The Milky Way and Andromeda are both large spiral galaxies. These galaxies have a swollen central halo, a disk, and spiral arms. Interstellar material is scattered between the disks of spiral galaxies. Bright nebulae and hot young stars are located in the spiral arms, showing that stars are still being born. Discs are usually dusty.

Open star clusters can be seen in the arms of the closer spirals, and globular clusters can be seen in the halo. Spiral galaxies are a mix of young and old stars. About two-thirds of nearby spiral galaxies have peanut-shaped, box-shaped stellar bars that orbit the center. The Milky Way also has a bar. Spiral arms start rotating from the end of the rod. In barless and barred spiral galaxies, we see a variety of shapes.

Image of barred spiral galaxy NGC 1300 captured by the Hubble Space Telescope

The diameter of the bright parts of spiral galaxies varies from 20,000 to more than 100,000 light years. New research suggests that galactic material may flow beyond the edge of the galaxy. These materials are cold and thin gases that are difficult to detect in observations.

According to the available data, the mass of the visible parts of spiral galaxies varies between 1 billion and one trillion solar masses. The total luminosity of most spiral galaxies is between 100 million and 100 billion times that of the Sun. The Milky Way and the M31 (Andromeda) galaxies are considered large and massive spiral galaxies. There is also a significant amount of dark matter in and around galaxies; It is possible to find out the existence of dark matter based on the speed of movement of stars in the outer parts of the galaxy.

Vortex non-barred spiral galaxy and its accompanying satellite galaxy

Elliptical galaxies are composed almost entirely of old stars and are spherical or elliptical in shape (compacted spheres). These galaxies lack a spiral arm. Their light comes from old red stars or population 2 stars. In larger elliptical galaxies, many globular clusters are seen. Nebulae and dust are not prominent in elliptical galaxies, But they have a small amount of interstellar matter.

The degree of width of elliptical galaxies is different from each other. Some of them are nearly spherical systems like flat spiral galaxies. Giant elliptical galaxies are more than a few hundred thousand light-years in diameter and are significantly larger than spiral galaxies. Although the stars rotate independently around the center of the elliptical galaxy, their orbits are not in the same direction as in the spiral galaxy. As a result, elliptical galaxies do not rotate regularly, and for this reason, it is difficult to estimate their dark matter.

The dimensions of elliptical galaxies vary from giant galaxies to dwarf galaxies. Leo I is an example of a dwarf elliptical galaxy. The brightness of this type of dwarf is almost equal to the brightest global clusters. An intermediate galaxy between giant and dwarf systems such as M32 and M110 are the two companion galaxies of Andromeda. Although these galaxies are classified as elliptical dwarfs, they are significantly larger than galaxies like Leo I.

The giant elliptical galaxy ESO 325-G004

Irregular galaxy

Hubble classified the galaxies that do not have a regular shape in the irregular galaxy group. Irregular galaxies usually have less mass and luminosity than spiral galaxies and are disorganized. These galaxies contain young population of 1 star and an old population of 2 stars.

The galaxy NGC 1427A is an example of an irregular galaxy at a distance of 52 million light-years

Two examples of the most well-known irregular galaxies are the Small Magellanic Cloud and the Large Magellanic Cloud, which are located at a distance of slightly more than 160,000 light years and are among the closest extragalactic neighbors. The names of these galaxies are a symbol of Ferdinand Magellan and his crew’s journey around the world. They were the first Europeans to see the Magellanic Cloud in the night sky. Although these two systems are not visible from Europe and the United States, they are easily visible in the night sky from the Southern Hemisphere. The Magellanic Clouds provide astronomers with an excellent opportunity to study nebulae, star clusters, variable stars, and other key galactic objects.

