Formation of Galaxies, Stars & Planets – Cosmic Marvels – Geography Notes

Formation of Galaxy

• Galaxies are massive systems of millions or billions of stars, along with gas and dust, held together by gravitational attraction. They are considered the fundamental building blocks of the universe. 
• The size of a galaxy can vary greatly, ranging from the smallest ones with only about 100,000 stars to the largest ones with up to 3000 billion stars.

Classification of galaxies

Galaxies are usually classified based on their shape and are of three types :

• 1) Spiral
• 2) Elliptical
• 3) Irregular

Regular Galaxies

Spiral Galaxies	Elliptical Galaxies
The Milky Way is an example of a disc-shaped spiral galaxy that has a greater concentration of stars near its center. They consist of populations of old stars in the center and the youngest stars located in the arms.	Star distribution is nonuniform.
Spiral galaxies are well supplied with the interstellar gas in which new bright, young stars form.	Most of their member stars are very old and have no new star formation in them.
Smaller and less bright	The brightest galaxies in the universe are elliptical.

Irregular Galaxies

• The irregular galaxies comprise about one-tenth of all galaxies.
• The stars of irregular galaxies are generally very old.

Our Galaxy (the Milky Way)

• The thickness of the disc of the Milky Way can vary depending on the region. In the central bulge, it can be up to 10,000 light-years thick, while in the outer regions, it can be much thinner, around 500-2,000 light-years.
• The estimate of 100-400 billion stars in the Milky Way is based on observations and models, but the exact number is difficult to determine with certainty.
• The difference in orbital speed between stars closer to the center and those further out is known as differential rotation. It’s a common feature of spiral galaxies and is caused by the gravitational influence of the galaxy’s mass distribution.
• The Solar System is indeed located in the Orion Arm, which is a spiral arm of the Milky Way galaxy. We are about 26,000 light-years away from the center of the galaxy, which is roughly one-third of the way out from the center to the edge of the galaxy.
• The estimate for the time it takes for the Sun to complete one orbit of the Milky Way is actually closer to 225-250 million years, but the exact value is still a subject of ongoing research.
• Andromeda is indeed the closest spiral galaxy to the Milky Way, at a distance of about 2.5 million light-years. However, there are other galaxies (such as the Magellanic Clouds) that are closer to us and can also be seen in the night sky.

Formation of Stars

• Stars are massive, luminous spheres of plasma, which are held together by their own gravity. They emit energy in the form of heat and light, which is generated by nuclear fusion reactions that occur in their cores. In these reactions, hydrogen atoms are converted into helium, releasing vast amounts of energy in the process.
• The physical characteristics of stars, such as their size, color, brightness, and temperature, are used to classify them into different types. For example, larger stars tend to be hotter and more luminous than smaller stars, and different chemical elements in the star’s outer layers can give it a different color. 

Nebula

• Nebulas are clouds of gas and dust in space, primarily composed of hydrogen and helium, which can span hundreds of light-years across. 
• They are the birthplaces of stars, where the gravitational forces of the gas and dust cause them to collapse and eventually form a protostar.

Protostar:

• A protostar is a young star that has not yet reached the stage where nuclear fusion has started in its core. 
• It is formed when a cloud of gas and dust collapses under its own gravitational force, and as the material continues to fall inwards, the temperature and pressure at the center of the cloud increase.
• Once the temperature and pressure have reached a high enough level, nuclear fusion begins, and the protostar becomes a true star.
• During its protostar phase, the star continues to accrete material from the surrounding cloud, and it may also emit strong winds and jets of material as it continues to grow and evolve.

• Protostars are often surrounded by dense clouds of gas and dust, known as protostellar envelopes or protoplanetary disks, which can block most of the visible light emitted by the protostar. This makes it difficult to observe them directly in the visible spectrum.

T-Tauri star:

• T Tauri stars are often surrounded by a circumstellar disk of gas and dust, which is believed to be the remnant of the original protostellar cloud from which the star formed. 
• They are also known for their variable brightness, caused by changes in their accretion rate and the presence of star spots on their surface. 
• T Tauri stars are important objects of study for astronomers as they provide insights into the early stages of stellar evolution and the formation of planetary systems.

Main sequence stars:

• Main sequence stars are the most common type of stars in the universe and are characterized by the fact that they are fusing hydrogen into helium in their cores. 
• As a main sequence star exhausts the hydrogen in its core, it will eventually run out of fuel and begin to evolve into a red giant. The sun is a main sequence star.
• The outer layers of the star will expand outward, engulfing any nearby planets, before eventually being ejected as a planetary nebula. The core of the star will then shrink down to become a white dwarf.

