Jan 9, 2025 · NASA’s real-time science encyclopedia of deep space exploration. Our scientists and far-ranging robots explore the wild frontiers of our solar system.

Our Sun belongs to the generation of stars created 4.6 billion years ago, when our galaxy was roughly half its present age. A cloud of interstellar gas, dust, and ices containing several generations of material collapsed to form the nebula from which the Sun and the rest of …

Although the space between stars in our galaxy appears to be completely empty of matter, there exists a very dilute gas even in “empty” space. This interstellar (meaning “between the stars”) medium varies in density (the amount of matter within a certain volume). Typical densities in the interstellar medium are

Europa’s surface is made of water ice and so it reflects 5.5 times the sunlight than our Moon does. Europa orbits Jupiter at about 417,000 miles (671,000 kilometers) from the planet, which itself orbits the Sun at a distance of roughly 500 million miles (780 million kilometers), or 5.2 astronomical units (AU).

By actually totaling the masses of all visible stars in our galaxy and other galaxies, with some accuracy, the amount of bright (luminous) matter present is less than one percent of the amount required to halt the expansion of the universe.

Finally this month, it's a great time to try and spot the galaxy of Andromeda. Andromeda is also known as M31. It's a spiral galaxy similar in appearance to our own Milky Way, but slightly larger. Both contain hundreds of billions of stars, and (we think), trillions of planets.

Our solar system’s small bodies – asteroids, comets, and meteors – pack big surprises. These chunks of rock, ice, and metal are leftovers from the formation of our solar system 4.6 billion years ago.

Despite its proximity to the Sun, Mercury is not the hottest planet in our solar system – that title belongs to nearby Venus, thanks to its dense atmosphere. But Mercury is the fastest planet, zipping around the Sun every 88 Earth days.

outshine the entire galaxy the star was part of. Stars generate energy in their cores by nuclear fusion: four hydrogen nuclei fuse to create helium. Stars are so enormous (our sun has a mass almost a million times that of the Earth!) that the weight of a star’s outer layer presses down on the core, raising the density

NASA’s real-time science encyclopedia of deep space exploration. Our scientists and far-ranging robots explore the wild frontiers of our solar system.