydrogen are cooked into heavier atoms. Stars condense from interstellar gas and dust, which are composed mostly of hydrogen.  But the hydrogen was made in the Big Bang, the explosion that began the Cosmos. 

I am made of atoms. My elbow, which is resting on the table before me, is made of atoms.  The table is made of atoms.  But if atoms are so small and empty and the nuclei smaller still, why does the table hold me up?  What, as Arthur Eddington liked to ask, do the nuclei that compromise my elbow not slide effortlessly through the nuclei that compromise the table?  Why don’t I wind up on the floor?  Or fall straight through the Earth?  The answer is the electron cloud.  The outside of an atom in my elbow has a negative electrical charge.  So does every atom in the table.  But negative charges repel each other. My elbow does not slither through the table because atoms have electrons around their nuclei and because electrical forces are so strong.  Everyday life depends on the structure of the atom.  Turn off the electrical charges and everything crumbles to an invisible fine dust.  Without electrical forces, there would be no longer be a thing in the universe – merely diffuse clouds of electrons, protons, and neutrons, and gravitating spheres of elementary particles, the featureless remnants of worlds.

There are ninety two chemically distinct kinds of naturally occurring atoms. They are called the chemical elements and until recently constituted everything on our planet, although they are mainly found combined into molecules.  Water is a molecule made of hydrogen and oxygen atoms.  Air is made mostly of the atoms nitrogen (N), oxygen (O), carbon ©, hydrogen (H) and argon (Ar) in the molecular forms, N2,O2,CO2,H2O and AR.  The Earth itself is a very rich mixture of atoms, mostly silicon, oxygen, aluminum, magnesium, and iron.  Fire is not made of chemical elements at all.  It is radiating plasma in which the high temperature has striped some of the electrons from their nuclei.  Not one of the four ancient Ionian and alchemical “elements” is in the modern sense and element at all: one is a molecule, two are mixture of molecules, and the last is plasma.

The attraction between the unlike charges of electrons and protons is what holds the atom together. 

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars.  We are made of star stuff.

A supernova is the explosive death of a star, which unleashes a burst of light through the cosmos.  They can happen in other galaxies beside the Milky Way also.

The origin and evolution of life are connected in the most intimate ay with the origin and evolution of the stars.  First:  The very matter of which we are composed, the atoms that make life possible, were generated long ago and far away in giant red stars.  The relative abundance of the chemical elements found in the Cosmos matches the relative abundance of atoms generated in stars so well as to leave little doubt that red giants and supernovae are the ovens and crucibles in which matter has been forged.  The sun is a second or third generation star.  All the matter in it, all the matter you see around you, has been through one or two previous cycles of stellar alchemy.  Second:  The existence of certain varieties of heavy atoms on earth suggests that are was a nearby supernova explosion shortly before the solar system was formed.  But this is unlikely to be a mere coincidence; more likely, the shock wave produced by the supernova compressed interstellar gas and dust and triggered the condensation of the solar system.  Third:  When the sun turned on, its ultraviolet radiation poured into the atmosphere of the Earth; its warmth generated lighting; and these energy sources sparked the complex organic molecules that led to the origin of life.  Fourth: Life on earth runs almost exclusively on sunlight. Plants gather the photons and convert solar to chemical energy.  Animals parasitize the plants.  Farming is simply the methodical harvesting of sunlight, using plants as grudging intermediaries.  We are, almost all of us, solar powered.  Finally, the hereditary changes called mutations provide the raw materials for evolution. Mutations, form which nature selects its new inventory of life forms, are produced in part by coms rays – highly energy particles ejected almost at the speed of light in supernova explosions.  The evolution of life on earth is driven in part by the spectacular deaths of distance, massive suns.

On the average, a supernova occurs in a given galaxy about once every century.  During the lifetime of at typical galaxy, about ten billion years, a hundred million stars will have exploded – a great many, but still only about one star in a thousand. 

The discovery of the big bang and the recession of the galaxies came from a commonplace of nature called the Doppler Effect.  We are used to it in physics of sound.  An automobile driver speediny bus blows his horn.  Inside the car, the driver hears a characteristic change in pitch.  To us, the sound of the horn elides from high frequencies to low.  A racing car traveling at 200 kilometers per hour (120 miles per hour) is going almost one fifth the speed of sound.  Sound I a succession of waves in air, a crest and a trough, a crest and a trough.  The closer together the waves are, the higher the frequency or pitch; the farthest parte the waves are the lower the pitch.  If the car is racing away from us, it stretches out the sound waves, moving them, from our point of view, to a lower pitch and producing the characteristic sound from which we are all familiar.  If the car were racing toward us, the sound waves would be squashed together, the frequency would be increased and we could hear a high pitched wail.  If we knew what the ordinary pitch of the horn was when the car was at rest, we could deduce it sped blindfolded, form the change in pitch.

Light is also a wave.  It travels perfectly through a vacuum.

Doppler effect - The change in frequency of a wave (or other periodic event) for an observer moving relative to its source.  (

Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared to be similar to stars, rather than extended sources similar to galaxies.

The whale like all other animals on earth, has a gene library and a brain library.

Pg 298  - Whales, communication large distances, steam engine invention by humans, and whales can communicate as efficiently and effectively.

We humans use base ten arithmetic only because we have ten fingers on our hands.

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