A wide view of the Large Magellanic Cloud

The Small Magellanic Cloud is much less massive than the Large Magellanic Cloud and is six times as long as it is wide. This narrow strip is like an arrow towards our galaxy. The Small Magellanic Cloud has taken its current form due to the gravitational interaction with the Milky Way. A huge string of pebbles resulting from this reaction are stretched in the night sky and can be seen inside the gas clouds that move at a high speed. This set is called the Magellanic string. Based on the interaction of galaxies, the irregular shape of smaller galaxies can be justified.

In the next four billion years, the Milky Way will collide with its spiral neighbor Andromeda. The sun will probably wander, But it does not threaten the earth and the solar system. Andromeda, also known as M31, is 2.5 million light-years away from Earth, but it is moving towards the Milky Way at an incredible speed due to the common gravity between the two galaxies and the dark matter that surrounds them both.

Computer simulations of the Hubble data show that after the two galaxies approach, it will take two billion years for both galaxies to completely merge due to gravity. The Milky Way and Andromeda will form an elliptical galaxy after merging, which is one of the most common examples in the universe. According to the simulations, the solar system will probably move away from the center of the galaxy after the collision.

This firestorm in the sky is the result of the collision of two spiral galaxies that started around a few hundred million years ago. This collision has caused the compression of huge clouds of gas and dust and the beginning of a new wave of star births in the galaxy.

Some galaxies are classified as active galaxies if they contain an active galactic nucleus (AGN). In these galaxies, instead of stars, gas, and interstellar matter, a significant part of the energy is emitted by the galactic nucleus. There are several types of AGNs; But the nuclei that are in the low brightness spectrum are called Seyfert galaxies, while the nuclei with more brightness than the host galaxy are called quasi-stellar objects or quasars.

Active galactic core rays are the result of the gravitational energy of matter falling into the black hole from the disk. AGN luminosity depends on the mass of the supermassive black hole (SMBH) and the rate at which matter is infalling into it.

UGC 6093 is classified as an active galaxy, which means it hosts an active galactic nucleus.

Blazers and quasars

Ballars and quasars are two examples of common AGNs. A quasar is an active galactic nucleus that emits streams of particles outward from the center of the galaxy. The difference between quasars and blazars is their angle. The closest quasar to Earth is Markarian quasar 231 in the constellation Ursa Major. Quasars are so distant that they are thought to be from the early stages of the galaxy.

A quasar is only one type of active galactic nucleus, which includes blazars, radio galaxies, and Seyfert galaxies. The full name of a quasar is a quasi-stellar radio object. Quasars are actually supermassive black holes active in the center of galaxies, and they shine more than stars in the galaxy. These objects have an accretion disk of gas and dust and release electromagnetic radiation as matter falls into the quasar. Quasars are disabled after taking the augmentation pill. These objects are usually located in the center of distant galaxies.

All the quasars that have been seen so far are billions of light-years away from Earth, that’s why many scientists believe that these objects are the core of young galaxies. If we can see these objects today, they are probably completely calm, and like any other galaxy, no flow can be seen from them.

Quasar Markarian 231 in the constellation Ursa Major

It seems impossible to count all the galaxies. One of the problems is instrumental limitation. To achieve the best view, telescopes need a large aperture (the diameter of the main mirror or lens) and must be placed outside the Earth’s atmosphere to avoid distortion.

According to Mario Livio, an astrophysicist at the Space Telescope Science Institute in Baltimore, Maryland, the acceptable range of galaxies varies between 100 billion and 200 billion, although each expert offers several estimates. With the launch of the Jamesob telescope, it is expected that more information about the early galaxies of the universe will be obtained.

Regardless of the tool used, the method of estimating the number of galaxies is the same. The telescope takes pictures of a part of the sky. The total number of galaxies can then be estimated based on the obtained fraction.

Measuring the expansion of the universe shows that the age of the universe is approximately 13.8 billion years. As the age of the universe increases and those galaxies become larger, they become further away from each other. As a result, it becomes more difficult to see them with a telescope. This is where the visible world hypothesis comes into play. According to scientists, in the next one to two trillion years, the galaxies will go beyond the range of the Earth’s vision.

