How Did Galaxies Like Ours Come To Be?

Galaxies like the Milky Way have changed a lot over time, and will change a great deal more in the future.
Matthew S. Williams
360-degree panoramic image of the Milky Way.ESO

On a clear night, when the conditions are just right and there's not a lot of light to obscure the view, the starry sky is a breathtaking sight. If you live in a rural area, or are just taking a break from city-living, you'll be able to see a sky that is plump full of stars.

You might even be able to see a band of light running across the sky, one which look hazy (or "milky") in nature. Believe it or not, that is how our galaxy got its name. Thousands of years ago, astronomers looking up at the night sky noticed this same band and saw the resemblance to the beverage.

Over time, our understanding of the Milky Way grew. Not only did we realize that the Milky Way is actually a massive collection of stars held together by gravity, we learned that it is merely one of billions (or even trillions) in the Universe.

Eventually, astronomers and cosmologists came to understand that the Universe is breathtakingly vast, in terms of both time and space. And while we still don't know how far the Universe extends (or if it is in infinite), we have a pretty good idea of how long it has existed (roughly 13.8 billion years).

For this reason, astronomers have dedicated a great deal of time and energy in looking as far as they possibly can - through space and time - in order to see the earliest galaxies. By doing this, they hope to learn how galaxies like our own formed and evolved over the course of billions of years.

How Did Galaxies Like Ours Come To Be?
Spiral galaxy NGC 4414. Source: AURA/STScI/NASA

What are Galaxies?

Put simply, galaxies consist of massive groupings of gravitationally bound stars, gas, and dust. However, all of that is merely the part of galaxies that we can detect, because it either emits, absorbs or radiates light.

Beyond that, astronomers have theorized for decades that galaxies also include lots of dark matter, which is so-named because it is invisible as far as conventional detection is concerned.

The study of galaxies has led astronomer to group them based on their overall structure. While some galaxies conform to a basic shape, with a central "bulge" and "arms" extending out from the center in swirls, astronomers have noted different kinds of variations.

From this, astronomers have come to classify galaxies based on three main categories. This classification scheme is known as the Hubble Sequence, named after famed American astronomer Edwin Hubble.

Hubble's scheme divided regular galaxies into three broad classes –ellipticallenticular and spiral galaxies – based on their visual appearance. A fourth class contains galaxies with an irregular appearance.

First, there are spiral galaxies like the Milky Way, which are rich in gas and dust an still have stars forming in their arms. Then there are elliptical galaxies, which have relatively smooth, featureless light distributions. They are relatively devoid of gas and dust, have a low rate of star formation, and are so-named because they are more circular in structure.

There's also lenticular galaxies. These consist of a bright, central bulge surrounded by an extended, disk-like structure. Unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any great number. They include Messier 84 and the cartwheel galaxy.

How Did Galaxies Like Ours Come To Be?

The Hubble "tuning fork" Sequence. Source: NASA/ESA

Hubble's classification system also includes irregular galaxies. These are galaxies that do not fit into the Hubble sequence because they have no regular structure. Examples include the Magellanic Clouds and M82.

Galaxies can also be classified based on their sizes, which range from having a few hundred million stars (in the case of dwarf galaxies) to a hundred trillion stars (giant galaxies), each orbiting its galaxy's center.

"Loud" and "Quiet" Galaxies

Outside of this scheme, astronomers also differentiate between galaxies that have what is called an Active Galactic Nucleus (AGN) and those that don't. An AGN is a compact region at the center of a galaxy that has a much higher than normal luminosity. Much of the energy output of AGNs is non-stellar, and many AGN are strong emitters of X-rays, radio and ultraviolet radiation, as well as optical radiation. 

One theory is that the non-stellar radiation from an AGN is the result of the accretion of matter by a supermassive black hole (SMBH) at the center of its host galaxy. This causes the surrounding dust, gas and even stars to fall into an accretion disk around the black hole's outer edge (aka. event horizon). Over time, this matter is slowly fed (accreted) onto the face of the black hole.

