When we look beyond the stars of our own neighborhood, we realize that the universe is comprised of massive, sprawling cities of gas, dust, dark matter, and billions of stars bound together by immense gravity. These are galaxies. Legendary astronomer Edwin Hubble classified them into three main shapes in what is now known as the "Hubble Tuning Fork."
1. Spiral Galaxies
Like a giant cosmic pinwheel, Spiral Galaxies are arguably the most beautiful. They boast a bright central bulge packed with older, redder stars, surrounded by sprawling, sweeping "spiral arms" filled with cool gas, dust, and intensely hot young blue stars.
Our Home: The Milky Way is a "Barred Spiral" galaxy, meaning its central bulge has an elongated bar-shaped structure of stars cutting across it. Our closest large galactic neighbor, the Andromeda Galaxy, is also a spiral!
2. Elliptical Galaxies
Unlike to dynamic, starry pinwheels, Elliptical Galaxies look like gigantic, glowing, featureless spheres or footballs. They have no spiral arms and almost no interstellar dust or gas.
Because they lack the gas required to "give birth" to new stars, Ellipticals are almost entirely made up of very old, dim, red and yellow stars. Interestingly, these are often the largest and most massive galaxies in the known universe, often formed when two spiral galaxies collide and merge over billions of years.
3. Irregular Galaxies
Some galaxies just refuse to fit in. Irregular galaxies have no symmetrical shape, no spiral arms, and no central bulge. They are chaotic, messy clouds of stars and glowing gas.
Their messy shape is usually the result of intense gravitational trauma. Many irregular galaxies were once beautiful spirals that got too close to a massive elliptical galaxy, and their shapes got horribly distorted by the "gravitational tides." The Magellanic Clouds, two miniature galaxies orbiting our Milky Way, are famous irregular galaxies.
