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The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang. The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster , which is itself a component of the Laniakea Supercluster. Dark regions within the band, such as the Great Rift and the Coalsack , are areas where interstellar dust blocks light from distant stars. The area of sky that the Milky Way obscures is called the Zone of Avoidance. The Milky Way has a relatively low surface brightness.

Its visibility can be greatly reduced by background light, such as light pollution or moonlight. The sky needs to be darker than about As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations. From Sagittarius, the hazy band of white light appears to pass around to the galactic anticenter in Auriga.

The band then continues the rest of the way around the sky, back to Sagittarius, dividing the sky into two roughly equal hemispheres. Relative to the celestial equator , it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux , indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic, relative to the galactic plane. Because of this high inclination, depending on the time of night and year, the arch of the Milky Way may appear relatively low or relatively high in the sky. Estimates of the mass of the Milky Way vary, depending upon the method and data used.

The low end of the estimate range is 5. Much of the mass of the Milky Way seems to be dark matter , an unknown and invisible form of matter that interacts gravitationally with ordinary matter.

How Many Galaxies Are in the Universe? - Video

A dark matter halo is conjectured to spread out relatively uniformly to a distance beyond one hundred kiloparsecs kpc from the Galactic Center. Mathematical models of the Milky Way suggest that the mass of dark matter is 1—1.

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In March , astronomers reported that the mass of the Milky Way galaxy is 1. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion 10 12 stars. This disk has at least a comparable extent in radius to the stars, [66] 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. The disk of stars in the Milky Way does not have a sharp edge beyond which there are no stars. Rather, the concentration of stars decreases with distance from the center of the Milky Way.

For reasons that are not understood, beyond a radius of roughly 40, ly 13 kpc from the center, the number of stars per cubic parsec drops much faster with radius.

Hence, such objects would probably be ejected from the vicinity of the Milky Way. 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, [28] [73] and microlensing measurements indicate that there are more rogue planets not bound to host stars than there are stars. The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust and stars. A galactic quadrant, or quadrant of the Milky Way, refers to one of four circular sectors in the division of the Milky Way.

In astronomical practice, the delineation of the galactic quadrants is based upon the galactic coordinate system , which places the Sun as the origin of the mapping system. Quadrants are described using ordinals —for example, "1st galactic quadrant", [92] "second galactic quadrant", [93] or "third quadrant of the Milky Way". This value is estimated using geometric -based methods or by measuring selected astronomical objects that serve as standard candles , with different techniques yielding various values within this approximate range.

Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.

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In , two gigantic spherical bubbles of high energy emission were detected to the north and the south of the Milky Way core, using data from the Fermi Gamma-ray Space Telescope. The diameter of each of the bubbles is about 25, light-years 7.

Hubble Traces a Galaxy’s Outer Reaches | NASA

Outside the gravitational influence of the Galactic bar, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms. The Milky Way's spiral structure is uncertain, and there is currently no consensus on the nature of the Milky Way's spiral arms. 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. The rest of the arms contain excess gas but not excess old stars.

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  7. The explanation for this apparent discrepancy is unclear. The Near 3 kpc Arm also called Expanding 3 kpc Arm or simply 3 kpc Arm was discovered in the s by astronomer van Woerden and collaborators through centimeter radio measurements of HI atomic hydrogen. It is located in the fourth galactic quadrant at a distance of about 5. It is located in the first galactic quadrant at a distance of 3 kpc about 10, ly from the Galactic Center. A simulation published in suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.

    It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring , [] and the two patterns would be connected by the Cygnus arm.

    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. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way. Although the disk contains dust that obscures 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, as there is little gas cool to collapse into stars.

    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 farther than previously thought, [] the possibility of the disk of the Milky Way extending farther is apparent, and this is supported by evidence from the discovery of the Outer Arm extension of the Cygnus Arm [] [] and of a similar extension of the Scutum—Centaurus Arm. 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.

    The Sloan Digital Sky Survey of the northern sky shows a huge and diffuse structure spread out across an area around 5, 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 Milky Way.

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    The proposed likely interpretation is that a dwarf galaxy is merging with the Milky Way. In addition to the stellar halo, the Chandra X-ray Observatory , XMM-Newton , and Suzaku have provided evidence that there is a gaseous halo with a large amount of hot gas. The halo extends for hundreds of thousand of light-years, much farther than the stellar halo and close to the distance of the Large and Small Magellanic Clouds. The mass of this hot halo is nearly equivalent to the mass of the Milky Way itself. 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. There are about stars brighter than absolute magnitude 8. This illustrates the fact that there are far more faint stars than bright stars: in the entire sky, there are about stars brighter than apparent magnitude 4 but 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 Milky Way 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 passes through the Galactic plane approximately 2. These oscillations were until recently thought to coincide with mass lifeform extinction periods on Earth. At this speed, it takes around 1, years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU astronomical unit.

    The stars and gas in the Milky Way rotate about its center differentially , meaning that the rotation period varies with location. As is typical for spiral galaxies, the orbital speed of most stars in the Milky Way does not depend strongly on their distance from the center. 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 Milky Way the orbit speeds are too low, whereas beyond 7 kpcs the speeds are too high to match what would be expected from the universal law of gravitation. If the Milky Way 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. Alternatively, a minority of astronomers propose that a modification of the law of gravity may explain the observed rotation curve. The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang. Nearly half the matter in the Milky Way may have come from other distant galaxies.

    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. Since the first stars began to form, the Milky Way has grown through both galaxy mergers particularly early in the Milky Way's growth and accretion of gas directly from the Galactic halo.

    Direct accretion of gas is observed in high-velocity clouds like the Smith Cloud. 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. According to recent studies, the Milky Way as well as the Andromeda Galaxy 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 the Milky Way and the Andromeda Galaxy.