Welcome to Space Physics: Stars and the Universe!

Hello there! This chapter is where we truly look up and explore the enormous scale of the cosmos. Don't worry if these ideas seem massive—we'll break down everything from how our Sun works to the eventual fate of the most massive stars, and finally, how we know the entire Universe is expanding. Get ready to think big!

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1. The Sun as a Star (6.2.1)

Our Sun is the most crucial object in our Solar System, supplying almost all the energy that drives life on Earth.

1.1 Basic Properties of the Sun (Core)

  • The Sun is a star of medium size.
  • It consists primarily of two gases: Hydrogen (H) and Helium (He).
  • The Sun radiates energy across the electromagnetic spectrum, concentrating most of its output in three regions:
    • Infrared (IR)
    • Visible Light
    • Ultraviolet (UV)

1.2 How the Sun Generates Energy (Supplement)

Stars, including our Sun, are immensely powerful energy sources because of a process happening deep within their cores.

The energy that powers the Sun comes from nuclear reactions.

In a stable star like the Sun, these reactions involve nuclear fusion:

  1. Hydrogen nuclei are forced together under extreme heat and pressure.
  2. They fuse to form Helium nuclei.
  3. This process releases a colossal amount of energy (light and heat) according to Einstein's famous relationship, \(E = mc^2\).

Analogy: Think of nuclear fusion like smashing four tiny hydrogen LEGO bricks together so hard they form one slightly lighter helium brick, and the 'lost' mass is instantly turned into pure energy!

Key Takeaway

The Sun is a mid-sized, hydrogen/helium star, and its power source is the fusion of hydrogen into helium in its core.

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2. The Scale of Stars and Galaxies (6.2.2)

The universe is impossibly large, and we need special measurements to handle these enormous distances.

2.1 Galaxies and the Milky Way (Core)

  • A galaxy is a massive collection of stars, gas, dust, and dark matter held together by gravity.
  • Galaxies are made up of many billions of stars.
  • Our Sun is just one star in our galaxy, which is called the Milky Way.
  • The Milky Way is itself just one of many billions of galaxies that make up the entire Universe.
  • Other stars in the Milky Way are vastly further away from Earth than the Sun is.
  • The diameter of the Milky Way galaxy is approximately \(100\,000\) light-years. (This number helps illustrate the scale.)

2.2 Measuring Astronomical Distances (Core & Supplement)

We cannot use kilometres or metres to measure the distance to stars; the numbers would be too big! Instead, we use the light-year.

What is a Light-Year? (Core)

A light-year is the distance travelled in a vacuum by light in one year.

Remember, this is a unit of distance, not time!

  • Did you know? Light travels at \(3.0 \times 10^8 \text{ m/s}\). Since there are about 31,536,000 seconds in a year, a light-year is a huge distance.
  • Calculation (Supplement):

    $$1 \text{ light-year} = 9.5 \times 10^{15} \text{ m}$$

2.3 Calculating Travel Time for Light (Core)

We can use the speed of light (\(v\)) and the distance (\(d\)) to calculate the time (\(t\)) it takes for light to travel between objects in the Solar System or beyond, using the formula: \(t = d/v\).

Example: Light takes approximately 8 minutes to travel from the Sun to the Earth. This means we always see the Sun as it was 8 minutes ago.

Quick Review: Distance and the Sun
  • Unit of distance: The light-year.
  • Our star: The Sun (medium, H/He, fusion-powered).
  • Our galaxy: The Milky Way (100,000 light-years across).

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3. The Life Cycle of a Star (6.2.2 Supplement)

All stars have a beginning, a stable middle age, and an eventual death. The ultimate fate of a star is determined entirely by its initial mass.

Step-by-Step Stellar Evolution

1. Birth from a Nebula

A star begins as a cold, vast interstellar cloud of gas and dust, primarily containing hydrogen.

2. Protostar Formation

Due to internal gravitational attraction, parts of the cloud collapse. As the cloud shrinks, the material heats up dramatically, forming a dense, hot core called a protostar.

3. The Stable Star (Main Sequence)

When the core temperature is high enough (millions of degrees), hydrogen fusion begins. The star enters its stable phase (the "Main Sequence").

  • A stable star exists in a balance called hydrostatic equilibrium:

    Inward Force: Immense gravitational attraction trying to collapse the star.

    Outward Force: Pressure created by the high temperature and energy released by the nuclear fusion pushing outward.

4. Running Out of Fuel

All stars eventually exhaust the hydrogen fuel in their core, causing the fusion rate to drop. Gravity temporarily wins, causing the core to shrink and heat up again, leading to the dramatic final stages.

5. The Final Stages (Based on Mass)

A. Less Massive Stars (like the Sun):

  1. The star expands greatly to become a Red Giant (because the outer layers cool down).
  2. The outer layers drift away gently, forming an expanding shell of gas known as a Planetary Nebula.
  3. The remaining hot, dense core is left behind as a White Dwarf.

