Introduction to the Universe: Our Location in Space and Time - PDF

Introduction to the Universe: Our Location in Space and Time A star system is a star with several planets orbiting that star and moons orbiting those planets. The name of our home star system is the Solar System. There is only one star in our Solar System: the Sun. We live on planet Earth, the thir...

Introduction to the Universe: Our Location in Space and Time A star system is a star with several planets orbiting that star and moons orbiting those planets. The name of our home star system is the Solar System. There is only one star in our Solar System: the Sun. We live on planet Earth, the third planet orbiting the Sun. A galaxy is a collection of billions of star systems held together by gravity. The name of our home galaxy is the Milky Way Galaxy. There are roughly one hundred billion star systems that together compose the Milky Way Galaxy, and the Solar System is just one of these one hundred billion star systems within the Milky Way Galaxy. Galactic groups contain only a few dozen galaxies, while galactic clusters contain between several hundred and several thousand galaxies. Our Milky Way Galaxy is not a member of a galactic cluster; our Milky Way Galaxy is a member of a galactic group. The name of our home galactic group is the Local Galactic Group (or just the Local Group for short). The Local Galactic Group is composed of a few dozen galaxies, although most of them are small galaxies. There are only three major galaxies in the Local Galactic Group: our Milky Way Galaxy, the Andromeda Galaxy, and the Triangulum Galaxy. Galactic superclusters are enormous organizations composed of between tens of thousands and hundreds of thousands of galaxies. The name of our home galactic supercluster is the Laniakea Galactic Supercluster, and our Local Group is a small galactic group on the outskirts of the Laniakea Galactic Supercluster. Cosmic filaments are colossal organizations composed of several galactic superclusters, and hence cosmic filaments contain between a few hundred thousand and a few million galaxies. The name of our home cosmic filament is the Perseus-Pisces-Sculptor-Hercules Cosmic Filament, and our Laniakea Galactic Supercluster is one of several galactic superclusters within the Perseus-Pisces-Sculptor-Hercules Cosmic Filament. The observable universe contains hundreds of thousands, perhaps millions, of cosmic filaments. Since each cosmic filament contains between a few hundred thousand and a few million galaxies, the entire observable universe contains roughly one hundred billion galaxies. Assuming that each galaxy contains on average one hundred billion star systems just like our Milky Way Galaxy, then there are roughly ten sextillion star systems in the observable universe. (Please refer to the following multiplication table, where each number is one thousand times the previous number: one, one thousand, one million, one billion, one trillion, one quadrillion, one quintillion, one sextillion, one septillion, one octillion, one nonillion, one decillion. Caution: this multiplication table is only correct in American English. These same words are used for different numbers in British English.) We can summarize our location in the universe with our cosmic address. Whenever anyone asks for our mailing address, we provide a list of larger and larger organizations wherein we reside. After our name comes a house number, then a street/avenue/road/boulevard (which is a collection of houses), then a municipality (which is a collection of streets/avenues/roads/boulevards), then a county (which is a collection of municipalities), then a state/province (which is a collection of counties), and then a country (which is a collection of states/provinces). If we were to continue, we would then provide a continent (which is a collection of countries), then a planet (which is a collection of continents), then a star system (which is a collection of planets orbiting a star), then a galaxy (which is a collection of star systems), then a galactic group or a galactic cluster (which is a collection of galaxies), then a galactic supercluster (which is a collection of galactic groups and galactic clusters), then a cosmic filament (which is a collection of galactic superclusters), and finally a universe (which is a collection of cosmic filaments). Every person we have ever met or ever will meet and every person we have ever heard of or will ever hear of has the same cosmic address starting with planet Earth followed by the Solar System, the Milky Way Galaxy, the Local Galactic Group, the Laniakea Galactic Supercluster, the Perseus-Pisces-Sculptor-Hercules Cosmic Filament, and the observable universe. One light-year is the distance that light travels in a time of one year. We must never forget that a light-year is a length of distance, not a duration of time. This is easy to forget, since a year is a duration of time. Nevertheless, a light-year is not a duration of time; a light-year is a length of distance. How far is one light-year? Light travels roughly three hundred thousand kilometers every second through the vacuum of outer space. This is extraordinarily fast by human standards. Since three hundred thousand kilometers is the distance light travels in one second, we multiply this by sixty to find how far light travels in one minute, since there are sixty seconds in one minute. We multiply this result by another sixty to obtain how far light travels in one hour, since there are sixty minutes in one hour. We multiply this result by twenty-four to find how far light travels in one day, since there are twenty-four hours in one day. Finally, we multiply this result by 365.25 to obtain how far light travels in one year, since there are 365.25 