Structure of Universe

Since the beginning people on Earth were curious to understand the structure of the universe. What structures are present in our universe and their details? So in this article, I will try to mention the common structures present in the universe. I will try my best to explain the concepts very simpler so that one can easily understand it.

So, let’s start with stars. Stars are massive, luminous balls of gas held together by their own gravity. They are composed mainly of hydrogen and helium, the two lightest elements, with traces of heavier elements. Stars emit energy in the form of light and heat due to nuclear fusion reactions that take place in their cores. These reactions fuse hydrogen atoms together to form helium, releasing a tremendous amount of energy in the process. This energy is what makes stars shine and provides the energy that sustains life on planets orbiting around them. Like our sun whose diameter is 1.39 Million km. All planets revolve around the star. The neighboring star of the sun is Proxima Centauri. The largest discovered star Uy Scuti has a diameter of around 1.2 Billion km. Planets are large, spherical objects that orbit around stars and do not produce their own light. They are typically composed of rock, metal, or gas, and are classified into two main categories: terrestrial planets and gas giant planets. Terrestrial planets are those that are made of rocks like Mercury, Venus, Earth, and Mars. While gas giant planets are mostly made of gases like Jupiter, Saturn, Uranus, and Neptune. There are 8 planets in our solar system and one dwarf planet Pluto. The diameter of Earth( our homeland) is 12,700 km. While Jupiter is the largest planet in our solar system having a diameter of 139,820 km. The planets that are present outside our solar system or those planets that don’t revolve around the sun are exoplanets. The largest discovered exoplanet in the universe is WASP-17b It is 1.9 times the size of Jupiter. Moon revolves around planets. They are called the natural satellites of planets. they have different sizes and each planet has a different number of moons. Besides these asteroids and comets are present in the solar system. Asteroids are small, rocky objects that orbit the Sun. Like asteroids present in the asteroid belt present between Mars and Jupiter. The width of the asteroid belt is 149.6 million km. Comets are small, icy bodies that orbit the Sun in highly elliptical orbits. They are mostly present in the Kuiper belt ( present beyond Neptune) and Oort cloud. So. All the objects mentioned above combine forming a solar system. Our solar system consists of the sun, 8 planets their moons, and asteroids. It is estimated that there are roughly billions of solar systems present in the milky way galaxy. Now lets move out from the solar system. The Kuiper Belt is a region of the outer solar system beyond the orbit of Neptune that is home to a large number of small, icy objects known as Kuiper Belt Objects (KBOs). The Kuiper Belt is similar to the asteroid belt, but it is much larger and consists mainly of icy bodies rather than rocky ones. The Oort Cloud is a hypothetical, spherical cloud of icy bodies that is believed to be located at the outermost edge of the solar system, extending out to about 100,000 astronomical units (AU) from the Sun. An astronomical unit is the distance from the Earth to the Sun, which is approximately 93 million miles (150 million kilometers).

Many celestial bodies are made from Nebula. Nebulae are basically clouds of gas and dust. They are often star-forming regions. How stars and other bodies are formed I explained better in my other article “ Birth of Solar System”. There are four main types of nebulae Planetary Nebula, dark nebula, emission nebula, and reflection nebula. Planetary nebulae are formed by the expanding outer layers of a dying star. They are named as such because early astronomers thought they looked like planetary disks when viewed through telescopes. Planetary nebulae are typically composed of ionized gas and emit light in various colors. Despite their name, planetary nebulae have nothing to do with planets. Dark nebulae, as the name suggests, are clouds of dust and gas that are so dense that they block out the light of the stars behind them. They are also known as absorption nebulae because they absorb light rather than emitting it. Emission nebulae, on the other hand, are clouds of ionized gas that emit light of various colors. They are often associated with regions of active star formation, as the ultraviolet radiation from young stars can ionize the surrounding gas and cause it to emit light. Reflection nebulae are clouds of gas and dust that reflect the light of nearby stars, giving them a bluish appearance. Unlike emission nebulae, they do not emit light of their own. The most nebulae are Orion nebula, Ring nebula, Eagle nebula, Horse-head nebula, and Crab nebula. The size of nebulae can be from a few hundred to thousands of light years.

