School in a Book: Astronomy

Everyone loves space. Why? I don’t know. It just sort of blows our minds, I guess. The following will give you many of the main astronomical terms and ideas, but do also read The Fabric of the Cosmos by Brian Greene and Steven Hawking’s A Brief History of Time. There’s also a great memoir by Scott Kelly of living on the ISS for a year called Endurance, as well as many excellent space documentaries.


Space: All of the area outside the earth’s atmosphere. Space has no air. Its temperatures vary from far below freezing (such as areas blocked by planets to the rays of stars) to far above boiling water (such as areas not blocked from the rays of stars).

Universe: All of the billions of galaxies in existence. The Universe is held together by gravity and, at the same time, slowly expanding. It is mostly empty space, with matter like stars and planets at distances from each other that are comparable to the distances of particles in atoms. This is why collisions are infrequent, despite the many and varied paths taken by celestial bodies.

Star: A ball of very hot gas in space. Stars can be white, red, yellow or blue.

Sun: The only star in Earth’s solar system. It is medium-sized: one million times the size of Earth and ten times the size of Jupiter. On its surface, the sun is 5,500 degrees.

Planet: A spinning ball of rock or gas that travels around a star (or a black hole) in an orbit. We can only see a few planets outside our solar system.

Moon: A mini planet that revolves around a regular planet instead of revolving around a star. The earth’s moon is dry and dusty with many craters. It takes 27 days for the moon to spin once, and 27 days for it to orbit once around the earth, which is why it doesn’t seem to be spinning. It is always facing away from us, so we’ve never seen the other side. People have gone to the moon several times. It takes about three days to reach the moon and each crew spent about three days there.

Solar system: A group of planets revolving around a single star or a group of stars, or a small group of stars revolving around each other.

Galaxy: A group of solar systems which orbit around each other. Many galaxies contain millions of stars. Sometimes galaxies cross paths and collide. It’s likely that most or all galaxies have a black hole at their center. Many galaxies orbit other galaxies, but not all. It is difficult to determine what galaxies orbit, if anything, due to the slowness of their movement and limitations of technology.

Star cluster: Groups of stars that form together

Galaxy cluster: A group of galaxies

Orbit: The circular path taken by a planet, moon, star or other celestial body. Orbits can be maintained indefinitely because the gravity that pulls the orbiting object toward another object is balanced by the orbiting object’s momentum, which seeks to keep the orbiting object moving in a straight line. In empty space, friction, drag and other forces do not exist, so the object’s momentum is never lost.

One day: The unit of time marking one spin of the earth on its axis, which takes approximately 24 Earth hours. The part of the earth facing the sun has light, and the other doesn’t. It takes 365 days for the earth to orbit around the sun one time.

Light year: The distance light travels in one year. It is used as a measurement of distances in space.

The nine phases of the moon: The views of the earth’s moon from the earth, which change in a 29.5-day cycle. The nine phases are: new moon (no light); waxing crescent moon (getting more visible and in a crescent shape); first quarter moon (half moon); waxing gibbous (getting more visible and in a lopsided circle shape); full moon; waning gibbous (getting less visible); last quarter moon (half moon); waning crescent; new moon.

Solar eclipse: A celestial event during which the sun is entirely obscured from view for a short time due to the path of the moon, which brings it between Earth and the sun

Lunar eclipse: A celestial event during which the moon is entirely obscured from view for a short time due to the path of the Earth, which brings it between the moon and the sun. At that time, the moon sits in the Earth’s shadow and no light from the sun illuminates it.

Solar wind: The stream of charged particles in the form of plasma that make the air glow at Earth’s magnetic poles, creating the aurora borealis (the Northern Lights).

Solar mass: The mass of our sun. It is used as a standard unit of measurement of space bodies.

Our eight planets, in order from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Gas giants: Jupiter, Saturn, Uranus, Neptune. Rest are rock. Jupiter is largest, Mercury is smallest. Jupiter has the Great Red Spot, a permanent gas storm. Saturn is very light, light enough to float in water.

The Milky Way: The name of the galaxy our solar system is in. It is about 100,000 light years across. It contains one star, eight planets, many moons, and an asteroid belt. The Milky Way doesn’t orbit anything, but other galaxies orbit it and Andromeda, the closest neighboring galaxy.

Andromeda: The nearest large galaxy to the Milky Way

The Local Cluster: The galaxy cluster our galaxy is in

Supercluster: A group of galaxy clusters

Virgo Supercluster: The supercluster our galaxy is in

Asteroid: A big lump of rock or metal in space

Meteoroid: Dust or small space rocks (house-sized to coffee-ground sized) in orbit around the sun

Meteor/shooting star: A meteoroid that burns up in a planet’s atmosphere

Meteorite: A meteoroid that hits the surface of a planet

Comet: A ball of dirty ice floating around space. When close enough to the sun, the ice melts partway and the solar wind blows a trail of gas and dust behind it, making a tail.

Nebula: Big cloud of gas and dust that stars are formed in

Pulsar: A collapsing star that instead of becoming a black hole keeps spinning faster and faster and getting denser as it collapses. It gives off waves (pulses) of electrons.

Supernova: A very large star that has reached the end of its life (and its supply of gas) and is exploding

Red Giant: A smaller or medium-sized star that is near the end of its life and has swelled up and turned red

White dwarf: A star that results from the Red Giant’s exterior gas burning off. After a time, it cools and fades away.

Binary star:

Neutron star:

Black hole: A supercondensed, superheavy ball of matter and energy whose gravity pulls in everything near it and from which nothing, not even light, can escape. Some are the remains of very large stars that, instead of dying, collapsed. Some black holes are only a few miles across, while others are several million miles across. Black holes continuously draw in more matter and expand due to their huge gravitational force.

Event horizon: The boundary surrounding a black hole that, once matter crosses, no escape is possible–it will be pulled into the black hole

The Big Bang: The (theoretical) explosion that created the universe. This explosion may have happened approximately 15 billion years ago. It occurred when all energy and all matter in existence somehow gathered into a tiny speck with virtually no empty space between particles, then, under this enormous pressure, suddenly exploded and became randomly distributed in space. As the matter cooled, gravity caused larger bits to attract smaller bits and form large stars and planets.


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