The astronomer… is usually a very reverent man, because he can see, better than anyone else on Earth, the marvelous workings of the universe. With his scientific training, he can… appreciate the plan and order of the universe. He knows this law and order of the sky is not accidental, that it did not just happen. Even more than we, the astronomer can realize that this vast universe is the result and plan of Our Source — a Source of inconceivable intelligence and incomprehensible power.
– From Outer Space to You, 1959
Getting Oriented
- You are within a multi-part section. We make it easier to find what you need by showing you how the content is structured.
- To go “up” or “back” one level, see A Galactic Viewpoint – Overview & Outline of Contents.
- To view an outline of this section as a whole, see Exopolotics – Intro & Overview. Use the links to navigate quickly and efficiently.
- See also: Related Content at the bottom of the page.
Foundational Terminology
- Universe — Everything that exists. All of space and time including planets, stars, galaxies and other forms of matter and energy.
- Space — In astronomy, refers to the region that begins where the earth’s atmosphere ends. (WhatIs.com)
- Galaxy — A large group of stars, gas and dust held together by gravity. There are billions of galaxies in the Universe (categorized into four types: spiral, barred spiral, elliptical and irregular.) Galaxies vary greatly in size and shape and include stars, planets, moons, comets, asteroids, nebulae, dust, neutron stars and black holes. Some galaxies are visible without a telescope and can be seen from either the southern or northern hemisphere.
- Solar System — A planetary system whereby a star is being orbited by planets. Often, the phrase “solar system” refers only to the planetary system of which Earth is a part. However, more than 2,500 stars in our galaxy have planets orbiting them. (NASA SpacePlace)
- Planet — A celestial body in space that orbits a star. Other criteria includes 1) large enough that its gravity forces it into the shape of a sphere, 2) is not a satellite of another planet, 3) is not big enough to cause thermonuclear fusion (which would make it a star) and 4) has cleared its neighboring objects by its gravitational pull. (The Physics of the Universe)
- Asteroid / Dwarf Planet / Planetoid — A minor planet; a small, rocky body orbiting the Sun. Larger asteroids may be called dwarf planets or planetoids.
- Gas Giant — A large planet of relatively low density consisting mostly of hydrogen and helium, giving it a composition like that of the Sun.
- Star — A luminous ball of gas (mostly hydrogen and helium) held together by its own gravity. The Sun is the closest star to Earth. (See also: About the Sun) “Recent observations show that half a dozen new stars are produced each year in the Milky Way.” (Space.com)
- Star System / Star Cluster — Stars that are grouped closely together are called star systems. Larger groups of hundreds or thousands of stars are called star clusters. A system of two stars orbiting each other is a binary star.
- Constellation — A group of stars forming a recognizable pattern. Often named for its form or a mythological figure. The International Astronomical Union, or IAU (the world’s authority for assigning names to celestial objects) officially recognizes 88 constellations. For names and meanings of each, see Constellation Guide.
- Galaxy vs. Constellation — A galaxy is a collection of billions of stars; a constellation is a collection of only a few. There are billions of galaxies in the Universe; there are only 88 constellations. There are only a few galaxies visible to the naked eye; there are many visible constellations.
The distinction between dwarf planets, asteroids, meteoroids and comets is perhaps less clearly defined. Simplistically, asteroids are relatively small inactive bodies composed of rock or metals; dwarf planets are the largest asteroids; meteoroids are smaller particles of asteroids (called meteors or “shooting stars” when they burn up in the atmosphere, and meteorites if they manage to penetrate to the Earth’s surface); comets are mainly composed of dirt and ices rather than solid rock or metal, and tend to have dust and gas tails when close to the Sun.
– The Physics of the Universe
The Milky Way Galaxy
- The Milky Way is the name of the galaxy that contains our Solar System (and over 100 billion more stars).
- Our Sun is just one star among the hundreds of billions of stars in the Milky Way Galaxy.
- The Milky Way appears as a gigantic spiral disk with a bright, central bulge of gas, dust and stars.