Now let’s discuss Galaxies. A galaxy is a large system of stars, solar systems, nebulae, gas, dust, and dark matter, all bound together by gravity. Galaxies come in a wide variety of shapes and sizes. Galaxies are the building blocks of the universe, and they can interact and merge with each other over time. Our solar system is present in the Milky Way galaxy. There are three main types of galaxies Spiral, Elliptical, and Irregular galaxies.  Spiral galaxies: These are the most common type of galaxies, and they are characterized by a flat, rotating disk with arms that spiral outward from a central hub or nucleus. The arms are filled with gas, dust, and young stars, and they often have a blue color. Our own Milky Way galaxy is a spiral galaxy. Elliptical galaxies have a smooth, spherical, or elliptical shape, and they lack the distinct arms and disk-like structure of spiral galaxies. They contain mostly old stars and very little gas or dust, which means that new stars are not forming. Irregular galaxies are irregular shapes, with no particular structure or symmetry. They often contain a mix of old and young stars, as well as large amounts of gas and dust. Irregular galaxies are relatively rare compared to spiral and elliptical galaxies. The sizes of galaxies vary from each other. Milky Way has a diameter of about 100,000 light years. Our neighboring galaxy Andromeda has a diameter of about 220,000 light years. The largest discovered galaxy named IC1101 has a diameter of around 3.9 Million light years. It is estimated that there are roughly 2 trillion galaxies present in universe.

Now  I will discuss the galaxy group. A galactic group is a small collection of galaxies that are gravitationally bound together. These groups typically contain a few to dozens of galaxies, and they are the smallest known structures that are bound by gravity. Galaxies are held together by their mutual gravity or there is a possibility that the gravity of large galaxies may capture the smallest ones. The Milky Way is part of a galactic group called the Local Group, which contains more than 54 known galaxies, including the Andromeda Galaxy, the Triangulum Galaxy, and numerous smaller dwarf galaxies. Galactic groups can be found throughout the universe, and they are often arranged in larger structures such as clusters and superclusters. Galaxy Clusters are the most massive and tightly packed clusters, containing hundreds to thousands of galaxies that are concentrated in the central regions of the cluster. There are two main types of galaxy clusters regular and irregular clusters. Regular clusters have a symmetric, roughly spherical shape, and are tightly packed with galaxies at their center. The galaxies in these clusters are usually elliptical or lenticular, and are generally old and red in color. Regular clusters also tend to have a large amount of hot, ionized gas between the galaxies, which emits X-rays. Irregular clusters have a more diffuse and asymmetric shape, with a lower concentration of galaxies at their center compared to regular clusters. The galaxies in these clusters are often spirals, and there is generally less hot gas between the galaxies, which emits less X-ray radiation. Some common clusters are the Virgo cluster and the Coma cluster. The largest known galaxy cluster is the El Gordo cluster (ACT-CL J0102-4915), which is located about 7 billion light-years away from us. It has a mass of about 3 quadrillion times that of the Sun and is estimated to contain around 2,000 galaxies.

Supercluster is a group of galaxy clusters or galaxy groups typically consisting of 3 to 10 clusters and spanning as many as 200,000,000 light-years. They are one of the largest structures in the universe. The commonly known superclusters are Virgo superclusters, Laniakea supercluster, and Comma supercluster. The local group is a part of the Virgo supercluster. The clusters and galaxies in superclusters are bounded by gravity. The clusters and galaxies in superclusters are often connected to each other by filaments consisting of galaxies, gas dust clouds, and dark matter. They act like a highway or bridge that connects galaxies and clusters. They typically have a diameter of 100-200 million light-years and contain dozens of galaxy clusters, each of which contains hundreds or thousands of galaxies. Examples include the Shapley Supercluster and the Coma Supercluster, the Hercules-Corona Borealis Great Wall, which is the largest known structure in the universe, with a length of over 10 billion light-years, and the Sloan Great Wall which is over 1 billion light-years in length. In superclusters, the clusters present may share material with each other like dust and gas clouds, etc. This may happen because of gravitational influence.

The Great Nothing region, also known as the Zone of Avoidance, is a relatively empty region of space in the southern hemisphere of the Milky Way galaxy. It is so-called because it is obscured from view by the densest parts of the Milky Way's disk and bulge, as well as by dust and gas in our own galaxy. Despite its name, the Great Nothing region is not entirely devoid of objects. It contains a number of galaxy clusters, galaxy groups, and other structures. Scientists and researchers are currently studying about it and its under research.

