Abdul Wahab - science4fun

The universe is a sort of thing in which everything, which we know of, lives including humans, Earth, planets, stars, black holes, and galaxies etc. Our universe is full of mysteries and unexplained events that amaze scientists around the world regularly.

What is the Universe?

galaxies-in-observable-universe — Each bright object and point in the image is a galaxy.

The definition of the universe is still incomplete because there are many observations that are still unexplained. But, in short, you can consider the universe as a thing that has space, time, and other stuff.

The actual size of our universe is not known because there are certain physical phenomena, such as the speed of light, that are restricting us from observing the whole universe. The universe that scientists can observe with science instruments is called “observable universe”. The current diameter of the observable universe is found to be around 93 billion light-years (1 light year = 9461 billion kilometers).

What is inside the Universe?

Our universe contains many types of objects and mysterious things. Some of these things you may know very well, such as planets, moons, asteroids, comets, stars, galaxies, and black holes, etc. All of these objects are made of matter. There are also many mysterious things that, we know indirectly, are the part of the universe, such as dark matter and dark energy.

Birth of the Universe – The Big Bang Theory

Scientists have estimated that our universe was formed around 14 billion years ago. The formation of our universe is described by the theory of the big bang. The theory describes that our universe started its journey from the rapid expansion of something that was very small, dense, and hot. After the rapid expansion, the universe continued to expand and the formation of cosmic objects began until now.

One of the strange and interesting facts is that our universe is still expanding today at an accelerating rate. Scientists have explained the accelerated expansion by the presence of mysterious energy called dark energy. After the rapid expansion, it took around 380,000 years for the first atoms to be formed – that were the atoms of hydrogen. The first stars that illuminated the sky started to appear 200 million years after the rapid expansion of the universe.

Dark Matter and Dark Energy

Dark matter and dark energy are one of the least known things in our universe. Scientists don’t have direct evidence for their existence, they only know them indirectly. For example, the existence of dark matter was found from the gravitational force which they exert on our normal matter. The reason we don’t know much about dark matter is that dark matter doesn’t interact with electromagnetic forces. In other words, it doesn’t interact with light and can pass through normal matter without being noticed.

Dark energy, on the other hand, was found from the fact that our universe is still expanding at an accelerating rate. The unknown source of energy that is powering the accelerating expansion of the universe is dark energy. The word ‘dark’ can be a bit confusing, but it is used because we don’t know much about these things.

Facts

We get the clue of the expansion of the universe from the fact that galaxies are going away from each other with each passing moment.
Our universe has no center because all the galaxies are uniformly distributed in the observable universe.
The multiverse is a concept that is used to describe more than one universes.

The Sun is one of the stars that you see in the skies during night time, but it is the only star in our solar system. All the planets, including our Earth, orbit around the sun due to its immense gravity. Also, life is possible on Earth due to the presence of the Sun.

There are billions of stars in our galaxy and the Sun is categorized as a smaller star – also called a yellow dwarf. The sun is the largest and most massive object in our solar system which accounts for 99.86% of the total mass of our solar system. The planets, moons, asteroids, and comets together make-up around 0.14% of the solar system.

Formation

The age of our Sun is estimated to be 4.6 billion years, which means the Sun was formed that much ago. The formation of the Sun began when the molecules of hydrogen and helium gases in a giant cloud began to collapse. The collapse of molecules occurred due to the gravitational force. When enough mass of the gases was compressed, the fusion reaction started and the Sun was born.

The size of the newly born Sun was much smaller and its brightness was less than today’s sun. Because, as the Sun ages, its size and brightness increases. Our Sun is considered to have passed half of its life.

Structure

The Sun has a radius of 695,510 kilometers and has many layers that differ from each other due to various physical properties. But, the main layers of our Sun are core, radiative zone, tachocline, convective zone, photosphere, and atmosphere.

Let’s discuss these layers in more detail.

Core – The core is the vital layer of the Sun and any other star because this is the layer in which the energy is produced by the fusion reaction. The core occupies around 25% of the Sun’s radius and has a tremendous amount of pressure and temperature. The temperature of the Sun in this layer is around 15 million degrees Centigrade (or 27 million Fahrenheit).

