Star-struck eyes gather the distant light

Galaxy Andromeda. Photo by PavelSmilyk/iStock/Getty Images

If you’ve ever wondered how far can you see, the answer hovers in the night sky of late November. Step outside and look up – straight up. Under a clear sky away from light pollution, you can spot an object whose distance can’t be comprehended; only quantified.

Let’s start with distances we can comprehend. We resolve letters on an eye chart from a distance measured in feet; words and symbols on a road sign from a distance measured in yards; the outline of a city skyline or mountain range from a distance measured in miles. We Contra Costa County folk are graced with a magnificent long-distance object: Mt. Diablo, about 10 miles west of downtown Brentwood. Let’s use the mountain as a point of reference.

A little elevation – say, the crest of Round Valley’s Hardy Canyon Trail – rewards us with a view of an object 10 times farther than Mt. Diablo: the granite majesty of the Sierra Nevada Range to our east. A greater challenge to the imagination is the view of our Moon sinking into the west behind Mt. Diablo. The Moon: 24,000 times more distant than the mountain – though not nearly as impressive as the Sun: 9 million times the distance of the mountain.

Our next step takes us into interstellar space. The nearest bright star in our late-autumn sky, found southeast of the constellation Orion in Canis Major, is the glinting diamond we call Sirius, a whopping nine light years from Earth. Now, if nine light years doesn’t sound impressively remote … it should.

A light year is a measure not of time but distance: the distance light travels in one year. Once we leave our tiny solar system, the space between stars, and galaxies of stars, becomes so enormous that astronomers describe distance in light years instead of miles. It’s hard to wrap the mind around a number ending in 18 zeroes.

How far is a light year? Well, if you could hitch a ride on a wave of light, if you could go 186,000 miles per second – seven times around Earth in one second – it would take you 8½ minutes to reach our Sun and nine years to reach Sirius.

But in the scale of the cosmos, Sirius is our next-door neighbor. The main rectangle stars above Sirius in Orion – Betelgeuse, Bellatrix, Rigel and Saiph – range from 240 to 900 light years away. Hanging below Orion’s belt is M42, the Orion Nebula, at a distance of 1,350 light years.

But 1,350 light years is a piece of cake. You can see farther than the Orion Nebula – a lot farther. All the stars you can spot with your naked eye reside within our home galaxy, the pinwheel of between 200 and 400 billion stars we call the Milky Way. But there’s a naked-eye object out there that’s well beyond our galaxy. And that would be another galaxy.

Graphic by Ger Erickson

Labeled M31, the Andromeda Galaxy floats in our November evening sky a staggering 2½ million light years away. Expressed in miles, that’s 12,900,000,000,000,000,000. What the heck, round it up to 13 quintillion miles. At that distance, the cumulative light of Andromeda’s trillion stars strikes your retinas with a few thousand photons per second – more than enough to flip the switch of your optical apparatus.

And more than enough to flip the switch of your imagination. When you finally resolve that gossamer oval, preferably through binoculars or a telescope, keep in mind that you’re not viewing Andromeda in the present; you’re viewing it as it was 2½ million years ago. Andromeda isn’t merely the most distant object you can see with your naked eye; it’s the most ancient. 

The next time you squint at your optometrist’s Snellen chart and lament what’s become of the 20/20 vision of your youth, take heart. You might not be able to resolve that P in line 8, but there’s another object you can resolve.

“By the way, Doc. I stepped outside last night and saw something really far away.”

“Yah? How far?”

“Oh, about 13 quintillion miles,” you say with an air of scientific detachment.