How have galaxies like ours arrived?

On a clear day, when conditions are ideal and there is not much light to obscure the view, the starry sky is a breathtaking sight. If you live in a rural area or if you stop for a city break, you will see a sky covered with stars.

You might even be able to see a band of light running in the sky, which has foggy (or "milky") air in nature. Believe it or not, that's how our galaxy got its name. Thousands of years ago, astronomers observed the same band and observed the resemblance with the drink.

Over time, our understanding of the Milky Way has increased. Not only do we understand that the Milky Way is actually a massive collection of gravity-related stars, but we've also learned that it's only billions (or even billions) of billion) in the universe.

Astronomers and cosmologists have come to understand that the Universe was breathtaking, both in terms of time and space. And even though we still do not know until where the Universe expands (or it's infinite), we have a pretty good idea of ​​the duration of its existence (about 13, 8 billion years).

For this reason, astronomers have spent a lot of time and energy looking as far as they can – in space and time – to see the first galaxies. In doing so, they hope to learn how galaxies like ours have formed and evolved over billions of years.

What are galaxies?

In simple terms, galaxies consist of massive groups of stars, gas, and gravitational dust. However, all this is only part of the galaxies that we can detect because it emits, absorbs or emits light.

Beyond this, astronomers have theorized for decades that galaxies also include a lot of dark matter, so named because it is invisible with respect to conventional detection.

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

Astronomers have come to classify galaxies into three main categories. This classification system is known as the Hubble Sequence, named after the famous American astronomer Edwin Hubble.

Hubble's plan divided regular galaxies into three major classes –elliptical, lenticular and spiral galaxies – based on their visual appearance. A fourth class contains galaxies with a irregular appearance.

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

There is also lenticular galaxies. These consist of a bright central bulb surrounded by an extensive structure resembling a disk. Unlike spiral galaxies, the discs of lenticular galaxies have no visible spiral structure and do not actively form stars in large numbers. They include Messier 84 and the galaxy at the wheel.

The Hubble classification system also includes irregular galaxies. These are galaxies that do not match the Hubble sequence because they do not have a regular structure. Examples include the Magellan and M82 clouds.

Galaxies can also be classified according to their sizes, which range from a few hundred million stars (in the case of dwarf galaxies) to one hundred trillion stars (giant galaxies), each gravitating around the center. of his galaxy.

"Strong" and "calm" galaxies

Outside of this pattern, astronomers also distinguish galaxies that have what is called a galactic active nucleus (AGN) from those that are not. An AGN is a compact region in the center of a galaxy which has a lot higher than normal brightness. Much of the energy of AGNs is non-stellar and many of them are powerful transmitters of X-rays, radio and ultraviolet rays, as well as optical rays.

One theory is that the non-stellar radiation of an AGN is the result of the accumulation of matter by a a supermassive black hole (SMBH) in the center of his host galaxy. This causes dust, gases and even surrounding stars to fall into an accretion disk around the outer edge of the black hole (also called the event horizon). Over time, this material feeds slowly (accrues) on 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 huge amount of electromagnetic energy and radiation. This appears in the radio, microwave, infrared, optical, ultraviolet, X – ray and gamma wavelengths.

SMBHs are also known for their rotating magnetic fields, which interact with their accretion disks to create powerful magnetic jets. The material contained in these jets can reach a fraction of the speed of light (also called relativistic velocities), which allows them to reach hundreds of thousands of light-years away.

AGNs can be divided into two categories according to their streams: "radio-silent" and "radio-loud" nuclei. Radio-loud AGNs are those whose radio emissions are produced by their accretion disk and jets, while silent AGNs produce negligible jet-related emissions.

The Milky Way

As stated, the Milky Way is a spiral galaxy with a relatively inactive galactic core. According to the latest estimates, the Milky Way has a diameter of between 150 000 and 200 000 light-years and a thickness of 1000 light-years.

It is also estimated that it has between 100 and 400 billion stars and more than 100 billion planets. At its center, the central bulb is about 10 000 light years in diameter.

This is the central region of our Milky Way and is also "barred" – which means it contains a central bar-shaped structure of stars. The size of this bar is the subject of debate, with estimates ranging from 3,000 to 16,000 light-years.

The center of the Milky Way contains an intense radio source called Sagittarius A * (pronounced Sagittarius A-star). It is thought that this is a SMBH that represents more than 4 million times the mass of our sun.

