Discovery Space: The milky way system
The solar system is also part of a galaxy. Together with billions of stars it travels along its path around the center of the Milky Way system.
Our home star system is known as the Milky Way galaxy. On a clear night, with good visibility, we can make out a shimmering band at diffuse light crossing the sky. When this happens, we are looking directly into the disk section of our galaxy.
This band of light is known as the Milky Way, but the term is also used to describe the entire galaxy, of which our sun is a part. We cannot see the Milky Way galaxy from the outside, but we can try to measure the distances between its stars with the best possible precision.
The structure of the Milky Way galaxy
Because opaque interstellar dust clouds complicate this process, the structure of the Milky Way was not understood until the previous century when it became possible to use infrared and radio frequencies to make astronomical observations. As it turned out, the Milky Way system is a relatively large spiral galaxy. Our solar system is located in one of the spiral arms, near the outer region.
The outer layer of the galaxy is called a galactic halo, consisting of widely scattered aggregations of ancient stars, so-called globular clusters. Star types can be distinguished based on their composition, age, and distribution in space. The youngest are located near the symmetrical sections of the spiral arms and have a relatively high concentration of heavy chemical elements. These elements were produced by older generations of stars in the fusion of their nuclear fuel.
As they weakened, some of their matter was ejected into space, and was later incorporated in the formation of new generations of stars. Middle-aged stars are found near the galactic disk. The old stars in the globular clusters contain the lowest percentage of heavy chemical elements. It is likely that they were already formed when the galactic matter aggregated. To locate the center of the Milky Way galaxy, we have to look toward the constellation Sagittarius.
The center is shrouded in thick, interstellar matter and is not visible to us. However, its structure can still be explored based on radio frequency and infrared and x-ray radiation. Around the central area, stars are distributed more thickly than they are in the outer regions. In the direct center, it appears that the equivalent of approximately three million sun-sized masses is concentrated into a tight space. It is suspected that an extremely large concentration of matter has accumulated here, a so-called black hole.
THE LOCAL GROUP
1 tie Milky Way galaxy is part of a group of mora than 30 galaxies held together by gravitational forces. The Milky Way galaxy and the Andromeda galaxy are the largest representatives of this so-called Local Group and are about 2.5 million light years away from each other
They are surrounded by a cluster of smaller galaxies, including the Large and the Small Magellanic Clouds. At approximately 150.000 and 180,000 light-years away, there are relatively close to the Milky Way.
BASICS
THE MILKY WAY system has a diameter of about 100.000 light-years. It consists of approximately 100 to 400 billion stars. The sun is about 27,000 light-years away from the galactic center.
It orbits around the center at 124 miles per sec- ond (200 km/sec). One revolution takes 240 million years.