Radiative Zone – Radiative zone begins after the core and occupies around 45% of the Sun’s radius. The energy produced by the core transfer through this layer. The transfer of energy from this layer occurs by the method of radiation, instead of convection. It takes around 100,000 years for a photon to travel from this layer and reach the surface of the Sun.

Tachocline – Tachocline is a very thin layer and is simply a transition layer in which certain physical processes occur that separate radiative zone from the convective zone.

Convective zone – This layer of the sun extends from the tachocline layer and meets with the surface of the sun (the photosphere). The convective zone has a lower density as compared to the radiative zone, which makes it easier to transfer the heat by convection.

Photosphere – This is the layer that is visible and is considered as the surface of the Sun. The temperature here drops to 5,500 °C and radiation from this layer finally escapes the Sun. The thickness of the photosphere changes throughout the Sun and may change from 10 km to 100 km.

Atmosphere – Atmosphere is the layer that surrounds the surface of the Sun. It is composed of various other layers which include chromosphere, transition region, corona, and heliosphere. The atmosphere of the Sun can also be seen with naked eyes during a solar eclipse.

Composition

At the time of formation, the Sun was majorly made of hydrogen and a small amount of helium gas. But, since the Sun fuses hydrogen atoms to make helium atoms in a process called fusion. At the present time, the hydrogen accounts for 75% of the Sun, while helium is 23.8%. The remaining mass of the Sun contains other elements such as oxygen, iron, and carbon.

The fusion of hydrogen into helium gas happens at the core of the Sun. The fusion reaction consumes hydrogen and produces helium which has increased the quantity of helium in the core of the Sun. It is estimated that newly born Sun may have 24% helium at the core, but now helium accounts for 60% of the core. Whereas, the hydrogen remains at 40%.

Facts

It is estimated that, when the Sun will reach near its death in about 7 billion years, it will expand so much that it will gulp the orbit of our Earth.
The sun along with whole solar system orbits around the Milky Way – much like planets around the Sun. It takes around 250 million years to complete one orbit around the center of the Milky Way.
“UY Scuti” is the largest star in our observable universe. It is so big that if you replace this star with our sun, its diameter alone will cross the orbit of Jupiter; its radius is around 1700 times larger than our Sun’s radius.

During night time, the sky is full of twinkling white stars that illuminate all night. The stars that you can during the night are a few light-years to millions of light-years away from the Earth. Our Sun is also a star and belongs to a group of smaller sized stars.

What is a Star?

Stars are the giant balls of gas that burn very hot and bright that they become visible to us during the night. Most of the stars are made of hydrogen and helium gas. They stay brighter and hotter due to the nuclear fusion of hydrogen gas into helium.

There are several hundred billion stars in a galaxy, and there are around a few hundred billion galaxies that scientists have found in our universe. You may have noticed during the night time that stars twinkle; their brightness changes between dim and full. This is actually not the brightness of a star that dims or brightens, but it is due to the Earth’s atmospheric effects.

Formation of Stars

The formation of a star begins in a cloud of dust that mostly contains hydrogen and helium gases. The high-density regions of the cloud start to collapse and merge with each other due to their own gravitational force.

Due to collapsing, the density and temperature of that region increases; the center has the highest temperature. The material that collapses takes the shape of a sphere which is very hot, also called a protostar. The compression process continues in the protostar and its temperature and density gradually rise.

When the temperature of a protostar reaches around 1 million degrees Celsius, nuclear fusion starts and a new star is born. In nuclear fusion, hydrogen atoms fuse together to form a helium atom. The fusion of hydrogen into helium produces a large amount of energy that powers a star.

Types of Stars

There are many types of stars according to their formation, size, and some other physical properties. But for simplicity, there are 3 main types of stars; giants, dwarfs, and neutron stars.

Giants Stars – Giant stars are the largest of all the types of stars and some of them can even grow to the size of our entire solar system. The lifespan of a giant star is less as compared to dwarf stars. It is because, due to the large size, they have tremendous pressure and temperature at the center. So, they consume fuel for nuclear fusion faster than the dwarf star.

