If you get a chance to go night camping at a place like Arches National Park in the US, Bodmin Moor in the UK, or one of the other certified International Dark Sky Parks, don’t you miss it, because at both of these sites you may witness one of the most spectacular sights in the night sky? These areas offer a unique opportunity (weather permitting) to view a hazy, milky streak across the sky that looks like a big cloud with bright light coming out of it. This milky band in the sky is the Milky Way Galaxy, home to billions of solar systems, including ours.
In fact, all the stars that we can see in the sky with the naked eye are part of the Milky Way. The name Milky Way is a translation of Latin via lactea, which is taken from the Greek term galaxias kyklos which literally means a milky circle. Spanning around 100,000 light-years in space, the Milky Way holds several mysteries of our universe within itself.
What is a galaxy?
Before going deep into the Milky Way, it is crucial to understand the concept of a galaxy. In the 1600s, French astronomer Charles Messier was the first to identify and catalog galaxies, but at that time he did not know what they were. Up until the 19th-century, galaxies were generally referred to as spiral-shaped nebulae. It was astronomer Edwin Hubble who first realized, in 1923, that the spiral nebula Andromeda is actually a galaxy, and that the Milky Way is just one of many galaxies in the universe.
Today it is well-known that galaxies are actually vast cosmic entities (and larger than nebulae) made up of millions and billions of stars, held together by gravity. Much of a galaxy is empty space with five light-years as the average distance between each star. Galaxies differ enormously in size, shape, and mass, but they tend to fall in some basic shapes, like a spiral, barred spiral, elliptical, etc. Most galaxies are billions of years old, although it is difficult to know their exact age. Also, almost every galaxy has a black hole at its center.
Understanding our galaxy, the Milky Way
The Milky way is a gigantic, barred spiral galaxy, which means it is flat for the most part and has several arms that spiral out of a central bar of stars. A very valid question is how we know its shape if we are inside the galaxy. Radio-astronomer Dr. Alastair Gunn explains that though we cannot see its spiral shape, we have enough clues that point towards this shape.
The first clue is that there is a high concentration of stars along a single plane which suggests a disc-like shape when seen from its edge. The concentration is even higher in the Sagittarius constellation making it look like the central bulge that other galaxies have. The second clue is that the galaxy’s stars follow a rotational motion pattern, similar to those seen in spiral galaxies.
The third and most convincing clue is that when the distances of these stars are measured, it’s revealed that they are clearly concentrated along the arms of a spiral. It is difficult to determine how many stars are there in the Milky Way, and researchers use different models to estimate this value. This involves either calculating the number of stars in a small patch and then extrapolating this, or estimating the mass of the galaxy and then estimating how many stars would be required to make up that much mass.
The answers differ based on what is assumed as the average mass of a star, but the number is generally in the range of 100 to 400 billion stars. Until Edwin Hubble announced in 1924 that the spiral nebula Andromeda is actually a galaxy, astronomers believed that the Milky Way encompasses the whole universe.
There are many minor galaxies near the Milky Way, but Andromeda (also called Messier31, or M31) is the closest large galaxy to the Milky Way. Just as our Sun has its planets revolving around it, both these galaxies have many satellite galaxies orbiting them. Both the galaxies and their satellite galaxies are part of a collection of galaxies called “The Local Group.”
The galaxies of the Local Group are all located within roughly 5 million light-years of space around us. The Local Group contains not only the Milky Way and Andromeda but the smaller Triangulum galaxy and 50 or so smaller, dwarf galaxies.
However, that is not the end of it as it was discovered that the Local Group is part of a giant supercluster of galaxies known as the Virgo Supercluster, which has at least 100 such galaxy groups and clusters and spans around 10 times the diameter of the Local Group.
Let’s look at some more interesting facts about the Milky Way :
The Milky Way is not actually flat
The Milky Way being like a flat disc is an oversimplification. Since the mid 20th century, researchers have known that the Milky Way is warped, similar to a Pringles potato chip, and subsequent research has shown that this galactic warping feature is common among other spiral galaxies.
Moreover, another research published in Nature suggests that this phenomenon of the Milky Way may be caused by satellite galaxies, namely to the large and small Magellanic clouds - two dwarf galaxies thought to be orbiting the Milky Way. They argue that these two dwarf galaxies may be pulling on the dark matter in our galaxy, creating a wake that enhances their gravitational influence on the disk and producing the warp.
Our Milky Way is a Cannibal
Mergers of galaxies are common, and research suggests that 5-25% of the galaxies are actually undergoing a merger at any given time. The Milky Way is the product of past mergers, and billions of years from now, the Milky Way will merge with the Andromeda Galaxy, eventually forming one big galaxy.
By studying the data received from European Space Agency’s Gaia (Global Astrometric Interferometer for Astrophysics) scientists have found that there are two distinct sets of stars in the Milky Way. One set is comprised of “redder stars” that are believed to have formed in a larger, metal-rich galaxy ('metal' in astronomical terms refers to any chemical elements heavier than hydrogen or helium), and the other set is of “bluer stars” that could have emerged from a smaller, metal-poor galaxy.
These findings suggest that the current Milky Way was formed when it consumed a smaller galaxy, referred to as Gaia-Enceladus. Even at present, the Milky Way is pulling stars from the Canis Major Dwarf and the Sagittarius Dwarf Spheroidal Galaxies, which are the closest and second closest galaxies to the Milky Way, respectively. Next on its plate are the large and small Magellanic clouds.
Our galaxy consists of mysterious cosmic bubbles
The Fermi Gamma-ray Space Telescope was launched into space in June 2008 to carry-out gamma-ray observations. In 2010, observations using the telescope helped uncover previously unknown giant spherical structures of gas and magnetic fields emerging from the center of the Milky Way. These structures stretched 25,000 light-years above and below the plane of the galaxy and were termed Fermi Bubbles.
A study published in the University of Chinese Academy of Sciences suggests that these bubbles may have formed from vast amounts of energy released from Sagittarius A*, the black hole at the center of the galaxy.
Astronomers suggest that our universe is 14 billion years old, whereas the Milky Way has been here for around 13.6 billion years, making it one of the oldest galaxies of our universe. Home to billions of stars and planets, the Milky Way is also always in motion, moving at a speed of approximately 130 miles per second. However, despite being so vast and holding so many mysteries, our understanding of its origins and makeup increases all the time.
