Everything about Milky Way Galaxy totally explained
The
Milky Way (a translation of the
Latin Via Lactea, in turn derived from the
Greek Γαλαξίας (Galaxias) sometimes referred to simply as "
the Galaxy"), is a
barred spiral galaxy that's part of the
Local Group of galaxies. Although the Milky Way is one of billions of galaxies in the
observable universe, the Galaxy has special significance to humanity as it's the home galaxy of the planet
Earth. The plane of the Milky Way galaxy is visible from Earth as a band of light in the night sky, and it's the appearance of this band of light which has inspired the name for our galaxy.
Some sources hold that, strictly speaking, the term
Milky Way should refer exclusively to the observation of the band of light, while the full name
Milky Way Galaxy, or alternatively
the Galaxy should be used to describe our galaxy as an astrophysical whole. It is unclear how widespread the usage of this convention is, however, and the term
Milky Way is routinely used in either
context.
View from Earth
The Milky Way galaxy, as viewed from the
Earth, itself situated on one of the spiral arms of the galaxy (see
Sun's location), appears as a hazy band of
white light in the night sky arching across the entire
celestial sphere originating from
stars and other material which lie within the galactic plane.
The Milky Way looks brightest in the direction of the
constellation of
Sagittarius, toward the
galactic center. 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. The fact that the Milky Way divides the night sky into two roughly equal
hemispheres indicates that our Solar System lies close to the
galactic plane. The Milky Way has a relatively low
surface brightness, making it difficult to see from any
urban or
suburban location suffering from
light pollution.
Image:Milkyway pan1.jpg|360-degree photographic panorama of the galaxy.
Image:Deathvalleysky nps big.jpg|The Milky Way as seen from Death Valley, 2007. This is a panoramic picture.
Size
The stellar disk of the Milky Way galaxy is approximately 100,000 light years in diameter, and is believed to be, on average, about 1,000 light years thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass stars, which is highly uncertain. Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12,000 light years - twice the previously accepted value. As a guide to the relative physical
scale of the Milky Way, if it were reduced to 130 km (80 mi) in diameter, the Solar System would be a mere 2 mm (0.08 inches) in width.
The
Galactic Halo extends outward, but is limited in size by the orbits of the two Milky Way satellites, the Large and the Small
Magellanic Clouds, whose
perigalacticon is at ~180,000
light-years.
Age
It is extremely difficult to define the age at which the Milky Way formed, but the age of
the oldest star in the Galaxy yet discovered is estimated to be about 13.2 billion years, nearly as
old as the Universe itself.
This estimate is based on research by a team of astronomers in 2004 using the UV-Visual Echelle Spectrograph of the
Very Large Telescope to
measure, for the first time, the
beryllium content of two stars in
globular cluster NGC 6397. . From this research, the elapsed time between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster was deduced to be 200 million to 300 million years. By including the estimated age of the stars in the globular cluster (13.4 ± 0.8 billion years), they estimated the age of the oldest stars in the Milky Way at 13.6 ± 0.8 billion years. Based upon this emerging science, the Galactic thin disk is estimated to have been formed between 6.5 and 10.1 billion years ago.
Composition and structure
Observations by the
Spitzer Space Telescope in 2005 confirmed the nature of the Milky Way as a
barred spiral galaxy. The Galaxy consists of a bar-shaped core region surrounded by a disk of
gas, dust and stars forming four distinct arm structures spiralling outward in a
logarithmic spiral shape (see
Spiral arms). The mass distribution within the Galaxy closely resembles the Sbc
Hubble classification, which is a spiral galaxy with relatively loosely-wound arms. Astronomers in the 1980s first began to suspect that the Milky Way is a barred spiral galaxy rather than an ordinary
spiral galaxy and their suspicions were confirmed by the
Spitzer Space Telescope observations in 2005 comprising 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be −20.9. Most of the mass of the Galaxy is thought to be
dark matter, forming a
dark matter halo of an estimated 600–3000 billion
solar masses (M
☉) which is spread out relatively evenly.
Galactic center
The galactic disc, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000
light-years. The distance from the
Sun to the galactic center is now estimated at 26,000 ± 1400 light-years, while older estimates could put the Sun as far as 35,000 light-years from the central bulge.
