The Cosmos Book: A Guide to the Universe

The Cosmos Book: A Guide to the Universe is a great resource for anyone interested in learning about the cosmos. It covers a wide range of topics, from the Big Bang to black holes.

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Welcome to “The Cosmos Book: A Guide to the Universe.” In this guide, we will explore the vastness of the cosmos and attempt to make sense of it all. We’ll start by discussing some of the basic principles of astronomy, then move on to the different components of the universe. Finally, we’ll discuss some of the major cosmological theories that attempt to explain everything that we see.

The Solar System

The solar system is the sun and all the objects that orbit around it. The largest object in the solar system is the sun. It is so huge that it contains more than 99.8% of the mass in the solar system. The planets that orbit around the sun are (in order from closest to farthest away from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. There are also many smaller objects orbiting around the sun such as asteroids and comets.

The word “planet” comes from the Greek word for “wanderer”. The planets wanders against the background of stars. ancient Greeks were able to see 5 planets with their eyes: Mercury, Venus, Mars, Jupiter and Saturn. Uranus and Neptune were discovered by using a telescope. Pluto was found in 1930 by using a special type of telescope called a blink comparator which allowed astronomers to take two photographs of a section of space a few nights apart and then compare them

The Sun

The sun is the star at the center of the solar system. It is the Earth’s primary source of light and heat, and drives the Earth’s climate and weather. The sun is a medium-sized star and is about halfway through its life. It will continue to burn for another 5 billion years before it expands and cools to become a red giant. Eventually, it will shed its outer layers, creating a planetary nebula, before cooling to become a white dwarf.

The planets

The solar system is made up of the sun, eight planets, five dwarf planets, satellites (such as our moon), asteroids, meteoroids, and comets.

The planets that orbit our sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn Uranus and Neptune. Additionally there are five dwarf planets: Ceres, Pluto, Haumea Makemake and Eris.

Mercury is the closest planet to the sun. It is small and rocky with no atmosphere. It has a day that is longer than its year!
Venus is the second closest planet to the sun. It is covered in clouds of sulfuric acid. The atmospheric pressure on Venus is 90 times that of Earth.
Earth is the third planet from the sun. It is unique as it is the only planet known to support life.
Mars is the fourth planet from the sun. It is known as the Red Planet because of its red rusty appearance. Mars has two moons Phobos and Deimos which orbit close to the surface of Mars.
Jupiter is fifth out from the sun and by far the largest planet in our solar system – it’s two and a half times wider than all the other planets combined! Jupiter also has at least 67 moons orbiting it! The most famous ones are Io, Europa, Ganymede and Callisto which are also known as The Galilean Moons as they were first seen by Galileo Galilei in 1610 through his telescope.
Saturn was first seen by Galileo in 1610 but he could not make out its rings. In 1655 Christiaan Huygens was able to confirm their existence using a better telescope. Saturn has at least 62 moons including Titan which is larger than Mercury!
Uranus was discovered by William Herschel in 1781 using a telescope he had built himself . Uranus was originally thought to be a star but was later classified as a planet . It has 27 moons including Miranda which has some very strange features such as canyons , volcanoes and ice castles !
Neptune was only discovered in 1846 after scientists noticed irregularities in Uranus’ orbit . Neptune has 14 moons including Triton which orbits Neptune backwards !  
Ceres was discovered in 1801 and classified as a planet but later re-classified as a ‘dwarf planet’ when further discoveries were made . Ceres orbits between Mars and Jupiter and takes 4 years to go around the Sun once . Pluto was discovered in 1930 but its status as a ‘planet’ came into question after further discoveries were made in 2005 . In 2006 it was decided that Pluto would be re-classified as a ‘dwarf planet’ along with Ceres . Haumea , Makemake and Eris were all also classified as ‘dwarf planets’ in 2006 .

The moons

The moons are the natural satellites of the planets in our solar system. They orbit their parent planet, and are held in place by the gravitational pull of that planet. There are over 150 moons in our solar system, and they come in a variety of shapes and sizes.

