Maya 2012: The Cosmos, Galaxy and Universe

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The Milky Way Galaxy

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 is part of the Local Group of galaxies. Although the Milky Way is one of billions of galaxies in the observable universe,[4] the Galaxy has special significance to humanity as it is 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 is 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.

Milky Way Galaxy View from Earth

The Milky Way galaxy, as viewed from the Earth, itself situated on one of the spiral arms of the galaxy, 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.

Milky Way Galaxy

360-degree photographic panorama of the galaxy

Size of the Milky Way Galaxy

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,[10] 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.

Milky Way Galaxy

The Milky Way as seen from Death Valley, 2007. This is a panoramic picture

Age of the Milky Way Galaxy

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 done in 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.

Milky Way Galaxy

A green and red Perseid meteor is striking the sky just below the
Milky Way in August 2007

Milky Way Composition and structure

NGC 7331 is often referred to as "the Milky Way's twin." This is what observers from another galaxy might see when looking at our own. IC 342 is another galaxy that may resemble the Milky Way.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. 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 which showed the Galaxy's central bar to be larger than previously suspected. As of 2006, the Milky Way's mass is thought to be about 5.8×1011 M☉ 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.

Milky Way Galaxy

NGC 7331 is often referred to as "the Milky Way's twin." This is what observers
from another galaxy might see when looking at our own. IC 342 is another galaxy
that may resemble the Milky Way

Galactic Center of the Milky Way Galaxy

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. 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.

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