Dennis Mammana

Stargazers

We live in a world defined by inches, feet and miles (or centimeters, meters and kilometers, if you prefer), and it’s not difficult for most of us to comprehend sizes and distances given in these units. But astronomers deal with a universe much grander than our workaday world, and we’ve long ago realized that these distance units are much too small to help us.

For example, watch the full moon rising over the east-southeastern horizon early on the evening of Thursday, July 10; at that time, the moon will lie at a distance of some 242,156 miles from Earth. I’d argue that no human can comprehend on a gut level such an immense scale, even though the moon is our nearest cosmic neighbor.

The planets of our solar system are considerably farther — millions and billions of miles away — so to help keep the numbers smaller, astronomers have invented the “astronomical unit,” or AU. One AU is defined as the average distance of the Earth from the sun, about 93 million miles. We can say that the planet Neptune, for example, lies about 2.8 billion miles, or 30.1 AU, away.

But the solar system in which we live is a relatively small place in the cosmos.

What about the stars? They are much like the sun, and it’s not tough to imagine that they appear so small and faint because they’re farther away. Much farther!

Even the nearest stars are trillions of miles away, and they make up a giant disc we know as the Milky Way galaxy, some six hundred thousand trillion (600,000,000,000,000,000 miles) miles across. Even using AU for these distances would soon become unworkable, with the Milky Way diameter measuring some 6.3 billion AU. Astronomers need an even larger unit, and we have it: a “light-year.”

Though it sounds like a unit of time, a light-year is the distance that light travels in one year — about 6 trillion miles.

Take the brightest star high in the eastern sky tonight (Vega); it lies about 150 trillion miles (or 1.6 million AU). To make this journey, a beam of light would require 25 years, so we say that Vega is 25 light-years away. Much more workable, wouldn’t you say?

Of course, Vega is just one of the closest stars to us, and there are hundreds of billions that make up the Milky Way.

From side to side, our galactic disc spans about 100,000 light-years, and we believe that the universe is populated with hundreds of billions of such galaxies that fill a space some 93 billion light-years across.

Right back to the huge numbers again, so astronomers have created an even larger unit: a “parsec,” equivalent to about 3.26 light-years. But even parsecs can be too small when discussing the largest cosmic structures, so we add prefixes such as “kilo-” (thousands), “mega-” (millions), or “giga-” (billions).

Now the diameter of the known universe can be described as 558 billion trillion miles (that’s 558 followed by 21 zeros) or 28 gigaparsecs. If you’re like most astronomers, I’m pretty sure which size numbers you’d prefer to deal with!