Imagine that gravity stopped. This is only a thought experiment, and no technology we have can switch gravity off or block it, but it is a useful way to ask what gravity holds together, beginning with the Sun. The Sun is a giant ball of hydrogen gas, and gravity holds all of that gas together, even though it would take almost 1,000 Jupiters to fill up the Sun and the Sun has about 332,900 times Earth's mass. Gravity pulls together the material that makes stars and planets, so without that pull, nothing would hold the gas of a star in one ball.

Gravity alone would only pull a star inward, and that is not what the Sun does, because the Sun's core is about 15 million degrees Celsius, hot enough for fusion, and the outward pressure this creates keeps the Sun's huge mass from collapsing. Inside the Sun, the inward pull of gravity is exactly balanced at each point by the outward push of gas pressure, called hydrostatic equilibrium. The Sun, like most stars, is stable: it is neither expanding nor contracting, because all the forces within it are balanced. Hydrostatic equilibrium is also self-correcting: if pressure falls short of the weight above, the star contracts and builds pressure; if it exceeds it, the star expands and releases pressure.

That balance has lasted a very long time, and it lets scientists read the inside of a star without ever going there. The Sun has put out roughly the same energy for billions of years, so its gravity has been balanced by pressure for a very long time. To hold up the Sun against gravity, gas at its center must be about 15 million kelvins; just from the fact the Sun is not shrinking we can tell it is hot enough for fusion. Models of the Sun give the same answer in more detail: the Sun's modeled central pressure is about 2.477 times 10 to the 11 bar, with a central temperature of about 15.7 million kelvins, what it takes to hold up its weight. The Sun turns about 4,260 million kilograms of mass into energy every second through fusion.

Leaving the pull of a planet or a star takes enormous speed. Escape velocity is the speed an object must be given to escape from Earth; the Moon's, at 2.38 kilometers per second, is much lower because its gravity is weaker. Earth's escape velocity, the speed needed to leave it for good, is about 11.2 kilometers per second, while the Sun's escape velocity is 617.6 kilometers per second, 55 times Earth's, and its surface gravity is 28 times Earth's. Earth's surface at the equator moves about 465 meters per second as Earth spins, far slower than the roughly 11,186 meters per second needed to escape Earth.

At the far end is a black hole, which packs so much mass into so little space that its gravity does not even let light escape. Gravity acts on light in gentler cases too, because a flashlight beam shone upward grows imperceptibly redder, a change scientists can measure. You may have heard that gravity pulls only on things with weight. In fact, gravity pulls on everything with mass, and even on light. Newton described gravity as a force between masses, while Einstein described it as curved space-time: his general theory of relativity treats gravity as the curving of space and time by mass and energy.

On a smaller scale, gravity holds Earth's own skin in place. Earth's mass is about 5.9722 times 10 to the 24 kilograms, and Earth's atmosphere has a total mass of about 5.1 times 10 to the 18 kilograms, while the oceans hold about 1.4 times 10 to the 21 kilograms. Earth's atmosphere is only about 1/1,200,000 the mass of Earth, a thin skin held on by gravity, and Mars, with less mass and weaker gravity, lost most of its original atmosphere, while even on Earth, some fast-moving molecules in the upper air escape the pull of gravity, over time scales of billions of years.

Earth's nearest neighbor shows the same pull at work. The Moon's average distance from Earth is about 384,400 kilometers, and the Moon has about 1.2 percent of Earth's mass, and its surface gravity is about 0.165 of Earth's. The Moon spins at the same rate that it circles Earth, so the same hemisphere always faces Earth. The Moon is slowly moving away from Earth at about 3.8 centimeters a year, and the Moon likely formed when a large chunk of rock smashed into the young Earth and the debris clumped together.

Einstein's theory predicted that gravity could ripple. Einstein predicted gravitational waves in 1916 in his general theory of relativity: ripples in space-time from massive accelerating objects such as orbiting black holes. Gravitational waves travel at the speed of light, 186,000 miles per second, and squeeze and stretch anything in their path. Each LIGO observatory has two arms more than 4 kilometers long; a passing wave changes their lengths slightly, and lasers and mirrors detect it. On September 14, 2015, LIGO sensed gravitational waves from two black holes colliding 1.3 billion light-years away, the first direct detection. By the time the first waves arrived, the space-time wobble was 10,000 times smaller than the nucleus of an atom.

Words to know

• equilibrium: a balance
• fusion: atoms joining
• hemisphere: half a ball
• kelvins: temperature units
• gravitational: of gravity