Milky Way

The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Galaxia Kyklos), sometimes referred to simply as "the Galaxy", is a barred spiral galaxy which forms part of the Local Group. Although the Milky Way is but one of millions of galaxies in the universe, the Galaxy has special significance to humanity as it is the home of the solar system, which is located near the Orion Arm. Democritus (460 BC - 370 BC) was the first known person to claim that Milky Way consists of distant stars.

Structure

As of 2005, the Milky Way is thought to comprise a large barred spiral galaxy of Hubble type SBbc (loosely wound barred spiral) with a total mass of about 1012 solar masses (M☉), comprising 200-400 billion stars http://www.eso.org/outreach/press-rel/pr-2004/pr-20-04.html.

Related Topics:
As of 2005 - Barred spiral galaxy - Hubble type - Mass - Solar mass - Billion

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

It was only in the 1980s that astronomers began to suspect that the Milky Way is a barred spiral rather than an ordinary spiral, which observations in 2005 with the Spitzer Space Telescope have since confirmed, showing that the galaxy's central bar is larger than previously suspected http://www.newscientistspace.com/article.ns?id=dn7854.

Related Topics:
Spiral - 2005 - Spitzer Space Telescope

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The galactic disk has an estimated diameter of about 100,000 light-years (see 1 E20 m for a list of comparable distances). The distance from the Sun to the galactic center is estimated at about 27,700 light-years. The disk bulges outward at the center.

Related Topics:
Light-year - 1 E20 m - Sun

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

As with most galaxies, it is suspected that the galactic center harbours a supermassive black hole, with Sagittarius A* being thought to be the most plausible candidate for the location of this extreme concentration of mass.

Related Topics:
Supermassive black hole - Sagittarius A*

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

As is typical for many galaxies, the distribution of mass in the Milky Way is such that the orbital speed of most stars in the galaxy does not depend strongly on its distance from the center. Away from the central bulge or outer rim, the typical stellar velocity is 210 and 240 km/s http://zebu.uoregon.edu/~imamura/123/lecture-2/mass.html. Hence the orbital period of the typical star is directly proportional only to the length of the path travelled. This is unlike in the solar system where different orbits are also expected to have significantly different velocities associated with them.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The galaxy's bar is thought to be about 27,000 light years long, running through the center of the galaxy at a 44±10 degree angle to the line between our sun and the center of the galaxy. It is composed primarily of red stars, believed to be ancient.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees (see Valle, below). 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 right:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• 2 and 8 - 3kpc and Perseus Arm
• 3 and 7 - Norma and Cygnus Arm (Along with a newly discovered extension - 6)
• 4 and 10 - Crux and Scutum Arm
• 5 and 9 - Carina and Sagittarius Arm

There are at least two smaller arms or spurs, including:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• 11 - Orion Arm (which contains the solar system and the Sun - 12)

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.

Related Topics:
Outer Ring - Monoceros Ring - Brian Yanny - Heidi Jo Newberg

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The galactic disk is surrounded by a spheroid halo of old stars and globular clusters. While the disk contains gas and dust obscuring the view in some wavelengths, the halo does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but not in the halo. Open clusters also occur primarily in the disk.

Related Topics:
Spheroid - Halo - Globular cluster - Star - Open cluster

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

X-ray image of Milky Way taken by Chandra X-ray Observatory

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~