B. More Massive Stars (Many times the mass of the Sun):

  1. The star expands into a much larger Red Supergiant.
  2. The core collapses rapidly, resulting in a catastrophic explosion called a Supernova.
  3. This explosion forms a new nebula containing hydrogen and newly created heavier elements (like iron or gold).
  4. The dense remnant left behind is either a Neutron Star or, if it was extremely massive, a Black Hole.

Encouraging Note: Don't worry if all these names are confusing! The key point is that massive stars die violently (Supernova -> Neutron Star/Black Hole), and less massive stars die calmly (Red Giant -> White Dwarf).

Key Takeaway

A star's life is a constant battle between gravity and the pressure from fusion. When fusion stops, gravity dictates whether the star becomes a white dwarf or explodes into a supernova.

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4. The Expanding Universe and Redshift (6.2.3 Core)

The biggest question is: How do we know the Universe is changing? We use light from distant objects to gather evidence.

4.1 What is Redshift?

When studying the light (electromagnetic radiation) emitted by distant stars and galaxies, we observe the Doppler effect for light, known as redshift.

  • Definition of Redshift: Redshift is an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies.
  • When an object is moving away from an observer, the waves it emits are stretched out, increasing their wavelength.
  • Since the red end of the visible spectrum has the longest wavelength, the light from distant galaxies appears shifted towards the red end (it is "redshifted").

Analogy: Think of an ambulance siren. When it moves toward you, the sound waves are compressed (higher pitch). When it moves away, the sound waves are stretched (lower pitch). Redshift is the 'lower pitch' for light!

4.2 Redshift as Evidence

Because the light emitted from virtually all distant galaxies is redshifted:

  • It tells us that these galaxies are receding (moving away) from Earth.
  • The greater the redshift, the faster the galaxy is moving away.
  • This motion is evidence that the Universe is expanding.
  • This expansion supports the Big Bang Theory, which states that the Universe started from a very hot, dense state and has been expanding ever since.
Key Takeaway

Distant galaxies show redshift, meaning their light waves are stretched because they are moving away. This proves the Universe is expanding.

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5. Further Evidence of the Big Bang (6.2.3 Supplement)

5.1 Cosmic Microwave Background Radiation (CMBR)

In addition to redshift, scientists observe a specific type of radiation coming uniformly from all directions in space.

  • This radiation is called Cosmic Microwave Background Radiation (CMBR). It is microwave radiation of a specific frequency observed at all points in space around us.
  • Explanation: The CMBR was produced shortly after the Universe began (The Big Bang) when the Universe was extremely hot and dense. As the Universe expanded over billions of years, this intense radiation expanded and stretched. Its wavelength stretched so much that it moved into the microwave region of the electromagnetic spectrum.
  • The CMBR is often called the "echo" or "afterglow" of the Big Bang, providing incredibly strong evidence for the theory.

5.2 Hubble's Law and the Age of the Universe

The speed (\(v\)) at which a distant galaxy is moving away (determined by its redshift) is directly proportional to its distance (\(d\)) from us. This relationship is summarized by the Hubble Constant (\(H_0\)).

Hubble Constant Definition:

The Hubble constant (\(H_0\)) is defined as the ratio of the speed (\(v\)) at which a galaxy is moving away from the Earth to its distance (\(d\)) from the Earth.

Equation for Hubble Constant:

$$H_0 = \frac{v}{d}$$

  • Current Estimate: The current estimate for \(H_0\) is approximately \(2.2 \times 10^{-18} \text{ per second}\) (\(s^{-1}\)).
Estimating the Age of the Universe

If the expansion rate (\(H_0\)) has been constant, we can calculate the time it took for the galaxies to reach their current distance by rearranging the equation. Since \(v = d/t\), substituting this into Hubble's equation gives:

$$t = \frac{1}{H_0}$$

  • This calculation provides an estimate for the age of the Universe (around 13.8 billion years).
  • This result strongly supports the idea that all the matter in the Universe was initially present at a single point, confirming the Big Bang model.

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Revision Checklist: Stars and the Universe

Key Concepts to Understand:
  • The Sun is a medium star powered by hydrogen fusion into helium.
  • Astronomical distances are measured in light-years (\(9.5 \times 10^{15} \text{ m}\)).
  • The stellar life cycle proceeds from Nebula -> Protostar -> Main Sequence.
  • Star death depends on mass:
    • Low mass: Red Giant -> White Dwarf.
    • High mass: Red Supergiant -> Supernova -> Neutron Star or Black Hole.
  • Redshift shows that distant galaxies are receding, proving the Universe is expanding.
  • CMBR is the microwave 'afterglow' of the Big Bang.
  • Hubble's constant links recession speed and distance: \(H_0 = v/d\).