- “Our galaxy is called the Milky Way because it appears as a milky band of light in the sky when you see it in a really dark area.” (NASA)
- At the center is a “supermassive” black hole called Sagittarius A. (LiveScience)
- The Milky Way Galaxy is part of a group of about 40 galaxies known as the Local Group, which is part of a larger “supercluster” called the Virgo cluster (containing over 2000 galaxies). (Sea and Sky)
- The Milky Way Galaxy is in the form of a “barred spiral” with two sets of “arms” or “spurs.” It is constantly rotating. (“There has been some debate over the years as to whether the Milky Way has two spiral arms or four. The latest data shows that it has four arms, as shown in the artist’s illustration here.” – NASA)
- Just as the Earth goes around the Sun, the Sun goes around the center of the Milky Way. It takes 250 million years for our Sun and Solar System to go all the way around the center of the Milky Way. (NASA)
The Solar System
- Our Solar System is about three-quarters of the way out from the center of the galaxy, in one of the spiral arms called the Orion Arm. “We live in the suburbs of our galaxy.” (NASA)
- The Solar System orbits around the center of the Milky Way Galaxy, taking about 230 million years to make one complete orbit. (NASA Starchild)
- It consists of the Sun and its orbiting planets (including Earth), along with numerous moons, asteroids, comet material, rocks, and dust. The Sun contains 99.8% of the mass of the Solar System. (Space.com)
- Let’s say that you are tracking the Solar System outward, beginning at the Sun. It goes past four inner planets (Mercury, Venus, Earth and Mars) and through the Asteroid Belt (between Mars and Jupiter). The Asteroid Belt includes the dwarf planet Ceres.
- The Solar System continues outward past four gas giants (the planets of Jupiter, Saturn, Uranus and Neptune). Between these planets are dozens of moons.
- It continues on to the disk-shaped Kuiper Belt (which includes the dwarf planet Pluto) and beyond to the edge of the Solar System, at about 9 billion miles / 15 billion kilometres from the Sun.
- The Oort Cloud lies far beyond the Kuiper Belt and is thought to surround the solar system. (Space.com)
- The Earth has one natural satellite: the Moon. Mars has two moons: Deimos and Phobos.
Other Galaxies
- The closest galaxy to the Milky Way is Canis Major Dwarf in the Canis Major constellation.
- The nearest “major galaxy” to our own is the Andromeda Galaxy.
- The nearest “giant galaxy” is Centaurus A.
- The other closest galaxies are Virgo Stellar Stream, Sagittarius Dwarf Elliptical Galaxy, Large Magellanic Cloud and Small Magellanic Cloud.
- The most distant galaxy is GN-z11 in the Ursa Major constellation.
A Few Key Stars & Star Systems
Galactic history speaks of events that have happened throughout the Universe, and of visitors that have come to Earth from various star systems including these:
- Altair / Alpha Aquilaie
- Andromeda Constellation & Andromeda Galaxy
- Arcturus
- Cassiopeia Constellation
- Centaurus Constellation & Centaurus A Galaxy
- Draco Constellation
- Lacerta Constellation / Little Cassiopeia
- Lyra Constellation & Vega
- Orion Constellation
- Pleiades Star Cluster
- Reticulum Constellation
- Sirius
Altair / Alpha Aquilae
- Altair is the 12th brightest star and one of the closest neighbors to Earth
- It’s the brightest star in the constellation, Aquila the Eagle, and so is also known as Alpha Aquilae.
- A distinctive feature is its rapid rotation. (EarthSky.org)
- It forms the southern point of the “Summer Triangle” that includes Vega and Deneb.
Andromeda Constellation & Andromeda Galaxy
- Located in the northern sky (in the first quadrant of the northern hemisphere, known as NQ1).
- Named after mythical princess Andromeda, wife of the Greek hero Perseus.
- Also known as Chained Maiden, Persea (wife of Perseus) or Cepheis (daughter of Cepheus).
- Neighboring constellations: Cassiopeia, Lacerta, Pegasus, Perseus, Pisces and Triangulum.
