The Center, Bulge, and Disk of the Milky Way

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For years, Skowron and her team watched these big, young stars periodically pulse—even monitoring the ones on the very fringes of our galaxy—using the OGLE instrument, which is mounted on a telescope in Chile, along with several other telescopes. And when Skowron and her colleagues plotted the precise distances to their 2, stars, the resulting 3-D galactic map was intriguingly kinked.

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Skowron and her colleagues also identified groups of Cepheids that had recently burst into existence, cosmically speaking. By contrast, the oldest known stars in the Milky Way are a whopping 10 to 13 billion years old. Colored dots represent older red and younger blue stars in the Milky Way belonging to three stellar groups that seem to have formed in relatively recent bursts. Robin notes that slowly moving regions of higher density can compress the gas and dust in interstellar clouds, triggering star formation, while Skowron suggests that a recent interaction with a passing dwarf galaxy could be the culprit.

Read Caption.

Milky Way Galaxy

The Milky Way is warped around the edges, new star map confirms If we could see it from outside, our home galaxy would not be a flat disk, but would look more like a poached egg sliding off a slotted spoon. By Nadia Drake. Stars Countless stars dot the night sky. Learn how these celestial objects form, how they are classified by brightness and temperature, and what happens when stars die. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy.

It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies. Bulges that have properties similar to those of elliptical galaxies are often called "classical bulges" due to their similarity to the historic view of bulges.

The distribution of light is described by a Sersic profile.

The Modern Picture of the Milky Way

Classical bulges are thought to be the result of collisions of smaller structures. Convulsing gravitational forces and torques disrupt the orbital paths of stars, resulting in the randomised bulge orbits. If either progenitor galaxy was gas-rich, the tidal forces can also cause inflows to the newly merged galaxy nucleus. Following a major merger , gas clouds are more likely to convert into stars, due to shocks see star formation.

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Many bulges have properties more similar to those of the central regions of spiral galaxies than elliptical galaxies. These bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the stars in the outer disk.

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  7. This contrasts greatly with elliptical galaxies. Subsequent studies using the Hubble Space Telescope show that the bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. Giant spiral galaxies are typically 2— times the size of those spirals that exist in bulges. Where they exist, these central spirals dominate the light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rate at which stars form in disk galaxies. Sometimes bulges contain nuclear rings that are forming stars at much higher rate per area than is typically found in outer disks, as shown in NGC see photo.

    Properties such as spiral structure and young stars suggest that some bulges did not form through the same process that made elliptical galaxies and classical bulges. Yet the theories for the formation of pseudobulges are less certain than those for classical bulges. Pseudobulges may be the result of extremely gas-rich mergers that happened more recently than those mergers that formed classical bulges within the last 5 billion years.

    However, it is difficult for disks to survive the merging process, casting doubt on this scenario. Many astronomers suggest that bulges that appear similar to disks form outside of the disk, and are not the product of a merging process. When left alone, disk galaxies can rearrange their stars and gas as a response to instabilities.

    The products of this process called secular evolution are often observed in such galaxies; both spiral disks and galactic bars can result from secular evolution of galaxy disks.

    Milky Way's Central Bulge

    Secular evolution is also expected to send gas and stars to the center of a galaxy. If this happens that would increase the density at the center of the galaxy, and thus make a bulge that has properties similar to those of disk galaxies. If secular evolution, or the slow, steady evolution of a galaxy, [9] is responsible for the formation of a significant number of bulges, then that many galaxies have not experienced a merger since the formation of their disk.

    This would then mean that current theories of galaxy formation and evolution greatly over-predict the number of mergers in the past few billion years. Most bulges and pseudo-bulges are thought to host a central relativistic compact mass, which is traditionally assumed to be a supermassive black hole. Such black holes by definition can not be observed directly light cannot escape them , but various pieces of evidence suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals.

    Milky Way Galaxy | Physical Geography

    The masses of the black holes correlate tightly with bulge properties. The M—sigma relation relates black hole mass to the velocity dispersion of bulge stars [11] [12] , while other correlations involve the total stellar mass or luminosity of the bulge [13] [14] [15] , the central concentration of stars in the bulge [16] , the richness of the globular cluster system orbiting in the galaxy's far outskirts [17] [18] , and the winding angle of the spiral arms. Until recently it was thought that one could not have a supermassive black hole without a surrounding bulge.