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Meanwhile Atlus continues to take a dump on Europe…

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Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun’s orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giant stars and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm.[65][66] In 2008, Robert Benjamin of the University of Wisconsin–Whitewater used this observation to suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess stars.[44]

A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.[75]

Another interesting aspect is the so-called “wind-up problem” of the spiral arms. If the inner parts of the arms rotate faster than the outer part, then the galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies; instead, astronomers propose that the spiral pattern is a density wave emanating from the Galactic Center. This can be likened to a moving traffic jam on a highway—the cars are all moving, but there is always a region of slow-moving cars. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular hydrogen and protostars form as a result.

Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago.

Halo[edit]

The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center.[76] However, a few globular clusters have been found farther, such as PAL 4 and AM1 at more than 200,000 light-years away from the Galactic Center. About 40% of the galaxy’s clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[77] The globular clusters can follow rosette orbits about the Galaxy, in contrast to the elliptical orbit of a planet around a star.[78]

The Chandra X-ray Observatory has provided evidence that the halo contains a large amount of hot gas. The halo extends for hundreds of thousand of light years. The mass of the halo is estimated to the mass of the stars in the galaxy. The factors of these depends on the amount of oxygen to hydrogen.[79] The temperature of this halo was said to be between 1 million and 2.5 million kelvin or a few hundred times hotter than the surface of the sun.[80]

While the disk contains gas and dust which obscure the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo. Open clusters also occur primarily in the disk.

Discoveries in the early 21st century have added dimension to the knowledge of the Milky Way’s structure. With the discovery that the disk of the Andromeda Galaxy (M31) extends much further than previously thought,[81] the possibility of the disk of the Milky Way Galaxy extending further is apparent, and this is supported by evidence from the 2004 discovery of the Outer Arm extension of the Cygnus Arm.[74][82] With the discovery of the Sagittarius Dwarf Elliptical Galaxy came the discovery of a ribbon of galactic debris as the polar orbit of the dwarf and its interaction with the Milky Way tears it apart. Similarly, with the discovery of the Canis Major Dwarf Galaxy, it was found that a ring of galactic debris from its interaction with the Milky Way encircles the Galactic disk.

On January 9, 2006, Mario Jurić and others of Princeton University announced that the Sloan Digital Sky Survey of the northern sky found a huge and diffuse structure (spread out across an area around 5,000 times the size of a full moon) within the Milky Way that does not seem to fit within current models. The collection of stars rises close to perpendicular to the plane of the spiral arms of the Galaxy. The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. This galaxy is tentatively named the Virgo Stellar Stream and is found in the direction of Virgo about 30,000 light-years (9 kpc) away.[83]

Observations of distant galaxies indicate that the Universe had about one-sixth as much baryonic (ordinary) matter as dark matter when it was just a few billion years old. However, only about half of those baryons are accounted for in the modern Universe based on observations of nearby galaxies like the Milky Way.[84] On September 24, 2012, a team of five astronomers working with the Chandra X-ray Observatory, along with data gathered by the XMM-Newton, and Suzaku (satellite) missions, announced that the halo had a mass nearly equivalent to the baryons in the galaxy itself. They also discovered that it reaches much farther then previously thought, with new estimates showing that it extends as far as the Large and Small Magellanic Clouds.[85][86] If these findings are confirmed it could be the identity of the missing baryons around the Milky Way.[84]

Illustration of the two gigantic X-ray/gamma-ray bubbles (blue-violet) of the Milky Way (center).
Gamma-ray bubbles[edit]

In 2010, two gigantic spherical bubbles of high energy emission were detected to the north and the south of the Milky Way core, using data of the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25,000 light-years (7.7 kpc); they stretch up to Grus and to Virgo on the night-sky of the southern hemisphere. Their origin remains unclear.[87][88]

Sun’s location and neighborhood[edit]

Diagram of the Sun’s location in the Milky Way Galaxy. The angles represent longitudes in the galactic coordinate system.

Diagram of the stars in the Solar neighborhood.

The Sun is near the inner rim of the Galaxy’s Orion Arm, within the Local Fluff of the Local Bubble, and in the Gould Belt, at a distance of 8.33 ± 0.35 kiloparsecs (27,200 ± 1,100 ly) from the Galactic Center.[8][47][89] The Sun is currently 5–30 parsecs (16–98 ly) from the central plane of the Galactic disk.[90] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years (2,000 pc).[91] The Sun, and thus the Solar System, is found in the Galactic habitable zone.

There are about 208 stars brighter than absolute magnitude 8.5 within a sphere with a radius of 15 parsecs (49 ly) from the Sun, giving a density of one star per 69 cubic parsec, or one star per 2,360 cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs (16 ly) of the Sun, giving a density of about one star per 8.2 cubic parsec, or one per 284 cubic light-year (from List of nearest stars), illustrating the fact that most stars are less bright than absolute magnitude 8.5.[citation needed][original research?]

