The interstellar medium is the "space" between stars in a galaxy.

Think of it as a very dilute soup 🍜 of mainly hydrogen atoms with a sprinkle of helium, a dash of carbon, oxygen, nitrogen, and a hint of cosmic dust and rays.

🌌 Ingredients

• Hydrogen Atoms: 🥄
• Helium: 🥄🥄
• Carbon, Oxygen, Nitrogen: 🥄🥄🥄
• Cosmic Dust & Rays: 🌌

Density of interstellar medium: Imagine squeezing 1 hydrogen atom into a sugar cube 🎲. That's roughly its density! Compare that to Earth's best vacuum which has 100,000 molecules in the same space!

How does a star come to life? The interstellar medium starts contracting due to gravity.

As it contracts, it heats up (like how a snowball becomes denser and icier as you pack it). This rise in temperature is thanks to the conversion of gravitational potential energy to thermal energy.

🤓 Imagine a bouncy ball 🎾. The more you drop it from a height, the more energy it has when it hits the ground!

Once this 'ball of gas' hits around 10 million degrees Kelvin (that's hotter than your oven by... a lot), nuclear fusions start. This newborn star is now a protostar!

The protostar becomes a main sequence star. Imagine it's like leveling up in a video game 🎮.

On the HR (Hertzsprung-Russell) diagram, our star will now find its spot based on its temperature and brightness.

Balance of Forces: While in the main sequence, the star is like a seesaw 🪀. On one side, there's gravity pulling it in, and on the other, there's thermal and radiation pressure pushing out.

⚖️ Equilibrium Achieved

• More Mass = More Gravity = More Pressure Needed.
• Hence, massive stars need hotter cores.

Brighter & Bigger Stars Burn Out Faster! 🌞 vs. 💥

• A big star, like one 10 times the Sun's mass, is a short-lived superstar, shining brightly for just about 30 million years!
• Our Sun? A middle-aged star, set to shine for around 10 billion years.