MILKYWAY GALAXY

The Milky Way has been described as a serpent or a river, and in Sweden they call our galaxy Vinter Gatan – ‘Winter Street’. According to poets it is the Winter Street that will connect us with the wonder of our nocturnal skies. The word Galaxy comes from the Greek word for our own galaxy, the root galakt means milk. A galaxy is a system made up of billions of stars, like our Sun, and there are probably more than one hundred billion galaxies in the observable universe .
The Milky Way consists of at least 200 and maybe as many as 400 billions stars. It is spread out as a thin disk, and from the outside it would look like a spiral galaxy. Recent evidence suggests that the Milky Way is a barred spiral galaxy; this is a type of spiral galaxy with a central bar-shaped structure consisting of stars. Barred spiral galaxies are common, and the bar structure is believed to be a stellar nursery fuelling star birth in the centres. The bar acts like a mechanism that channels gas inwards from the spiral arms towards the centre, and stars are born out of the interstellar cloud. It has been suggested that spiral galaxies without bars may suddenly develop them; the bars might come and go.
The Milky Way is about 100 000 light years across. The Sun is out midway out in the disk. The fact that it divides the night sky into two roughly equal hemispheres suggests that our Solar System lies close to galactic plane. Our galaxy is visible form Earth as a band of light in the night sky, but the low surface brightness makes it difficult to see due to light pollution.
Andromeda is the nearest spiral galaxy and it is about twice as big as the Milky Way. The Milky Way and Andromeda galaxy are on a collision course, and in a few billion years the two galaxies will make their first close encounter. This is within our Sun’s life time. Then over a span of 1 billion years or so, then they will merge to form a single giant elliptical galaxy in about 5 billion years. Andromeda is about 2.5 million light years away, but the gap is closing at 500 000 km/hour. It is predicted that our solar system will be thrown out into the merging galaxy’s outer rim (about 100 000 light years from the centre of the new galaxy). Our Sun will be an aging star and almost in the red-giant formation stage when this merger takes place.
The oldest star in Milky Way is estimated to 13.2 billion years, nearly as old as the Universe itself. The Universe isestimated to 13.7 billion years with an uncertainty of about 200 million years.
Suns orbit the Milky Way’s core at extremely high speeds – 4 800 meter per second. It would take our Earth 3 days only to orbit around the Sun at that speed. It has been suggested that a black lies in our galaxy’s core, and is responsible for accelerating stars to the high speeds. The centre of the Milky Way Galaxy is in the Constellation Sagittarius.
There should be hundreds of dwarfs, small galaxies, gravitationally bound to the Milky Way, but only about 20 dwarfs have been observed. Scientists have been puzzled about the missing galaxies, and now it has been suggested that the galaxies are there we simply cannot see them because they owe their existence to cold dark matter. This matter does not emit any light, so you cannot observe it directly.