🌌 The Milky Way & the Galaxy Zoo

What you’ll learn

1. Our Galaxy From InsideUnderstand what the Milky Way is, its major components, and how astronomers map it from within.The Milky Way is a barred spiral galaxy of 100 to 400 billion stars, about 100,000 light-years across, and we see it as a band of light because we sit inside its disk. It is built from a disk, a central bulge, and a vast halo, all wrapped in dark matter. Trapped within, astronomers map it using radio waves and missions like Gaia, which charted nearly two billion stars.
2. Our Address in the Milky WayLocate the solar system within the galaxy and understand the bar, the spiral arms, and our galactic orbit.The Sun lies about 26,000 light-years from the center on the minor Orion Arm, not at the core. The Milky Way is a barred spiral, with an elongated bar of stars feeding its center. The solar system orbits the galactic center at 220 km/s, yet a single 'galactic year' takes 225 to 250 million years.
3. The Monster at the CenterUnderstand Sagittarius A*, the stellar orbits that proved it, and the Event Horizon Telescope image.Sagittarius A* is the Milky Way's central supermassive black hole, about four million solar masses. Genzel and Ghez tracked stars orbiting it for decades, winning the 2020 Nobel Prize by proving its presence. In 2022 the Event Horizon Telescope imaged it directly as a bright ring around a dark shadow, confirming general relativity.
4. The Galaxy ZooLearn the main galaxy types, the Milky Way's Local Group neighbors, and how galaxies evolve and merge.Galaxies divide into star-forming spirals, smooth old ellipticals, and chaotic irregulars, as Hubble first classified. The Milky Way belongs to the Local Group alongside Andromeda and satellites like the Magellanic Clouds. Galaxies evolve through mergers, which scramble spirals into ellipticals and trigger bursts of star formation.
5. When Galaxies CollideUnderstand the coming Andromeda merger, why stars won't crash, and the giant galaxy it will create.Andromeda is approaching the Milky Way at about 110 km/s and is expected to merge with it in four to five billion years. Because stars are so far apart, essentially none will collide, though gas clouds will ignite new stars. The merger will likely transform both spirals into a single large elliptical galaxy.
6. The Invisible MajorityUnderstand galaxy rotation curves, the evidence for dark matter, and the honest limits of what we know about it.Vera Rubin's flat rotation curves showed galaxies contain far more mass than their visible matter, revealing dark matter. Multiple independent lines — lensing, cluster motions, the cosmic microwave background — confirm it, making up roughly five times ordinary matter. Its gravitational reality is well established, but its exact identity remains unknown.
7. The Grand ScaleGrasp how many galaxies exist, what the observable universe is, and our place within the cosmic web.Hubble Deep Field images reveal thousands of galaxies in tiny patches of sky, implying hundreds of billions to possibly trillions across the observable universe. That observable region spans about 46 billion light-years in every direction. The Milky Way is one modest galaxy in a vast cosmic web, all embedded in dark matter.

Questions this course answers

Grounded in trusted sources

• NASA
• European Space Agency
• Event Horizon Telescope Collaboration
• Space Telescope Science Institute

Every Wunder lesson is built from real, reputable sources — never invented.

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