The largest moon is Jupiter’s moon Ganymede, which is larger than Mercury. The smallest moon is Saturn’s moon Mimas, which is just 246 miles across.

Most moons are barren and airless, but some have atmospheres and even oceans of liquid water on their surfaces. The Earth’s moon, for example, has an atmosphere that is very thin but still sufficient to support human life.

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The moons can be divided into three main groups: the terrestrial moons, the gas giant moons, and the dwarf planet moons. The terrestrial moons are those that orbit the inner planets: Mercury, Venus, Earth, and Mars. These moons are small and rocky, and include our own moon (the Moon) as well as Mercury’s moon (Mercury II), Venus’s two small moons (Venus I and II), Mars’s two small satellites (Phobos and Deimos), and the large satellite of Mars (Mars I). The gas giant planets—Jupiter, Saturn, Uranus, Neptune—all have large numbers of much bigger Moons; in fact Jupiter alone has more than 60 natural satellites! Finally, there are the dwarf planet moons which orbit objects such as Pluto (Pluto I) or Ceres (Ceres I). There may be many more of these yet to be discovered; however they tend to be very small indeed.

Comets, asteroids, and meteoroids

Comets, asteroids, and meteoroids are small bodies that orbit the Sun. They are leftovers from the formation of the solar system and are made of the same stuff as planets and moons.

Comets are usually small, icy objects with long tails of gas and dust. They orbit the Sun in elliptical (oval-shaped) orbits and often have very long orbital periods. Asteroids are small rocky or metallic objects that orbit the Sun in elliptical orbits. They are usually found in a region called the asteroid belt, between the orbits of Mars and Jupiter. Meteoroids are small bodies (usually rocks or pieces of metal) that orbit the Sun or other bodies in our solar system. When they enter Earth’s atmosphere, they burn up and create streaks of light in the sky, which we call “shooting stars” or “meteors.”

There are millions of comet, asteroid, and meteoroid orbiting the Sun. Some of them have been known about for centuries, while others were only discovered recently.

The Kuiper Belt and Oort Cloud

The Kuiper Belt is a disc-shaped region of the solar system beyond the orbit of Neptune that is thought to contain thousands of planetary bodies, including Pluto. These bodies are thought to be left over from the formation of the solar system. The Oort Cloud is a spherical cloud of comets that surrounds the solar system and extends to about one light-year from the sun. It is thought to be the source of long-period comets, such as Halley’s Comet.

The interstellar medium

In astronomy, the interstellar medium (ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is also considered part of the interstellar medium.

The interstellar medium is found in every direction throughout the galaxy. Its density generally decreases with distance from the center of a galaxy; however, it is not uniform. The physical properties of the ISM vary greatly from one location to another within the Milky Way. Understanding these variations is crucial to understanding many astrophysical processes taking place throughout our galaxy.

Star formation

Star formation is the process by which dense regions of gas in molecular clouds collapse into individual stars.

The initial mass of a star controls its further evolution and eventual fate. The most massive stars are destined to end their lives in supernova explosions, while lower mass stars will gently fade away as white dwarf stars.

Stars form in groups known as star clusters, and these can be classified according to their age and appearance. Young star clusters contain hot, blue stars, while older star clusters are composed of redder, cooler stars.

Globular star clusters are very old, giant spherical collections of thousands to millions of stars that orbit around the centers of galaxies. In our own Milky Way galaxy there are around 150 known globular clusters.

Main-sequence stars

Most stars, including the Sun, are on the main sequence of the Hertzsprung–Russell diagram—a plot of absolute stellar luminosity versus surface temperature. They generate energy from the nuclear fusion of hydrogen atoms in their cores. Each second, more than four million tonnes of matter are converted into energy within the Sun’s core, producing neutrinos and gamma rays.

Red giants and white dwarfs

Red giants and white dwarfs are among the most interesting objects in the cosmos. They offer a tantalizing glimpse into the lives of stars and provide a link between the past and future of our universe.