The powerful gravity of the black hole causes the material to accelerate to the point where it begins to emit a tremendous amount of electromagnetic energy and radiation. This appears in the radio, microwave, infrared, optical, ultra-violet, x-rays and gamma-ray wavelengths.

SMBHs are also known for their rotating magnetic fields, which interacts with their accretion disks to create powerful magnetic jets. The material in these jets can reach a fraction of the speed of light (aka. relativistic speeds), which makes them able to reach hundreds of thousands of light-years in distance.

AGNs can be further divided into one of two categories based on their jets - “radio-quiet” and “radio-loud” nuclei. Radio-loud AGNs are those that have radio emissions produced by their accretion disk and jets while radio-quiet AGNs show negligible jet-related emissions.

How Did Galaxies Like Ours Come To Be?

Active galaxy M87 and its relativistic jet. Source: NASA/Hubble Heritage Team (STScI/AURA)

The Milky Way

As noted, the Milky Way is a spiral galaxy with a relatively inactive galactic nucleus. According to the latest estimates, the Milky Way is thought to measure between 150,000 and 200,000 light-years in diameter and 1000 light-years thick.

It is also estimated to be populated by between 100 and 400 billion stars, and more than 100 billion planets. At its center, measuring about 10,000 light-years in diameter is the central bulge.

This constitutes the core region of our Milky Way and is also "barred" - meaning that it contains a central, bar-shaped structure composed of stars. The size of this bar is the subject of debate, with estimates ranges from 3,000 to 16,000 light-years across.

The center of the Milky Way contains an intense radio source known as Sagittarius A*  (pronounced Sagittarius A-star). This is thought to be an SMBH that is more than 4 million times the mass of our Sun.

Extending from the center are several spiral arms containing billions of stars and interstellar gas and dust. The exact number and configuration of these arms is the subject of some debate, and changes depending on new information. 

Recent observations have revealed that there may be four main spiral arms - the Scutum–Centaurus Arm, the Carina-Sagittarius arm, the Norma and Outer Arm, and the Far-3 kiloparsec and Perseus arm. However, there are sometimes said to be only two major arms, the Scotum-Centaurus and Perseus, with the rest being minor. 

Our Sun lies near a small, partial arm called the Orion Arm, or Orion Spur (or Orion-Cygnus arm).

The existence of these arms was determined by observing parts of the Milky Way and other galaxies - not the result of direct observation.

This is an interesting fact about observing the galaxy: astronomers are actually able to determine the size, structure, and shape of galaxies that are millions (or billions) of light-years away with greater confidence than they are our own.

If the cosmos could be likened to a city, and the Solar System our own backyard, one would get the impression that our own neighborhood would be more familiar to us than those located on the other side of town. However, there is a good for this reason and it all comes down to our point of view.

Put simply, the Solar System is nestled into the disk of the Milky Way, which makes being able to get a sense of its true dimensions rather difficult. It is also difficult to see what's on the other side of the galaxy because of light interference from the central bulge.

It has also recently been theorized that the Milky Way is actually warped in shape. If viewed from the side, the spiral arms would resemble a record bent into an S shape.

To date, no robotic missions have been able to see the Milky Way from an external point of view. Hence why any image you see of a galaxy as a whole is either not the Milky Way, or is an artist's impression.

Where is the Solar System?

Our Sun is located in the Orion Arm of the Milky Way, a region of space that is between two major arms of our galaxy. It is situated about 27,000 light-years from the center of the galaxy and orbits around it with the rest of the stars in the disk.

The Sun takes about 240 million years to complete a single orbit in what is known as a galactic year (or cosmic year). By this reckoning, the Sun has completed just over 19 orbits since it formed about 4.6 billion years ago.

How Did Galaxies Like Ours Come To Be?
 The location of the Sun in the Milky Way Galaxy. Source: NASA/JPL-Caltech

Based on its spectra, our Sun is classified as a G-type yellow dwarf, which makes it somewhat uncommon in terms of our galaxy's stellar population. All told, roughly ten percent of the stars in the Milky Way Galaxy are yellow dwarfs, which works out to about 20 to 40 billion Sun-like stars.

