From the December 2013 issue

Balls of crushed fire

Seek out some strangely dense objects this winter.
By | Published: December 30, 2013 | Last updated on May 18, 2023

Matter arranges itself in wildly different ways. Museums display dense cannonball chunks of iron pyrite too heavy to lift, while wispy, barely-there hydrogen dominates the cosmos. It’s everywhere. You wouldn’t float in water if your body didn’t have more hydrogen atoms than anything else.

Why explore this topic? Simple: Some of the most famous weird-density objects now parade overhead.

We express a material’s density by revealing how much a cubic centimeter (cm3) of it would weigh. One cm3 is the size of a sugar cube, so picture a sugar cube composed of water, iron, or gold. If water, it weighs 1 gram, which is 1/28 of an ounce. If iron, the sugar cube weighs 7.87 grams. If it is made of gold, the little cube tips the scale at a whopping 19.28 grams.

We know the average density of planets by how quickly they make orbiting spacecraft or natural moons whip around them. Turns out, the three nearest worlds to the Sun — Mercury, Venus, and Earth — all share similar densities, between 5.2 and 5.5 grams per sugar cube. That’s more than five times the weight of water. They’re the densest objects in the solar system.

To achieve such a high average density, Earth’s fluffy surface items like oceans, oaks, and olives must be balanced by a very dense interior: its nickel and iron core. We know Earth can’t be solid gold beneath the surface because our overall density would then be 19 g/cm3 instead of the actual 5.5.

By contrast, the Sun and the planets from Jupiter outward have densities between 0.7 and 1.6 g/cm3. This isn’t surprising considering they’re mostly compressed hydrogen. The Sun is actually quite normal: Most star densities more or less resemble that of water.

Now we’re ready for the weird stuff.

We’ve known about Sirius the Dog Star’s little companion, Sirius B (affectionately called the Pup), since the Civil War. Small and compact, this “white dwarf” has collapsed to the size of Earth. Conveniently, it’s now a nice 10 arcseconds from Sirius, a separation that approaches the widest in the pair’s elliptical 50-year orbit. Steady nights and a good telescope let you glimpse the Pup firsthand.


Even easier is 40 Eridini B to the right of Orion’s foot star Rigel. The first white dwarf identified, its 9th-magnitude glow stands out easily through even the crummiest telescopes.

Both 40 Eridini B and the Pup are more than merely tiny stars that, oddly enough, spin slowly. They’re crushed down to a density of 1 million g/cm3 — a million times water’s density. A sugar cube of their material weighs a ton. Imagine needing a forklift to pick up a sugar cube. They would make excellent gag items at a novelty store: “Hey Mike, could you pass me those little dice?”

White dwarfs aren’t composed of exotic elements. They’re ordinary carbon and oxygen, crushed super-solid. Despite off-the-scale surface gravities, they stop imploding when Earth-sized.

Everything changes if a white dwarf possesses more than 1.4 times the Sun’s mass. Then when its nuclear furnace grows too weak to support its heavy outer layers, it keeps collapsing until it’s a ball just 12 miles (20 kilometers) wide — smaller than Arches National Park. The most famous such “neutron star” floats overhead on winter nights. You can find it next to Taurus the Bull’s left horn — the Crab Pulsar.

You need at least a 14-inch telescope to glimpse this faint 16th-magnitude pinpoint in the heart of the Crab Nebula. (Some say it’s a challenge even through a 20-incher.) If you spot it, you’ve beheld the smallest deep-space object anyone has ever seen. And among the fastest spinning. Neutron stars are strangely different from their white-dwarf cousins, which leisurely spin in about a day. Instead, these whirl dozens of times per second. One performs 716 spins in the same time you’d say “Mississippi.”

The Crab Pulsar’s density is 100 trillion g/cm3. A sugar cube of its material weighs a hundred million tons. Imagine taking a giant cruise ship and crushing it down until its size matches the ball in a ballpoint pen. We’re talking about a ballpoint containing thousands of tons of steel. Then alone you’d attain neutron star density.

Interestingly, this about matches the density of each proton and neutron in your body. So you yourself are already made of such stuff — some 30 octillion teensy specks of it. Who can doubt we live in a funny old universe?

Only a black hole could top a neutron star’s density. The nearest, V616 Monocerotis, is also out these nights — to the left of Orion’s Belt. But we can’t quantify its density. Theoretically, the density could be infinite, which has no physical meaning at all.

And when density reaches this level of strangeness, it’s time for a new topic.

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