Red giants are massive stars that have exhausted their supply of hydrogen fuel and are in the process of shedding their outer layers. This process, known as stellar death, is thought to be responsible for the creation of heavy elements like oxygen, carbon, and nitrogen. As these elements are expelled into space, they can be recycled into new stars and planets.

White dwarfs are the corpses of stars that have run out of fuel and collapsed under their own weight. These incredibly dense objects are incredibly fascinating because they offer a window into the future of our own sun. When our sun runs out of hydrogen fuel, it too will collapse and become a white dwarf.

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Neutron stars and black holes

Neutron stars and black holes are some of the most fascinating objects in the cosmos. Neutron stars are incredibly dense, with a mass greater than the Sun but compressed into a sphere just a few kilometers across. Black holes are even more massive and compact, with a gravitational pull so strong that nothing can escape from them, not even light.

In this chapter, we’ll explore what these objects are, how they form, and how they affect their surroundings. We’ll also see how scientists are using neutron stars and black holes to probe the nature of gravity and test the limits of our understanding of the universe.

The Milky Way

The Milky Way is our home galaxy. It is a huge spiral galaxy with a diameter of about 100,000 light years. It contains between 200 and 400 billion stars and is thought to have about as many planets. The Milky Way is just one of billions of galaxies in the universe. Our Solar System is located in a spiral arm called the Orion Spur, about 27,000 light years from the center of the Milky Way.


There are billions of galaxies in the universe, each one containing hundreds of billions of stars. Galaxies come in a wide variety of shapes and sizes, from tiny dwarf galaxies to massive elliptical galaxies. Our own Milky Way Galaxy is thought to contain between 100 and 400 billion stars.

Most galaxies are organized into groups or clusters, containing anywhere from a few dozen to a few thousand galaxies. Our Milky Way Galaxy is part of a group of about 54 galaxies known as the Local Group. The largest member of the Local Group is the Andromeda Galaxy, which is about twice the size of our own galaxy.

The large-scale structure of the universe

The large-scale structure of the universe is the distribution of matter on the grandest of scales. It deals with structures larger than galaxies, such as galaxy clusters and superclusters, and with overall features of the cosmos, such as voids. The primary observational tool for cosmologists studying this structure is mapping the locations and densities of galaxies. Other tracers can be used to map out far more diffuse gas, although this gas generally cannot be observed directly but must be inferred by its interaction with other matter (such as its effect on galaxy motions or its X-ray emission).

On the largest scales, cosmologists have discovered that galaxies are not evenly distributed throughout space. Instead, they form a “cosmic web” made up of filaments and voids. The filamentary nature of this web was first predicted in the 1980s by computer simulations of structure formation in the universe. These simulations showed that under the influence of gravity, matter tends to clump together into ever-denser structures, with filamentary webs forming around regions of high density.

The filaments themselves are made up of galaxies clustered together by gravity. These cluster tend to be elongated along the filamentary direction and are often known as “Finger-of-God” features due to their appearance when projected onto a two-dimensional map. At the intersections of filaments are relatively dense regions known as nodes, which are made up of galaxy clusters or groups. In between nodes are vast empty regions known as voids. Voids are important because they help cosmologists understand what matter is doing on large scales; specifically, they help constrain models of “dark energy”, an enigmatic form of energy that appears to be causing the expansion of the universe to accelerate.

The cosmological principle and the Copernican principle

The cosmological principle is the idea that the universe is homogeneous and isotropic. This means that on a large enough scale, it looks the same in all directions. The Copernican principle is the idea that we are not special observers of the universe. In other words, there is no center to the universe and no edge.

The expanding universe

In 1929, Edwin Hubble resolved a long-standing debate about the geometry of the universe.

He showed that distant galaxies are moving away from us at speeds proportional to their distances. This means that the farther away they are, the faster they are receding.

In an expanding universe, galaxies are getting further apart all the time. As they do so, the space between them stretches. This has some strange consequences.

For example, it means that light from distant galaxies is stretched as it travels to us. This makes the galaxies appear redder than they really are — a process called redshift.

