The short answer is that no one knows the exact number of galaxies in the entire universe. In the observable universe—the part whose light has had time to reach us since the Big Bang—modern estimates suggest there are likely hundreds of billions to perhaps around two trillion galaxies, depending on how faint and small a…
There are eight planets in the Solar System under the current definition used by the International Astronomical Union: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. That answer is simple, but the reason it is eight rather than nine is the result of both astronomical discovery and a formal decision about what counts as…
The Big Bang theory is the leading scientific explanation for how the universe began evolving from an extremely hot, dense early state and then expanded over billions of years into the cosmos we observe today. It does not describe an explosion into empty space. Instead, it describes space itself expanding, carrying galaxies apart on the…
The universe, as far as modern cosmology can tell, was not formed as an explosion into empty space. It began about 13.8 billion years ago in an extremely hot, dense early state, and space itself has been expanding and cooling ever since. Over time, simple particles formed, then atoms, then stars and galaxies, and much…
Astronomy is the science of the universe beyond Earth’s atmosphere. It studies objects such as the Moon, planets, stars, galaxies, gas clouds, black holes, and the large-scale structure of the cosmos, as well as the physical laws that govern them. In practice, astronomy combines careful observation with physics, mathematics, chemistry, and increasingly computing to understand…
Space is the vast physical realm beyond Earth’s atmosphere, but in science it is more than “empty nothingness.” It contains matter, radiation, magnetic fields, and the structure of spacetime itself. Space begins gradually rather than at a perfectly sharp boundary, and on the largest scales it includes everything from the region around Earth to galaxies,…
The best current estimate is that the universe is about 13.8 billion years old. More precisely, modern cosmology places its age at roughly 13.8 billion years based on measurements of how fast space is expanding and what the early universe looked like. This number does not come from a single clock, but from several independent…
The universe is, as far as observations allow us to say, at least about 93 billion light-years across in diameter for the observable universe. That does not mean the whole universe is only that size; it means light has had a finite time to reach us since the Big Bang, so there is a limit…
The planets in the Solar System are the eight major worlds that orbit the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. They are not just points of light in the sky, but distinct physical systems shaped by gravity, chemistry, radiation, impacts, and billions of years of evolution. Together they record how planetary…
Astronomy is the science of everything beyond Earth’s atmosphere: the Moon, planets, stars, galaxies, and the universe as a whole. For beginners, the subject can be understood as a way of asking simple questions with careful tools: What is out there, how does it move, how did it form, and could life exist elsewhere? Modern…