The Study of Galaxies

The study of galaxies goes back several millennia, though astronomers were not entirely aware of what they were observing until the modern era. Basically, it was not until the 17th century that the true nature of our galaxy was understood, and it was not until the 19th century that scientists understood that our galaxy is one of many.

The name "Milky Way", as applied to the central band of light in the night sky, is actually very time-honored. In ancient Rome, astronomers called it "Via Lactea" (lit. "Milky Way" in Latin) which was a translation of the Greek word for "milky circle" ("galaxías kýklos", γαλαξίας κύκλος).

Over time, astronomers began to speculate that the Milky Way was actually stars concentrated in a tight band. For example, in the 13th century, Persian astronomer Nasir al-Din al-Tusi provided the following description in his book, Tadhkira:

“The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color.”

In 1610, Galileo Galilee released his seminal work Sidereus Nuncius ("The Starry Messenger" in Latin), which contained his descriptions of the Moon, the Sun, and Jupiter. He also recorded his observations of "nebulous" stars that were contained in the Ptolemaic catalog.

Galileo's observations showed that these objects were actually countless stars that were so distant that they appeared to be clustered and could not be observed with the naked eye. Or as Galileo described them, they were "congeries of innumerable stars grouped together in clusters".

Much like Galileo's advocacy of the heliocentric model of the Universe (where the Sun is orbited by the planets), this revelation further demonstrated that the stars are actually much farther from Earth than previously thought.

By 1775, German philosopher Immanuel Kant took things a step further by proposing that the Milky Way was a large collection of stars held together by mutual gravity. He also hypothesized that the galaxy was laid out like the Solar System, with the stars rotating around a common center and flattened out in a disk.

In 1785, astronomer William Herschel attempted to map out the structure of the Milky Way to reveal its true shape. Unfortunately, his efforts came up short because of how large portions are obscured by gas and dust.

Another interesting development during this time was the publication of the Messier Catalog (1771 to 1781). This work was produced by Dutch astronomer Charles Messier, who began keeping records of "nebulous" objects he originally mistook for comets.

At the time, telescopes were not yet sophisticated enough to resolve these objects - most of which were stellar clusters or distant galaxies. However, by the 19th century, astronomers like William Henry Smyth (also an Admiral with the Royal Navy) were able to resolve individual stars in them.

How Did Galaxies Like Ours Come To Be?
The arrow of time, visualized. Source: NASA

By the 1920s, American astronomer Edwin Hubble finally provided evidence that spiral nebulae observed in the sky were actually other galaxies. This discovery also led astronomers to conclude what the Milky Way's true shape is (i.e. a barred, spiral galaxy).

It was also Hubble who demonstrated that most galaxies are actually moving away from our own. This led to the realization that the Universe is in a state of expansion. The rate at which it is expanding is known as the Hubble Constant, in honor of Hubble's discovery.

This find would dramatically alter our perception of the Universe and give rise to theories like the Big Bang and Dark Energy. With the beginning of the Space Age, our knowledge of the Universe and the galaxies has grown considerably.

Space telescopes, for example, are capable of observing distant objects free of atmospheric interference. Ground-based observatories have also improved considerably as a result of improvements in instruments, methods, and data-sharing.

The First Galaxies

According to the most widely-accepted cosmological models, the first stars formed when the Universe was just 100 million years old (ca. 13.7 billion years ago). By roughly 1 billion years after the Big Bag, these stars and other baryonic matter began to condense with dark matter halos to form the first galaxies.

Over the next few billion years, the denser regions of the Universe became gravitationally attracted to each other. This was known as the Structure Epoch when the large-scale structure of the Universe began to form.

How Did Galaxies Like Ours Come To Be?
The distance of galaxies in the Sloan Digital Sky Survey. Source: SDSS

It was during this period that things like globular clusters, galactic bulges, SMBHs, and other cosmic structures are thought to have formed. Stars, dust, and gas also fell into disk-shaped structures around the central bulges, and more material was added from intergalactic clouds and dwarf galaxies.

The formation of SMBHs is thought by many to have played a key role in regulating the growth of galaxies by limiting the amount of matter added. They also influenced the rate of star formation, since galaxies experienced a burst of star formation prior to their appearance.

