Milky Way

The Milky Way is a spiral galaxy in which we reside. To us, it appears as a glowing band within the night sky, which if tracked around the whole celestial sphere is seen to form a ring around us, our view of a disk-shaped region of stars within which we are embedded. The galaxy (i.e., its visible portion) is on the order of 100,000 light-years in diameter, about 1000 light-years in thickness and has 100-400 billion stars and likely at least that many extra-solar planets. Its mass is about a trillion solar masses. The solar system is very roughly half way between its center and its edge. It is the smaller of the Local Group's two largest galaxies, the larger being Andromeda. An orbit of the Sun around the galaxy lasts in the range of 225-250 million years (i.e., the galactic period aka galactic year or cosmic year). The Milky Way's oldest stars are on the order of 13 billion years old, more than 90% of the age of the universe. Surrounding the visible galaxy (of stars) is its galactic halo, a somewhat-spherically shaped region with associated material, which can be thought of as a less-visible portion of the galaxy.

The Milky Way is naturally a target for up-close study of a galaxy, specifically, a disk galaxy. A stellar population classification of Milky Way stars based upon metallicity measures and location divides them into Population I stars with high metallicity, which are found in the disk including the spiral arms, and Population II stars with lower metallicity found throughout the galaxy, but basically the only stars of the bulge, globular clusters, and the galactic halo. Both kinematics and metallicity distinguish a thin disk, where Population I stars generally reside, and a thick disk, with more Population II stars. Metallicity is presumed to grow with succeeding generations of stars, the metals being formed by stellar fusion and supernovae, the metals spread by the latter and by stellar wind, so older stars, necessarily leftovers from earlier generations, have lower metallicity. The Sun has a high metallicity but not the highest, falling in the middle of the thin disk's range. Stellar associations, the leftovers from recent star formation, can have higher.

The Milky Way is a typical spiral galaxy in some ways, but among its unusual characteristics, it is somewhat quiescent (a green valley galaxy), it is small given its mass, its outer part is unexpectedly metal-poor, and it has massive satellite galaxies.

The term Milky Way is used for the portion of the night sky that is slightly brighter than the majority, and before what we now know as galaxies were known, the word Galaxy meant the same thing. Telescopes revealed this brighter area to consist of many stars previously not discerned individually. Later, in the 20th century, when it was established that some fuzzy objects (nebulae) in the sky were huge groups of stars far beyond those we usually see, the terms extragalactic nebulae, island universes, and (other) galaxies were used for them, the last term eventually becoming the common term, and the term Milky Way was taken to mean our own galaxy.