Understanding the process of galaxy formation has been one of the most important research areas in astronomy since its inception. Many researchers have dedicated their entire careers to this specific topic.
As you may be imagining, a number of scenarios have been theorized about the formation of galaxies and stars, but only recently have scientists begun to agree on some provable facts.
The advances of recent years, achieved by Big Bang theorists , have shed new light on how, when and why the first galaxies in our universe originated.
In short, knowing the origin of our galaxy (the one we can observe most easily) is an essential point to begin to understand the universal dynamics, which have generated so many conflicting opinions throughout our history.
If you find this topic interesting, you'll surely enjoy our article on the origin of the universe and the Big Bang theory.
When was the formation of the first galaxies?
First of all, it is important to understand that despite their enormous size, when we compare them to the known size of the universe, galaxies are a tiny dot.
Which means that the universe contains hundreds of thousands of galaxies, while each galaxy contains hundreds of millions of stars, with their respective systems (like ours).
Only in recent years has a commonly accepted theory about the formation process of the first galaxies begun to be put together, and this has happened thanks to advances in the Big Bang theory, since it has been possible to determine in what era of the evolution of the universe the first galaxies began to be born.
It is estimated that the origin of galaxies was earlier than initially thought, and began in the last known phase of the Big Bang, only about 600.000 years after the first space-time singularity.
At that point in time, the universe was still very homogeneous, with a similar distribution of matter at every point in space.
However, cosmic clouds (matter) began to attract each other due to the expansion of gravitational fields, forming clusters of matter concentrated in space.
In this way the universe became a heterogeneous space, with an unequal distribution of the density of matter.
The oldest observable galaxy!
Although it has been determined that most galaxies formed during a relatively short parenthesis of time (about 800 million years), some older and some younger have been observed.
Observations from Hubble and other super telescopes have made it possible to identify the oldest galaxy to date.
In 2009 it was identified and named UDFy-38135539, the oldest galaxy ever observed by ground-based telescopes.
The observable photon trail from this cluster, believed to contain over 1.000 billion stars, is 13.100 billion years old . This means it formed only 500 million years after the Big Bang, just after the first atomic particles were created.
How do galaxies form?
The formation of galaxies has been a mystery to humanity for centuries – and to modern scientists for decades. In fact, there is still some disagreement about exactly how the birth of the first galaxies began.
The truth is that a consensus has been reached in determining that galaxies were created from the clusters of gases in cosmic clouds, as soon as the universe began to cool after the Big Bang.
However, even though most of the known galaxies formed at about the same time, they are not static.
All cosmic bodies evolve over time; they mutate and move, and this dynamic has become an interesting field of debate for astronomy.
In addition, these theories would also explain what conditions and what part of the formation process defines the types of galaxy that are formed in each singularity.
In short, there are mainly two accepted theories that could explain the formation of galaxies and stars.
Model #1 – Galactic Clusters
The most widely accepted theory by the scientific community.
It assumes that the formation of galaxies and the bodies they contain were the direct result of the collapse of clusters of cosmic mass, attracted to each other as a consequence of primordial fluctuations in mass density in the universe.
These changes in density distribution caused the matter existing at that stage—accumulations of cosmic gases—to be attracted to each other until they formed Superclusters of mass in particular regions of the universe.
The different concentrations of matter within each cluster area gave way to the formation of more complex (and more compact) bodies that are organized hierarchically into: galactic clusters, galaxies, star clusters and stars.
This model would perfectly explain the reason for the distribution of galaxies in concentrated clusters and not simply distributed throughout the space of the known universe in a "regular" way.
Model #2 – The protogalaxies
Protogalaxies, or "primitive galaxies" as they are also known, correspond to the first formations of cosmic matter after the cooling period of the Big Bang.
Some theories about the origin of galaxies assume that the galaxy clusters we see today were created as a result of the collapse of massive protogalaxies about 500 million years after the Big Bang.
However, this theory has suffered some setbacks recently, as modern observations have identified really ancient galaxies (formed only 500 million years after the Big Bang), which would leave a very short time frame for the formation and collapse of protogalaxies to be possible.
What types of galaxies exist?
The reason that determines what type of galaxy forms remains a mystery and will likely remain so until we fully decipher the origin of galaxies.
The truth is that the research and observations achieved since the second half of the 20th century, thanks to powerful super telescopes, have allowed us to identify the different types of galaxies that exist (or at least that we know of so far).
