The Milky Way Galaxy
Recent estimates put the total mass of the Milky Way Galaxy in the range one to two trillion solar masses, including a large but uncertain amount of dark matter in the dark halo. The Milky Way is the second largest member of the Local Group, after the Andromeda Galaxy.
Disk and spiral armsThe disk of the Milky Way Galaxy is home to the spiral arms. These arms contain many ordinary, intermediate-age disk stars, such as the Sun, together with the more showy Population I objects, in the form of young, hot stars, stellar associations, open clusters, diffuse nebulae, and the bulk of the interstellar matter from which future stars will form.
Because we have no way (at present!) of viewing of our galaxy from the outside, the disk and the arms can only be mapped using observations made from our solar vantage point. Astronomers began this mapping in the 1950s based on radio wave observations of neutral hydrogen gas. These early data suggested the existence of four major star-forming arms, called the Norma, Scutum-Centaurus, Sagittarius, and Perseus Arm. The sun appeared to lie in a small, partial arm known as the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms.
This traditional four-arm scheme has now been challenged by observations made in the infrared part of the spectrum. First, in the 1990s, infrared sky surveys led to the discovery of a large bar of stars stretching across the center of the Milky Way. In 2005, infrared measurements made with the Spitzer Space Telescope showed that the Milky Way's bar extends even farther out from the center of the galaxy than had previously been suspected. In 2008, new infrared imagery from Spitzer, encompassing 110 million stars and a swath of the Milky Way stretching 130 degrees across the sky and one degree above and below the galaxy's mid-plane, made a strong case for the existence of just two major spiral arms in the Milky Way – a common structure for barred galaxies. These major arms, the Scutum-Centaurus and Perseus arms, have the greatest densities of both young, bright stars, and older, so-called red-giant stars. The two minor arms, Sagittarius and Norma, are filled with gas and pockets of young stars. The two major arms appear to join up neatly with the near and far ends of the central bar.
The Sun is located 27,700 light-years from the center of the Milky Way Galaxy.
Bulge and haloThe galactic bulge and the much larger galactic halo contain Population II objects – mostly old stars and roughly 200 globular clusters, of which about 150 are known. These globulars are strongly concentrated toward the galactic nucleus.
The center of the Milky Way Galaxy
The nucleus of the Milky Way contains a complex of gas, dust, stars, supernova remnants, magnetic filaments, and, almost certainly, a supermassive black hole at the very center. It lies in the direction of Sagittarius, around R.A. 17h 46m and Dec. -28° 56'. Lying dead center in the Galaxy is the Sagittarius A Complex, which is believed to be associated with the black hole, material in orbit around this object, and a nearby supernova remnant. Surrounding the galactic center are narrow threads known as nonthermal filaments (NTFs), the most prominent of which are called the Arc, the Pelican, and the Snake. These seem to consist of magnetic flux tubes filled with relativistic electrons, beaming synchrotron radiation, that have been swept up from adjacent molecular clouds and hurled along the field lines at incredible speeds. Another unusual structure in the nucleus is catalogued as 359.1-00.5 and appears to be a superbubble with a cluster of as many as 200 newborn stars at its heart.
Satellite galaxiesThe Milky Way Galaxy has a number of satellite galaxies, which are showed in the illustration and table below. (See individual entries for more details on these galaxies.)
Twins of the Milky Way
Computer simulations of how galaxies form and evolve tend to produce few examples that match the Milky Way and its retinue, suggesting that they're rarities. Robotham and his team have quantified this rarity. They found that about 3% of galaxies similar to the Milky Way have companion galaxies like the Magellanic Clouds – 14 systems in total – an of these, only two were an almost exact match. They also found that although big satellite galaxies like the Magellanic Clouds are rare, when they are found they're usually near a galaxy that closely resembles the Milky Way.
Related category GALAXIES
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