Astronomy 101 - Big Numbers

Lesson 4: It's A Big Universe

At 2.5 million light-years, the Andromeda Galaxy is the closest spiral galaxy to the Milky Way. The term "light-year" was invented to handle the immense distances between objects in the universe. Later, "parsec" was developed for truly huge distances. Adam Evans/Wikimedia Commons.

Our universe is huge, larger than most of us can even imagine. In fact, our solar system is beyond the grasp of most of us to truly visualize in our mind's eye. The systems of measurement we use just don't stand up to the truly immense numbers involved in gauging the size of the universe, the distances involved, and the masses and sizes of the objects it contains. However, there are some shortcuts to understanding those numbers, particularly those for distance. Let's take a look at measurement units that help put the immensity of the cosmos into perspective. 

Distances in the Solar System

In perhaps a nod to our old belief of Earth as the center of the universe, our first unit of measurement is based on the distance of our home to the sun. We are 149 million kilometers (93 million miles) from the Sun, but it's much simpler to say we're one astronomical unit (AU). In our solar system, the distance from the Sun to the other planets can be measured in astronomical units as well. For example, Jupiter is 5.2 AU away from Earth. Pluto is about 30 AU from the Sun. The outer "edge" of the solar system is at the boundary where the Sun's influence meets the interstellar medium. That lies about 50 AU away. That's about 7.5 billion kilometers away from us. 

Distances to the Stars

The AU works great within our own solar system, but once we start looking at objects outside our Sun's influence the distances get very hard to manage in terms of numbers and units. That's why we created a unit of measure based on the distance that light travels in a year. We call these units "light-years," of course. A light-year is 9 trillion kilometers (6 trillion miles).

The closest star to our solar system is actually a system of three stars called the Alpha Centauri system, consisting of Alpha Centauri, Rigil Kentaurus, and Proxima Centauri, which is actually slightly closer than her sisters. Alpha Centauri is 4.3 light-years from Earth.

If we want to move beyond our "neighborhood," our nearest neighboring spiral galaxy is Andromeda. At roughly 2.5 million light-years, it's the most distant object that we can see without a telescope. There are two closer irregular galaxies called the Large and Small Magellanic Clouds; they lie at 158,000 and 200,000 light-years, respectively. 

That distance of 2.5 million light years is a huge one, but merely a drop in the bucket compared to the size of our universe. In order to measure larger distances, the parsec (parallax second) was invented. A parsec is approximately 3.258 light-years. Along with the parsec, larger distances are measured in kiloparsecs (thousand parsecs) and megaparsecs (million parsecs).

One other way to denote very large numbers is something called scientific notation. This system is based on the number ten and is written like this 1 × 101. This number equals 10. The small 1 located to the right of the 10 indicates how many times 10 is used as a multiplier. In this case once, so the number equals 10. So, 1 × 102 would be the same as 1 × (10 × 10) or 100. An easy way to figure a scientific notation number out is to add the same number of zeros at the end as the small number to the right of 10. So, 1 × 105 would be 100,000. Small numbers can be written this way as well by using a negative power (the number to the right of 10). In that case, the number will tell you how many places to move the decimal point to the left. An example: 2 × 10-2 equals .02.

Edited by Carolyn Collins Petersen.