- The Andromeda Galaxy (named from its location in the constellation of Andromeda) is the nearest major galaxy to the Milky Way. It’s about the same size at the Milky Way and is on track to eventually collide with it. (Laboratory News)
Arcturus
- Red giant star in northern hemisphere.
- In the constellation Bootes.
- One of the brightest stars seen from Earth.
- “Follow the arc to Arcturus:” 5-min video.
Cassiopeia Constellation
- Located in the the northern sky.
- Recognizable because of its distinctive “W” shape.
- Named after Cassiopeia, the vain and boastful queen in Greek mythology. (Constellation Guide)
Centaurus Constellation & Centaurus A Galaxy
- Located in the southern sky.
- One of the largest constellations.
- Represents the centaur (half-man, half horse) from Greek mythology.
- The Centaurus constellation contains two of the top ten brightest stars in the sky: Alpha Centauri and Beta Centauri.
- The Centaurus A Galaxy, one of the brightest galaxies, is located here.
Draco Constellation
- Located in the northern sky.
- One of the largest constellations.
- “Draco” means “dragon” in Latin. Named after Ladon, the dragon that guarded the gardens of the Hesperides in Greek mythology. (Constellation Guide)
Lacerta Constellation / Little Cassiopeia
- Located in the northern sky between Andromeda and Cygnus.
- Its name means “the lizard” in Latin.
- Sometimes called Little Cassiopeia because its brightest stars form a “W” shape as in the larger Cassiopeia constellation. (Constellation Guide)
Lyra Constellation & Vega
- Located in the northern sky. (in the fourth quadrant of the northern hemisphere, known as NQ4).
- A small constellation.
- Named after the lyre, a musical instrument with strings. Associated with the Greek musician and poet Orpeheus. (Constellation Guide)
- The Lyra constellation contains Vega, the fifth brightest star in the sky.
- “Vega was the North Star several thousand years ago, and it will regain that status in about 12,000 years.” (Space.com)
Orion Constellation
- Located on the celestial equator.
- One of the most prominent and recognizable constellations in the sky. Can be seen throughout the world. (Space.com)
Pleiades Star Cluster
- Visible from virtually every part of the globe.
- Also known as the Seven Sisters or M45
Reticulum Constellation
- Located in the southern sky.
- The name means “small net” in Latin and represents a part of the telescope that makes it possible to measure star positions. (Constellation Guide)
Sirius
- Located in the Canis Major constellation.
- From any part of Earth, the brightest visible star is Sirius.
- Also called the Dog Star.
Sources & Resources
- Byrd, Deborah, EarthSky.org — Sirius is Dog Star and Brightest Star
- Choi, Charles Q., Space.com — Solar System Facts
- Constellation Guide — Constellation Names
- Grifantini, Kristina, LiveScience — What’s at the Center of the Milky Way?
- Howell, Elizabeth, Space.om — Vega: The North Star of the Past and the Future
- Laboratory News — Milky Way Steps Up in Gravitational Arms Race
- NASA — Imagine the Universe
- NASA Starchild — Question of the Month
- The Physics of the Universe — What is a Planet? What is a Dwarf Planet?
- Greshko, Michael, National Geographic — Galaxies, Explained
- McClure, Bruce, EarthSky.org — Pleiades Star Cluster, aka Seven Sisters
- Night Sky Network — Solar System, Galaxy, Universe: What’s the Difference?
- Redd, Nola Taylor, Space.com — Milky Way Galaxy: Facts About Our Galactic Home
- Sea and Sky — Celestial Objects
- Sessions, Larry, EarthSky.org — Altair is the Bright Star of the Eagle
- Star Gazers — Follow the Arc to Arcturus— 7 min video
- Than, Ker, Space.com — Astronomers Had it Wrong: Most Stars are Single
- University of Cambridge Institute of Astronomy — How Many Solar Systems are There?
- WhatIs.com — Space
- Zimmermann, Kim Ann, Space.com — Orion Constellation: Facts About the Hunter
