what use are 21-cm radio waves to galactic astronomers?

Learning Objectives

Past the end of this section, you lot will be able to:

• Draw the structure of the Galaxy Milky way and how astronomers discovered it
• Compare theoretical models for the formation of screw arms in disk galaxies

Astronomers were able to make tremendous progress in mapping the spiral structure of the Milky Way after the discovery of the 21-cm line that comes from cool hydrogen (see Between the Stars: Gas and Grit in Infinite). Remember that the obscuring issue of interstellar dust prevents us from seeing stars at large distances in the disk at visible wavelengths. However, radio waves of 21-cm wavelength pass right through the dust, enabling astronomers to detect hydrogen atoms throughout the Galaxy. More recent surveys of the infrared emission from stars in the disk have provided a like dust-costless perspective of our Galaxy's stellar distribution. Despite all this progress over the past fifty years, we are even so just beginning to pin down the precise structure of our Galaxy.

The Arms of the Milky Way

Our radio observations of the disk'southward gaseous component indicate that the Galaxy has two major spiral arms that sally from the bar and several fainter arms and shorter spurs. You can come across a recently assembled map of our Galaxy's arm structure—derived from studies in the infrared—in Figure 1.

Effigy 1. Milky Way Bar and Arms: Here, we encounter the Milky Fashion Galaxy every bit information technology would await from higher up. This image, assembled from data from NASA'southward WISE mission, shows that the Milky Way Milky way has a modest bar in its fundamental regions. Two spiral artillery, Scutum-Centaurus and Perseus, emerge from the ends of the bar and wrap around the bulge. The Sagittarius and Outer arms take fewer stars than the other two arms. (credit: modification of work by NASA/JPL-Caltech/R. Hurt (SSC/Caltech))

The Sun is virtually the inner border of a short arm called the Orion Spur, which is about x,000 light-years long and contains such conspicuous features as the Cygnus Rift (the great dark nebula in the summertime Galaxy) and the bright Orion Nebula. Figure two shows a few other objects that share this small section of the Milky way with us and are easy to see. Remember, the farther abroad we try to look from our own arm, the more the dust in the Galaxy builds upwardly and makes information technology hard to run across with visible light.

Figure 2. Orion Spur: The Sun is located in the Orion Spur, which is a minor spiral arm located between two other arms. In this diagram, the white lines point to some other noteworthy objects that share this characteristic of the Milky Style Galaxy with the Sun. (credit: modification of work by NASA/JPL-Caltech)

Formation of Spiral Structure

At the Sun'due south distance from its center, the Milky way does not rotate like a solid wheel or a CD inside your player. Instead, the way private objects turn around the heart of the Galaxy is more like the solar system. Stars, besides as the clouds of gas and dust, obey Kepler's tertiary police force. Objects farther from the center take longer to complete an orbit around the Galaxy than practice those closer to the middle. In other words, stars (and interstellar matter) in larger orbits in the Milky way trail backside those in smaller ones. This upshot is chosen differential galactic rotation.

Differential rotation would appear to explain why so much of the material in the deejay of the Milky Manner is concentrated into elongated features that resemble spiral arms. No matter what the original distribution of the cloth might be, the differential rotation of the Milky way tin can stretch it out into spiral features. Figure 3 shows the evolution of spiral arms from two irregular blobs of interstellar matter. Observe that as the portions of the blobs closest to the galactic center movement faster, those farther out trail behind.

Figure 3. Simplified Model for the Formation of Screw Arms: This sketch shows how spiral arms might grade from irregular clouds of interstellar cloth stretched out by the different rotation rates throughout the Galaxy. The regions farthest from the galactic eye take longer to complete their orbits and thus lag behind the inner regions. If this were the only mechanism for creating screw arms, and so over time the spiral arms would completely current of air upwards and disappear. Since many galaxies have spiral arms, they must be long-lived, and there must be other processes at work to maintain them.

But this flick of screw arms presents astronomers with an immediate problem. If that'southward all in that location were to the story, differential rotation—over the roughly 13-billion-twelvemonth history of the Galaxy—would have wound the Galaxy'south arms tighter and tighter until all semblance of spiral construction had disappeared. But did the Milky Way really have spiral arms when it formed 13 billion years agone? And practise screw arms, one time formed, last for that long a time?

With the advent of the Hubble Space Telescope, it has become possible to find the structure of very afar galaxies and to see what they were like shortly after they began to form more than than thirteen billion years ago. What the observations show is that galaxies in their infancy had bright, clumpy star-forming regions, just no regular screw construction.

Over the side by side few billion years, the galaxies began to "settle down." The galaxies that were to become spirals lost their massive clumps and adult a central bulge. The turbulence in these galaxies decreased, rotation began to boss the motions of the stars and gas, and stars began to form in a much quieter disk. Smaller star-forming clumps began to class fuzzy, not-very-singled-out screw arms. Bright, well-defined spiral arms began to appear but when the galaxies were nearly 3.6 billion years quondam. Initially, at that place were two well-defined arms. Multi-armed structures in galaxies like we see in the Galaxy appeared just when the universe was near 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 tin can get the sense that the spiral structures we now detect in mature galaxies have come up along later in the full story of how things develop in the universe.

Scientists have used supercomputer calculations to model the formation and development of the arms. These calculations follow the motions of up to 100 million "star particles" to run across whether gravitational forces can crusade them to course screw structure. 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 artillery. These artillery then become self-perpetuating and can survive for at least several billion years. The arms may change their effulgence over fourth dimension equally star germination comes and goes, merely 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 fourth dimension.

Fundamental Concepts and Summary

The gaseous distribution in the Galaxy's deejay has ii main screw arms that sally from the ends of the central bar, forth with several fainter arms and brusque spurs; the Sun is located in one of those spurs. Measurements show that the Galaxy does non rotate as a solid body, simply instead its stars and gas follow differential rotation, such that the material closer to the galactic middle completes its orbit more quickly. Observations show that galaxies like the Milky Mode have several billion years after they began to grade to develop spiral structure.

Glossary

differential galactic rotation:

the thought that unlike parts of the Galaxy turn at different rates, since the parts of the Milky way follow Kepler's tertiary law: more afar objects take longer to complete one full orbit effectually the eye of the Galaxy

spiral arm:

a spiral-shaped region, characterized by relatively dumbo interstellar material and young stars, that is observed in the disks of spiral galaxies

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Source: https://courses.lumenlearning.com/astronomy/chapter/spiral-structure/

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