milky-way

The Milky Way did not grow peacefully. It reached its current size by absorbing smaller galaxies — tearing them apart with tidal forces, stripping their stars into long streams, and incorporating their material into the disk and halo. One major collision 10 billion years ago reshaped the Milky Way's entire inner halo. Another is happening right now. A third, larger one is coming in 4 billion years, and there is nothing that can stop it.

Four million solar masses packed into a region smaller than our solar system, sitting 27,000 light-years from Earth. For three decades, astronomers tracked stars swinging around it at nearly 3% of the speed of light. Then, in 2022, they photographed it.

In 2010, NASA's Fermi telescope found two colossal gamma-ray structures erupting from the center of the Milky Way, each 25,000 light-years tall. In 2020, an X-ray telescope found even larger versions of the same structures. Nobody fully agrees on what made them.

Within 500 parsecs of the Milky Way's center lies a region unlike anything else in the galaxy — the Central Molecular Zone. It contains roughly 10% of the entire galaxy's molecular gas compressed into less than 0.1% of its volume. Star formation there should be rampant, but it is strangely quiet. Giant molecular clouds orbit at velocities that defy simple physics. A cloud called the Brick should have already collapsed into thousands of stars, yet it has not. Understanding the CMZ means understanding what keeps the center of our galaxy from exploding with new stars — and what occasionally makes it erupt instead.

In ~4.5 billion years, Andromeda will collide with the Milky Way. Here's what will actually happen — and why no stars will hit each other.

Galaxies are not eternal — they are born, grow through mergers, and eventually die. Here's how the universe built these island universes.

The Milky Way is a barred spiral galaxy with roughly 200 to 400 billion stars. Its disk, central bar, bulge, stellar halo, and dark matter halo are mapped from the inside by Gaia and radio surveys. At the center sits Sagittarius A*, a black hole of about 4.3 million solar masses, imaged by the Event Horizon Telescope in 2022. The Sun orbits about 26,000 to 27,000 light-years out, completing a lap in roughly 200 to 230 million years.

The Andromeda Galaxy is approaching the Milky Way at ~110 km/s and will collide in approximately 4.5 billion years. The two galaxies will merge over several billion years of passes, ultimately forming a giant elliptical galaxy — sometimes called 'Milkomeda.' Despite the collision of two galaxies each containing hundreds of billions of stars, individual stellar collisions are extremely rare. Earth will likely survive, though in a dramatically different night sky.

Globular clusters are the oldest and densest stellar systems in the universe — spherical swarms of hundreds of thousands of stars bound together for billions of years. Here's how they form, what they reveal about galactic history, and why they remain one of astronomy's most productive laboratories.