Galaxies also change over time. For example, as we said in the previous section, the Milky Way galaxy will collide with the Andromeda galaxy in the distant future, and both will merge within 4 billion years. The inhabitants of distant galaxies will face the world of darkness.

In the late 1970s, astronomer Vera Rubin discovered dark matter. He was studying the rotation of galaxies when he noticed the strange rotation of Andromeda. The speed of the materials at the edge of the galaxy was as high as the materials in its center, and this phenomenon undermined Newton’s and Kepler’s laws. Although much of the mass was concentrated in the center; an invisible mass called dark matter seemed to hold the galaxy together. Rubin soon discovered a huge halo of dark matter in the Andromeda galaxy.

Although almost half a century has passed since this discovery, no one still knows what the nature of dark matter is. However, this strange and invisible matter makes up nearly 84% of the mass of the universe, and its pervasive presence affects stars and galaxies and the density of matter in the early universe.

Some of the best evidence for dark matter comes from the galaxy cluster 1E 0657-556, or the Bullet Cluster. This cluster was formed by the collision of two larger galaxy clusters, the most energetic event since the Big Bang. Because the main parts of the cluster pair, the stars, gas, and dark matter, behaved differently during the collision, scientists were able to study them separately.

The stars of the globular cluster galaxies observed by the Magellan and Hubble telescopes in visible light were not affected by the collision and passed by each other. The hot gas from the collision of the two clusters was observed at X-ray wavelengths by the Chandra X-ray Observatory. This gas contains a large fraction of the normal matter of the cluster pair. Because the gases react electromagnetically with each other, the gas of both clusters has a slower speed than the stars. The third element of this collision, i.e. dark matter, was also discovered indirectly using the gravitational lensing of background objects.

By definition, dark matter does not have any electromagnetic reaction, especially light reaction. This material is dark as its name suggests. As a result, the masses of dark matter of the two clusters passed by each other like stars during the collision and left behind hot gas. If the hot gas was the most massive material of the clusters, the gravitational lensing effect would not be seen. As a result, these observations were the first evidence to prove dark matter.

Located in the constellation of the Southern Triangle, ESO 137-001 looks like a starfish swimming in a sea of ​​stars. This galaxy is a barred spiral galaxy. In addition to the bar, there are twists and trails in this galaxy. These trails are streams of stars that appear to be deflected from the galaxy like the tails of mermaids.

According to NASA, these stars form a tail of gas and dust that flows outward from ESO 137-001. The formation process of this galaxy remains a mystery because the gases inside the tail must be hotter than these for star formation.

The rotation speed of a massive disk galaxy like MACS 2129-1 is twice that of the Milky Way, But it is not as active as that. Hubble’s observations of this distant galaxy show that no new star has been born in this galaxy since 10 billion years ago.

MACS 2129 is also known as a dead galaxy because there are no new stars in this galaxy. According to scientists, such galaxies may have been formed over time due to the merger of smaller galaxies; But the stars in MAC 212901 are not the result of explosive mergers, but were formed in the main disk of the galaxy. The findings of this research show that dead galaxies change their internal structure because they merge with other galaxies over time and their shape changes.

Some galaxies are cannibals. The Andromeda Galaxy, Earth’s closest neighbor, has been swallowing other galaxies for the past ten billion years. This galaxy and the Milky Way will collide in about 4.5 billion years, although by then the Sun will reach the end of its life, and life on Earth will become impossible.

Three hundred million light years away, a giant frog swims in space. The Frog Galaxy has a tail that is 500,000 light-years long and ten times longer than the Milky Way; But what caused the formation of this strange galaxy? According to researchers in 2018, a cosmic collision is the cause of this strange shape. Two galactic disks are stretched over a smaller dwarf galaxy so that stars are piled up on one side of the head of the galaxy, and on the other side is a long tail.

