Smaller star-forming clumps began to form fuzzy, not-very-distinct spiral arms. Bright, well-defined spiral arms began to appear only when the galaxies were about 3. Initially, there were two well-defined arms. Multi-armed structures in galaxies like we see in the Milky Way appeared only when the universe was about 8 billion years old. We will discuss the history of galaxies in more detail in The Evolution and Distribution of Galaxies.
But, even from our brief discussion, you can get the sense that the spiral structures we now observe in mature galaxies have come along later in the full story of how things develop in the universe. Scientists have used supercomputer calculations to model the formation and evolution of the arms. What these calculations show is that giant molecular clouds which we discussed in Between the Stars: Gas and Dust in Space have enough gravitational influence over their surroundings to initiate the formation of structures that look like spiral arms.
These arms then become self-perpetuating and can survive for at least several billion years. The arms may change their brightness over time as star formation comes and goes, but they are not temporary features.
The concentration of matter in the arms exerts sufficient gravitational force to keep the arms together over long periods of time. Measurements show that the Galaxy does not rotate as a solid body, but instead its stars and gas follow differential rotation, such that the material closer to the galactic center completes its orbit more quickly. Observations show that galaxies like the Milky Way take several billion years after they began to form to develop spiral structure.
Skip to main content. The Milky Way Galaxy. Search for:. Spiral Structure Learning Objectives By the end of this section, you will be able to: Describe the structure of the Milky Way Galaxy and how astronomers discovered it Compare theoretical models for the formation of spiral arms in disk galaxies.
Spiral galaxies make up roughly 72 percent of the galaxies that scientists have observed, according to a Hubble Space Telescope survey. Most spiral galaxies contain a central bulge surrounded by a flat, rotating disk of stars.
The bulge in the center is made up of older, dimmer stars, and is thought to contain a supermassive black hole. Approximately two-thirds of spiral galaxies also contain a bar structure through their center, as does the Milky Way. The disk of stars orbiting the bulge separates into arms that circle the galaxy. These spiral arms contain a wealth of gas and dust and younger stars that shine brightly before their quick demise.
How the spiral arms form continues to puzzle scientists. One theory suggests the galaxy arms could be the result of density waves traveling through the outer disk. This indicates that the bulges and halos of spiral galaxies probably formed through the primordial collapse of individual gas clouds early in the history of the Universe. While this accounts for the almost exclusively old ages of halo stars, it cannot be the whole story for spiral bulges which also contain young and intermediate age stars.
The most reasonable explanation for the presence of these younger stars is that after the spiral bulges of these galaxies had formed through primordial collapse, they also experienced some form of secular evolution — through accretion processes or the actions of spiral arms or a central bar.
While the above scenario appears to account well for the stellar populations found in the bulges of Sa and Sb galaxies, the bulge populations for Sc and Sd galaxies are actually the same as those found in the disks of these galaxies. This would suggest that the bulges of extremely late-type spiral galaxies are formed almost entirely through secular evolution processes.
Optical image of the Milky Way. Credit : A. The stars in the disks of spiral galaxies are generally younger than the majority of stars found in the bulge and halo. For this reason, disks are thought to form after the primordial collapse event responsible for the formation of the spheroidal bulge and halo, possibly through the cooling of the hot gas contained within the halo of the newly formed galaxy.
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