Spiral Galaxies
Spiral galaxies, like our own (the Milky Way) , are the most common and also the best known. In fact, their shape corresponds to the graphic representation of a galaxy that we commonly see everywhere.
It is believed that the formation of spiral galaxy types corresponded to the abrupt, rather than gradual, collapse of gaseous formations.
In the first stage of its birth, a cluster of super dense matter agglomerates, which corresponds to the nucleus of the galaxy, generally formed by a concentration of old stars, around whose gravitational field the rest of the galactic mass revolves.
Discs would later form, which are made up of less dense stars, planets, asteroids, and cosmic cloud bodies.
It is also believed that some of the matter that makes up the galactic halo is formed from the remains of " dwarf galaxies" that orbit a larger galaxy and collapse, merging into the body of the larger one.
This theory arises as a result of the discovery of a small galaxy that orbits around our milky way and that could be slowly being "devoured" by ours, until it completely incorporates.
Spiral galaxies are characterized by:
- They have a formation disc with flat extension
- Its "arms" are made up mainly of interstellar dust and young stars.
- Its core or bulge is made up of large groups of old stars with low metallicity.
- Most spiral galaxies are thought to contain a black hole at the center of the bulge.
elliptical galaxies
Recent Hubble observations suggest that most elliptical galaxies have formed from the collision and massive merger of multiple galaxies, varying enormously in size, composition, and brightness from one another.
Despite this, it has been observed that giant elliptical galaxies (the largest in the observable universe so far), are composed mostly of old stars with low concentrations of metallicity.
In some of these galaxies, however, younger and smaller stars have been observed, but this could be the product of the collision with other galaxies.
Another very common physical characteristic of this type of galaxy is the low accumulation of cosmic gases, probably depleted by the formation of new stars and planets.
There are two types of elliptical galaxies according to the distribution of their mass in space:
Square elliptical galaxies , which generally correspond to massive galaxies.
These show an erratic movement between their groups of stars, without any definite pattern between them.
Square galaxies also do not seem to show a higher concentration of light in their nucleus, as is the case with other types of galaxies, such as spirals.
The second group corresponds to disk-elliptical galaxies , which show a more organized arrangement and move at much higher speeds through space.
These galaxies are made up of smaller, younger stars. Also, they show much higher luminosity levels at their core, but not too much.
What are stars made of?
Star formation does not only refer to the origin of stars. Researchers in astronomy and astrophysics have also widely discussed the composition of stars.
In other words, what are stars made of?
Stars are made up primarily of gases such as hydrogen and helium , and to a lesser extent of other elements such as oxygen, nitrogen, lithium, iron, and carbon.
Stars are made up of various chemical components, the variety and concentration of which varies greatly from one star to another, that is why there are different types of stars: Supergiants, giants, subgiants, dwarfs, white dwarfs, etc.
The distribution of the composition of the stars varies according to their age, which in turn varies their size, color, luminosity and classification on our scale.
The first thesis on the composition of the celestial bodies in space was proposed at the beginning of the XNUMXth century by Cecilia Payne-Gaposchkin.
The composition of the stars according to Cecilia Payne-Gaposchkin
It is impressive to think about the accuracy of the conclusions proposed in Cecilia Payne's doctoral thesis, if we consider the research tools available in the year 1925.
As a curious fact, Cecilia was the first person (man or woman) to obtain a doctorate in Astrophysics at Harvard University.
In his thesis he suggested that stars are made up mostly of hydrogen , a discovery that at the time was revolutionary for the global astronomy community.
His study served as a cornerstone for subsequent researchers to complement the thesis, discovering new elements in the stellar composition.
What happens when a star runs out of hydrogen?
The stars, like everything in the universe, are not eternal even though they seem to be.
The average age of a star like our sun is thought to be around 10.000 billion years. Currently our sun has consumed, more or less, half of its useful life, so some theorists estimate that it has between 5.000 and 7.000 million years before collapsing.
Since its energy and luminosity arise from nuclear fusions between hydrogen nuclei to form helium particles, its useful life is limited and depends on the hydrogen charge in its composition, that is, its fuel.
When the concentration of hydrogen drops too low in a star, fusions on its surface stop happening, preventing the concentrated energy from being released.
The excess pressure caused by the unreleased hydrogen particles causes an increase in the star's mass, turning it into a red giant.
When the fusion processes on the surface finally run out, the pressure on the star's core becomes excessive, causing it to contract until it implodes due to the compression of its mass. After that, a cooling process will begin that will last another couple of thousand years, until it becomes a white dwarf , a dead star.
