THE SUN

Size: About 1.3 million Earths could fit into the sun. Its diameter is about 864,000 miles.

Distance from Earth: 93 million miles.

Age: Dating the sun is not an exact science, it turns out. Some estimates say the sun is 4.5 billion years old or 5 billion years old.

How it was formed: From Space.com: “Although it may look empty, space is filled with gas and dust. Most of the material was hydrogen and helium, but some of it was made up of leftover remnants from the violent deaths of stars. Waves of energy traveling through space pressed clouds of such particles closer together, and gravity caused them to collapse in on themselves. As the material drew together, gravity caused it to spin. The spin caused the cloud to flatten into a disk like a pancake. In the center, the material clumped together to form a protostar that would eventually become the sun. … Over the course of about 50 million years, the temperature and pressure of the material inside increased, jump-starting the fusion of hydrogen that drives the sun today.”

Composition: It is usually said that the sun is a fiery ball of gases, but, actually, most of it is plasma, which isn’t quite the same thing, although hot gas and plasma are often used interchangeably. Plasma is created when the atoms in gas become so hot that they separate and the electrons and protons coexist individually. Nearly 75 percent of the charged particles are hydrogen, about 25 percent are helium, and there are trace amounts of oxygen, carbon, neon and iron.

Temperature: It varies. In the core, it can reach 27 million degrees Fahrenheit, but it cools down toward the surface, where it is about 10,000 degrees Fahrenheit. Then, as you move away from the surface, it heats up again, and the corona can be a few millions degrees Fahrenheit.

Why it matters to Earth: The sun is necessary for life as we know it on Earth. It heats Earth’s surface and atmosphere, its energy gives plants what they need to create nutrients necessary for animals to survive.

Will it last forever? Apparently nothing does, and the sun surely won’t. In about 5 billion years or so, the hydrogen and helium in the sun will have been used up and the process of fusion will be done. The sun will first swell so big that it will swallow up Earth, assuming it is still around, and then shrink again down to its core, becoming what is known as a white dwarf.

THE MOON

Size: “If Earth were the size of a nickel, the moon would be about as big as a coffee bean,” NASA says.

Distance from Earth: 238,900 miles.

Age: About 4.5 billion years old.

Temperature: The surface of the moon can reach 253 degrees Fahrenheit when the sun is shining light on it. On the side of the moon that gets no sunlight, temperatures can dive to minus-243 degrees Fahrenheit.

How it was formed: This has been a scientific mystery. The most widely accepted theory, which research in the past few years helps support, is the giant impact hypothesis, which says that the moon was formed from material left when still-forming Earth slammed into a rock about half its size called Theia.

Geography: The moon is rocky, and, like Earth, has a core, mantle and crust, but its geology is different from our planet. It has a weak atmosphere and a weak external magnetic field compared with our planet.

How often it orbits Earth: Every 27 Earth days, but because both the moon and Earth are spinning while orbiting, it seems to orbit Earth every 29 days.

If the moon disappeared tomorrow: Scientists believe that life may not have evolved the way it did without the moon’s gravitational pull on the oceans’ tides, but that doesn’t mean it would not have evolved or that life today would instantly disappear, too. How long it could survive is not clear, though. The moon keeps Earth tilting steadily on its axis at 23.5 degrees, but without the moon, Earth could tilt a lot — or not at all — drastically changing the weather from the possibility of no seasons to extreme seasons, according to NASA.

The far side of the moon: It’s the part of the moon that people on Earth don’t ever see. This is so because the moon revolves around Earth at the same rate that it rotates, leaving the same side facing Earth.

Number of humans who have walked on the moon: Twelve people — all men, all Americans.

Why astronauts took leaping strides on the moon: Gravity on the moon is much weaker than on Earth, so people weigh less, about one-sixth of their weight on Earth.

Future: There are a number of scenarios astronomers have proposed for the future of the moon, all of them, ultimately, ending in its demise.