From the center, several spiral arms contain billions of stars, as well as interstellar gas and dust. The exact number and configuration of these arms are debated and changed based on new information.

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

Our sun is located near a small partial arm called 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 – and not the result of direct observation.

It is an interesting fact about the observation of the galaxy: astronomers are able to determine the size, structure and shape of galaxies distant from several millions (or billions ) light years with more confidence than ours.

If the cosmos could be compared to a city and the solar system was our own backyard, we would feel that our own neighborhood would be more familiar to us than those located on the other side of the city. However, there is an advantage for this reason and it all depends on our point of view.

In simple terms, the solar system is tucked into the Milky Way disk, making it difficult to understand its true dimensions. It is also difficult to see what is on the other side of the galaxy because of interferences of light from the central bulb.

It has also been recently theorized that the Milky Way is actually distorted. Seen from the side, the spiral arms would look like an S-shaped folded disc.

To date, no robotic mission has been able to see the Milky Way from an external point of view. Hence the reason why any image of a galaxy as a whole does not correspond to the Milky Way or is an impression of the artist.

Where is the solar system?

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

It takes about 240 million years for the Sun to achieve a unique orbit in what is called a galactic year (or cosmic year). According to this calculation, the Sun has completed just over 19 orbits since its appearance about 4.6 billion years ago.

According to its spectra, our Sun is classified in the category of yellow dwarfs of type G, which makes it rather rare in terms of stellar population of our galaxy. In all, about ten percent of the stars in the Milky Way are yellow dwarfs, which gives about 20 to 40 billion stars similar to those of the Sun.

The study of galaxies

The study of galaxies goes back several millennia, even though astronomers were not quite aware of what they were observing until the modern era. Basically, it was only 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 was one of many.

The name "Milky Way", as it is applied to the central band of light in the night sky, actually has a long tradition. In ancient Rome, astronomers called it "Milky Way" "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 a concentrated star in a narrow band. For example, in the 13th century, The Persian astronomer Nasir al-Din al-Tusi provided the following description in his book, Tadhkira:

"The Milky Way, or galaxy, is made up of a very large number of closely grouped small stars, which, because of their concentration and small size, appear to be cloudy. For this reason, it has been compared to a milk color. "

In 1610, Galileo Galileo publishes 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 on the "nebulous" stars contained in the Ptolemaic catalog.

Galileo's observations showed that these objects were actually countless stars so far apart that they seemed to be clusters and could not be observed at the naked eye. Or, as Galileo described them, it was "countless stars grouped in clusters".

Just like Galileo's advocacy for the heliocentric model of the Universe (where the Sun revolves around the planets), this revelation has also shown that stars are actually far more distant from the Earth than one does. thought it before.

In 1775, the German philosopher Emmanuel Kant took a step further by proposing that the Milky Way be a vast set of stars held together by mutual gravity. He also hypothesized that the galaxy was arranged as the solar system, stars revolving around a common center and flattened into a disk.

In 1785, astronomer William Herschel attempted to map the structure of the Milky Way to reveal its true form. Unfortunately, his efforts were in vain because of the large amount of gas and dust that mask large amounts of particles.

Another interesting development at this time is the publication of the Messier catalog (1771 to 1781). This work was produced by the Dutch astronomer Charles Messier, who began recording "nebulous" objects that he had confused with comets.

At the time, the telescopes were not yet sophisticated enough to solve these objects, most of which were stellar clusters or distant galaxies. However, in the 19th century, astronomers like William Henry Smyth (also an admiral of the Royal Navy) were able to solve the problem of each of their stars.

In the 1920s, American astronomer Edwin Hubble finally brought evidence that the spiral nebulae observed in the sky were actually other galaxies. This discovery also led astronomers to conclude what was the true form of the Milky Way (ie a barred spiral galaxy).

It's also Hubble who has shown that most galaxies really distance us from ours. This led to the realization that the Universe was expanding. Its pace of expansion is called the Hubble Constant, in the honor of Hubble 's discovery.

This discovery would radically alter our perception of the universe and give rise to theories such as the Big Bang and Black Energy. With the beginning of the space age, our knowledge of the universe and galaxies is greatly increased.

Space telescopes, for example, are able to observe distant objects without atmospheric interference. Ground based observatories have also improved considerably thanks to improvements in instruments, methods and data sharing.

