The Milky Way is the most easily recognisable feature of our night sky. Its band of far denser clouds of light spans the heavens. It is not so much that there are more stars in that part of the sky compared to the rest of the observable universe, just that we are looking through the disc of our own galaxy towards its centre some 25 000 light years away.
The Milky Way is a disc of stars, dust, gas and intergalactic medium some 2 000 light years thick, and has a diameter of between 150 000 light years, perhaps to as much as 200 000 light years.
The Milky Way began to form 13.5 billion years ago. It contains at least 100 billion stars (though some estimates suggest as high as 400 billion), and it is estimated that six billion of them must have planetary systems.
Like most galaxies the Milky way has a black hole at its centre of some 3 million solar masses. It is a “Supermassive” black hole 25 million kilometres across. It was discovered on 13th February 1974 as a very strong and compact radio source, which astronomers called Sagittarius A*. This feature is embedded in a bubble of hot ionised gas, with the possible remnants of a supernova. Sagittarius A* has no orbital motion and is considered to be the centre of our galaxy. However, it does rotate once every 11 minutes. Nearby stars orbit this chaotic centre at speeds of up to 4.8 million kilometres an hour.
The name of our galaxy is so called because of its dim “milky” band that glows across the night sky, as from our perspective, we are looking towards the centre of the galaxy along its plane. The name derives from Latin (via lactea – from where we get the word ‘lactose’, a sugar in milk). In turn the name is derived from the Greek for “milky circle”.
The Milky Way is a “barred spiral galaxy”. In terms of its visible mass it is made up of 97% stars and 3% gas.
It was not known until 1953 that the Milky Way galaxy had a spiral structure, and that more recently that it also had a bar in the centre.
It is shaped in a flat spiral, with curved arms that spin round the central point. Our Sun sits near the edge of one of these arms.
If we were able to view our galaxy from above it would look like a Catherine wheel, with several spiral arms trailing behind as it rotates. There are four of these arms, all stretching round within the main disc of the galaxy, trailing out a number of spurs. The brightest is the Carinus-Sagittarius Arm beyond which, from our point of view, lies the galactic nucleus. Sagittarius A* is the region of the black hole at the centre. The arms are named after the constellation in which they are most prominent. Opinion is divided as to how these arms should be defined or categorised. Indeed, astronomers cannot agree on the number of arms there are, nor their precise configuration. Because we are part of this galaxy it is very difficult to create an accurate diagram. From various sources the following is suggested:
Perseus and Scutum-Centaurus are the two major spiral arms emanating from the ends of the galactic bar. These arms contain a greater density of stars, young and old.
The minor arms are Sagittarius and Norma. They have and overabundance of gas and dust, and pockets of star-forming activity. They are less distinct and are located between the major arms.
The solar system is confined to an offshoot from the Sagittarius Arm, the Orion-Cygnus spur.
According to Nasa there is a new spiral arm called the “Far-3 kiloparsec arm,” (3KPC). It was discovered during a radio-telescope survey of gas in the Milky Way. This arm is shorter than the two major arms and lies along the bar of the galaxy.
That is the best I can do for the moment. In one sense you could see astronomy as the art of discrepancy!
|The spiral structure of the Milky Way galaxy|
|Norma and Outer Arm||Minor|
|The Orion-Cygnus spur||Spur|
|The 3KPC spur||Spur|
There are two forms of star clusters in the Milky Way: open clusters and globular clusters.
Open clusters contain relatively young stars (Population I) and reside in the spiral arms. They are so named because the individual stars are easily resolved through a telescope. They are sometimes called “galactic clusters” as they are usually located in the arms of spiral galaxies. There are about 20 000 open clusters in the galaxy.
The nearest open star cluster to Earth is Hyades at 153 light years distant. It is roughly spherical and contains hundreds of stars. The Pleiades is another notable example, at about 440 light years distant. It contains more than 1 000 confirmed stars. Finally, The Beehive, an open cluster in the Constellation of Cancer. Like all open clusters the stars are young—in this case only 600 million years old. It also contains about 1 000 stars. Distance to The Beehive is 520 to 610 light years.
Stars in an open cluster have formed from the same initial giant molecular cloud. These clusters typically contain a few hundred stars—though some may contain up to a few thousand. Stars within an open cluster are only loosely bound by gravity. As the cluster orbits around the galactic centre it eventually disperses due to gravitational perturbations within the galaxy.
Outside the main spiral there are about 200 globular clusters that reside in the halo of the Milky Way. These contain very old stars (Population II), and their grouping is far more stable. They also hold far more stars than open clusters, from several thousand to as much as one million. These clusters are spherically distributed about the galactic centre.
There are two good examples. Omega Centauri is the most luminous of its type in the galaxy. It is 16 000 light years away and is estimated to contain 10 million stars. The next brightest is 47 Tucanae, another globular cluster, and about 15 000 light years away.
Fairly modern research has suggested that about 25% of the clusters in the Milky Way are “foreign”. Indeed many stars, as well as clusters, have arrived over the last billion years, according to this theory.
The Milky Way is part of the “Local Group”, which is home to more than 54 galaxies. This group has a diameter of 10 million light years. The three largest members of the group are the Andromeda Galaxy (the biggest), the Milky Way, and then the Triangulum Galaxy, ranked at number three. The rest are dwarf satellite galaxies. Incidentally, the Milky Way “ate” one called the Sagittarius Dwarf. As a matter of fact most galaxies in the universe are smaller than the Milky Way.
The Large Magellanic Cloud and Small Magellanic Cloud are also in this galactic bubble. They are satellites of the Milky Way. The Large Magellanic Cloud is 166 000 light years distant from Earth, and a little further out is the Small Magellanic Cloud at 200 000 light years.
The solar system is travelling at a speed of 800 000 kilometres an hour on its orbit around the Milky Way galaxy. It takes 225 million years to complete. Since it was born the sun has only gone round 20 times.
Our Sun lives in the Orion Spur about 25 000 light years from the galactic centre.
The solar system is also being whisked through the universe as part of the Milky Way Galaxy at about 600 kilometres a second (or a bit over two million kilometres an hour).
The nearest galaxy is the Andromeda Galaxy, which is 2.5 million light years away. It is 220 000 light years in diameter. Like the Milky Way Andromeda is a barred spiral galaxy. It is home to an astounding one trillion stars. The Milky Way is travelling at 400 million kilometres an hour towards Andromeda and the two will collide in three billion years.
And it does happen! The Bullet Cluster is a well-known example where two galaxies collided some 150 million years ago. Gases account for a greater mass than the rest of matter in this galaxy formation. The hot gases have slowed by the drag effect of one cluster passing through the other, and they are now in separated globs. It is one of the hottest known cluster of galaxies. The stars of these two galaxies were not greatly affected by the collision, and most passed right through safe and sound.
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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.
Last updated: Sunday, 21 March 2021