Seeing the Sun at Night (Well, Sort of)

By Dennis Mammana

October 11, 2018 4 min read

Week of Oct. 14-20, 2018

The stars we see at night are like the sun, astronomers tell us. But just how true is that?

Well, all are thermonuclear furnaces. In most cases, the cores of these stars are packed with hydrogen atoms that, under unimaginably high temperatures and pressures, slam into one another, fuse together and create helium atoms. In this process, they release a tremendous amount of energy — some of which prevents the star from collapsing upon itself, and some of which leaks out into space as light and heat.

In this way, stars and our sun are similar. But in another, they're quite different. To illustrate this for yourself, try a thought experiment, or a gedankenexperiment, as Einstein used to call it.

Our brilliant sun lies 93 million miles from us, but imagine how its appearance would change if we could drag it farther away. As it recedes, it would become smaller and dimmer, smaller and dimmer ... until eventually it would look much like the stars we see at night.

Just how far is that? The answer is many trillions of miles.

Suppose you wanted to see how the sun would look against the other stars of the night sky. Well, you can't, of course, but you can see a star like the sun. Its name is Eta Cassiopeiae, and it lies within the W-shaped star grouping now high in the northeastern sky after dark. After the moon becomes too bright later this week, you may not be even be able to spot it without binoculars.

Eta Cass, as astronomers know it, lies a mere 114 trillion miles — or about 19.4 light-years — from us. Considering that our Milky Way galaxy spans at least 100,000 light-years, this is a very close neighbor. If it were much farther, we could never see it with the unaided eye.

So why is it so faint compared with most of the other stars in our night sky? Fact is, they're either larger or more luminous (or both) than our sun.

Take Altair, for example, the southernmost bright star in the large Summer Triangle high overhead after dark. It's only 98 trillion miles away, and its light takes 16.7 light-years to reach us (we say it's 16.7 light-years distant). It's almost twice as large and nearly 11 times more luminous than our sun.

And what about Deneb, on the northeasternmost corner of the Summer Triangle? It lies at least 9,000 trillion miles, or about 1,500 light-years, from us. This is a true supergiant with a diameter more than 100 times greater and a luminosity more than 200,000 times that of our sun.

In fact, virtually all the stars we see at night are larger and more luminous than our sun. That doesn't mean there aren't a lot of sunlike stars out there. There are plenty! But much like looking around a sandy beach, we easily see the largest rocks and boulders strewn about, not the millions of grains of sand that make up the beach.

So, the next time you gaze skyward on a clear, dark night and marvel at all the suns you see, think about all those you can't see!

(SET CAPTION) See the sun at night this week (well, sort of). (END CAPTION)

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