The first galaxies

According to the most widely accepted cosmological models, the first stars were formed while the universe only had 100 million years ago (about 13.7 billion years ago) . About a billion years after the Big Bag, these stars and other baryonic materials began to condense with dark matter halos to form the first galaxies.

Over the billions of years that followed, the densest regions of the Universe were gravitated to one another. This was called the era of structure when the large-scale structure of the universe began to form.

It was during this period that elements such as globular clusters, galactic bulbs, SMBHs and other cosmic structures would have formed. Stars, dust, and gas also fell into disc-shaped structures around central bulges, and more materials were added from intergalactic clouds and dwarf galaxies.

Many believe that the formation of SMBH has played a key role in regulating the growth of galaxies by limiting the amount of material added. They also influenced the rate of star formation, as galaxies experienced an explosion of star formation before they appeared.

When the first stars began to extinguish, it is theorized that they released heavier elements in the interstellar medium. As a result, subsequent generations of stars were becoming increasingly rich in metals, providing astronomers with an essential tool for producing age estimates.

Over time, this would have increased the abundance of heavy elements in the galaxies, which would have allowed the formation of planets and moons, while the remains of matter would become asteroids and comets forming belts around their stars.

How have they evolved since?

Through surveys by space telescopes such as Hubble and ground observatories like the Atacama Large Magnetic Table / Submillimeter (ALMA), astronomers have been able to see what galaxies looked like there are billions of dollars. ; years.

This, combined with more recent observations, has given astronomers a good idea of ​​the evolution of galaxies over time. For example, the first galaxies appeared to be elliptical in shape and smaller. Over time, galactic fusions led to the growth and complexity of galaxies.

Gradually, it is thought that the least amount of material accelerated their rotation. In the case of the Milky Way, many astronomers have come to believe that fusions with dwarf galaxies were quite common – and that this process is still ongoing.

In fact, the closest galaxy to ours is the Canis Major dwarf galaxy, located 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). Some onestronomers believe that the dwarf galaxy is being separated by the gravitational field of the galaxy of the Milky Way more massive. Disturbance of the tides leaves behind a long filament of stars in orbit around the Milky Way, forming a complex structure resembling a ring, sometimes called the Monoceros Ring, which surrounds our galaxy three times.

For nearly 9 billion years after the Big Bang, it is thought that the force of mutual gravitational attraction has predominated and that, as a result, the cosmos has developed very slowly. As a result, galactic mergers could have been very common over the first billion years after the Big Bang.

However, the expansion of the cosmos eventually resulted in galaxies becoming more distant from each other; It is then assumed that the influence of dark energy has begun to be felt.

Many think that this is what led to the era of cosmic acceleration (about 5 billion years ago), where the cosmos began to grow at a rate accelerated. At this point, galactic fusions have become a lot rarer, but the process is still known to happen … and it will happen to us!

The future of our galaxy and the cosmos

As Hubble has observed, the vast majority of neighboring galaxies are moving away from ours. However, two of them are heading towards us: the neighbors Andromeda (aka Messier 31) and Triangulum Galaxy (Messier 33).

According to current estimates, the galaxies of the Milky Way and Andromeda are approaching each other at a speed of about 130 km / s. At this rate, they will collide in about 4.5 billion years.

When that happens, they could form a giant elliptical or lenticular galaxy (nicknamed "Milkomeda" or "Milkdromeda ") The tidal disturbances caused by the meltdown could lead to the overthrow of some stars and the SMBH merger.

It is unclear what will be the impact on the solar system. However, it is theorized that our Sun will have exhausted its hydrogen fuel by then and will become a red giant – which will cause it to expand and engulf the Earth, and perhaps the entire system. solar.

It is assumed that these types of fusions become rarer as the cosmos continues to develop and galaxies are further and further apart. Eventually, the Universe galaxies will become darker and more red as the shorter stars start to disappear.

These include everything from blue giants and supergiants (types O and B) to blue-white (types A and F), to yellow and orange dwarf stars (types G and K). In the end, only the red dwarf stars of type M – which have the longest natural life span (up to 10,000 billion years) – will remain.

Eventually, the galaxies will become so far apart that not all the intelligent life forms of the Milky Way could see any other galaxy. It's the same for the inhabitants of any other galaxy, who looks up at the night sky and sees only faint red stars.

Over time, the galaxies themselves will die with the decline of the last stars and the darkness of the universe. Fortunately for us, this should not happen before billions of years. At this point, humanity will have disappeared or evolved far beyond anything that could be considered a human being.

Further reading: