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.
Channel your inner superpower by looking up at the night sky precisely when a dazzling blaze of light is beamed to Earth from outer space.
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.
Comets develop tails as they approach perihelion—the place in their orbits when they are closest to the sun. The sun’s heat causes some of the material in a comet to vaporize, which in turn releases dust particles that were trapped in the ice.
The Sun is more than 330,000 times as massive than the Earth. It has a diameter of nearly 1.4 million kilometers (865,000 miles), and its volume could enclose about 1.3 million Earths.
There are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
The Sun’s outer atmosphere, the corona, is the source of the solar wind, a steady outflow of charged particles from the Sun.
When it comes to both mass and volume, Jupiter is the largest planet in the solar system, while Mercury is the smallest.
Meteor showers occur when Earth passes through a stream of meteoric material. The brief streaks of luminescence we call meteors are caused by meteoroids burning up as they pass through the atmosphere.
The practices of astrology and astronomy have common roots, but they have evolved into two separate fields. Astronomy studies positions, motions, and properties of celestial objects. Astrology attempts to study how those positions, motions, and properties affect people and events on Earth.
Radio astronomy is the study of the universe through analysis of very long-wavelength emission from celestial objects.
Look a little deeper into the summer sky with our new e-book, Summer Deep-Sky Observing. We'll have you happily busy at your telescope all season long!
When a meteor shower is coming up, have you thought of trying your hand at meteor photography? Here are some techniques to help you on your way.
Download our free guide to the heavens and learn how to observe all the wonders overhead.
A nova visible in good binoculars was spotted July 7, 2012, by observers in Japan.
Both Pluto and the star are 14th magnitude, but observers with big telescopes and sufficient video capability should try to record this important event.