It also means that objects in the universe were closer together in the past than they are today. In fact, if we go back far enough in time, all the matter in the universe was once concentrated in a very small space indeed — perhaps even smaller than an atom!

The Big Bang

The Big Bang is the scientific theory that describes the universe’s origins. According to this theory, the universe began as an incredibly hot, dense point known as a singularity. About 13.8 billion years ago, the singularity expanded and cooled, forming the first hydrogen atoms. This expansion continues today, and the universe is now much cooler and much less dense than it was in its early stages. The Big Bang created not only all of the matter and energy in the universe, but also time and space itself.

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The early universe

The early universe was an extremely hot and dense place. It is thought to have begun with a very powerful explosion called the Big Bang. This event created space and time, and set in motion the process that would eventually lead to the formation of galaxies, stars, and planets.

The Universe today is still expanding, but at a much slower rate. It is thought to be billions of years old and is filled with billions of galaxies. Each galaxy contains millions or billions of stars. Our own Milky Way Galaxy contains about 200 billion stars.

The inflationary universe

In ballooning terms, the inflationary universe is like a balloon that starts out small, but as more air is pumped into it, the balloon expands rapidly. In the inflationary model, space itself expands very rapidly in the first split second of the Big Bang. This expansion stretches any matter that happens to be around at that time (such as photons) so that it becomes extremely diluted. In fact, if we could look back in time far enough, we would see that the entire universe was once compressed into a tiny region much smaller than an atom.

Inflation solves several problems with the Big Bang theory. One is the problem of the horizon. If space were infinite at t=0 (as in the Big Bang theory), then there would be no apparent reason why different regions of space should have uniform temperatures. The inflationary model explains why different regions of space have uniform temperature even though they are too far apart to have been in contact with each other.

Inflation also explains why there is more matter than antimatter in our universe. According to inflationary theory, when the universe was very small, there were equal amounts of matter and antimatter. But as the universe expanded rapidly during inflation, matter and antimatter became separated from each other so that today there is far more matter than antimatter in our corner of the cosmos.

Dark matter and dark energy

Dark matter is an unidentified type of matter that makes up most of the mass in the universe. It cannot be seen with telescopes but astronomers know it to be there because of its gravitational effects on visible matter. Dark energy is a mysterious force that is causing the universe to expand at an accelerating rate. It makes up most of the contents of the universe and its nature is not well understood.

The end of the universe

The end of the universe is a topic of great interest to astrophysicists and cosmologists. There are many theories about how the universe will end, but no one knows for sure what will happen.

The most popular theory is that the universe will eventually stop expanding and begin to contract. This theory is known as the Big Crunch. According to this theory, all matter in the universe will eventually be pulled back together by gravity, and the universe will collapse in on itself.

Another popular theory is that the universe will continue to expand forever. This theory is known as the Big Freeze. In this scenario, the universe will become so large that all matter will become extremely dilute, and stars will stop forming. Eventually, all matter will lose energy and fall into a state of equilibrium. The universe will become dark and cold, and time will come to a standstill.

No one knows for sure what will happen to the universe in the future, but it is clear that it will be an exciting time for astrophysics and cosmology!

Life in the universe

Life in the universe is a topic of astrobiology and includes the study of the origin of life, the distribution and evolution of life throughout the universe, and the possible existence of extraterrestrial life. Astrobiology addresses three central questions: Where do we come from? Are we alone? Where are we going?

In addressing these questions, astrobiology considers a wide range of topics, including cosmology, astronomy, biology, geology and planetary science. One simple way to understand astrobiology is to consider it as a study that attempts to answer three very big questions:
-Where does life come from?
-Is there intelligent life elsewhere in the Universe besides Earth?
-What is our future as a species and what is the future of intelligent life in general within our Cosmos?


The Cosmos is a vast and beautiful place, full of mystery and wonder. It is our home, and it is a part of us. We are its stewards, and it is our duty to protect it. We must learn as much as we can about the Cosmos, so that we can make informed decisions about its future. Thank you for joining me on this journey. I hope that you have found it as fascinating and enriching as I have.

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