Astronomy, part 09 : Vocabulary.

09 Astronomy-themed Science Vocabulary


Large, weird-shaped object that orbits Earth. A bunch of them hang out between Mars and Jupiter; a zone known appropriately as “The Asteroid Belt.” They’re very similar to a comet, except they’re are made out of rock or solid material (like metal) rather than gas. If you had a choice between Earth getting hit by either an asteroid or a comet, you should definitely go with the latter.

Astronomical unit (AU)

An astronomical unit that is equivalent to 93 million miles, which is not a random number. Hint: it’s the approximate distance from Earth to the Sun. Hint: that was the answer, not a hint. It might be the first tangible inkling you have of how huge the universe is: remember, one AU is equivalent to how far away our sun is. So...


The air and gas that blankets objects in space. Earth’s atmosphere completely surrounds it up to about 600 miles and is made up of mostly nitrogen and oxygen. Earth’s atmosphere has six layers that play a vital role in the water cycle, carbon cycle, and nitrogen cycle. Ours is unique because it sustains life by allowing just the right mix of sunlight to warm and cool, while filtering out the sun’s most dangerous radiation. So it keeps us alive and safe. For now.


An orbiting object in space made up up of dust, ice, and gas. Sometimes described as “a dirty snowball,” or more accurately, “a snowy dirtball.” Our solar system has billions; the most famous is Halley’s Comet.


Group of stars that form a pattern or outline. They are often named after an animal, mythological creature, god, or other super cool icon. Modern science has divided up the ones we know into 88 different ones. Famous ones include Aries, Canus Major, Orion, and Ursa Major (the Big Dipper).  

Dwarf planet

Like a big planet, only smaller. Sad.

Dwarf star

A small star with that emits a sad and pathetic amount of light.


A twice-a-year occurrence that happens when the sun crosses directly over the equator. The hours of daylight and darkness are the same all over the earth. March 20 (vernal) and September 22 (autumn), mark your calendars.

Giant / Supergiant

When a star has used up all its hydrogen and going on celestial hospice, the heavier elements, including helium, release more and more energy and the star expands. Which means it gets bigger. Like a giant, or the opposite of a dwarf star. They burn bright and die fast. The James Dean of celestial objects. No fizzling for them.

A supergiant is way way bigger than a giant. Basically a super gigantic clump of hydrogen and helium that could swallow up ten of our suns.


The sun’s light takes approximately 500 seconds to reach earth.

Light year

A unit of measurement that gets mind boggling very fast. One light year is equivalent to the distance traveled by light in one year.  One light year is roughly equivalent to 63,000 astronomical units; one astronomical unit being 93 million miles. So...things add up fast. For a sense of scale: our galaxy, the Milky Way, is about 100,000 light years across. For one more sense of scale, one light year travels about six trillion miles. Yes, that was a “t” before the “illion.” The farthest galaxy away from us that we’ve measured is 13 billion light years. Do the math. Goodbye.


Referring to the moon.

Main sequence stars

Stars that have helium cores fused with hydrogen atoms. Ninety percent of the stars in the universe are main sequence. Starting off as clouds of dust and gas, gravity pulls them together and eventually, that gas and dust burns hot enough that hydrogen fuses into helium, creating pressure that counterbalances gravity and lets it float around in harmony with everything around it. Until it dies. Which could be a short time off, or a long while. We just don’t know. We’re all gonna die. Life life well.

Meteroid, meteor, meteorite

Chunk of rock broken off an asteroid and hurling through space. Here we go:

Meteor is the flash of light when that chunk gets burned up blazing through our atmosphere.

Meteoroid is the debris itself that usually gets vaporized and never makes it to earth.

Meteorite is the lucky one that runs the atmosphere gauntlet and lands on earth. Hopefully not on your head. But it might. Don’t look up.


A big giant sexy cloud of dust and gas in space. Gravity draws them together into clumps that get bigger and bigger...and eventually collapses from its own gravity and forms the beginnings of...a star. Beautiful.

