Everyone knows our solar system is a part of the milky way galaxy, have you wondered what the milky way galaxy is or how it formed? In this post, we dive into the depths of our home galaxy and get to know it better. Before that, let us answer a few simple questions!
What is a galaxy and how they form?
What is a galaxy?
A galaxy is this magnificent arrangement of gas, dust, stars, remnants from dead stars, interstellar medium and dark matter (Yes! Dark matter is real) all bound together by gravity. All this matter rotates around the galactic centre which may or may not contain a supermassive black hole. Our solar system is a part of the galaxy we call the milky way.
We derive the word galaxy from the Greek word 'galaxias', it means 'milky'.
The ancient Greeks looked up the night sky and discerned the obscure, hazy band of light as the milky way. In 1610, this view changed, when Galileo Galilei viewed the cosmos through his telescope lens. He was the first to discern the hazy band across the night sky as individual stars. For many years from there, until the 1920s, People believed that the milky way was the entire universe; they thought there was nothing beyond that. But modern science has taken us far beyond the naïve understandings of the universe.
What we now know is astounding compared to the ancient Greek's knowledge of the cosmos.
We now know the milky way is a galaxy that contains a hundred thousand billion stars with their own planetary systems. We have learned that the Milky Way is a spiral galaxy with an estimated visible diameter of almost 1.9 million light-years. It is a part of the local group called the Virgo supercluster among 700 similar galaxies. And it is one of the 100 billion such galaxies in the observable universe.
We now know we are not even microbial specks in the grand scale of the universe, yet we contemplate the wonders of the cosmos. Take a moment to relish this fact, and we shall learn about all the types of galaxies that exist in the Universe.
What are the different types of galaxies?
We classify galaxies based on their size, shape and structure. Dwarf galaxies are the most common type, they contain around a few million stars. Dwarves are the most commonly occurring type. Giant galaxies are less common and can possess more than a trillion stars.
We also classify galaxies based on their shape and appearance, the main types are elliptical, spiral and irregular.
One-third the galaxies in the visible universe are elliptical; they can appear almost circular or very elongated. These galaxies have little interstellar material, and therefore less active. Scientists suspect them to have formed because of the collision of two are more galaxies.
A shell galaxy is a type of elliptical galaxy where the stars arrange themselves in concentric shells. We see Shell structures only in elliptical galaxies but never in spiral galaxies. The shell-like arrangements form when a larger one swallows its smaller companion galaxy.
Spiral galaxies appear flat even though in reality they look like a sunny side up egg. The centre has a bulge of stars, gas and dust. A band of stars run through this bulging centre of some spiral galaxies, we call these the barred spiral galaxy; the milky way belongs to this type. Spiral galaxies contain a lot of interstellar material hence actively form stars.
Irregular galaxies are neither elliptical nor spiral. They have noticeably less interstellar medium and are less active. These are the earliest galaxies to have appeared in the universe.
How galaxies form?
Things in the universe take time (a LOT of time!) to happen, it is hard for us to witness anything forming from its initial state, within the human lifetime.
Even medium-sized stars like the sun take tens of millions of years to form; colliding galaxies take billions of years to merge. Despite all this, we have a pretty clear understanding of how the universe works. We can look out and see different versions of similar events happening in various places. All this is possible because light takes time to get here. We simply have to look out to see the history of the universe written all over it.
We know that from the big bang; the universe started at a remarkably orderly, homogenous state. The early universe had mostly large clumps of dark matter. Even though we have made a ton of observations over the past few decades, scientists haven't concluded the exact mechanism that explains galaxy formation. But they put forward two not so different approaches to deal with this.
The ELS theory put forward by Olin Eggen, Donald Lyden Bell and Allan Sandage suggests that the galaxies form when a gigantic cloud of gas collapses. This is the top-down approach.
The other theory suggests that larger clouds collapse into smaller clouds and these smaller gas clouds develop into individual protogalaxies. Many protogalaxies merge to form the larger galaxies we see today. This is the bottom-up approach to galaxy formation.
There is also a third possibility which we will discuss next.
The below visualization shows small galaxies forming, interacting, and merging to form Milky Way-type galaxies with spiral arms.
