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| The Realm of the Very Large |
The Realm of the Very Large
The realm of the very large spans the universe beyond the Earth. It includes objects from the size of the Earth to the size of the whole universe (several billion lightyears) ; and time scales from 100 years to the age of the universe (about 13.7 billion years).
Our solar system is centred on the Sun with eight planets in orbit around it: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune . Most of the planets have one or more moons. There are also a number of dwarf planets, thousands of asteroids, and thousands of comets. The Sun has a diameter of about 1.4 million km and contains over 99% of the mass of the entire solar system. Most of the rest of the solar system lies within a disc about 12 light-hours in thickness.
Beyond the solar system is a myriad of other stars. Some are 10 times more massive and many times brighter than the Sun; most are about the same size, or smaller and fainter than the Sun. While a few hundred of the brightest stars visible in the sky have been named, most are just given a number in one of the many star catalogues. All of the stars in the sky appear much fainter than the Sun because they are so far away.
The nearest star to the Sun is Proxima, with a distance of 4.2 light-years. Most stars have nearby companions, or occur in clusters with hundreds of members. The nearest 100 billion stars are grouped together and orbit around a central black hole to form the Milky Way galaxy. The Milky Way is disc shaped and about 300 000 light-years in diameter. The Sun is about 25 000 light-years from the centre of the disc, on the inner edge of the Orion arm.
Beyond the Milky Way, there are billions of other galaxies. The nearest galaxy comparable to the Milky Way is the Andromeda galaxy, with a distance of about 2.9 million light-years. Even though the Andromeda galaxy contains billions of stars, it is so far away that it appears as just a faint smudge in the night sky. Even galaxies tend to occur in clusters. The Milky Way and the Andromeda galaxies dominate the Local Group, which consists of 30 galaxies. The Virgo cluster contains the Local Group and about 2000 other large galaxies. It has a diameter of about 60 million light-years. The largest telescopes on Earth, or in space, have yet to detect an end to the distribution of galaxies in space.
The universe is not static. The Sun ascends and descends in the disc of the Milky Way once about every 50 million years, and revolves around the centre of the galaxy every 250 million years. Individual stars are born, evolve through predictable stages, and die. Stars similar to the Sun have an estimated life span of about 10 billion years before they exhaust all their energy and become brown dwarfs. Stars larger than the Sun age more rapidly. Smaller stars age more slowly. The stars in a region of rich interplanetary clouds can pass through several generations. Whole galaxies pass through evolutionary stages as the stars within them consume primordial hydrogen and radiate energy into the surrounding universe. The galaxies in a cluster move about each other, and a few galaxies have been observed in collision with each other.
A number of investigations have estimated the age of the universe at 13.7 billion years, so the radius of the knowable universe is just the distance that light could travel in that time, or 13.7 billion light-years.
The realm of the very large spans the universe beyond the Earth. It includes objects from the size of the Earth to the size of the whole universe (several billion lightyears) ; and time scales from 100 years to the age of the universe (about 13.7 billion years).
Our solar system is centred on the Sun with eight planets in orbit around it: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune . Most of the planets have one or more moons. There are also a number of dwarf planets, thousands of asteroids, and thousands of comets. The Sun has a diameter of about 1.4 million km and contains over 99% of the mass of the entire solar system. Most of the rest of the solar system lies within a disc about 12 light-hours in thickness.
Beyond the solar system is a myriad of other stars. Some are 10 times more massive and many times brighter than the Sun; most are about the same size, or smaller and fainter than the Sun. While a few hundred of the brightest stars visible in the sky have been named, most are just given a number in one of the many star catalogues. All of the stars in the sky appear much fainter than the Sun because they are so far away.
The nearest star to the Sun is Proxima, with a distance of 4.2 light-years. Most stars have nearby companions, or occur in clusters with hundreds of members. The nearest 100 billion stars are grouped together and orbit around a central black hole to form the Milky Way galaxy. The Milky Way is disc shaped and about 300 000 light-years in diameter. The Sun is about 25 000 light-years from the centre of the disc, on the inner edge of the Orion arm.
Beyond the Milky Way, there are billions of other galaxies. The nearest galaxy comparable to the Milky Way is the Andromeda galaxy, with a distance of about 2.9 million light-years. Even though the Andromeda galaxy contains billions of stars, it is so far away that it appears as just a faint smudge in the night sky. Even galaxies tend to occur in clusters. The Milky Way and the Andromeda galaxies dominate the Local Group, which consists of 30 galaxies. The Virgo cluster contains the Local Group and about 2000 other large galaxies. It has a diameter of about 60 million light-years. The largest telescopes on Earth, or in space, have yet to detect an end to the distribution of galaxies in space.
The universe is not static. The Sun ascends and descends in the disc of the Milky Way once about every 50 million years, and revolves around the centre of the galaxy every 250 million years. Individual stars are born, evolve through predictable stages, and die. Stars similar to the Sun have an estimated life span of about 10 billion years before they exhaust all their energy and become brown dwarfs. Stars larger than the Sun age more rapidly. Smaller stars age more slowly. The stars in a region of rich interplanetary clouds can pass through several generations. Whole galaxies pass through evolutionary stages as the stars within them consume primordial hydrogen and radiate energy into the surrounding universe. The galaxies in a cluster move about each other, and a few galaxies have been observed in collision with each other.
A number of investigations have estimated the age of the universe at 13.7 billion years, so the radius of the knowable universe is just the distance that light could travel in that time, or 13.7 billion light-years.
