May 16, 20199 min read

The tale of galaxies

Updated: Jan 24, 2020

The barred spiral galaxy NGC 1672/ NASA, ESA

Light has got amazing properties. But none so prominent as its speed. But to say that light travels fast is probably a mockery of its speed. Trains and rockets travel fast. The speed of light is in a league of its own. It’s so bafflingly quick that in a second, it can travel around the earth about eight times! But the cosmic scales are so big even for something as fast as light. For instance, when a photon leaves the surface of the Sun, it takes just about 8 minutes for it to crash into our eyes. That might sound quick, but if you could pack your bags and go to Neptune[1], the same photon will take about 4.1 hours to approach your eyes. Not bad, but once a beam of light leaves the solar system, even it starts to feel slow. The hours turn into months, and months turn into years.

Light-year is the distance traveled by light in a year, and it is what’s used when we talk about astronomical distances. The light from Sirius[2], the brightest star in the night sky, has to travel about 8.6 light-years to get here, which means that the light that falls into your eyes when you spot Sirius is about eight and a half years old. But when we scale up from a few stars to a billion, and then some, a spiral structure begins to take shape. When we zoom out enough, we see our galactic home, the Milky Way, and when we talk about galaxies, the speed of light suddenly becomes inconsequential.

The Sun and Earth are separated by 149 million kilometers [3] of space, which might sound like a lot. But this is nothing compared to the size of a galaxy! The end to end size of the Milky Way is about 6.3 billion times the distance between Earth and the Sun, and it takes light, a staggering, 100,000 years to reach from one end to the other.

Distance between Earth and the Sun, otherwise referred to as the Astronomical unit — Distance between Earth and the Sun, referred to as the Astronomical unit

But the size of the Milky way could possibly be even larger than the commonly used size. A ring of stars[4] exists outside the plane of the Milky way and, if we were to include this filament as a part of the galaxy, the diameter would swell up to about 150,000-180,000 light-years! But galaxies can get bigger. In fact, in the galactic neighborhood, our galaxy is trumped by our sister galaxy, Andromeda[5], and on a broader scale, at a distance of about 1 billion light-years from us, lies one of the largest galaxies ever detected. IC 1101[6], a supergiant galaxy is estimated to be about 2 million light-years in diameter!

How do galaxies form?

This question stems back to a time when the universe was a very different place from what it is today. The infancy of the universe is characterized by the creation of matter from a hot soup of particles[7]. And it is widely believed that it is this matter which clumped together over time under gravity to produce stars, galaxies, and clusters. The firstly emerged stars after the big bang, supposedly went supernovae, emitting clouds of gas and dust into the universe. Some of these massive stars could have turned into black holes that accreted mass from its surroundings to form even more massive supermassive black holes.

Supermassive black holes are called that for a reason, and some of them even consist of billions of times[8] the mass of our sun! The supermassive black holes attract matter, and the accreted matter orbits the event horizon of the black hole at incredible speeds, generating immense friction, lighting up the structures, to produce what are known as quasars.

Artist’s impression of distant quasar; Credit: ESA/Hubble, NASA, M. Kornmesser

Quasars[9] are easily the most luminous objects in the universe, and they are also thought to be the active galactic nuclei[10] of active galaxies, galaxies that emit a substantial amount of energy than normal galaxies. But soon, the energy released by quasars becomes so large that it expels enough gas and matter out, thereby ceasing star formation and black hole growth. It was estimated that supermassive blackholes still exist in the center of most galaxies. In 2019, the event horizon telescope[11] validated these predictions when it found a supermassive black hole in the heart of a galaxy known as Messier 87[12], which was the first black hole to ever been photographed.

Galactic mysteries

Just like how the orbital velocities of planets reduce the further away you go from the sun, the objects which are farther away from the center of the galaxy too, should demonstrate a similar pattern. But when Vera Rubin[13] was studying the orbital speeds of stars in the outer extents of galaxies, the results she found refused to adhere to this understanding. Though the farthest regions of the galactic spirals were less crowded than the regions near to the center, there, the stars were moving at a constant speed contrary to what was expected from Newton’s laws. This meant that there was something else causing the gravity of the galaxy to be constant throughout its disk. Something that our telescopes could not detect. Something dark and invisible. The mysterious entity was coined dark matter[14], a name suggested in 1933 by Fritz Zwicky[15], a Swiss astronomer studying a similar behavior in galaxy clusters.