Credit: NASA/Goddard Space Flight Center and the Advanced Visualization Laboratory at the National Center for Supercomputing and B. O'Shea, M. Norman
Why is there a black hole at the centre of every galaxy?
Whether galaxies came together as gas and then began forming stars or whether stars formed from little gas clumps and then assembled into galaxies is uncertain. But there is another possibility we need to consider.
We know that black holes result from stellar death. When stars with core mass greater than 8 solar masses, collapse under their own gravity, resulting in black holes. These are the most common types of black holes whose mechanism most scientists agree upon. Scientists believe that there are ten million to a billion stellar black holes in the Milky Way alone. But these are not the only black holes found in the universe.
Apparently, we have supermassive black holes, which are millions to billions, of times as massive as the sun. Supermassive black holes lie at the centre of almost every galaxy including our own Milky Way.
We believe the formation of supermassive black holes involves a chain reaction of collisions of stars in dense star clusters that results in the evolution of immensely massive stars, which then collapse to form intermediate-mass black holes. The star clusters nearby then sink into the galactic centre where the intermediate-mass black holes fuse to form a supermassive black hole.
Another possibility is that black holes could have appeared before the galaxies formed. The black holes that emerged before the galaxies could have swept up material all around forming galaxies we see today. And galaxies formed from the surrounding gas that did not get sucked in by the supermassive black holes.
The centre of the Milky Way galaxy, with the supermassive black hole Sagittarius A* (Sgr A*) located in the middle, is revealed in this image.
(Credit X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI)
What are satellite galaxies?
In the universe, not only planets possess their own satellites, but galaxies have them too. Galaxy's satellites are not boring old rocks but they are entire galaxies themselves.
Not only billions of stars in a galaxy orbit the galactic centre, sometimes dwarf galaxies of lesser mass go around the giant galaxies; astronomers call these satellite galaxies. A satellite galaxy is a smaller partner galaxy that travels on bound orbits within the gravitational range of a more massive primary galaxy. Satellite galaxies are bound to their host or primary galaxy, much like the moon is gravitationally bound to Earth. About fifty satellite galaxies orbit the milky way galaxy, the Large Magellanic Cloud being the hugest among them.
Below is an animation illustrating the discovery history of satellite galaxies of the Milky Way over the last 100 years. The classical satellite galaxies are in blue and more recent discoveries are in green.
What happens when galaxies collide?
Galactic collisions are quite common in our universe and happen all the time. In this sense, the word collision does not take the literal meaning but refers to the gravitational interaction between two or more galaxies.
This collision may have different results depending on the size of the colliding galaxies.
When larger galaxies interact with their satellite galaxies, they might become locked with one of its spiral arms.
If two galaxies collide and do not have enough momentum to continue travelling after the collision, they merge and become one galaxy; they plunge back into each other and ultimately merge into one larger galaxy.
If one of the colliding galaxies is much larger than the other, the bigger one will look the same, while the smaller galax gets shredded apart and becomes part of the larger one.
Astronomers have predicted the Milky Way Galaxy will collide with its galactic neighbour the Andromeda Galaxy in about 4.5 billion years. The two spiral galaxies will ultimately unite to grow into an elliptical or possibly a massive disk-like galaxy.
The distance between stars would make it improbable that any of them will individually collide, but the collision will eject some stars from the resulting galaxy, nicknamed Milkomeda or Milkdromeda.
How do galaxies die?
We know galaxies are nothing but a vast collection of stars and star systems. Does a galaxy die when all its stars die? Not exactly, but we consider a galaxy dead when it runs out of fuel to produce new stars. Most elliptical galaxies have burnt out their star-making fuel and are prominent examples of fossil galaxies.
Elliptical galaxies burnt up all their reserves of star-forming gas, and all that is left are the longer-lasting stars known as red giants. Eventually, over extensive lengths of time, those stars will blink out one after the other, and the whole thing reaches the background temperature of the universe.
As long as galaxies have gas for star formation, they will keep on flourishing. Once the fuel runs out or a dramatic merger uses all their star-forming gas, they are gone!