The Milky Way Galaxy – our home in the universe

New satellite data will reframe our understanding of this fascinating collection of stars

Our solar system resides in the spiral Milky Way (MW) Galaxy. American astronomer Edwin Hubble (1889-1953) is credited with discovering the shape and scope of it, so named because it looks like a hazy band of light where many of the stars cannot be individually distinguished.

The word galaxy is derived from the Greek word for milk and the Greeks called our galaxy the Milky Circle. Recent data from the Gaia satellite shows that the MW Galaxy may be up to five times lower in mass than was thought, a discovery that will surely revise our understanding of our galaxy’s 13.6 billion year history.

A galaxy is a large collection of gas, dust and billions of stars (about 100 thousand million) and their solar systems, all held together by gravity. The universe contains at least 100 billion galaxies. Our solar system, in which planet Earth resides, consists of a central sun around which eight planets rotate – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (in order from the sun).

About 60 per cent of the observable universe is comprised of spiral galaxies where the stars, gas and dust are gathered in spiral arms that radiate outwards from the galaxy centre. Our MW Galaxy has four spiral arms and our solar system lies in one arm nearly two thirds of the way (about 26,000 light years) from the centre. The stars in the galaxy arms burn brighter, picking out the arms and highlighting the galaxy’s characteristic shape. Overall, the geometry of the MW Galaxy approximates to a flattened disc – 100,000 light years across but only 1,000 light years thick.


A gigantic black hole called Sagittarius A, more than four million times the mass of the sun, hides behind clouds of gas and dust, at the centre of the MW Galaxy – it is thought that huge black holes reside at the centres of most galaxies.

About 4 billion years from now our MW Galaxy will collide with its neighbouring Andromeda galaxy, and the collision may last about 5 billion years. The two galaxies presently speed towards each other at 402,340km per hour. The collision will not be as catastrophic as one might imagine and, probably, few stars will be destroyed.

All the galaxies we can see from Earth rotate about their centres because of the gravitational pull of objects within the galaxy. They also move through space because the universe is expanding. The MW Galaxy disc is rotating at about 270km per second. This rotational force counteracts the inward pull of gravity. If galaxies didn’t rotate they would collapse inward and everything would merge into the black hole at the centre. It takes our solar system about 250 million years to rotate once around the MW Galaxy. Last time we were where we are now in our circumnavigation, dinosaurs had not yet arisen on Earth.

The stars near the edge of the MW Galaxy speed around the centre of the galaxy so fast they should fly off into space, but they don’t because a cosmic “glue” called “dark matter” holds everything together. Dark matter makes up more than 90 per cent of the mass of the MW Galaxy and about 27 per cent of the total matter in the universe, but its nature remains unknown. It is invisible and undetectable directly. We know of its existence only because of its gravitational effects on nearby visible objects. The even more mysterious entity of “dark energy” makes up about 68 per cent of the universe. Ordinary matter, the only type of matter science presently understands, makes up less than 5 per cent of the universe!

The European Space Agency launched a satellite called Gaia in 2013 to make a 3-D map of the MW Galaxy to better understand how the galaxy was formed. Using data from Gaia scientists calculated that the mass of the MW Galaxy is four to five times lower than previous estimates. Stars at the edge of MW galaxy were also seen to be rotating more slowly than expected, suggesting the galaxy is missing some dark matter.

Scientists don’t yet know why the MW Galaxy is so much lighter than expected. It seems something happened we don’t yet know about in the galaxy’s long history, such as a collision long ago with another galaxy.

William Reville is an emeritus professor of biochemistry at UCC