Now let’s discuss the mysterious objects in the universe the black holes and the neutron star. When a star that is more massive than the  Sun has burned all of its nuclear fuel, its core will begin to contract under the force of gravity. This contraction generates heat, which causes the surrounding layers of the star to expand and cool. As the core continues to contract, the temperature and pressure in the core will increase. Eventually, the core will become hot and dense enough to ignite helium fusion, where two helium nuclei combine to form beryllium. This process generates a lot of energy, which causes the star to expand and become brighter. Once helium fusion begins, the star will enter a new phase of its life known as the red giant phase. During this phase, the star will continue to expand and cool, eventually becoming so large that it may engulf nearby planets. As the star continues to fuse heavier elements, the core will eventually become composed of iron, which cannot undergo further fusion without absorbing energy instead of releasing it. Since the core cannot support itself, it will collapse rapidly under the force of gravity, producing an enormous amount of heat and pressure. This sudden collapse of the core will generate a shock wave that travels outward through the layers of the star. When the shock wave reaches the outer layers of the star, it will cause a massive explosion known as a supernova. The supernova explosion will release an enormous amount of energy, including light, heat, and radiation, and it will also produce heavy elements such as gold and platinum. The remaining core of the star will either collapse into a neutron star or a black hole, depending on its mass.

As the radius of neutron stars due to dense gravity is less so according to the law of conservation of angular momentum their speed increases. Their rotational speed is very large it can rotate several hundred times in a second and also has a strong magnetic field.  It's believed that stars with initial masses between roughly 8 and 25 times that of the Sun will end their lives as neutron stars, while stars with initial masses above 25-30 solar masses will likely form black holes. Black holes have much stronger gravity than neutron stars.  A black hole is a region of space where the gravitational pull is so strong that nothing, not even light, can escape from it. In astrophysics, a singularity refers to a point in space-time where the laws of physics as we know them break down. It is a mathematical concept that describes a region of space-time with infinite density, where the curvature of space-time becomes infinite. In the case of a black hole, the singularity is located at the center of the black hole, where the gravitational field is so strong that it becomes impossible for anything, including light, to escape. The singularity is hidden behind the black hole's event horizon, making it inaccessible to observation. The event horizon of a black hole is the boundary around the black hole beyond which nothing, not even light, can escape the gravitational pull of the black hole. It is the point of no return, beyond which any matter or radiation that gets too close to the black hole is inevitably pulled into it. An accretion disk is mostly composed of dust and gas clouds that orbit a black hole These clouds are captured by other nearby stars by the black hole's gravity. They move faster around the black holes releasing radiation including X-rays. They appear black because they do not emit light and light from it cannot escape.  As many people thought that black holes are holes they are not holes fact they are black objects. They are very dense you can imagine that the radius of a black hole formed from a star with 20 times the mass of the sun is estimated to be around 59 km. Afterward, the size of black holes increases through the process of accretion. The matter from the accretion disk falls into black holes resulting in an increase in the mass of black holes so its size continues to increase. The stars close to black holes are torn by the gravity of black holes.  Stars that are less massive like mass ranging from 0.07 to 8 times the mass of the Sun, exhaust their nuclear fuel and start to run out of energy. As the star's core contracts, the outer layers of the star expand and cool, forming a red giant. The red giant will eventually shed its outer layers, leaving behind a small, hot core known as a white dwarf. The white dwarf is incredibly dense and hot, with temperatures reaching up to 100,000 Kelvin. The white dwarf will slowly cool over time as it radiates its energy away into space. Eventually, after trillions of years, the white dwarf will become a cold, dark object known as a black dwarf. Quasars (or quasi-stellar objects) are extremely luminous and distant celestial objects that emit huge amounts of energy, primarily in the form of light and radio waves. They are believed to be powered by supermassive black holes at the centers of galaxies. As matter falls into the black hole, it becomes extremely hot and emits radiation, including X-rays and gamma rays, which are the primary sources of energy for quasars.

So this was all about the structure of the universe I tries to explain the concepts simply.