Our universe is composed of celestial bodies such as stars, planets, asteroids, and comets. All of these celestial bodies possess specific structural characteristics. They can also organize themselves into different structures. Stars, for example, arrange themselves to form larger structures known as galaxies. Our galaxy, the Milky Way, contains between 200 and 400 billion stars and spans an area with a diameter of 100,000 light years. Galaxies, influenced by gravity, also organize among themselves to form galaxy groups, and these groups come together to create galaxy clusters. It is estimated that there are approximately 25 billion galaxy clusters in our entire universe.
Galaxy Groups
Structures formed by nearby galaxies organizing themselves under the influence of gravity are called galaxy groups. Galaxy groups consist of a few galaxies that can interact with each other and sometimes even merge. The best-known galaxy group is the Local Group, which includes our own galaxy. The Local Group was first recognized by Edwin Hubble during his initial distance measurements and redshift observations. One of its most important members is M31, also known as the Andromeda Galaxy, which is the galaxy closest to us. Additionally, there are two satellite galaxies, M32 and M110.
Astronomers are now confident that they have almost fully understood the relationship between our galaxy and its galactic neighbors. Our galaxy is the second largest member of the Local Group, which comprises more than 50 galaxies spread over an area of roughly 10 million light years. The Local Group also includes the Triangulum Galaxy (M33), Leo I, and NGC 6822. The center of this group lies somewhere between the Milky Way and M31. M31 and the Milky Way are the largest galaxies in the group, while M33 is the third largest. Both the Milky Way and M31 have their own associated dwarf galaxies. The Local Group is part of the larger Virgo Cluster.
The dynamics of the Local Group are constantly changing. It is predicted that the Milky Way and the Andromeda galaxies will eventually collide, and this collision is expected to result in the formation of a giant elliptical galaxy.
The distances between the galaxies in the Local Group are extremely vast. The distance between the Milky Way and the Andromeda Galaxy is approximately 2.54 million light years, which means there are significant gaps between galaxies. In fact, it can be said that about 90% of the universe consists of empty space. Various methods are used to calculate distances on this scale. For these calculations, Cepheid variable stars are employed. Edwin Hubble used one such star in the Andromeda Galaxy to determine that the galaxy is more than one million light years away, which is far beyond the outer reaches of our own galaxy. There are many methods to calculate these distances, and we will briefly touch upon the three most well-known ones.
Trigonometric Parallax Method
To understand how this technique works, imagine your index finger as a star. Extend your arm with your index finger pointing forward, close one eye, and place your finger in front of an object—this object could be a picture frame or the corner of your room. Now, open your closed eye and close the other one. You will notice that your finger appears to shift its position. Then, repeat this exercise while gradually bringing your finger closer to your face. Does your finger appear to shift more or less compared to before?
Hopefully, you have observed that your finger shifts more when it is closer. A nearby object appears to shift much more than a distant object when viewed from two different perspectives (in this case, your eyes). Astronomers use a similar technique when observing a star at six-month intervals. A nearby star appears to shift much more than a distant one, and we convert the angle of this apparent shift into a distance using trigonometry.
Moving Cluster Method
Many stars are not close enough to exhibit a useful trigonometric parallax. However, when stars are part of a stable star cluster with unchanging physical dimensions—such as the Ülker star cluster—the apparent motions of the stars within the cluster can be used to determine the cluster’s distance.
The moving cluster method relies on observing the proper motions and Doppler shifts of every member of a known star group. The stars in the cluster move almost at the same speed in nearly parallel directions, much like a flock. The idea is that these stars seem to be moving toward a common vanishing point, which is essentially a perspective effect.
Statistical Parallax Method
The statistical parallax method is generally used to measure the distances of very distant stars. This method involves developing a model based on the positional differences and motions observed among multiple stars. It makes use of the statistical data of these stars.
For instance, distances can be estimated using parameters such as the overall motion of the galaxy, the distribution of stars, and their velocities. This approach is particularly useful at extremely large distances where traditional parallax methods become insufficient. In summary, the statistical parallax method attempts to calculate the distances of more remote stars and galaxies by employing statistical data and modeling rather than relying on direct parallax measurements.
