We don’t usually think about the thickness of things, but it intrudes on us nonetheless. especially when our region gets clobbered by a passing hurricane every decade or two, and we experience destructive gales or floods. The damage we suffer is almost always worse from the water than from the winds. The reason is simply that water is 816 times denser than air. So the force of a few feet of flooding pushing against a house, even if it’s only flowing at five m.p.h., results in much greater damage than that of 100 m.p.h. hurricane gusts.
More commonly our encounters with density variations may range from the thinness of mountain air to the high density of a steel chain. Rarely do we encounter Earth’s highest-density elements, since we’ve probably never been asked to lift a bucket of lead pellets or had the unlikely fun of hefting a bar of gold.
The standard for assessing density is water. A full aquarium measuring one foot on each side weighs 62.4 pounds, not easy to tote. A gallon of water or milk weighs eight pounds. It’s noticeably heavier than any other common fluid in our lives: a gallon of gas, for example, weighs two pounds less.
Stars like our Sun have around the same overall density as water. But when a star collapses in its old age, and its material gets compressed by runaway gravity, the result is a brilliant white sphere whose gases get packed to an unbelievable degree.
This is where we rewind to 1844, when wobbles in the motion of the Dog Star Sirius led German astronomer Friedrich Bessel to conclude that it had an unseen companion tugging at it. In 1862, the world’s premier telescope-maker, Alvan Clark, testing out his latest instrument, saw a faint dot next to the dazzle of Sirius. It was soon named Sirius B, but is often called “the Pup” since it accompanies the Dog Star.
It’s now called a “white dwarf,” and dozens more have been found in the Sun’s neighborhood. Thanks to its binary system membership, both Sirius and the Pup have had their weights precisely determined, and the Pup weighs exactly the same as our Sun.
In 2005, its size was very accurately measured too – at 7500 miles, almost exactly the same as Earth. Having the Sun’s enormous mass packed into a tiny Earth-sized ball means that material in the Pup is over 100,000 times denser than steel. A piece of that star the size of a deck of cards would outweigh ten cement trucks.
It’s very cool and easy to remember that the Pup matches our Sun’s weight and Earth’s size. And though its tiny diameter makes it it’s too faint to see without a good telescope, you can easily locate it by looking at its close companion, dazzling blue-white Sirius. Conspicuous on any clear night from now throughout the winter by simply following Orion’s belt down and to the left, which takes you to the night’s brightest “star” (and ignoring slightly-brighter yellowish Jupiter, which outshines all the true stars). But as long as we’re mentioning planets, we must mention Saturn, whose lack of density is what’s remarkable. Having only 0.7 the thickness of water, Saturn would float like a cork if thrown into a large enough lake.
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