From the August 2005 issue

Bob Berman’s strange universe: The masked ball

August 2005: The sphere is the universe's most common shape. Why don't stars instead look like disks, cigars, or cubes?
By | Published: August 1, 2005 | Last updated on May 18, 2023
Bob Berman
Some of us will take breaks from this month’s midnight meteors to look through telescopes. We’ll peer at all sorts of shapes.

For untold millennia, people gazed at a sky dominated by two perfectly round objects. But now, through telescopes, we rarely see anything truly round. Jupiter’s wild 28,000-mph rotation gives its equator a pronounced bulge. Saturn is even more oval. Venus and Mars currently show gibbous phases. Clusters, stars, galaxies — none of them looks perfectly round either.

Despite all that, the sphere is the universe’s most common shape. Why don’t stars instead look like disks, cigars, or cubes? Answer: A sphere alone has a surface where every spot lies the same distance from the center. Its exterior has the smallest possible area. You need more paint to cover a cube or diamond than a ball of equal weight.

So when a gas cloud or solid body contracts from its own self-gravity, it pulls itself into the smallest possible form, and that’s always a ball. Only things with little mass, like tiny moons or asteroids, lack enough gravity to do the job.

If the object spins quickly, its equator may bulge out and it’ll be thrown out of the ball club. The Sun’s strangely slow 4-week rotation lets it keep its lovely round figure. Our lethargic Moon (which also has a 4-week spin) differs from a perfect sphere by just one part in 500. To the eye, the Full Moon is indistinguishable from a perfect disk. It’s not the slightest bit out-of-round.

The night is dark because the universe is young.
This pleased ancient cultures, who regarded the circle as nature’s ideal shape. Yet you never found flawless disks on Earth. Only in the heavens, with the Sun and Moon, was such perfection beheld. And both of those appeared the same size! The Moon would even sometimes place itself precisely over the Sun to produce the total eclipse.

What magic was that!

Ancient cultures also noticed the Moon’s disk shifts against the background stars in 1 hour. The Sun’s disk moves its diameter in 1 day, and again in 1 night. It all seemed so perfect.

Nowadays, we know that the most perfect spheres are neutron stars, the nearest of which is 300 light-years away. Each of these ultraheavy balls has a density equal to a loaded freight train crushed down to the size of the period at the end of this sentence. Flawless superhard globes 12 miles wide, their solid glowing surfaces cannot be blemished by any possible method. Hit one with nuclear missiles and it wouldn’t scratch or dent. They are indestructible.

Math-o-phobics might confront their fear by tackling the formula for a ball’s volume. It’s 4.19 × radius3. Use it to find the volume of the Sun quickly, or how much space exists in the vast sphere between Earth and some galaxy. The main result is that as you look farther and farther away, the amount of space increases wildly, and so does the number of stars or galaxies.

Let’s try an example. There happen to be 100 stars nearer to us than 20 light-years. Say we want to know how many stars lie within 2,000 light-years, roughly as far as the star Wezen in Canis Major. Easy. 2,000 light-years is 100 times farther than 20 light-years. So we cube 100 (100 x 100 x 100) and find the new volume of space is a million times greater than the old. Since 100 stars lie within 20 light-years, 100 million stars thus should exist between Wezen and us. (Most are invisibly faint.)

Two quick conclusions emerge from this. First, most of the universe lies very far away. Second, because the image or light from the newborn Earth is only now reaching a distance or radius of 4.5 billion light-years from here, more than 95 percent of the universe’s stars and planets cannot possibly see us no matter what kind of telescope aliens might build there, because our light hasn’t yet reached them! We’re invisible to most of the cosmos.

Pondering the volume of the sphere of space centered on Earth, German astronomer Heinrich Olbers realized 2 centuries ago that stars increase with distance cubed, while star-brightness only fades with distance squared. Bottom line: The huge numbers of faraway stars should more than make up for their dimmer appearance, and the night sky should blaze with brilliance. Stars ought to fill every tiny piece of sky. The dome of night should have the same brightness as the Sun’s surface.

Neither Olbers nor anybody for the next several lifetimes could figure out why the sky is dark, though lots of people went around in circles trying. Finally, we have the answer.

Mostly, it’s the same reason much of the universe cannot see us: There hasn’t been time for light from virtually all the universe’s stars to get here. The night is dark because the universe is young.

It’s one enormous masked ball.