Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Space science and astronomy → The universe and its components ↓


Stars and galaxies


The universe is a vast and fascinating place, filled with many wonders and mysteries. Two of the most fascinating components of the universe are stars and galaxies. Understanding these celestial phenomena helps us appreciate the vast scale and beauty of the universe. In this explainer, we will explore what stars and galaxies are, how they form, and their importance in space science and astronomy.

What are stars?

Stars are massive, luminous celestial bodies composed mostly of hydrogen and helium. They are held together by gravity, and they produce energy through nuclear fusion reactions that take place in their cores. This fusion process converts hydrogen into helium, releasing enormous amounts of energy that radiates outward, making the star shine.

Stars are born in clouds of gas and dust called nebulae. When an area of the nebula collapses due to its own gravity, it may form a protostar. Over time, if conditions are right, the protostar will continue to collapse and heat up, eventually reaching the temperature and pressure necessary to initiate nuclear fusion.

Stars come in different sizes, colors, and temperatures. These characteristics depend on their mass and the stage of their life cycle. Here is a simple classification based on temperature and color:

Blue Stars: Very hot and massive stars.
White Stars: Moderately hot stars.
Yellow Stars: Medium temperature stars. Our Sun is an example.
Red Stars: Cooler and often older stars.

Life cycle of a star

Stars go through a lifecycle that involves different phases. This lifecycle can span millions or even billions of years. Here is a basic outline of a star's lifecycle:

1. Stellar nebula

A stellar nebula is where stars form. It is a cloud of gas and dust that collapses under gravity to form a protostar.

2. Main sequence

When the protostar gets hot enough to begin nuclear fusion, it enters the main sequence phase. During this longest phase of a star's life, it converts hydrogen into helium in its core.

3. Red giant or supergiant

When a star runs out of hydrogen fuel, it turns into a red giant or a supergiant, depending on its initial mass. Larger stars become supergiants.

4. The final step

The ultimate fate of a star depends on its mass:

  • Low-mass stars: These stars, like our Sun, become white dwarfs after becoming red giants by shedding their outer layers.
  • High-mass stars: These stars can explode as supernovae, leaving behind neutron stars or black holes.

What are galaxies?

Galaxies are huge collections of stars, gas, dust, and dark matter bound together by gravity. They contain billions of stars and can take on different shapes and sizes. Galaxies are the building blocks of the universe, and they come in different types. Here are the three main types of galaxies:

1. Spiral galaxies

Spiral galaxies, such as our Milky Way, have a flat, spinning disk with spiral arms. These galaxies often have a central bulge surrounded by the disk. Spiral galaxies are rich in gas and dust, making them regions of active star formation.

2. Elliptical galaxies

Elliptical galaxies have a smooth, oval shape and contain old stars and very little gas and dust, resulting in minimal star formation. They range from nearly spherical to highly elongated structures.

3. Irregular galaxies

Irregular galaxies have no definite shape and look chaotic. They are often formed as a result of gravitational interactions or collisions between galaxies.

Formation of galaxies

Soon after the Big Bang, galaxies formed in the early universe. As gravity pulled matter together, small structures began to emerge. Over time, these small structures merged and turned into galaxies. The process of galaxy formation is complex and involves interactions between dark matter and baryonic matter (normal matter).

Milky Way galaxy

The Milky Way is the galaxy that contains our solar system. It is a spiral galaxy with a central bulge, a rotating disk with spiral arms, and a circular halo. The Sun is located in one of the spiral arms, called the Orion Arm, about 26,000 light-years from the center.

Like other galaxies, the Milky Way is a dynamic and evolving structure. Stars are born and die, clouds of gas and dust move and interact, and gravity shapes the galaxy's overall structure.

The relationship between stars and galaxies

Stars are the basic building blocks of galaxies. While individual stars are fascinating in their own right, their collective arrangement in galaxies reveals a deeper picture of cosmic structures. Studying stars and galaxies together helps scientists understand the formation and evolution of the universe.

Conclusion

Stars and galaxies are integral parts of the universe. Stars illuminate the sky with their bright glow, while galaxies organize these stars into stunning cosmic structures. Understanding stars and galaxies gives us a glimpse into the vastness of the universe and the complex dynamics going on within it. As we continue to explore these celestial phenomena, we learn more about the origin, nature, and ultimate destiny of the universe.


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Stars and galaxies

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