Milky Way

Milky Way is our Home Galaxy - the galaxy of which our Sun is a member. It is a disk-shaped system, with a diameter of between 80000 and 100000 light-years and a thickness of about 2000 light-years, that contains more than a hundred billion (10¹¹) stars. The core, or nucleus, of the Milky Way is surrounded by an ellipsoidal central bulge, containing relatively closely spaced stars, that measures some 15000 light-years in diameter and about 6000 light-years in the direction perpendicular to the plane of the disk. Surrounding the bulge and extending in a near spherical distribution above and below the galactic plane (the plane of the disk) is the galactic halo. The halo contains about 200 globular clusters (near-spherical clusters that contain between 10,000 and 1000,000 stars) and an extremely thinly scattered population of individual stars.

The Milky Way Galaxy derives its name from the Milky Way, a misty band of starlight that stretches across the sky and which may be seen with the unaided eye on a clear dark night. The Milky Way represents the combined light of millions upon millions of stars concentrated close to the galactic plane.

Like many other galaxies, the Milky Way Galaxy has a spiral structure. Within its disk, bright young stars, nebulae, and clouds of gas and dust are concentrated into curved ‘arms’ that appear to radiate from the central bulge in a spiral pattern. The Galaxy’s spiral pattern consists of several major arms and a number of shorter segments, one of which - the Orion arm, or ‘spur’ - contains the Sun and the Orion star-forming region.

The Sun is located just over half way out from the center to the edge of the disk, estimates of its distance from the center ranging from about 22000 to about 29000 light-years, with 25000 light-years being a reasonable working value. In common with other stars, the Sun revolves round the galactic center. The orbital velocity of the Sun is about 220 km s⁻¹ and its orbital period is about 225 million years. Overall, the Galaxy exhibits differential rotation, stars and gas clouds closer to the center having shorter orbital periods than those that are located further out.

Stars in the Galaxy are divided into two principal categories, or populations, Population II stars being old stars that formed early in the history of the Galaxy and population I stars being second, or later, generation stars that formed from gas clouds that had been seeded with heavier elements generated in, and expelled from, earlier generations of stars. The halo is composed of Population II objects and the disk, where clouds of gas and dust are concentrated and where ongoing star formation is taking place, is dominated by Population I objects. The central bulge contains both populations but its light output is dominated by radiation from old red giants.

The exact center, or nucleus, of the Galaxy coincides with a strong source of radio emission, called Sagittarius A*, that is less than 15 astronomical units in diameter. This in turn is strong evidence for the presence of a super massive Black Hole at the center of the Milky Way.

Questions to Ponder

• How do we know the Milky Way is a spiral galaxy?
• Do super-massive black holes cause galaxy rotation?
• How do stars move in the Galaxy?