The Apex of the Sun’s Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun’s Galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun’s orbit about the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the Galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the Galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth.[92] However, a reanalysis of the effects of the Sun’s transit through the spiral structure based on CO data has failed to find a correlation.[93]

It takes the Solar System about 225–250 million years to complete one orbit of the Galaxy (a Galactic year),[94] so the Sun is thought to have completed 18–20 orbits during its lifetime and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Galaxy is approximately 220 km/s or 0.073% of the speed of light. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).[95]

Galaxy rotation curve for the Milky Way. Vertical axis is speed of rotation about the Galactic Center. Horizontal axis is distance from the Galactic Center in kpcs. The Sun is marked with a yellow ball. The observed curve of speed of rotation is blue. The predicted curve based upon stellar mass and gas in the Milky Way is red. Scatter in observations roughly indicated by gray bars. The difference is due to dark matter.[20][96][97]
Galactic rotation[edit]

The stars and gas in the Galaxy rotate about its center differentially, meaning that the rotation period varies with location. As is typical for spiral galaxies, the distribution of mass in the Milky Way Galaxy is such that the orbital speed of most stars in the Galaxy does not depend strongly on their distance from the center. Away from the central bulge or outer rim, the typical stellar orbital speed is between 210 and 240 km/s.[98] Hence the orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike the situation within the Solar System, where two-body gravitational dynamics dominate and different orbits have significantly different velocities associated with them. The rotation curve (shown in the figure) describes this rotation. Toward the center of the galaxy the orbit speeds are too low while beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation.

If the Galaxy contained only the mass observed in stars, gas, and other baryonic (ordinary) matter, the rotation speed would decrease with distance from the center. However, the observed curve is relatively flat, indicating that there is additional mass that cannot be detected directly with electromagnetic radiation. This inconsistency is attributed to dark matter.[20] Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve.[99]

Formation[edit]

Main article: Galaxy formation and evolution

The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang. Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. These stars and clusters now comprise the stellar halo of the Galaxy. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.[100][101]

Since the first stars began to form, the Milky Way has grown through both galaxy mergers (particularly early in the Galaxy’s growth) and accretion of gas directly from the Galactic halo.[101] The Milky Way is currently accreting material from two of its nearest satellite galaxies, the Large and Small Magellanic Clouds, through the Magellanic Stream. Direct accretion of gas is observed in high velocity clouds like the Smith Cloud.[102][103] However, properties of the Milky Way such as stellar mass, angular momentum, and metallicity in its outermost regions suggest it has suffered no mergers with large galaxies in the last 10 billion years. This lack of recent major mergers is unusual among similar spiral galaxies; its neighbour the Andromeda Galaxy appears to have a more typical history shaped by more recent mergers with relatively large galaxies.[104][105]

According to recent studies, the Milky Way as well as Andromeda lie in what in the galaxy color-magnitude diagram is known as the green valley, a region populated by galaxies in transition from the blue cloud (galaxies actively forming new stars) to the red sequence (galaxies that lack star formation). Star formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium. In simulated galaxies with similar properties, star formation will typically have been extinguished within about five billion years from now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between both our galaxy and M31.[106] In fact, measurements of other galaxies similar to our own suggest it’s among the reddest and brightest spiral galaxies that are still forming new stars and it’s just slightly bluer than the bluest red sequence galaxies.[107]

Age[edit]

The ages of individual stars in the Milky Way can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238, then comparing the results to estimates of their original abundance, a technique called nucleocosmochronology. These yield values of about 12.5 ± 3 billion years (Ga) for CS 31082-001[108] and 13.8 ± 4 billion years for BD+17° 3248.[109][not in citation given] Once a white dwarf star is formed, it begins to undergo radiative cooling and the surface temperature steadily drops. By measuring the temperatures of the coolest of these white dwarfs and comparing them to their expected initial temperature, an age estimate can be made. With this technique, the age of the globular cluster M4 was estimated as 12.7 ± 0.7 billion years. Globular clusters are among the oldest objects in the Milky Way Galaxy, which thus set a lower limit on the age of the galaxy. Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years.[110]

In 2007, a star in the galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old, ≈0.5 billion years less than the age of the universe. As the oldest known object in the Milky Way at that time, this measurement placed a lower limit on the age of the Milky Way.[6] This estimate was determined using the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure the relative strengths of spectral lines caused by the presence of thorium and other elements created by the R-process. The line strengths yield abundances of different elemental isotopes, from which an estimate of the age of the star can be derived using nucleocosmochronology.[6]

The age of stars in the galactic thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 ± 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the galactic halo and the thin disk.[111]

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Nintendo win