Locally, spacetime is curved by the presence of massive objects. The total mass and energy density of the universe also has an effect on the overall curvature of space.
There was no “before the Big Bang"—the Big Bang created both time and space as we know it.
Dark matter is a mysterious type of matter that doesn't interact with any form of electromagnetic radiation, i.e., light. Although we’ve never detected dark matter directly, a large amount of evidence points to its existence.
Just how quickly is the universe expanding? Cosmologists attempt to answer this question in terms of the Hubble Constant, but the exact value of this constant is still a topic of debate.
Is the universe infinite, or just really, really big? How can we know? To answer these questions, we examine the possible shapes of the universe.
Barring the Sun, the closest star to Earth is a triple system called Alpha Centauri, which is over four light years away.
Because black holes severely warp the fabric of spacetime, they have a curious effect on the passage of time as seen by an outside observer.
Black holes are singularities: points of infinitely small volume with infinite density. However, the amount of a mass concentrated in a black hole varies, and the mass determines how wide the black hole's sphere of influence is.
Different types of black holes form through different processes.
The concept of a black hole was first contrived in by John Michell 1783. For a long time, many notable scientists, including Albert Einstein, believed black holes were merely theoretical. However, in the last century, astronomers have gathered a good deal of observational evidence for the existence of black holes.
A black hole is a region of space where the force of gravity is so strong that the escape velocity exceeds the speed of light.
The Big Bang marked the beginning of the universe's expansion from a singularity — a single point that was infinitely small, infinitely hot, and infinitely dense. Cosmologists have designated several distinct eras for the universe's evolution from the first moments after the Big Bang to a billion years later.
Cosmologists have invoked the concept of dark energy to explain the accelerated expansion of the universe, but the nature of dark energy remains one of the most pressing questions facing modern cosmology.
The universe began as a singularity that started expanding in the Big Bang. But the Big Bang was no regular explosion. Rather, space itself expanded, so there is no center of the entire universe. The observable universe, on the other hand, is a different story.
Astronomers determine the number of galaxies in the universe by counting up the number visible in a tiny portion of the sky, and then accounting for all the regions of the observable universe. A 2013 study estimates that there are 225 billion galaxies in the observable universe.
Determining the age of the universe requires a knowledge of the universe's expansion rate, as well as its density and composition. Cosmologists currently set the age of the universe at about 13.77 billion years.
During the latter half of the 20th century, cosmologists narrowed the universe’s fate to three possibilities, and they all depend on its density.
In 1930, the International Astronomical Union (IAU) divided the sky into 88 constellations. Each constellation is defined by an imaginary boundary on the sky and named after a classical star pattern within those boundaries. So when we say a star is “in” a particular constellation, we mean it lies within the IAU-defined boundaries of...
By measuring the number and luminosity of observable galaxies, astronomers put current estimates of the total stellar population of the observable universe at roughly 70 billion trillion.
The brightest star in the sky is Sirius, also known as the “Dog Star” or, more officially, Alpha Canis Majoris, for its position in the constellation Canis Major.
Both the life and death of a star depend on its mass. Generally speaking, the more massive a star, the faster it burns its fuel and the shorter its life. The most massive stars meet their end in a supernova explosion after only a few million years of fusion, while the tiniest stars continue...
A star is a luminous ball of gas, mostly hydrogen and helium, held together by its own gravity. Nuclear fusion reactions in its core support the star against gravity and produce photons and heat, as well as small amounts of heavier elements. The Sun is the closest star to Earth.
Though it wouldn’t work so well in the nursery rhyme, a star’s twinkling actually has a technical term, astronomical scintillation: the effect of our planet’s atmosphere on starlight.
Asteroids are rocky objects leftover from the solar system's formation, found primarily in the asteroid belt, a region of the solar system in between the orbits of Mars and Jupiter.
Like asteroids, comets are suspected to be remnants of planet formation in the solar system about 4.6 billion years ago. But while asteroids are generally comprised of rock and metal, comets are more akin dirty snowballs. Comets primarily originate in the Kuiper Belt and the Oort Cloud.