Red dwarf:

• Red dwarfs are the most common type of star in the Milky Way galaxy, making up about 75% of all stars. 
• They have a mass range of about 0.08 to 0.5 times that of the Sun, and a surface temperature range of about 2,200 to 3,500 K. 
• They are also much cooler and less luminous than the Sun, which is why they are difficult to observe with the naked eye. Despite their small size, some red dwarfs can be very active and emit powerful flares and coronal mass ejections.
• Proxima Centauri is a red dwarf star located in the Alpha Centauri star system, which is the closest star system to our Solar System.

Red giant: 

• Red giants have diameters between 10 and 100 times that of the Sun.
• Even though red giants have a lower surface temperature than the Sun, they are much larger, which gives them a higher luminosity.
• A red giant is formed during the later stages of its evolution as it runs out of hydrogen fuel at its center.
• As the core of the red giant runs out of hydrogen, it contracts and heats up, causing the outer layers of the star to expand and cool down. The energy released by the hydrogen fusion in the shell around the core creates a strong outward pressure that pushes the outer layers of the star outwards, causing the star to expand in size. This expansion causes the star to cool down and its surface temperature to decrease, making it appear redder in color, hence the name “red giant”.
• Red giants can fuse helium into heavier elements like carbon, and some may even go on to fuse heavier elements up to iron. However, as you mentioned, most stars are not massive enough to produce the high pressures and temperatures necessary for these processes, so fusion eventually stops and the star’s core will contract and heat up. 
• The outer layers of the star will be expelled, forming a planetary nebula, and the core will either become a white dwarf or, in more massive stars, may continue to contract and heat up to become a neutron star or black hole.

Red Supergiant:

• As the red giant star condenses, it heats up even further, burning the last of its hydrogen and causing the star’s outer layers to expand outward.
• At this stage, the star becomes a large red giant. A very large red giant is often called Red Supergiant.
•  It’s the collapse and subsequent expansion of the outer layers that cause the red giant phase, not the condensation of the star.

Planetary Nebula: 

• The outer layers will expand outward, forming a red giant. Eventually, the red giant will shed its outer layers and leave behind a hot, dense core known as a white dwarf. 
• The ejected outer layers form a planetary nebula, which can be visible as a colorful cloud in space. 
• The name “planetary nebula” is actually a misnomer, as they have nothing to do with planets, but were so named because early astronomers thought they resembled the disks of Uranus and Neptune seen through telescopes.

White dwarf: 

• A white dwarf is a very small, hot star, the last stage in the life cycle of a star like the Sun.
• White dwarfs are the end stage of evolution for low- to medium-mass stars, such as our Sun, after they have exhausted all of their nuclear fuel.
• The high density is due to the fact that the electrons in the white dwarf have been compressed to such a degree that they no longer occupy distinct energy levels, but instead form a continuous “sea” of matter known as degenerate matter. This leads to unusual properties such as the fact that the more massive a white dwarf is, the smaller its radius becomes.

Nova: 

• The sudden brightening of the star in a nova event is due to the explosive fusion of the accumulated hydrogen on the surface of the white dwarf. 
• This fusion releases an enormous amount of energy and blows off the outer layers of the star into space, creating a visible shell of gas and dust known as a nova remnant.
• After the explosion, the white dwarf remains intact, and the process of accumulating hydrogen from the companion star may start again, leading to the possibility of future Nova events.

Supernova:

• The brightness of a supernova can be up to 100 million times that of the Sun, making it one of the most energetic events in the universe.
• The extremely luminous burst of radiation expels much or all of a star’s material at a great velocity, driving a shock wave into the surrounding interstellar medium.
• The shock waves generated by a supernova can trigger the condensation of gas and dust in a nebula, which can lead to the formation of a new star.
• Supernovae are one of the primary sources of cosmic rays, which are high-energy particles that originate from outside the Earth’s atmosphere. 

Supernovae can be triggered in one of two ways:

Type I supernova or Type Ia supernova (read as one-a)

• Type Ia supernovae occur in binary star systems where a white dwarf star is orbiting a companion star. As the companion star evolves, it can expand and start transferring material onto the white dwarf
• In a binary star system, a white dwarf can accrete material from its companion star, which can cause the white dwarf to become more massive. As the mass of the white dwarf increases, its core temperature and pressure also increase, eventually reaching a point where carbon fusion is triggered and also triggers runaway nuclear fusion, completely disrupting the star.

