Spiral Galaxy

Spiral galaxies are magnificent disk-shaped collections of billions of stars, gas, and dust that showcase beautiful curved spiral arms sweeping outward from a central bulge. These cosmic pinwheels are among the most recognizable and photographed galaxies in the universe, including our own Milky Way home.</p><p>The spiral arms aren't rigid structures but rather density waves where gas compresses, triggering the birth of brilliant blue stars that illuminate these regions like cosmic streetlights. This active star formation, fueled by abundant hydrogen gas comprising 10-15% of the galaxy's mass, makes spiral galaxies vibrant stellar nurseries. As massive stars age and die within just millions of years, they fade, creating the illusion that the spiral pattern itself rotates.</p><p>Our Milky Way spans 100,000 light-years across with over 200 billion stars, while the nearby Andromeda Galaxy (M31) stretches even larger at 220,000 light-years. The iconic Whirlpool Galaxy (M51) perfectly demonstrates the classic spiral structure with its two prominent arms.</p><p>First systematically catalogued by astronomer Edwin Hubble in the 1920s, spiral galaxies represent about 60% of observed galaxies in the nearby universe. Their study revealed that galaxies rotate—with our solar system orbiting the Milky Way's center every 225 million years—fundamentally changing our understanding of cosmic structure and motion.