THE MILKY WAY
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 which forms part of the Local Group. Although the Milky Way is but one of billions of galaxies in the universe, the Galaxy has special significance to humanity as it is the home of the Solar system. Democritus (450 BC – 370 BC) was the first known person to claim that the Milky Way consists of distant stars.
The term “milky” originates from the hazy band of white light appearing across the celestial sphere visible from Earth, which comprises stars and other material lying within the galactic plane. The galaxy appears brightest in the direction of Sagittarius, towards the galactic center.
The main disk of the Milky Way Galaxy is about 80,000 to 100,000 light years in diameter, about 250-300 thousand light years in circumference, and outside the Galactic core, about 1,000 light years in thickness. It is composed of 200 to 400 billion stars. As a guide to the relative physical scale of the Milky Way, if the galaxy were reduced to 130 km (80 mi) in diameter, the solar system would be a mere 2 mm (0.08 in) in width. The Galactic Halo extends out to 250,000 to 400,000 light years in diameter. As detailed in the Structure section below, new discoveries indicate that the disk extends much farther than previously thought.
The Milky Way’s absolute magnitude, which cannot be measured directly.
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 26,000 light-years. The disk bulges outward at the center.
The galactic center harbours a compact object of very large mass, strongly suspected to be a supermassive black hole. Most galaxies are believed to have a supermassive black hole at their center.