The Milky Way Galaxy is a dense disk of stars, gas and interstellar dust about 100,000 light-years wide. It contains at least 250 billion stars. A bulge of stars, about 12,000 light-years wide, protrudes from either side of the disk’s center. A faint spherical halo of old stars and tight-knit balls of stars, known as globular clusters, surrounds the whole galaxy. A supermassive black hole lies at the galaxy's center.

The oldest stars in the Milky Way are about 13 billion years old, which means that they formed soon after the Universe was created by the Big Bang, 13.7 billion years ago. The Big Bang created a hot, superdense fireball that gradually expanded and then cooled. This fireball was not completely uniform. It had many dense patches that eventually became the galaxies we see today.

The Milky Way is a spiral galaxy, with bright spiral arms composed of dense concentrations of stars swirling through the disk. Large spiral galaxies like this are uncommon; most galaxies are small, faint and blob-like. The spiral shapes do not reflect the paths of the young stars within them, which follow circular orbits around the galactic center. Scientists believe that the spiral pattern is created by a kind of pressure disturbance called a density wave. Orbiting stars and gas clouds periodically pass into spiral pressure waves that stretch from the galactic center to the edge of the disk. The waves compress gas, which triggers the formation of many bright new stars. Astronomers do not know what causes the density waves in the first place.

Most of the Milky Way is invisible. The gravity of a mysterious amount of “dark matter,” which is in a giant ball enclosing the galactic disk and bulge, seems to influence the motions of stars. Astronomers are not sure about what makes up dark matter. They are almost certain that it contains some ordinary matter, such as stars that are too dim to see. But theory suggests that dark matter also includes some type of exotic matter that is yet undiscovered, such as Weakly Interacting Massive Particles (WIMPs), which are completely incapable of emitting light. If the Solar System has any dark matter, it cannot be very common, because it does not show a noticeable gravitational pull on the planets.

The Milky Way’s four fattest stars, identified at the Lowell Observatory in Flagstaff, Arizona in January 2003, are KW Sagittarii, V354 Cephei, KY Cygni and Mu Cephei. Based on measurements of their temperature and brightness, astronomers estimate that each of these stars is about 1500 times as wide as the Sun, and weighs about 25 times that of the Sun.

The most massive stars in the galaxy weigh up to 150 solar masses. One of these supermassive stars is Eta Carinae. Nuclear reactions inside Eta Carinae are generating radiation so intense that it exerts an outward pressure similar to the gravity that holds the star together. Because of this, Eta Carinae is violently unstable, with its brightness changing dramatically from year to year, or from decade to decade, sometimes one of the brightest stars in the night sky, sometimes invisible to the naked eye. In the nineteenth century, astronomers witnessed a giant explosion of Eta Carinae, yet somehow the star survived.

In early 2005, the fastest star in the galaxy was discovered. Traveling at 700 kilometers per second, more than three times as fast as the Sun’s speed around the galactic center, it is moving away from the galactic center so quickly that it will eventually escape the Milky Way’s gravitational pull. Astronomers speculate that the star was catapulted out by the supermassive black hole at the center of the galaxy. It is possible that the black hole has disrupted a binary star system so that one star went into orbit around the black hole, or got sucked in completely, while the other star was thrown in the opposite direction.

In the past, other galaxies have collided with the Milky Way. In a galaxy, stars or multiple star systems are typically three light-years apart, so when galaxies collide, the stars themselves rarely hit each other, although they change orbits. Interstellar clouds within galaxies do collide, then collapse under gravity. This triggers the formation of new stars. The best way to track down alien galaxies that have been gobbled up by the Milky Way is to look for stars that do not fit in, such as stars with unusual chemical compositions or unexpected ages, or a large group of stars that is moving in a different direction from its neighbors. The Sagittarius dwarf elliptical galaxy has been identified as an alien galaxy that has been consumed by the Milky Way. It contains about 30 million stars, mostly old, yellowish ones, and 15 times that much mass in dark matter. This dwarf galaxy is moving through the Milky Way at a speed of about 250 kilometers per second.

The structure of the Milky Way’s disk implies that a very large collision occurred in the distant past. Within the Milky Way’s thin disk, which consists of stars of all ages, is a thicker one that contains only old stars. This could mean that at least 10 billion years ago, the Milky Way cannibalized another galaxy about one tenth of its size, or several smaller ones.