The difference between Nova and Type I supernova

Nova	Type I supernova
In a nova, the system can shine up to a million times brighter than normal.	A supernova is a violent stellar explosion that can shine as brightly as an entire galaxy of billions of normal stars.
As long as it continues to take gas from its companion star, the white dwarf can produce nova outbursts at regular intervals.	If enough gas piles up on the surface of the white dwarf, a runaway thermonuclear explosion blasts the star to bits.

Type II supernova

• A Type II supernova occurs when the core of a massive star collapses, triggering a catastrophic explosion that ejects the outer layers of the star into space.
• Type II supernovae are often associated with the death of massive stars, including red supergiants, blue supergiants, and Wolf-Rayet stars. These stars have short lifetimes and end their lives in spectacular fashion as Type II supernovae.

Importance of supernova:

• As a star’s core runs out of hydrogen, it will start to expand and cool, eventually becoming a red giant. In the core of the red giant, the temperature and pressure become high enough to allow helium fusion to occur.
• More massive stars begin a further series of nuclear burning. The elements formed in these stages range from oxygen to iron.
• During a supernova explosion is neutron capture, which involves the absorption of neutrons by atomic nuclei. This can produce heavy isotopes of elements, including those beyond iron, such as gold, platinum, and uranium.
• The explosion of a supernova can eject the newly synthesized elements, along with the remains of the star, out into interstellar space. These materials are then available to become part of new stars and planets, allowing for the recycling of matter in the universe.

Black dwarf

• A black dwarf is a theoretical endpoint for the evolution of some stars, it’s important to note that other possibilities exist depending on the mass of the star.
• A black dwarf is a theoretical object that would be formed from a white dwarf that has cooled to the point where it no longer emits any significant amount of heat or light.
• Since the estimated time required for a white dwarf to cool down to become a black dwarf is longer than the current age of the universe, which is estimated to be around 13.8 billion years, no black dwarfs are expected to exist in the universe yet.

Brown Dwarfs

• Brown dwarfs are objects which are too large to be called planets and too small to be stars.
• Brown dwarfs are thought to form in the same way that stars do – from a collapsing cloud of gas and dust.
• However, as the cloud collapses, the core is not dense enough to trigger nuclear fusion.

Neutron stars

• Neutron stars are extremely dense celestial objects that are composed almost entirely of neutrons. They are produced when the core of a massive star collapses during a supernova explosion, and the protons and electrons in the core combine to form neutrons.
• Neutron stars are incredibly dense objects, with a mass that can be several times that of the sun packed into a sphere just 10 to 20 kilometers in diameter. 
• If its mass is any greater, its gravity will be so strong that it will shrink further to become a black hole.

Black holes

• A black hole is an object with such a strong gravitational field that even light cannot escape from its surface.
• The gravity is so strong because matter has been squeezed into a tiny space, This can happen when a star is dying.
• Black holes usually cannot be observed directly, but they can be “observed” by the effects of their enormous gravitational fields on nearby matter.
• Black holes distort the space around them and can suck neighboring matter into them including stars.
• Gravitational lensing is a phenomenon where the gravitational field of a massive object, such as a black hole or a galaxy, bends the path of light passing nearby.

Frequently Asked Questions (FAQs)

1. How are galaxies formed?

Answer: Galaxies are formed through a process called cosmic evolution. It begins with the gravitational collapse of a region within a large cosmic structure, such as a filament or a supercluster. Over time, as matter accumulates due to gravity, the region becomes denser and heats up. Eventually, a protogalactic cloud forms, which further fragments into smaller clumps that eventually evolve into galaxies. The interplay between dark matter, gas, and stellar feedback processes influences the size, shape, and composition of galaxies.

2. What is the life cycle of a star?

Answer: Stars go through a life cycle that begins with the gravitational collapse of a region within a large molecular cloud. This collapse leads to the formation of a protostar, a hot and dense core surrounded by a rotating disk of gas and dust. As the protostar accumulates mass, nuclear fusion ignites in its core, and it becomes a main-sequence star. The star’s fate is determined by its mass: smaller stars like our Sun eventually expand into red giants and then become white dwarfs, while massive stars undergo supernova explosions, leaving behind neutron stars or black holes.

3. How do planets form in our solar system?

Answer: Planets form through a process known as accretion within a protoplanetary disk surrounding a young star. The process begins with the coagulation of small particles, such as dust and ice, into larger planetesimals. These planetesimals then collide and merge to form protoplanets. In our solar system, the gas giants (like Jupiter and Saturn) formed farther from the Sun, where the disk contained more ice, while the terrestrial planets (like Earth) formed closer to the Sun, where it was hotter and primarily composed of metals and silicates. The leftover debris in the disk can sometimes result in the formation of smaller celestial bodies like asteroids and comets.

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