c. 13.3 Billion BCE

Milky Way

Astronomers define a galaxy as a gravitationally bound system of stars, gas, dust, and other more mysterious components (see Dark Matter), all moving collectively through the cosmos as if they were a single object. Once the first stars had formed, it was only a matter of time—not much time, in fact—that many of them would inevitably become attracted by each other’s gravity and form clusters, then clusters of clusters, and eventually huge congregations of stars orbiting their common center of gravity.

Our own Milky Way galaxy consists of an estimated 400 billion stars and has a structure that is typical of the class of so-called barred spiral galaxies seen throughout the universe (see Spiral Galaxies). The Milky Way has a crowded central semispherical bulge of stars surrounded by a flatter, spiral-shaped disk of stars (including the Sun), gas, and dust, all of which is surrounded by a diffuse spherical halo of older stars, star clusters, and two smaller companion galaxies. It’s an enormous structure, nearly 100,000 light-years (the distance light travels in a year, or about a billion billion miles) wide and 1,000 light-years thick in the disk. Our Sun is about halfway out from the galactic center, and one galactic-year orbit takes about 250 million Earth years.

Astronomers don’t know exactly when the Milky Way was formed. The oldest known stars in the galaxy are in the halo and are about 13.2 billion years old. The oldest stars in the disk are younger—about 8–9 billion years old. It is likely that the different parts of the Milky Way formed at different times, although the basic structure appears to have been set in motion very early.

Our ancient ancestors were awed by the bright whitish band that dominated their night sky, often envisioning it in creation myths as a river of light and life. Though we now know that we are inside a massive, choreographed gathering of stars looking out, it’s still easy to find awe in the scale and majesty of our home galaxy.

Wide-angle photo of the Milky Way galaxy’s Sagittarius arm. Light from billions of stars causes the bright, diffuse glow of the galaxy; dark dust in the disk blocks some of that starlight from our view. A meteor can be seen streaking through the scene near the bottom.