Our sun is not the only one in the Galaxy. There are at least 100 billion stars beyond our solar system, a spiral galaxy about 100,000 light-years across.

Our star, the Sun. Image Credit: NASA

Our star along with its planets float around the milky way galaxy. Stars beyond our Solar System are arranged in a pinwheel pattern with four major arms, and we live about two-thirds of the way up one of them. Almost every star hosts their own family of planets. More than a thousand of these exoplanets have been discovered and thousands more are awaiting confirmation.

Supermassive black hole in the middle of our galaxy. Image Credit: NASA

At the center of our galaxy lies a supermassive black hole, Sagittarius A*. All the stars in the Milky Way orbit it. This supermassive black hole is estimated to be some 4 million times as massive as our sun. Fortunately, it is within a safe distance of around 28,000 light years away from Earth. The Milky Way zips along a galactic orbit at an average speed of about 514,000 miles per hour (828,000 km/hr). It takes about 230 million years for our solar system to make one revolution around the galactic center.

 

Beyond Our Galaxy

Source: Robert Williams and the Hubble Deep Field Team (STScI) and NASA

Our galaxy is part of the Local Group, a neighborhood about 10 million light-years across. It consists of more than 30 galaxies that are gravitationally bound to each other. The most massive one in this group is Andromeda. It appears to be on course to collide with the Milky Way in about 4 billion years.

Scientists that study galaxies observed that the stars in the outer parts are orbiting the galactic centers just as quickly as the stars further in. They deduced that something other than the stars and clouds of gas and dust, known to comprise galaxies, was providing extra gravity. They calculated that there must be five times as much of this mysterious dark matter, detectable only by its gravitational pull, as there is of the matter we already knew about.

The Local Group is only one of many, many clusters of galaxies. They are all moving away from each other as more and more space comes into being between them. This means the universe, itself, is expanding. That discovery is what led to the theory of the Big Bang origin of the universe.

Scientists expected that the gravitational attraction of everything in the universe would put the brakes on the rate of expansion, and eventually, the expansion would stop or even reverse. But in the 1990s, scientists discovered that the expansion is actually getting faster. The force responsible for this surprising acceleration was named dark energy.

Scientists calculate that there are at least 100 billion galaxies in the observable universe, each one brimming with stars. On a very large scale, they form a bubbly structure, in which vast sheets and filaments of galaxies surround gargantuan voids.