The Powers of Pressure

Feeling pressured? Don't worry, so too is everyone else across the globe. It's a fact. Each day, every day, all day, Earthlings wake up under pressure. What's more, for most of us, it's inescapable—even when we sleep. Of course, most of us wouldn't want it any other way, because without this kind of pressure—air pressure, that is—life, as we now enjoy it, simply wouldn't exist.

It's Everywhere!

Ever feel like you're walking on air? Well, you do it every day. From the tip of your head to the bottom of your feet, air surrounds you. Air molecules are invisible, but they still have weight and take up space—and their weight is pressing against you. You're so used to this air pressure that you don't feel it.

If we could weigh a column of air one inch square that extended all the way to the “top” of the atmosphere (about 500 miles), it would weigh approximately 14.7 pounds at sea level. Thus, atmospheric pressure at sea level is approximately 14.7 pounds per square inch (psi), which equals one atmosphere. Pressure does not stop at sea level. It increases as depth below sea level increases. (The opposite, of course, happens when you climb in altitude—by approximately 1.0 psi for every 2,343 feet.)

The pressures inside the Earth are tremendous—so tremendous that we have to leave behind human experience and try to picture the forces with our imaginations. Consider, for instance, that oxygen—a simple, colorless, odorless gas at sea level—would crystallize under a pressure of 55,000 atmospheres! If you could descend about two miles toward the geometrical center of the Earth, you would feel a thousand times more pressure on you than you would at sea level. If you could journey about 3,700 miles to the very core of the Earth, you'd experience a pressure of some 3.5 million atmospheres! You wouldn't have to worry about gym class anymore, because even the thought of doing a sit-up under these conditions would be painfully impossible.

Light Pressure

You could escape “air pressure” by fleeing into space, which is a vacuum. But that doesn't mean that air particles aren't zipping around space, any more than being in a vacuum means that you've escaped “pressure.”

Just look at a comet. It has two tails: a dust tail and a gas tail. Both always point away from the sun. Why? The pressure of sunlight, weak as it is, can exert a pressure on dust particles flaking off a comet, blowing them downwind to form its gently curving dust tail. The comet's gas tail, on the other hand, is composed of gas being blown straight behind the comet by the solar wind—a flow of charged particles (ions, electrons, and neutrons) that continuously streams out from the sun at speeds of about one million miles per hour (about 400 kilometers per second).

Pressure: It's Waaaay Out There!

Speaking of the sun, did you know that its very stability is a consequence of a battle between pressures? In any given layer of a star, there is a balance between the pressure of gravity trying to compress the star, and radiation pressure (an outward flow of hot gases) trying to expand it.

But this balancing act doesn't last forever. Once a star uses up its nuclear fuel—turning its hydrogen into helium—gravitational contraction wins out over the radiation pressure and the star collapses. At the end of its life, a star like our sun will ultimately collapse into a white dwarf—a tiny sphere about the size of our Earth, but with a mass a million times greater than it. A star such as our sun is not massive enough to collapse further.

Now, if you could stand on the surface of a white dwarf, the gravitational pressure you'd experience is the stuff that sci-fi movies are made of. A teaspoonful of white dwarf material would weigh five and a half tons; one cup of white dwarf stuff would outweigh 24 elephants!

A star four to eight times more massive than our sun is under even greater pressure. When its core collapses, the gravity is so intense that all the electrons and protons that form normal matter are squeezed further into neutrons and other exotic subatomic particles. The result is what astronomers call a neutron star—the densest form of matter known to exist. So a neutron star is supported by a repulsion pressure equal to a million billion times that of water. The pressure is exerted by neutrons in the nuclei of the heavy atoms forged in the neutron star's core.

Neutron stars measure only about 9 miles across, the size of a tiny city. A teaspoonful of neutron star stuff would weigh about 10 million tons! Theoretically, you could hold in the palm of your hand a piece of neutron star surface weighing as much as a fleet of battleships. If you could stand on this star, the gravitational pressure would be immense—300,000 times that of Earth.

What is the ultimate pressure machine? You might say a black hole. Indeed, a black hole is so massive that when its core collapses there is no pressure that we know of—not even on the subatomic level—that can prevent the core from collapsing to infinity, a mind-boggling concept indeed. A black hole's gravitational pressure is so intense that not even light can escape its pull.

But what about a supermassive black hole! That's the big kahuna! While a black hole measures big on the stellar scale, it's peanuts to a supermassive black hole, which we measure on a galactic scale! Take M87, a galaxy in Virgo, for instance. The Hubble Space Telescope has found evidence for a supermassive black hole at its core—one so massive that it weighs as much as three billion suns, but is concentrated into a space no larger than our solar system! And M87 is but one of countless other galaxies—including our own Milky Way—that might contain such a “central beast.”

It's useless to even try to fathom the pressure that our galaxy and its star stuff are under. So the next time you stumble out of bed, and feel so heavy that you can hardly shuffle your feet across the floor, just be thankful that your floor is on the surface…of Earth!


To apply.

The act of driving back or repelling.

subatomic particle:
Any of various units of matter that are smaller than the size of an atom.

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  1. What are the two pressures at work on the Sun? How do these pressures work together?
  2. What is a white dwarf?
  3. What are two things that change when a star becomes a white dwarf?
  4. Why would a teaspoonful of a neutron star weigh so much?