The local group of Galaxy

Galaxies tent to cluster together. The Milky Way galaxy belongs to a small cluster, consisting of at least 25 galaxies. Four are visible to the naked eye. One is a small spiral that can just be seen under good conditions in Triangulum. The others are the Andromeda galaxy and the large and Small Magellanic Clouds.

 The Local Group of galaxies is dominated by two spirals, the Milky Way galaxy and the Andromeda Galaxy, and the two are often compared. Both are larger than the average spiral, but Andromeda is twice as broad as ours and the largest member of the Local Group. It is approaching our galaxy at a speed of 275 Km/Sec – negligible speed in comparison with its distance of over two million light years.

The Local group contains hundred trillion members. They contain little gas, and a few young stars, so they are predominantly reddish in color. There are no large elliptical galaxies in the Local Group, but some small ones are satellite galaxies of Andromeda.  The large and Small Magellanic clouds are satellite galaxies of the Milky Way. They are visible as two patches of light close to the Milky Way in the Southern Sky. The Large Magellanic Cloud is 170,000 light years distance and 30,000 light years. The Small Magellanic Cloud is 190,000 light years away and 16,000 light years diameter. Both are classified as spiral, though they have been heavily distorted by the Milky Way.

Evolution of Galaxy

 The solar system is located in a spiral galaxy known as the Milky Way. The galaxy formed from an ill defined globe of hydrogen and helium gas approximately 12 to 14 billion years ago. This proto galaxy gradually shrank under its own gravitational attraction. As it did so, smaller concentrations of gas condensed within it. These were the basis of globular clusters of stars, each orbiting the centre of what was to become the galaxy. Each in turn broke onto hundreds of thousands of smaller knots of gas, which evolved into stars.

 The bulk of the proto galaxy collapsed into the centre without forming part of a globular cluster. As the gas collapsed, it whirled faster round the centre, as water whirls down a plughole. Most of the matter formed a huge flattened globe – the galaxy’s central bulge. Most of this gas broke up into stars, but some was spun out into a great rotating disc, 100,000 light years across. The rotation of the disc slowed the condensing of gas into stars, which continues today.