The Language of Astronomy

An Introduction to Astronomy--A Few Terms at a Time

Astronomy and astronomy objects have a vocabular all their own. Carolyn Collins Petersen

Learn the Words Astronomers Use

Astronomers are the people who study the stars. Like any technical discipline, such as medicine or engineering, astronomers have terminology all their own. We often hear them speak of "light-years" and "exoplanets" and "galaxy collisions", and those words invoke fascinating thoughts about the vastness of the cosmos that we explore. Take "light-years" for example. It is used as a measure of distance . It's based on how far light travels in a year, at a speed of 186,252 miles (299,000 km) per second. The nearest star is to the Sun currently is Proxima Centauri, at 4.2 light-years away. The nearest galaxies — the Large and Small Magellanic Clouds — are more than 158,000 light-years away.  The closest spiral is the Andromeda Galaxy, at an approximate distance of 2.5 million light-years away. 

Understanding Distance Terminology

It's interesting to think about these distances and what they mean. When we see the light from the nearby star Proxima Centauri, we are seeing it as it WAS 4.2 years ago. The vision of Andromeda that we see is 2.5 million years old. When Hubble Space Telescope spots galaxies that lie 13 billion light-years away from us, it shows us an image of them as they were, 13 billion years ago. So, in a sense, the distance of an object lets us look back in time.  It took 4.2 years for that light to reach our eyes from Proxima Centauri, and so that's how we see it: 4.2 years old. And, so it goes for greater and greater distances. The farther across space you look, the further back in time you are "seeing". 

Within the solar system, astronomers don't use terms such as "light-year." It's easier to use the distance between Earth and the Sun as a convenient distance marker. That term is called "astronomical unit" (or AU for short).  The Sun-Earth distance is one astronomical unit, while the distance to Mars is around 1.5 astronomical units. Jupiter is 5.2 AU away, and Pluto is 29 AU distant. 

Describing Other Worlds

Another term you sometimes hear astronomers use is "exoplanet". It refers to a planet orbiting another star. They are also called "extrasolar planets". There are more than 1,900 confirmed exoplanets and nearly 4,000 more candidates to be determined. The study of exoplanets is a story of what they are, how they formed, and even how our own solar system developed. 

Galactic Activity

"Galaxy collisions" are often referred to as "galaxy interactions" or "galaxy mergers".  They are how galaxies develop in the universe. These have occurred throughout nearly all of the universe's 13.8 billion-year history. They happen when two or more galaxies get close enough to mingle stars and gases. Sometimes one galaxy gobbles another one up (occasionally referred to as "galactic cannibalism"). This is occurring right now as the Milky Way "ingests" two or more dwarf galaxies. It has been doing this its entire existence. 

Often, two galaxies collide in a rather violent way, and they will take on interesting shapes, with warped arms and streams of gas stretching out across space. It's very likely that the Milky Way and the Andromeda Galaxy will collide in the next 10 billion years, and the end result has been nicknamed the "Milkdromeda Galaxy."

Earth-based Astronomy Terms

Did you know that terms we commonly see on the calendar are also astronomy-based? "Month" comes from the word "moon", and lasts about as long as it takes for the Moon to go through one cycle of phases.  Watching and charting the Moon's apparent change of shape is a great skywatching activity to do with kids. 

You may also have heard about "solstice" and "equinox". When the Sun rises due east and sets due west, that's the day of the equinox. These occur in March and September. When the Sun rises sets farthest south (for those of us in the northern hemisphere), that's the day of the December (winter) solstice. It rises and sets farthest north on the June solstice.

Astronomy isn't just a science; it's a human and cultural activity that helps us understand the cosmos. It comes down to us from the earliest stargazers thousands of years ago. For them, the sky WAS a calendar. To us today, it's a place to explore.