Galaxy Clusters and Galaxy Groups
We previously mentioned that galaxies and galaxy groups come together to form galaxy clusters. Although there is no specific number of galaxies that distinguishes a galaxy cluster from a galaxy group, clusters generally contain over a hundred galaxies. Galaxy clusters are the largest stable structures in the universe, and their typical properties are as follows:
- They contain between 50 and 1000 galaxies.
- They include hot gas and large amounts of dark matter.
- Their masses range from about 10¹⁴ to 10¹⁵ solar masses.
- Their radii are between 2 and 6 Megaparsecs.
- The velocity dispersion of their member galaxies is approximately 800–1000 km/s.
Galaxy clusters have played a key role in reshaping our understanding of the large-scale universe. Dark matter was first detected in galaxy clusters. These clusters are also very bright X-ray sources, with the X-rays emitted by gas within the cluster that can reach temperatures of 10⁷ to 10⁸ Kelvin.
When observed, galaxy clusters appear as collections of galaxies held together by mutual gravitational attraction. However, their velocities are so high that they cannot remain gravitationally bound solely by the visible mass. This suggests that, in addition to gravity, there must be an extra component of invisible mass or an additional attractive force. In a typical cluster, galaxies account for about 5% of the total mass, X-ray-emitting hot gas makes up roughly 10%, and the remainder is dark matter.
Some notable galaxy clusters include:
- Virgo Cluster: Discovered by Charles Messier in 1781, the Virgo Cluster is the largest cluster near the Local Group—which includes our galaxy—and contains between 1300 and 2000 galaxies. This cluster forms the core of the supercluster to which we belong.
- JKCS 041 Cluster: The JKCS 041 Cluster is the most distant galaxy cluster observed to date. It is located 10.2 billion light years away, which is 1 billion light years farther than the most distant galaxy known at the time.
- El Gordo Cluster: The largest known galaxy cluster in the universe is called El Gordo (officially ACT-CL J0102-4915). Discovered in 2014, El Gordo is currently the largest observed galaxy cluster. Its diameter is approximately 2.5 million light years, and its mass is about 3×10¹⁴ solar masses.
Giant Structures in the Universe: Superclusters
Superclusters are enormous cosmic structures that encompass galaxies, galaxy groups, galaxy clusters, and other cosmic formations. Generally, astronomers decide which galaxy clusters form a supercluster by examining the distances between them. This method works well when superclusters are clearly separated. However, if galaxy clusters are found along the entire expanse between the centers of two neighboring superclusters—which is often the case—it can be difficult to determine whether a particular galaxy cluster, located roughly midway between the supercluster centers, belongs to one supercluster or the other. Galaxies and galaxy clusters are not uniformly distributed throughout the universe; rather, they congregate into vast clusters interspersed with enormous voids where very few galaxies exist.
The average mass of superclusters is about 6 million billion times the mass of the Sun, and their typical size is around 200 million light years. To put this into perspective, these superclusters are roughly 2,000 times larger than our own Milky Way galaxy. Imagine that if the Milky Way were represented by a 1-lira coin standing in the middle of a soccer field, then a supercluster would be equivalent to the entire soccer field. In terms of mass, if we equate the mass of the Sun to that of a golf ball, the mass of a supercluster would be comparable to that of Mount Everest. These analogies vividly illustrate just how enormous superclusters are.
Although superclusters contain a significant amount of mass, that mass is spread over an enormous volume, making superclusters less dense than individual galaxies. Nonetheless, their density is sufficient for their gravitational pull—including that of dark matter—to influence the motions of the matter within them. Some of the best-known superclusters are as follows:
- Virgo Supercluster:
One of the nearest superclusters in the universe, it spans an area with a diameter of about 2 million light years. This supercluster is located within the Laniakea Supercluster and is one of its important components. It also includes the galaxy group known as the Local Group and contains many large galaxies, including the Milky Way and Andromeda, making it a critical structure on a cosmic scale. - Einasto Supercluster:
Together with the Perseus-Pisces Supercluster, the Einasto Supercluster can be considered a part of the Local Supercluster. It consists of a large number of galaxy clusters and is approximately 100 million light years wide. Like other superclusters, the Einasto Supercluster forms an important framework for understanding the large-scale distribution of galaxies, serving as a reference point for astronomical studies of cosmic structures and the cosmic web. - Serpent-Bull Supercluster:
The Serpent-Bull Supercluster is the supercluster closest to the Virgo Supercluster. The Serpent Supercluster on its own is not very large, so it is studied together with the Bull Supercluster, even though these two are distinct structures. Together, they form the pair of superclusters nearest to Virgo. This combined supercluster spans roughly 100 million light years and contains a single rich galaxy cluster, also known as the Serpent Cluster.