As the earliest stars began to die out, it is theorized that they released heavier elements into the interstellar medium. Because of this, subsequent generations of stars were increasingly metal-rich, which provides astronomers with a vital tool in producing age estimates.

Over time, this increased the abundance of heavy elements in galaxies is thought to have allowed for the formation of planets and moons, while the leftover matter became asteroids and comets that formed into belts around their stars.

How Have They Evolved Since?

Thanks to surveys performed by space telescopes like Hubble and ground-based observatories like the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have been able to see what galaxies looked like billions of years ago.

This, combined with more-recent observations, has given astronomers a good idea of how galaxies have changed over time. For example, the earliest galaxies appeared to be elliptical in shape, and smaller. Over time, galactic mergers caused galaxies to grow and become more complex.

Gradually, the infall of material is thought to have caused their rotation to speed up. In the case of the Milky Way Galaxy, many astronomers have come to think that mergers with dwarf galaxies were quite common - and is a process that is still ongoing.

In fact, the closest galaxy to our own is the Canis Major dwarf galaxy, which lies at a distance of about 25,000 light-years from our Solar System and 42,000 light-years from the center of the Milky Way. Until recently, astronomers were unaware of its existence, because it was obscured by cosmic dust.

However, in 2003, an international team of astronomers detected it as part of the Two Micron All Sky Survey (2MASS) infrared survey. Some astronomers believe that the dwarf galaxy is in the process of being pulled apart by the gravitational field of the more massive Milky Way galaxy. Tidal disruption causes a long filament of stars to trail behind it as it orbits the Milky Way, forming a complex ringlike structure sometimes referred to as the Monoceros Ring, which wraps around our galaxy three times. 

For almost 9 billion years after the Big Bang, it is thought that the force of mutual gravitational attraction predominated and as a consequence, the cosmos expanded very slowly. As a result, galactic mergers may have been very common during the first few billion years after the Big Bang.

However, the expansion of the cosmos eventually resulted in galaxies becoming spaced farther apart; at which point it is hypothesized that the influence of Dark Energy began to be felt.

This is thought by many to be what led to the Cosmic Acceleration Epoch (ca. 5 billion years ago), where the cosmos began to expand at an accelerating rate. At this point, galactic mergers became much rarer, but the process is still known to happen... and will happen to us!

The Future of our Galaxy and the Cosmos

As Hubble observed, the vast majority of neighboring galaxies are moving away from our own. However, there are two that are moving towards us: the neighboring Andromeda (aka. Messier 31) and Triangulum Galaxy (Messier 33).

Based on current estimates, the Milky Way and Andromeda galaxies are moving toward each other at a velocity of about 130 km/s. At this rate, they will collide with each other in about 4.5 billion years.

When this happens, they could form a giant elliptical or lenticular galaxy (nicknamed "Milkomeda" or "Milkdromeda"). Tidal disruptions caused by the merger could cause some stars to be kicked out and a merging of SMBHs.

It is unknown how this will impact on the Solar System. However, it is theorized that our Sun will have exhausted its hydrogen fuel by then and become a red giant - which will result in it expanding and swallowing up the Earth, and maybe the entire Solar System.

These types of mergers are hypothesized to become rarer as the cosmos continues to expand and galaxies are pushed further and further apart. Eventually, the galaxies of the Universe will become darker and redder as shorter-lived stars begin to die out.

These include everything from blue giants and supergiants (O-type and B-type) to blue-white (A-type and F-type), yellow and orange dwarf (G-type and K-type) stars. Eventually, only M-type red dwarf stars - which have the longest natural lifespan (up to 10 trillion years) - will remain.

Eventually, the galaxies will become so far apart that any intelligent life forms in the Milky Way would not be able to see any other galaxies. The same holds true for the residents of any other galaxy, which will look up at the night sky and see only faint red stars.

In time, the galaxies themselves will die as the last stars decay and the entire Universe goes dark. Luckily for us, that is not expected to happen for trillions of years. At that point, humanity will either have gone extinct or will have evolved well-beyond anything that could be considered human.

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