The universe is populated by galaxies —huge collections of stars. The galaxies shown here belong to a group of five known as Stephan’s Quintet. The bright stars in view are closer and belong to the Milky Way Galaxy.
The universe started in an event known as the Big Bang, which occurred about 13.7 billion years ago. It was a type of explosion that produced everything in today’s universe—all energy, matter, and space —and marked the start of time.
Back then, the universe looked nothing like it does today, but everything that exists now existed in some form then. Although the amount of material and energy the universe is made of has remained the same, it has been cooling, expanding, and changing ever since it came into being.
No one knows what came before the Big Bang, or why it occurred, but we have put together the story since almost the instant of the universe’s creation.
The universe was created in a tiny fraction of a second. It was then an exceptionally hot and an immensely dense ball of radiation energy. It was also microscopically small, but within a trillionth of a second it ballooned to about the size of a soccer pitch, before settling down to a slower rate of expansion.
The first atoms formed when the universe was 300,000 years old. Hydrogen and helium nuclei joined with protons and electrons, which are other tiny particles, to make atoms.
This ordinary matter consisted of 76 percent hydrogen and 24 percent helium, with a trace of lithium. The hydrogen and helium would go on to produce all the elements found in today’s universe.
At the time that the first atoms were forming, the universe changed from being opaque to being transparent. In places, hydrogen and helium gas and dark matter began to concentrate into clumps.
Over tens of millions of years, the first galaxies formed in these denser regions, as dark matter settled into huge haloes around rotating disks of gas. Within these, the first stars were born.
The universe is everything we know about, as well as everything we have yet to discover. It includes all space and time as well as everything we see or detect in other ways.
Parts of the universe, such as the planets, stars, and galaxies, are familiar to us, but only make up a small amount of it. The vast majority of the universe remains unknown.
There are at least 125 billion galaxies in the universe. Each consists of a huge number of stars, vast amounts of gas and dust, and dark matter, all bound together by gravity. Galaxies come in four main shapes and in a range of sizes.
Dwarf galaxies measure a few thousand light-years across and have about 10 million stars, while a giant galaxy is typically 300,000 light-years wide with 1,000 billion stars. The very center of a galaxy is known as its nucleus or core, and most galaxies, if not all, have a supermassive black hole lying there.
THE MILKY WAY
The Milky Way Galaxy is our galactic home. It is a disk-shaped system of gas and dust, and about 500 billion stars. It is classified as a barred spiral galaxy.
Along with the rest of the solar system, we live about 27,000 light-years from the galaxy’s center—a little more than halfway to the outer edge. From our position inside the galaxy, we see it as a milky path of light across Earth’s nighttime sky, which is why we call it the Milky Way.
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