Galaxies are constantly interacting with each other, changing the shape of their neighbors, stealing stars, and doing other mischief. The brightest known galaxy in the universe is one of these thieves. In 2018, scientists reported the observation of the galaxy W2246-0526, which has stolen half of the mass of its three neighboring galaxies.

Probably the cutest galaxy is the Little Lion Cub in the constellation Ursa Major. This dwarf galaxy has been inactive since the Big Bang, meaning it contains unchanged molecules from moments after the Big Bang explosion, about 8.13 billion years ago. The little galactic lion cub is doomed; Because a spiral galaxy called NGC 3359 is devouring its material. However, it is possible to understand the effects of the molecules of the early universe by examining the flow of its stellar gases.

The galaxy ESO 381-12 appears to be flourishing. This galaxy is located at a distance of 70 million light-years from Earth in the constellation Centaurus. This galaxy is a lenticular type, which is a combination of a spiral galaxy like the Milky Way and an elongated elliptical galaxy. What makes this galaxy strange are the uneven petal-like blooms that extend outward from the main body of the galaxy. Astronomers are not entirely sure what causes these structures, But the blooms may be shock waves from a cosmic collision that set the stage for new tectonics to form.

Messier 83 is a massive spiral galaxy with a bar-shaped center similar to the Milky Way. This galaxy is located at a distance of 15 million light-years from Earth in the constellation of the Serpent. Messier 83 is strange in several ways. It primarily has a dual-core in the center. It seems that two supermassive black holes are located in the center of this galaxy.

Secondly, it is full of supernovae. Astronomers have so far identified six of these stellar explosions and the remnants of more than 300 of them. In this way, track 83 ranks second in terms of the number of supernovae after the galaxy NGC 6946, which has the highest number of recorded supernovae.

This image seems to be of a dandelion rather than a cosmic phenomenon. This image, recorded by the Hubble Space Telescope, belongs to a galaxy called the Bug; Because its light jets prevent it from examining nearby stars.

The disk of the spiral galaxy IC 2163 appears to be scanning the sky with a large eye. This eye-like galaxy is actually a huge string of stars and dust. According to astronomers, these eye features only last for a few million years, which is based on the lifespan of galaxies like the blink of an eye.

The researchers found that the characteristic optical gases are moving towards the center of IC 2163 at a speed of 100 km/s and become more disordered and slower as they approach the center of the galaxy. The deceleration causes the gas to gather and compress, which itself provides the basis for the formation of new stars.

Most galaxies probably have a massive black hole in their center, But some of them also have two black holes. One of these galaxies, NGC 7674, is a spiral galaxy centered on two supermassive black holes, one light-year apart. This galaxy probably absorbed its black hole during a merger with another galaxy. The only other known galaxy with two central black holes is the massive galaxy 379+0402.

If you are a galaxy, you will probably either swallow other galaxies or die. The galaxy NGC 1277 chose the latter. This galaxy, which was first observed in 2018, is 240 million light-years away from Earth. During the last ten billion years, no new stars have been formed in this galaxy and therefore it is considered a dead galaxy.

Astronomers believe that NGC 1277 is so amazing that it is moving at such a high speed to swallow up other galaxies. This galaxy moves in space at a speed of 3.2 million kilometers per hour. NGC 1277 would not be able to form stars without the gas and dust of other galaxies. Some astronomers think that most galaxies look like NGC 1277 and have grown through mergers with other galaxies.

According to scientists, most galaxies are moving away from Earth because space is continuously expanding; But Messier 90 is not like this. This spiral galaxy, which is 60 million light-years away from Earth, is moving in the direction of the Milky Way.

Astronomers can measure the galaxy’s motion because its light is deflected toward the blue end of the light spectrum. Objects moving away from Earth tend to be red, that is, their light emission goes towards the red. Messier 90 is part of a large group of galaxies called the Virgo cluster. This galaxy can be seen in May with a telescope or binoculars in the Northern Hemisphere between the constellations of Virgo and Leo.