The galactic center harbors a compact object of very large mass (named
Sagittarius A*), strongly suspected to be a
supermassive black hole. Most galaxies are believed to have a supermassive black hole at their center.
The Galaxy's bar is thought to be about 27,000 light-years long, running through its center at a 44 ± 10 degree angle to the line between the Sun and the center of the Galaxy. It is composed primarily of red stars, believed to be ancient (see
red dwarf,
red giant). The bar is surrounded by a ring called the "5-
kpc ring" that contains a large fraction of the molecular hydrogen present in the Galaxy, as well as most of the Milky Way's
star formation activity. Viewed from the
Andromeda Galaxy, it would be the brightest feature of our own galaxy.
Spiral arms
Each spiral arm describes a
logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. There are believed to be four major spiral arms which all start at the Galaxy's center. These are named as follows, according to the image at left:
Outside of the major spiral arms is the
Outer Ring or
Monoceros Ring, a ring of stars around the Milky Way proposed by astronomers Brian Yanny and Heidi Jo Newberg, which consists of gas and stars torn from other galaxies billions of years ago.
As is typical for many galaxies, the distribution of mass in the Milky Way Galaxy is such that the
orbital speed of most stars in the Galaxy doesn't depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is between 210 and 240 km/s. Hence the
orbital period of the typical star is directly proportional only to the length of the path traveled. This is unlike in the Solar System where different orbits are also expected to have significantly different velocities associated with them, and is one of the major pieces of evidence for the existence of
dark matter. Another interesting aspect is the so-called "wind-up problem" of the spiral arms. If one believes that 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 isn't what is observed in spiral galaxies; instead, astronomers propose that the spiral arms form as a result of a matter-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's always a region of slow-moving cars. Thus this results in several spiral arms where there are a lot of stars and gas. 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.
In a large area from
Central Asia to
Africa, the name for the Milky Way is related to the word for
straw. It has been claimed that this was spread by
Arabs who in turn borrowed the word from
Armenian. In several
Uralic,
Turkic languages,
Fenno-Ugric languages and in the
Baltic languages the Milky Way is called the "Birds' Path". The
Chinese name "Silver River" (銀河) is used throughout
East Asia, including
Korea and
Japan. An alternative name for the Milky Way in ancient China, especially in poems, is "Heavenly River of
Han"(天汉). In
Japanese, "Silver River" (銀河
ginga) means galaxies in general and the Milky Way is called the "Silver River System" (銀河系
gingakei) or the "River of Heaven" (天の川
ama no kawa). In
Swedish, it's called
Vintergatan, or "Winter Street", because the stars in the belt were used to predict time of the approaching winter.
Discovery
» See also:Galaxy-Observation history
As
Aristotle informs us in
Meteorologica (DK 59 A80), the
Greek philosophers
Anaxagoras (ca. 500 BC–428 BC) and
Democritus (450–370 BC) proposed that the Milky Way might consist of distant
stars. The
Persian astronomer Abū Rayhān al-Bīrūnī (973-1048 AD) further proposed the Milky Way
galaxy to be a collection of countless
nebulous stars. Actual proof of this came in 1610 when
Galileo Galilei used a
telescope to study the Milky Way and discovered that it was composed of a huge number of faint stars. In a treatise in 1755,
Immanuel Kant, drawing on earlier work by
Thomas Wright, speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by
gravitational forces akin to the Solar System but on much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the
nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own.
The first attempt to describe the shape of the Milky Way and the position of the
Sun within it was carried out by
William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. He produced a diagram of the shape of the Galaxy with the Solar System close to the center.
In 1845,
Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral-shaped nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.
In 1917,
Heber Curtis had observed the nova
S Andromedae within the "Great
Andromeda Nebula" (
Messier object M31). Searching the photographic record, he found 11 more
novae. Curtis noticed that these novae were, on average, 10
magnitudes fainter than those that occurred within our galaxy. As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the "island universes" hypothesis, which held that the spiral nebulae were actually independent galaxies. In 1920 the
Great Debate took place between
Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant
Doppler shift.
The matter was conclusively settled by
Edwin Hubble in the early 1920s using a new telescope. He was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some
Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way. In 1936 Hubble produced a classification system for galaxies that's used to this day, the
Hubble sequence.
Further Information
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