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Table 23.1 to compare some crucial properties of the three straightforward components the the Galaxy. Is there some evolutionary script that have the right to naturally account for the Galactic structure we check out today? The price is the there is, and it takes us all the way back come the birth of our Galaxy, 10—15 billion year ago. Not all the details space agreed top top by every astronomers, but the overall photo is now relatively widely accepted. For simplicity us confine our discussion here come the Galactic disk and halo. In numerous ways the bulge is intermediary in the properties in between these two extremes.

Highly flattened
TABLE 23.1 as whole Properties that the Galactic Disk, Halo, and Bulge
GALACTIC DISKGALACTIC HALOGALACTIC BULGE
Roughly spherical—mildly flattenedSomewhat flattened and also elongated in the airplane of the disk ("football shaped")
Contains both young and also old starsContains old stars onlyContains both young and old stars; more old stars in ~ greater distances from the center
Contains gas and also dustContains no gas and also dustContains gas and dust, specifically in the inside regions
Site of recurring star formationNo star formation during the critical 10 billion yearsOngoing star formation in the within regions
Gas and stars relocate in one orbits in the Galactic planeStars have random orbits in 3 dimensionsStars have actually largely arbitrarily orbits however with part net rotation around the Galactic center
Spiral armsNo apparent substructureRing the gas and dust close to center; Galactic nucleus
Overall white coloration, through blue spiral armsReddish in shade Yellow-white

Figure 23.14 illustrates the current view of ours Galaxy"s evolution, starting (not unlike the star-formation script outlined in thing 19) native a contracting cloud the pregalactic gas. (Sec. 19.1) once the first Galactic stars and also globular swarm formed, the gas in our Galaxy had not yet collected into a thin disk. Instead, the was spread out over an irregular, and also quite extended, an ar of space, spanning many tens of kiloparsecs in every directions. As soon as the first stars formed, castle were distributed throughout this volume. Their circulation today (the Galactic halo) reflects that fact—it is one imprint of their birth. Countless astronomers believe that the very very first stars formed even earlier, in smaller systems the later linked to produce our Galaxy (Figure 23.14a); the contemporary halo would look the same in one of two people case.

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Figure 23.14 (a) The Milky means Galaxy possibly developed via the merger of number of smaller systems. (b) Astronomers reason that, early on, ours Galaxy was irregularly shaped, v gas dispersed throughout the volume. Once stars formed during this stage, there was no desired direction in which castle moved and also no preferred location in i beg your pardon they to be found. (c) In time, rotation led to the gas and also dust to fall to the Galactic aircraft and form a rotate disk. The stars that had currently formed were left behind, creating the halo. (d) new stars forming in the decaying inherit its all at once rotation and so orbit the Galactic facility on ordered, one orbits.

During the previous 10—15 exchange rate years, rotation has actually flattened the gas in our Galaxy right into a reasonably thin disk. Physically, this process is similar to the flattening that the solar nebula during the formation of the solar system, as explained in chapter 15, except on a vastly larger scale. (Sec. 15.2) Star formation in the halo end billions of years ago when the raw materials dropped to the Galactic plane. Continuous star formation in the disk offers it the bluish tint, but the halo"s short-lived blue stars have long since burned out, leaving only the long-lived red stars that offer it its characteristic pinkish glow. The Galactic gloriole is ancient, whereas the disc is complete of youthful activity. The thick disk, with its intermediate-age stars, may represent one intermediate stage of star formation that occurred while the gas to be still flattening into the plane.

The chaotic orbits that the halo stars are additionally explained through this theory. As soon as the aur developed, the irregularly shame Galaxy to be rotating only really slowly, therefore there was no strongly preferred direction in which issue tended to move. As a result, aur stars were free to travel along nearly any path once they developed (or as soon as their parent systems merged). Together the Galactic disk formed, however, preservation of angular momentum brought about it to spin much more rapidly. Stars creating from the gas and dust that the decaying inherit that rotational motion and so move on well-defined, circular orbits. Again, the special disk"s properties imply that it developed while gas was still sinking to the Galaxy"s midplane and also had no yet reached its last (present-day) rotation rate.

In principle, the framework of our Galaxy bear witness come the conditions that produced it. In practice, however, the translate of the observations is made difficult by the sheer intricacy of the mechanism we inhabit and also by the many completing physical procedures that have actually modified that appearance due to the fact that it formed. As a result, the early on stages that the Milky way are still an extremely poorly understood.


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Us will return to the topic of galaxy development in thing 24.