MSc Physics , 1st semester
The Universe is all of time and space and its contents. The Universe includes planets, stars, galaxies, the contents of intergalactic space, the smallest subatomic particles, and all matter and energy. The observable universe is about 91 billion light-years in diameter at the present time. The size of the whole Universe is not known and may be infinite.
But where are we in the place in the universe? Our Earth feels like all there is, but we know that it’s just a tiny planet in a vast Solar System. And our Solar System is just one member of a vast Milky Way galaxy with 200 to 400 billion stars. The current estimation is that there are 100 to 200 billion galaxies in the Universe, each of which has hundreds of billions of stars.
The Milky Way galaxy is one of a few dozen galaxies known as the Local Group. Astronomers have discovered that our Local Group is on the outskirts of a giant cluster of several thousand galaxies – which astronomers call the Virgo Cluster. We also know of an irregular super cluster of galaxies, which contains the Virgo Cluster, which in turn contains our Local Group, which in turn contains our Milky Way galaxy and the nearby and Andromeda galaxy. At least 100 galaxy groups and clusters are located within this Virgo Supercluster. Its diameter is thought to be about 110 million light-years. Yet the Virgo Supercluster is one of millions of superclusters in the observable universe.
Now just focus on our position on Milky Way.
The Milky Way is a barred spiral galaxy that measures about 100,000 light-years across, and is thought to contain between 100 and 400 billion stars. The Solar System (and Earth) is located about 25,000 light-years to the galactic center and 25,000 light-years away from the rim. So, imagine a dinner plate as the Milky Way, and draw an imaginary line from the center of the plate to the outside. We’re located about halfway along that line.
Astronomers have recently agreed that the Milky Way probably has two major spiral arms – Perseus arm and the Scutum-Centaurus arm – with several smaller arms and spurs. The Solar System is located in a region in between the two arms called the Orion-Cygnus arm. This arm measures 3,500 light-years across and is 10,000 light-years in length, where it breaks off from the Sagittarius Arm.
You might be surprised to learn that it takes the Sun 250 million years to complete one rotation around the Milky Way. The last time the Solar System was in this position in the Milky Way, there were dinosaurs on Earth.