Small galaxies like the Large and Small Magellanic Clouds are among the closest galaxies to the Milky Way. Although many small and dwarf galaxies are close to the Milky Way, the Andromeda Galaxy is the closest large spiral galaxy to the Milky Way. The Andromeda Galaxy is the brightest object in the night sky, located 2.5 million light years from Earth; It means the farthest thing that a human can see with the naked eye.

Astronomers sometimes call Andromeda Messier 31 or M31. Andromeda is the 31st object in the famous list analyzed by the French astronomer Charles Messier (1817-1731). Today, amateur astronomers can see these galaxies with telescopes and binoculars. The diameter of Andromeda, like the Milky Way, reaches 100,000 light years.

The Milky Way galaxy is our home and for this reason, it has the greatest research importance. Looking at the night sky, you can see a band of light that many ancients called a river, lion, or path. This attractive line of light is actually the center of the Milky Way as seen from its outer arms.

Understanding the structure of the Milky Way is a bit challenging. The solar system is located right at the outer edges of one of the arms of the Milky Way. The position of the Sun in the galactic disk surrounded by dust is one of the obstacles to observing the structure of the Milky Way. The Milky Way is a spiral bar with a diameter of 100 thousand light years. If you look at it from above, you will see a central bulge surrounded by four large spiral arms. Spiral galaxies make up two-thirds of the galaxies in the universe.

A barred spiral galaxy, unlike a normal spiral galaxy, contains a bar-like structure in the central part and has two main arms. The Milky Way has two small arms as well as two smaller sequences. One sequence, called the Orion arm, contains the Sun and the Solar System. Orion’s arm is located between the two big arms of Perseus and Sagittarius.

The Milky Way is continuously rotating. Its arms move in space and the sun and the solar system move along with it. The solar system moves through space at an average speed of 828,000 kilometers per hour, and even with this high speed, it takes 230 million years to travel the entire Milky Way.

The arc of the Milky Way in the expanse of the night sky. This composite panorama image was recorded at the Paranal Observatory in northern Chile

The Milky Way is surrounded by a huge halo of hot gas that stretches for hundreds of thousands of light years. This halo of gas is as heavy as all the stars in the Milky Way and is rotating at a high speed like the Milky Way itself. The spiral arms around the center of the galaxy contain large amounts of gas and dust. New stars are continuously formed in these arms. These arms are called galactic disks. The thickness of the galactic disk is only 1000 light years.

In the center of the galaxy is the galactic bulge. The heart of the Milky Way is full of gas, dust, and stars. The gas and dust in this bulge is so thick that you cannot see beyond it. According to the hypotheses, there is a supermassive black hole at the center of the Milky Way, which is billions of times heavier than the Sun. This megamass black hole was initially small, But it grew bigger over time by feeding on the surrounding gas and dust. Although black holes cannot be seen directly, they can be identified based on the traces they leave behind. Most galaxies in the universe contain a black hole at their center.

In the early 1900s, many astronomers believed that the entire universe was located in the Milky Way galaxy. Others, such as Harlow Shipley, a scientist and head of the Harvard College Observatory, believed that there were spiral-shaped bubbles of gas and dust and called them island worlds.

It was in 1924 that Edwin Hubble first discovered some pulsating stars called Cephasian variables and realized that these stars are outside the range of the Milky Way. These astronomical objects were unique collections of stars far away from the Milky Way.

After measuring the distances, Hubble measured the Doppler effect, which is the stretching of the light of the galaxies due to motion. He discovered that the galaxies around the Milky Way are moving away from us at incredible speeds. The more distant the galaxies, the faster they escape. For this reason, Hubble noticed that the whole universe is expanding, and years later astronomers noticed the increase in the speed of this expansion.

Images recorded by the Hubble Space Telescope