ROME, July 25 — For as long as humans have looked up at the night sky, the pale, milky band stretching across the darkness has inspired wonder and myth. That band — our home galaxy, the Milky Way — is far vaster and more complex than our ancestors imagined, a sprawling barred spiral disk of stars, planets and dark matter that holds the Solar System within its arms. The Milky Way is a barred spiral galaxy.
Its D25 isophotal diameter — a standard measure of the visible disk — is roughly 87,400 light-years across, give or take about 3,600 light-years. Yet the galaxy is remarkably thin: only about 1,000 light-years thick at the spiral arms, though slightly thicker at the central bar.
Recent simulations suggest that a region of dark matter, also containing some visible stars, may extend the galaxy’s effective reach to a diameter of almost 2 million light-years. That would make it far larger than the bright disk we see. The galaxy holds between 100 billion and 400 billion stars, and at least as many planets.
The Solar System sits about 27,000 light-years from the Galactic Center, on the inner edge of the Orion Arm. That center, a dense bulge studded with older stars and one or more bars, hosts an intense radio source named Sagittarius A* — a supermassive black hole with a mass of about 4.1 million times that of our Sun.
The oldest stars in the Milky Way are nearly as old as the universe itself.
From myth to science
The name “Milky Way” goes back millennia. In Western culture, it comes from the dim, unresolved, milky appearance of the band. The term is a translation of the Classical Latin via lactea, which itself comes from the Hellenistic Greek galaxías — the root of our word “galaxy.” Greek mythology told that the band was milk spilled from Hera’s breast while she nursed Heracles. In the Babylonian epic Enūma Eliš, the Milky Way was created from the severed tail of the primordial dragon Tiamat, set in the sky by Marduk. The English term can be traced to a story by Geoffrey Chaucer around the year 1380.
The scientific understanding came much later. In 1610, Galileo Galilei turned his telescope toward the hazy band and resolved it into individual stars — a moment that foreshadowed centuries of discovery. Until the early 1920s, most astronomers believed the Milky Way contained all the stars in the universe. That view was shattered during the Great Debate of 1920 between Harlow Sh