Nuclear fusion

The short version is this: two little tiny cute nuclei band together to make a heavier nucleus. A nucleus being the center of an atom. This nuclear reaction releases a huge amount of energy. In astronomy, it’s what stars do. Hydrogen fuses together and makes helium, which releases huge amounts of energy, which is what powers the star’s light and heat.

Some elements can’t be fused. Most notably, iron (metal) doesn’t fuse. Stars keep joining all of their atoms and fusing away to make heavier and heavier atoms until they begin to make iron. At that point, the iron can’t fuse with other nuclei. So the whole star starts to die. Sad. Cycle of life. And death.


Astronomical unit. One parsec equals around 3.3 light years.


A big object that orbits a star. For example, Earth is a planet that orbits the Sun, which is a star. What’s the difference? Two things: planets are smaller and they don’t make light.

Our solar system has eight planets. Used to be eight, until Pluto got demoted. Sad.

In order from the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.


The movement of a planet around another object. For example, Earth revolves around the Sun. One time around is one revolution. Do not ever confuse with rotate. Nobody does that.


Any type of vehicle that’s pushed along by an engine. Specifically, a rocket engine, and it may be manned or unmanned. Or humanned or unhumanned. Examples include missiles, satellites, spacecraft, and aircraft. They’re super great for helping get big objects into space.


The movement of a planet or object spinning around one time on its north-to-south axis. So moving on an imaginary internal axis.


An object that orbits another object. There’s natural satellites, such as the moon. There’s human-made satellites, such as communications and weather ones, that orbit Earth


Earth has four seasons. They’re all the result of its axis tilt, which is approximately 23 degrees. As it revolves around the sun, different parts get different amounts of light (or shade) because of that tilt, so that’s how we end up with different seasons: warmer weather (summer) when the rays are hitting at a direct angle, colder (winter) when rays hit at an extreme angle.


Referring to the sun.


The two days a year when the sun is at its farthest north or south in relation to the Earth. The shortest day of the year is Winter Solstice (December 21 or 22). The longest days is Summer Solstice (June 21).

Space probe

A small aircraft without humans that either orbits or lands on a planet in order to study and learn more about it.

Space shuttle

Spacecraft used to carry astronauts and cargo into space. Pretty cool because they can be used more than once. Unless they blow up or get taken over by a superintelligent computer.

Space station

A human-built structure in space for astronauts and cosmonauts to live and do experiments and other space stuff. The most famous is the International Space Station.


A big ball of heat and light that’s powered by nuclear fusion and held together by gravity. The Sun is the closest star to Earth.

Scientists estimate there are a few hundred billion stars in our galaxy (the Milky Way). Multiply that number by a few more hundred billion and that gets us conservatively into the sextillions, which is the almost-unfathomable number of stars estimated to be in the universe.

A grouping of stars is known as a constellation.


The light given off by...stars. It’s one of the main ways we have of observing and learning about the universe (via telescopes).


Giant stars often times end their lives as supernovas, which is a killer way to go. Dark humor intended. Going out with a burst and a blast.


Super important scientific tool that makes things look bigger and brighter and closer. The first good one for astronomy was invented by Galileo.


The fancy name is obliquity. Earth’s tilt is a steady 23.5 degrees and usually points toward Polaris, the North Star. This is how we get our seasons: for half the year, one hemisphere is tilted toward the sun and the hemisphere away, and vice versa.


One of the most vital elements for sustaining life...possibly anywhere in the universe, but definitely on Earth and for humans. Water is incredible because it can exist as all three states of matter and supports life in uncountable ways, from rain in the atmosphere, to oceans regulating climate, to promoting plant growth and making it possible for animals - including humans - to function. Scientists believe that for life to exist anywhere, some sort of liquid has to exist, and suspect that water may be the only viable option. But who knows? Perhaps someday we will.

FOOTNOTE: Introduction to a ten-unit survey of Astronomy and the foundations of the universe beyond our world.

Education for most ages and for all curious people. Written by Joseph Ivan Long with curiosity, humbleness, and a big grin.