Spiral galaxy

Some star groups look like swirls.

These are called spiral galaxies.

They have long, thin arms. These arms go out from the center. They have many bright stars.

These stars are very hot.

The arms also have gas and dust. This gas helps make new stars. Most of these groups have a bright center.

This center is a big group of stars.

Some have a bar shape in the middle. Our own home has this bar too. It is hard to see from here.

Spiral galaxies are very common. They make up much of the universe. They are beautiful to see.

A spiral galaxy is a large group of stars.

Most have a flat, spinning disk. This disk holds stars, gas, and dust. The center has a bright group of stars called a bulge.

Many galaxies also have a halo. A halo is a large, round area of stars around the disk. Some stars live in small, tight groups called globular clusters.

Many spirals have long arms. These arms curve out from the center. The arms are very bright. This is because they make many new stars. These stars are young and hot.

About two-thirds of these galaxies have a bar. A bar is a straight line of stars in the center. Our own Milky Way has a bar.

How do the arms stay shaped like spirals? Scientists think about density waves. A density wave is a region with more mass. As gas moves into this wave, it gets squeezed. This squeeze makes the gas collapse to make new stars.

This way keeps the arms looking bright and clear.

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

These galaxies are special because they have a flat, spinning disk. In the middle of this disk is a bright group of stars called a bulge.

Many spirals also have a faint, round halo of stars around them. This halo often contains tight groups of stars known as globular clusters. Spiral galaxies and irregular galaxies make up about 60% of the galaxies in our universe today. They are usually found in areas where galaxies are not crowded together.

Most spiral galaxies have long arms that curve out from the center.

These arms are very bright because they are sites where new stars are born. These new stars are often young, hot, and blue. Many spirals also have a bar-like structure made of stars. This bar stretches from the center bulge toward the outer arms.

About two-thirds of all spiral galaxies have this bar. Even our own Milky Way is a barred spiral galaxy. It is hard to see our own bar because we live inside the galactic disk.

Scientists have wondered how these arms stay in a spiral shape for so long.

In 1925, Bertil Lindblad studied how galaxies rotate. He found a problem called the winding problem. If stars stayed in fixed arms, the spinning disk would make the arms wind too tight. To explain this, C. C. Lin and Frank Shu suggested a density wave model in 1964.

They thought the arms are waves of higher density. Stars and gas move in and out of these waves as they orbit. This keeps the spiral shape looking steady over time.

As gas clouds move into a density wave, they get squeezed together.

This squeeze makes the gas collapse to form new stars. This process makes the spiral arms look much brighter than the rest of the disk. Some scientists also suggest a model called stochastic self-propagating star formation. In this model, shock waves from exploding stars help trigger more star formation. These waves can spread through the gas and keep the cycle going. This helps explain why the arms stay so bright and active.

We can see how much galaxies change by looking at very old ones.

The oldest known spiral galaxy is called BRI 1335-0417. Its light took 12.4 billion years to reach us. Another very old galaxy is named BX442, which is 11 billion years old. We also see that bars in galaxies have become more common over time. Only 10% of spirals had bars about 8 billion years ago. Today, over two-thirds of the visible universe has these barred spirals.

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.