Spiral galaxy

A spiral galaxy is a massive, organized collection of stars, gas, and dust.

These galaxies are defined by their distinct spiral structures that extend from a central core into a flat, rotating disk. They are a major component of our universe. Together with irregular galaxies, spiral galaxies make up approximately 60% of all galaxies today. They are most commonly found in low-density regions of space. They are quite rare in the crowded centers of galaxy clusters.

Most spiral galaxies consist of several distinct parts. The most prominent feature is the rotating disk. This disk is relatively flat, with a thickness-to-diameter ratio of about 0.2. At the center sits a stellar bulge, which is a tightly packed group of stars. This bulge can look like a small elliptical galaxy. Many bulges are thought to host a supermassive black hole at their very center. For example, our own Milky Way has a black hole called Sagittarius A*. Surrounding the entire galaxy is a near-spherical halo. This halo contains older stars and many globular clusters.

Many spirals also feature a bar-shaped distribution of stars. This structure is called a galactic bar. It extends from the central bulge toward the outer edges. Roughly two-thirds of all spiral galaxies observed today have this bar component. The presence of bars has changed significantly over cosmic time. Only about 10% of spirals had bars 8 billion years ago. That number rose to one-quarter 2.5 billion years ago. Now, over two-thirds of the visible universe contains barred spirals. Our Milky Way is a barred spiral, though the bar is hard to see from our position inside the disk.

The spiral arms are the most beautiful part of these galaxies. These long, thin regions are sites of intense star formation. Because they contain many young, hot, and massive OB stars, the arms appear much brighter than the rest of the disk.

Different galaxies have different arm styles. In the Hubble sequence, Sa and SBa galaxies have tightly wrapped arms. In contrast, Sc and SBc galaxies have much "loose" arms. The arms are not permanent structures made of fixed stars. Instead, stars and gas move through these regions as they orbit the galactic center.

Scientists once struggled to explain how these arms stay stable. In 1925, Bertil Lindblad studied galactic rotation. He identified the "winding problem." Because the disk rotates at different speeds at different distances, a fixed arm would quickly wind too tight. To solve this, C. C. Lin and Frank Shu proposed the density wave model in 1964.

They suggested that arms are actually waves of higher density. Stars travel in slightly elliptical orbits.

These orbits cluster together in certain areas, creating the appearance of an arm. Stars simply pass through the density wave as they orbit.

These density waves directly cause new stars to form. As gas clouds enter a high-density wave, they get squeezed together. This compression makes it much more likely for the clouds to collapse. This process is known as Jeans instability. The compression triggers star formation on the leading edge of the spiral arms. Another theory is the stochastic self-propagating star formation model. This suggests that shock waves from previous star formation, like supernovae, trigger new star births. These shock waves move through the interstellar medium and sustain the cycle.

We can learn about the history of the universe by studying very old spiral galaxies. The oldest known spiral galaxy is BRI 1335-0417. Its light took 12.4 billion years to reach Earth. Another candidate for the oldest is Zhúlóng. The oldest "grand design" spiral is BX442, which is 11 billion years old.

This galaxy's shape may be caused by the gravity of a nearby dwarf galaxy. We also see very old spirals like A1689B11. This galaxy formed only 2.6 billion years after the Big Bang. Studying these distant objects helps us understand how galaxies evolved over billions of years.