days in one year. The final result of this calculation is that one light-year is roughly 9.5 trillion kilometers. This is close enough to ten trillion kilometers that throughout this course we will assume that one light-year is roughly ten trillion kilometers as a satisfactory approximation. Our universe is so enormous that it actually takes years for light to travel from one star system to a neighboring star system. It is for this reason that astronomers consider telescopes to be time machines. When we observe a star that is one hundred light-years distant for example, we see that star as it appeared one hundred years ago (one hundred years in the past), since it took that long for its light to travel from that star to us. The only way to know how that star actually appears at this moment is to wait another one hundred years for that light to travel to us. We see a star one thousand light-years distant as it appeared one thousand years ago (one thousand years in the past), since it took that long for its light to travel from that star to us. The only way to know how that star actually appears at this moment is to wait another one thousand years for that light to travel to us. Not only are telescopes time machines, but the human eye is itself also a time machine. If we look at the Sun (which we must never do since that would cause permanent blindness), we see the Sun as it appeared roughly eight minutes ago (roughly eight minutes in the past), since it takes that long for light to travel from the Sun to the Earth. The only way to know how our Sun actually appears at this moment is to wait another eight minutes for that light to travel to us. If we look at the Moon, we see it as it appeared roughly one second ago (roughly one second in the past), since it takes that long for light to travel from the Moon to the Earth. The only way to know how our Moon actually appears at this moment is to wait another second for that light to travel to us. When we look at the tables and chairs around us or even these words we are reading, we see those tables and chairs and these words as they appeared a few nanoseconds ago (a few nanoseconds in the past), since it takes that long for light to travel from the tables and chairs and even these words to our eyes. Obviously, these time delays are so tiny in everyday life that we do not notice them at all, but they are real nevertheless. The universe is so enormous that it takes a few years for light to travel from one star system to a neighboring star system, it often takes millions of years for light to travel from one galaxy to a neighboring galaxy, and it takes billions of years for light to travel from one side of the observable universe to the other side of the observable universe. For example, our Milky Way Galaxy has a diameter of roughly one hundred thousand light-years, and the Andromeda Galaxy is more than two million light-years from our Milky Way Galaxy. The universe is roughly fourteen billion years old. This is unimaginably old by human standards. The human mind can comprehend seconds, minutes, hours, days, weeks, months, and years of time. Ten years is called a decade. Ten decades (which is one hundred years) is called a century, which is roughly how long most humans live. Ten centuries (which is one hundred decades or one thousand years) is called a millennium. Ten millennia is almost twice as long as all of recorded human history, but it is still minuscule compared to the age of the universe. One hundred millennia is almost how long our species Homo sapiens has existed, and one thousand millennia (which is one million years) is almost how long our genus Homo has existed. Ten million years is almost how long our family of hominids has existed. One hundred million years ago, there were no hominids; dinosaurs roamed the Earth. One billion years is approaching the age of our planet Earth, but this is still not near the age of the universe. Ten billion years is roughly the age of our Milky Way Galaxy, and this is finally approaching the fourteen-billion-year age of the entire universe. We can gain a greater appreciation for the age of the universe through the cosmic calendar. In the cosmic calendar, we pretend that the entire history of the universe fits into one calendar year. In other words, when we use the cosmic calendar we pretend that the creation of the universe occurred at the very beginning of the calendar year on January 01st at midnight, and we pretend that the present day is at the very end of the calendar year on December 31st immediately before midnight. If the creation of the universe occurred on January 01st at midnight, we must wait until September before the Earth forms! We must then wait another month (October) before the most primitive unicellular microorganisms appear on Earth! We must wait another month (November) before multicellular but still microscopic organisms appear on Earth! We must wait another month (mid-December) before macroscopic but still invertebrate animals appear on Earth! The most primitive vertebrate animals (fishes) appear on roughly December 20th, amphibians appear on roughly December 22nd, and reptiles appear on roughly December 24th. The age of reptiles, commonly known as the age of dinosaurs, lasts from roughly December 25th to roughly December 30th, when the age of mammals begins. Hominids do not appear until December 31st at roughly 05:30 p.m., and Homo sapiens do not appear until December 31st at roughly 11:52 p.m.! Finally, all of recorded human history lasts for roughly fifteen seconds, beginning on December 31st at roughly 11:59:45 p.m.! Compared with the recorded history of our species and even compared with the entire history of our species (unrecorded and recorded), the universe is unimaginably old.