Dwarfs Stars – Dwarfs are smaller stars and there are 3 types of dwarfs, which are brown dwarfs, white dwarfs, and yellow dwarfs. Among these three dwarfs, yellow dwarfs are larger and our Sun belongs to this category of stars. White dwarfs are smaller and are formed when a star crosses the red giant stage of its life. The last ones are brown dwarfs, they are simply failed stars that resemble mostly with gas giant planets, such as Jupiter.

Neutron Stars – Neutrons stars are very different from giants and dwarf stars. They are made of subatomic particles called neutrons. They have tremendous density but are very smaller in size. Neutron stars are roughly around 20 kilometers in diameter, but their weight is more than our Sun.

Lifecycle of Stars

Birth Stage – In this state, the formation of a star begins until nuclear fusion starts.

Main Sequence Stage – This stage covers a large portion of a star’s life and the gradual consumption of hydrogen into helium continues. The main sequence stage begins after the birth stage is completed and ends when a star has consumed most of its nuclear fuel.

Red Giant Stage – When smaller and medium-sized stars consume its nuclear fuel (e.g. hydrogen), then they go to the red giant stage. In this stage, the core of the star collapses and its outer part expands.

White Dwarf Stage – A White dwarf is the remaining core of the star after it has passed the red giant stage. It is a dead star because no nuclear fusion occurs in it. But due to previous nuclear fusion, the white dwarf still has hot temperatures and emits light. The white dwarf slowly faints as its stored energy is radiated away with time.

Supernova – The death of large stars occurs very differently as compared to smaller stars. When a star consumes all of its nuclear fuel, it is collapsed under its own gravity and suddenly an explosion occurs. The explosion is so big that, it outshines the whole galaxy in which it lived. This explosion is called a supernova and is the biggest explosion in the universe known to humanity.

Facts

A neutron star is the densest known object and has gravity so strong that it can capture the light.
The smaller stars have a larger age as compared to big stars. It is because, small stars consume nuclear fuel slowly, whereas big stars consume nuclear fuel very quickly.
Our Sun has crossed half of its life span in around 4.5 billion years. After the next 4.5 billion years, our Sun will enter the red giant stage and will expand beyond the orbit of Mars.

Our solar system is a set of the Sun and the objects that orbit around it. The objects that orbit around the Sun include planets, dwarf planets, moons, asteroids, and comets. The sun and these objects are bound together with gravitational force.

Objects In The Solar System

Let’s discuss the objects in our solar system in more detail.

Sun – Our solar system has one star called the Sun. It is located at the center of the solar system and is the largest of all the objects combined together. The Sun also plays a central role in keeping all the objects in the orbit and making life possible on the Earth.

Planets – A formal definition of a planet is: an object that is large and has enough gravitation to make itself round and has its own apparent motion. Well, according to this definition, there are around 8 planets in our solar system which are:

Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune

Dwarf Planets – Dwarf planet are those objects that are large but are not considered as full planets due to various reasons, such as no apparent motion or a clear orbit of its own. Pluto was once considered a planet, but now it has been categorized as a dwarf planet.

Moons Or Natural Satellites – Moons are the objects that orbit around a planet instead of the Sun. Our Earth has one moon, called Luna. The largest number of moons in our solar system orbit around planet Jupiter; it has 67 moons, out of which 5 are large.

Asteroids – These are smaller and irregular rocky objects that orbit around the Sun in our solar system. Most of the asteroids are found in a region called asteroid belt that is located in a space between Mars and Jupiter. However, there are also individual asteroids that wander in the solar system and many larger ones have hit our Earth several times.

Comets – Comets are made of ice, or sometimes rock covered with ice. They orbit around the sun and are different from asteroids because they have a visible tail of gases behind them. The tail of the comets is formed when sunlight warms ice and causes it to release gases.

How The Solar System Was Formed?

Our solar system was formed around 4.5 billion years ago from the solar nebula. The solar nebula was a giant cloud of gases and dust which, due to gravitation, flattened and most of the gases accumulated at its center. The accumulation of gases at the center increased the pressure and temperature enough that the Sun was born.