Laniakea Supercluster: The Giant Structure of the Universe
Take our galaxy, multiply it by 100,000, add one million smaller galaxies, and you have Laniakea! A team of scientists gathered data from more than 8,000 galaxies around us and succeeded in mapping their motions and velocities in space. In doing so, they demonstrated that the Milky Way is part of a much larger galactic system—a supercluster they named Laniakea. The name Laniakea means “limitless, eternal, vast paradise” in Hawaiian. This name was given by Nawaʻa Napoleon, an assistant professor of the Hawaiian language at Kapiʻolani Community College.
The Milky Way is located at the very edge—the outermost part—of this structure. The entire universe can be envisioned as a complex network of galaxies, akin to a cosmic web. Superclusters are the largest known structures in the universe. The total mass of the Laniakea Supercluster is approximately 10^15 solar masses. Laniakea spans 160 megaparsecs (520 million light years) and contains over 100,000 galaxies. Due to its low brightness relative to the number of stars it contains, it is believed that a large portion of this supercluster consists of dark matter. Having discussed the general characteristics of Laniakea, let us continue our journey by focusing on the Great Attractor, which lies at the center of Laniakea and has the greatest influence on its members.
Great Attractor
In the early 1970s, the first signs of a deviation from the assumption of a uniformly expanding universe were detected. In 1986, a region—later named the “Great Attractor”—was identified. This gravitational anomaly is located at the center of Laniakea, near the Hydra-Centaurus Supercluster, and is very difficult to observe because it lies just behind the Milky Way’s galactic plane. The Great Attractor can be indirectly detected through its influence on the motions of galaxies and clusters.
While the expansion of the universe explains why galaxies are receding from each other, massive concentrations of matter can slow this motion. It is known that, over billions of years, our galaxy and all the galaxies around us have been drawn toward the Great Attractor. Various theories exist regarding the nature of the Great Attractor; for example, it is thought to be due to an extreme overdensity of matter.
Between us and the Great Attractor lie numerous star clusters, as well as clouds of gas and dust. Consequently, the light that should reach us is obscured, making it difficult for scientists to observe. The only certainty is that our galaxy—and everything in our supercluster—is steadily moving toward the Great Attractor.
The Importance of Superclusters in Understanding Our Universe
Superclusters are of great importance in helping us understand the structure of the universe. Observing the expansion of the universe is possible on the scale of superclusters. It is difficult to see this expansion by looking at galaxies or galaxy clusters because their gravitational forces are strong enough to resist the universe’s expansion. However, on the scale of superclusters, the gravitational attraction between them is not as strong since the distances between them are unimaginably vast. This allows us to observe the expansion of the universe much more clearly by studying superclusters.
Earth’s Place in Our Universe
It may be difficult to visualize this structure in your mind at first. Comparing it to familiar surroundings on Earth can make this process easier. Imagine that the solar system is your home. The Sun and planets would be the rooms. The exoplanetary systems discovered so far by the Kepler Space Telescope and others would be the other houses on your street—separate but very close to each other. If you look at your street from farther away, what you would see is your town or city. A galaxy is a city of stars, meaning the Milky Way can be thought of as our hometown in space. Our solar system is located in one of the outer arms of the Milky Way Galaxy. If we consider the center of our galaxy as the city center, then our location can be imagined as a rural area.
Just as cities on Earth come together to form countries, galaxies in space form structures known as clusters. Countries are located on continents, which consist of vast landmasses. Similarly, galaxy clusters are part of enormous groups called superclusters. Just as Earth is made up of continents, the observable universe is made up of superclusters. So now we know that we are living on a small, blue planet called Earth, orbiting a star named the Sun, located on the edge of a galaxy called the Milky Way, which itself is part of a supercluster called Laniakea!
To study galaxy clusters, astronomers use methods based on the Hubble–Lemaître law. This law describes the distribution of clusters in the universe and makes it possible to measure the distances of very distant objects. It states that the speed at which galaxies move away from each other is proportional to the distance between them. In other words, the farther away a galaxy is, the faster it is receding from us. Superclusters are important not only because they allow us to study the expansion of the universe but also because they help us understand how matter is organized in space.