Components of the Milky Way
We think that the Milky Way galaxy is composed of 3 parts; the disk, the bulge, and the halo.
Most of the stars in the Milky Way are located in a disk about 2kpc thick and about 40kpc across. The disc is primarily composed of hot, young stars, and contains lots of dust and gas.
The Galactic Bulge
The bulge lies in the center of the galaxy. It is about 6kpc across and extends above and below the disk. There is very little gas dust (the material required to form new stars). Therefore, the bulge is mainly made up of old stars.
Galactic Center of the Milky Way
The Galactic Center is the rotational center of the Milky Way, which is a supermassive black hole of 4.100 ± 0.034 million solar masses that powers the compact radio source Sagittarius A*. It is 26,700 ± 1,300 ly from the Earth in the direction of the constellation Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest.
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The Halo
The halo is primarily made up of globular clusters and is spherical in shape. It is centered in the center of the galaxy and is a 30-40kpc radius. Again, there is very little dust and gas. The halo is primarily composed of old stars. Interestingly, most of these stars are metallicity stars, which means that they have even fewer metals than the Sun.
The Disc
The disk of interstellar material is rotating above a very dense center. The disc is the thinnest immediately outside the central bulge and thickens slowly towards the edge. Stars are still forming the disc, and the younger objects form a spiral pattern. Around the disc, there is a large diffuse halo.
In the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars, whereas the thickness of the gas layer ranges from hundreds of light-years for the colder gas to thousands of light-years for warmer gas.
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The stars and star clusters in the halo represent the older generation of stars. The stellar orbits in the disc are fairly circular and align with the disc. The orbits of stars in the halo are very eccentric and randomly oriented. The stars of the oldest populations are very metal-poor, the younger population stars and the interstellar medium have a high metal abundance.
The space distribution, kinematics, and chemical abundances of the different populations have been interpreted within a unified model of the evolution of the galaxy.
The Formation of the Milky Way Galaxy
About 15,000 million years ago, the Milky Way was a large turbulent cloud made of hydrogen and helium. The first massive stars to form in it produced heavier elements and exploded as Supernovae. The shock waves from the explosions speeded up the formation of a further generation of stars.
All this time, the Milky Way cloud was contracting under its internal gravity. Its originally slow rotation accelerated because of the contraction since the angular momentum has to remain constant. As a result, at the later stages, the cloud collapsed almost entirely along its rotational axis, while in the plane of rotation the contraction stopped. The disc of the Milky Way was formed.
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The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Surrounding the galactic disk is a spherical Galactic Halo of stars and globular clusters that extends farther outward, but is limited in size by the orbits of two Milky Way satellites, the Large and Small Magellanic Clouds, whose closest approach to the Galactic Center is about 1,80,000ly. At this distance or beyond, the orbits of most halo objects would probably be ejected from the vicinity of the Milky Way.
The low-mass stars of the first generation still exist in the halo and particularly in the globular clusters. Their randomly oriented, elongated orbits reflect the turbulent state of the primordial cloud.
The increasing metal abundances of the younger stellar populations give information about the production of elements inside the stars and the enrichment of the interstellar medium by the ejection of enriched gas from a new generation of stars.
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Number of Stars
The Milky Way contains between 100 and 400 billion stars and at least 100 billion planets. An exact figure would depend on counting the number of very low-mass stars, which are difficult to detect, especially at distances of more than 300ly from the Sun.
The Milky Way may contain 10 billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes.
The Planets
Both gravitational microlensing and planetary transit observations indicate that there may be at least as many planets bound to stars as there are stars in the Milky Way, and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars.
The Milky Way contains at least one planet per star, resulting in 100-400 billion planets, according to a 2013 study with the Kepler space observatory. A different analysis of the Kepler data estimated that at least 17 billion Earth-sized exoplanets reside in the Milky Way. Besides exoplanets, “exocomets”, comets beyond the Solar System, have also been detected and may be common in the Milky Way.
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