The planets, including our Earth, and other objects in the solar system were formed from the remaining gases and dust particles. There are around four rocky planets, also called terrestrial planets, which include Mercury, Venus, Earth and Moon. These terrestrial planets occupy the inner region of the solar system, whereas the outer region of the solar system is occupied by other four planets, also called gas giants, which include Jupiter, Saturn, Uranus, and Neptune.

The gas giants are located farthest from the Sun because of strong solar winds from newly born Sun. The solar winds pushed lighter gases (i.e. hydrogen and helium) farther away from which the four gas giants were formed.

Facts

Our Solar system is a tiny part of the galaxy, called the Milky Way. It is estimated that the Milky Way contains around 200 billion stars. Many of these stars have their own system of planets.
Our solar system has a radius of around 39.5 AU (astronomical units). One astronomical unit is equal to the average distance of Earth from the Sun, which is 150 million kilometers.
The Sun accounts for 99.85% of the total mass of the solar system. That means, all the planets and other objects in the solar system account for only 0.15% weight of the solar system.

During the night time in a clear sky, you can see a lot of stars scattered all around. Most of the stars that you can see belong to our own galaxy called “Milky Way”. There are many other galaxies that are distributed throughout the universe. You can watch some galaxies clearly with the help of a hobbyist telescope, the other galaxies can be seen with large astronomical telescopes.

What is a Galaxy?

Galaxies are the home of stars and other cosmic objects. You can simply consider them as a cluster of stars, planets, asteroids, dust particles, black holes, and dark matter. All the members of a galaxy are held together by gravitational force – much like planets around the Sun. An average-sized galaxy has around a few hundred billion stars; large ones have trillions of stars. Many of those stars have their own solar system that houses many planets and their moons.

The gravity that holds all the stuff in a galaxy comes from a large black hole at the center of a galaxy. But, a black hole alone is not sufficient, dark matter also takes part in holding and shaping a galaxy. Scientists have estimated that there are more than 100 billion galaxies in the observable universe.

How Galaxies Are Formed?

The formation of first stars in the universe happened around 180 million years after the big bang. Astronomers think that early galaxies were the groups of roughly a million stars. But, how exactly the galaxies were formed is still an open question. Because there are many factors that took part in the formation of galaxies other than matter. For example, dark matter also took part in the formation and shaping of galaxies. The findings from the Hubble telescope suggest that the early galaxies started to form around 1 billion years after the big bang.

Types of Galaxies

Milky-way-galaxy-spiral-shaped — Milky way is a barred-shape spiral galaxy.

Galaxies have various shapes and physical properties but all of them can be divided into 4 types. These are elliptical, spiral, barred spiral, and irregular galaxies.

Elliptical Galaxies – These are the galaxies that come close to the shape of an ellipse. The rate of new star formation is slower in elliptical galaxies as compared to other types. The typical size of an elliptical galaxy is larger than a spiral galaxy.

Spiral Galaxies – Spiral galaxies are circular in shape and have a center from which spiraling arms begin. The center of a spiral galaxy has mostly older stars as compared to its arms. It is estimated that spiral galaxies have more dark matter than our ordinary matter. The dark matter is considered responsible for keeping the spiral galaxies in shape.

Barred Spiral Galaxies – These galaxies have a rectangular type of bar in the center, and spiraling arms beginning from the ends of the bar. Our own galaxy, Milky Way, is also barred spiral galaxy.

Irregular Galaxies – Galaxies that don’t fit in the above discussed 3 types are placed in the list of irregular galaxies. Most of the time, irregular galaxies are formed when two galaxies collide with each other.

Facts

Our solar system is itself bound in an orbit around the center of our galaxy – the Milky Way. It takes around 200 million years to our solar system to complete one orbit and is called galactic year.
Our Milky Way galaxy has a diameter of around 100,000 light-years and weighs around 1.5 trillion times the mass of the Sun.
The closest neighbor to our Milky Way galaxy is the Andromeda galaxy, which is around 2.6 million light-years away. Furthermore, the Andromeda galaxy is also on the course to collide with our Milky Way in about 4.5 billion years.