Science (Single Award) | Astrophysics

Welcome to Astrophysics! Studying the Stars

Hello future astrophysicist! Don't worry if this chapter seems intimidating. Astrophysics is just a fancy word for studying space, stars, and the entire Universe. We’ll be breaking down huge concepts—like the birth and death of stars and the origins of everything—into simple, manageable chunks.
Ready to explore? Let's begin our journey!

Section 1: Our Cosmic Neighbourhood

1.1 The Structures of the Universe

When we look up, we see many things, but these objects are organised into gigantic structures. Understanding the scale is the first step!

• Solar System: This is our home! It includes the Sun (our star), eight planets (like Earth), moons, asteroids, and comets, all held together by the Sun's gravity.
• Galaxy (The Milky Way): A galaxy is a massive collection of billions of stars, planets, gas, and dust, all rotating around a central point. Our Solar System is just one tiny corner of the Milky Way Galaxy.
• Universe: Everything! The Universe contains billions of galaxies, separated by vast empty spaces.

Think of it this way: Your Solar System is your house, the Milky Way is your city, and the Universe is the entire country. It puts the scale into perspective!

1.2 Defining the Light-Year

The distances in space are so enormous that using kilometres or miles would give us huge, impractical numbers. That’s why astronomers use the light-year.

The definition of a light-year: It is the distance that light travels in one year.

!! Common Mistake Alert !!
Students often think a light-year measures time. It does not! It measures distance.
Since light is the fastest thing we know, this unit helps us measure how far away objects truly are.

Did you know? Light from the Sun takes about 8 minutes to reach Earth. So, the Sun is 8 light-minutes away. The next closest star, Proxima Centauri, is about 4.2 light-years away!

Section 2: The Expanding Universe – Evidence

How do we know the Universe is so vast and that it's actually changing? Scientists rely on two major pieces of evidence related to light and waves.

2.1 Red-Shift (Evidence of Expansion)

Don't worry if this concept sounds tricky; let's use an analogy: the Doppler Effect (often heard with sound waves).

When an ambulance siren moves towards you, the sound waves are squashed, making the pitch higher. When it moves away from you, the sound waves are stretched, making the pitch lower.

Light behaves similarly. Light waves from objects moving:

• Towards us: The light waves are squashed (shorter wavelength) -> Blue-Shift.
• Away from us: The light waves are stretched (longer wavelength) -> Red-Shift.

When astronomers look at light from distant galaxies, they observe that the light is almost always Red-Shifted.

What does this mean? Almost all galaxies are moving away from us! The further away a galaxy is, the greater the red-shift, meaning it is moving away faster. This is strong evidence that the Universe is expanding.

2.2 Cosmic Microwave Background Radiation (CMBR)

If the Universe started from one very small, hot, dense point (the Big Bang), there should be heat left over from that initial explosion.

The Cosmic Microwave Background Radiation (CMBR) is that leftover heat.

It is faint, low-energy radiation (in the microwave part of the spectrum) that comes equally from every direction in space. It is often described as the "echo" or residual glow from the very early Universe. Finding the CMBR confirms the Big Bang theory’s prediction that the Universe started extremely hot and has cooled down significantly as it expanded.

Section 3: The Life Cycle of a Star (Like Our Sun)

Stars are born, live for billions of years, and eventually die. The life cycle depends massively on the star's starting mass. For Single Award Science, we focus on the path taken by stars similar to our Sun (medium mass).

3.1 Birth of a Star (Stages 1 & 2)

1. Nebula: A star begins as a gigantic cloud of gas (mostly hydrogen) and dust in space. Gravity starts pulling this material inwards.
2. Protostar: As the cloud shrinks, the gravitational energy is converted to heat, and the temperature rises dramatically. This hot, dense lump is called a protostar.

3.2 The Adult Star (Stage 3)

3. Main Sequence Star: When the core temperature reaches about 15 million °C, nuclear fusion begins. Hydrogen atoms fuse to form Helium, releasing massive amounts of energy.
   • During this long phase (billions of years), the star is stable. The outward pressure from the fusion energy balances the inward pull of gravity. This is the stage our Sun is in right now.

3.3 The Death of a Medium-Sized Star (Stages 4, 5, & 6)

When a star like the Sun runs out of hydrogen fuel in its core, its stability ends.

4. Red Giant: The fusion stops, and gravity starts crushing the core. This makes the core heat up, which in turn causes the outer layers of the star to expand dramatically and cool down (making them look red). This expanded star is a Red Giant.
5. White Dwarf: Eventually, the outer layers drift away into space, leaving behind the small, extremely hot, and dense core. This is a White Dwarf. No more fusion occurs here; it just glows brightly from leftover heat.
6. Black Dwarf: Over tens of billions of years, the white dwarf will cool down until it stops emitting light and becomes cold and dark—a theoretical Black Dwarf.

Memory Aid for the Sun's Life Cycle:
N -> P -> M -> R -> W -> B
Nebula, Protostar, Main Sequence, Red Giant, White Dwarf, Black Dwarf.

Section 4: The Big Bang Theory (Origin of the Universe)

The Big Bang theory is the leading scientific explanation for how the Universe began.

4.1 The Core Idea

The theory suggests that approximately 13.8 billion years ago, the entire Universe was concentrated in a single, incredibly hot, and dense point.

• This point suddenly began to expand and cool rapidly.
• It is still expanding today, which is why distant galaxies are moving away from us (as shown by Red-Shift).

4.2 Evidence Supporting the Big Bang

We already covered the two main pillars of evidence that make this theory so strong:

1. Red-Shift (Expansion): The observation that almost all galaxies are moving away from us provides clear evidence that space itself is stretching, confirming the ongoing expansion predicted by the Big Bang model.
2. CMBR (Leftover Heat): The uniform microwave radiation detected everywhere in space is the cooled-down remnant energy from the initial, extremely hot starting point of the Universe.

Don't worry if the idea of the Universe starting small is confusing. The important thing is to remember that the theory is strongly supported by physical observations (Red-Shift and CMBR).