Dark matter exists as clumps within a dark halo[16], which is a basic unit of cosmological structures like galaxies and galaxy clusters, according to our current cosmological models. Scientists are still unraveling the nature of dark matter, but are almost certain about its existence in the universe. In fact, according to the Lambda Cold Dark Matter Cosmological Model[17], a widely accepted model in cosmology, there is about 5 times more dark matter in the universe than ordinary matter. Some theorists suggests that it is the interactions of dark matter with the baryonic matter, within dark matter halos, which forms galaxies.

Galaxy structure

The galaxies in the universe do not look the same. Our home galaxy contains spirals, which is synonymous to Andromeda as well. But galaxies such as Messier 87 look ellipsoidal[18], and featureless when compared to the Milky Way. The shapeless structures like the large and small Magellanic clouds which hover around our galaxy are also considered as galaxies. The tuning fork diagram, otherwise known as the Hubble sequence,[19] invented by Edwin Hubble, is a widely used classification for galaxies. Depending on the visual factors such as shape, bulge, and spirals, the galaxies are categorized into spiral galaxies, elliptical galaxies, and irregular galaxies. Milky Way and Andromeda, and numerous other galaxies are spiral galaxies, and it is arguably the most common type of galaxy there exists.

Why do they get so big?

We now know that there are different types of galaxies floating around in the depths of space, and in an expanding universe, these structures are supposed to be moving away from each other in line with the Hubble flow[20]. But this is not what the observations tell us. They tend to move closer to one another in a peculiar way, even in superclusters. This is thought to be the result of gravitational fluctuations in space. Gravity acts as the common denominator which overcomes the accelerated expansion of the universe.

When galaxies attract one other, they are bound to collide, and when large galaxies collide, the collisions happen on such a magnificent scale that giant interstellar clouds accrete, forming star factories. The merging[21] of galaxies is thought to produce thousands of solar masses of new stars in a single year! For perspective, the milky way has a star formation rate of about 3 solar masses per year, which means that it produces just about 3 ‘sun-like’ stars in a year[22].

Two galaxies in the process of merging to form one giant galaxy/ Credit: NASA, Holland Ford (JHU), the ACS Science Team and ESA

Certain theories suggest that most of the large galaxies grew in size by completely acquiring dwarf galaxies, and possibly even through mergers as well. While it is certain the Milky way had undergone such processes in a bygone era, it is still evolving, being attracted to other galaxies on different scales. In fact, all galaxies in clusters and superclusters are moving towards the Norma[23] cluster, to a mysterious region in space, hidden behind the plane of our galaxy. Known as the great attractor[24], this region is thought to be a collection of dense galaxy clusters whose combined mutual gravity is tugging on everything else around them. The region could even be the core of the great Laniakea supercluster[25], a super-structure which hosts about 100,000 galaxies, and spans around 520 million light-years!

What of the Milky way?

What indeed? What will happen to it, if it too is part of this great attraction? While it is unclear what will happen on a very distant cosmic timescale, it is almost certain that within 3-5 billion years[26], probably long after life had gone from earth, but still short in the lifespans of galactic events, the Milky way will coalesce with Andromeda in an incredible galactic collision. The local group[27] consists of over 30 galaxies, whose center lies somewhere in between Andromeda and the Milky way, the two massive galaxies in the group.

Andromeda is the farthest object visible in the night sky, and when viewed with the means of a telescope, it appears blue. This is because Andromeda is heading towards us at a rate[28] of 110 km/s, which is fast enough to overcome the overall expansion of the universe, and its light is thus blue shifted. In about 4 billion years, the night sky on earth will play host to a magnificent light display, as the galaxies pass through one another. During this time, vast interstellar clouds from both galaxies will collide to form stars at a rapid rate. About 6 billion years from now, the galaxies will fall into one another, this time merging to form one giant elliptical galaxy[29]. Since the size of stars is small compared to the space between them, it is likely that no stars or planets will collide with one other during the galactic merger.

Series of illustrations showing how the night sky on Earth will change as Milky Way and Andromeda slowly merges to form one/ (Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas, and A. Mellinger)

But this collision might lead to something even more spectacular. When 'Milkdomeda' forms, the supermassive black holes in the center of both galaxies could merge to become an even larger one. Large scale mergers such as this are thought to trigger[30] the supermassive black holes, kick-starting the galactic center of the newly formed galaxy. The supermassive black hole, as if awoken from a long slumber, will start to accrete matter towards it, and this could even result in a quasar in earth’s backyard! Scientists believe that most large galaxies like the Milky Way could have been an active galaxy in the virgin universe. But as time passes, the quasar too shall dissipate, and the silence will ensue.

By the time this event takes place, Earth will be barren and devoid of life. But it might yet be alive to see the beauty of our new galaxy, only if it doesn't get vaporized by the enraging sun. But as for us, who knows, maybe we will be watching the view too, from a different planet, perhaps from our new home.