Combined Science (9204) | Space physics

🌌 Welcome to Space Physics! 🚀

Hello future astrophysicists! Don't worry if the universe seems vast and complicated—we’re going to break it down into easy, understandable steps. This chapter is all about understanding our place in the cosmos, how things move in space, and the dramatic lives (and deaths) of stars.

We will stick strictly to what you need for your Combined Science exam, making sure you grasp the key vocabulary and essential processes. Let's start our journey!

Section 1: Our Place in the Universe – The Cosmic Hierarchy

The universe is enormous, but everything fits together in a specific structure, like a set of Russian nesting dolls. We need to know the basic components, from smallest to largest, and understand where they belong.

Key Components of the Universe

• Moon: A natural satellite that orbits a planet. Example: Our Moon orbits Earth.
• Planet: A large object orbiting a star. Example: Earth is a planet orbiting the Sun.
• Star: A huge ball of extremely hot gas that produces light and heat through nuclear fusion. Example: The Sun.
• Solar System: A star and all the planets, moons, asteroids, and comets that orbit around it. Example: Our Solar System.
• Galaxy: A massive collection of billions of stars, dust, and gas, all held together by gravity. Example: The Milky Way Galaxy (this is our galaxy).
• Universe: Everything that exists—all the galaxies, the space between them, and all matter and energy.

Memory Aid: Think of the order of increasing size: My Pet Squirrel Gets Upset (Moon, Planet, Star/Solar System, Galaxy, Universe).

Section 2: Gravity and Orbits

Why don't planets just fly off into space? The answer is gravity. Gravity is the invisible force that holds everything together and controls all movement in space.

What is Gravity?

Gravity is an attractive force between any two objects that have mass. The bigger the object's mass, and the closer the objects are, the stronger the gravitational force is.

Simply put: Big, heavy objects pull small objects towards them.

Gravity provides the necessary force for objects to follow curved paths (orbits) rather than straight lines.

Understanding Orbits

An orbit is the curved path taken by an object (like a planet or a satellite) around a much larger object (like a star or a planet).

How does an object stay in orbit?
It's a delicate balancing act between two factors:

1. Velocity (Speed): The object has a forward speed that tries to make it move in a straight line.
2. Gravity: The attractive force pulling the object inwards toward the centre of the orbit (e.g., pulling Earth toward the Sun).

If the object were too slow, gravity would win, and it would crash into the central body. If the object were too fast, it would escape gravity and fly away. Since these two forces are perfectly balanced, the object constantly "falls" towards the centre but its forward speed keeps missing, resulting in a stable, elliptical (almost circular) path.

Analogy: Imagine throwing a ball horizontally. If you throw it softly, it lands quickly. If you could throw it so fast that by the time it falls a small distance, the curvature of the Earth has dropped away by the same distance, the ball would be constantly falling around the Earth—that is an orbit!

Key Takeaway: Orbits are determined by gravity and the speed of the orbiting object.

Section 3: The Life Cycle of a Star

Stars are not eternal; they are born, they live, and they die. The destiny of a star is determined entirely by its starting mass. Let's look at the stages.

Step 1: Birth – The Nebula

Every star starts as a Nebula, which is a vast cloud of dust and gas (mostly hydrogen) floating in space.

Gravity causes these particles in the nebula to pull closer together. As the cloud collapses, the gravitational potential energy turns into heat, and the centre heats up intensely. This hot, dense core is called a Protostar.

Step 2: Adulthood – The Main Sequence Star

When the core of the protostar reaches about 15 million °C, nuclear fusion begins.

Nuclear Fusion: This is the process where hydrogen atoms combine (fuse) to form helium atoms, releasing enormous amounts of energy (light and heat). This energy pushes outward, perfectly balancing the inward pull of gravity.

A star that is stable and undergoing fusion is called a Main Sequence Star. Our Sun is currently a main sequence star and will be for billions of years to come.

Step 3: Death – When the Hydrogen Runs Out

Once the star runs out of hydrogen fuel in its core, the outward fusion pressure stops, and gravity starts to win. The core collapses, and the outer layers expand dramatically. The star’s fate depends on its initial mass:

A. The Fate of Small/Medium Stars (like the Sun)

1. Red Giant: The outer layers expand and cool down, making the star look larger and redder.
2. White Dwarf: Eventually, the outer layers drift away (forming a planetary nebula), leaving behind a small, hot, dense core called a White Dwarf. It no longer undergoes fusion but glows due to residual heat.
3. Black Dwarf: After billions of years, the white dwarf will cool completely and stop glowing. This cold, dark object is called a Black Dwarf (though none exist yet, as the universe is not old enough).

B. The Fate of Large Stars (much heavier than the Sun)

1. Red Supergiant: The massive star expands much further than a red giant, becoming a Red Supergiant. These stars are hot enough to fuse heavier elements (like carbon, oxygen, and iron) in their cores.
2. Supernova: When the core runs out of fuel and collapses completely, it causes a catastrophic explosion called a Supernova. Supernovae are incredibly bright and can briefly outshine an entire galaxy.
3. Remnants: What is left depends on the mass of the core after the explosion:
   • If the core is dense but not too heavy, it becomes a Neutron Star (a tiny, incredibly dense object).
   • If the core is extremely massive, gravity totally overwhelms everything, crushing the matter into an infinitely dense point called a Black Hole (an object whose gravity is so strong that nothing, not even light, can escape).

Common Mistake Alert! Be careful with the difference between a Nebula and a Planetary Nebula. A Nebula is where stars are born. A Planetary Nebula is the cloud of gas shed by a dying star (a Red Giant).

Section 4: The Expanding Universe – Evidence for the Big Bang

The Big Bang Theory is the most accepted explanation for the start of the universe. It suggests that the universe originated from a tiny, extremely hot, and dense point, and has been expanding and cooling ever since.

The key piece of evidence you must know that supports the expansion of the Universe is Redshift.

Understanding Redshift

To understand redshift, we first need to understand the Doppler Effect.

Analogy: Think of an ambulance siren.
When the siren moves towards you, the sound waves are compressed, making the pitch sound higher (shorter wavelength).
When the siren moves away from you, the sound waves are stretched out, making the pitch sound lower (longer wavelength).

Light behaves in a similar way. Light waves have different wavelengths, which we perceive as different colours. Red light has the longest wavelength, and blue light has the shortest visible wavelength.

• If a light source (like a galaxy) is moving towards Earth, the wavelengths of light are compressed, shifting them towards the blue end of the spectrum (Blueshift).
• If a light source is moving away from Earth, the wavelengths of light are stretched out, shifting them towards the red end of the spectrum (Redshift).

The Key Discovery

When scientists look at the light coming from nearly all distant galaxies, they observe that the light is always redshifted.

The Conclusion:
Since the light is redshifted, it means that the galaxies are all moving away from us, and the galaxies that are furthest away are moving the fastest. This observation is the fundamental evidence for an expanding universe.