Astrophysics

Astrophysics is the branch of physics that applies physical principles and laws to understand the universe and the celestial objects within it, such as stars, galaxies, and nebulae.

### Measuring the Universe

To comprehend the vast scales of the cosmos, we use specialised units. The Astronomical Unit (AU) is the average distance from the Earth to the Sun. For greater distances, we use the light-year, the distance light travels in one year. For professional astronomy, the parsec (pc) is the preferred unit.

The primary method for measuring distances to nearby stars is trigonometric parallax. This is the apparent shift in a star's position against a background of more distant stars as the Earth orbits the Sun. The parallax angle (p), measured in arcseconds, is inversely proportional to the distance (d) in parsecs.

Formula: d = 1/p

### Stellar Luminosity and Brightness

A star's intrinsic brightness is its Luminosity (L), which is the total power it radiates in all directions, measured in Watts. How bright a star appears from Earth is its apparent brightness (b) or flux, measured in W/m².

Apparent brightness depends on both the star's luminosity and its distance from us, following the inverse square law. As light travels outwards from a star, it spreads over the surface of a sphere. The surface area of this sphere is 4πd², so the brightness diminishes with the square of the distance.

Formula: b = L / (4πd²)

This relationship is crucial as it allows us to calculate a star's luminosity if we can measure its apparent brightness and its distance.

### Stellar Classification and the H-R Diagram

Stars are classified based on their surface temperature, which also determines their colour and spectral characteristics. The spectral classes are arranged from hottest to coolest: O, B, A, F, G, K, M. O-type stars are hot and blue, while M-type stars are cool and red. Our Sun is a G-type star.

The Hertzsprung-Russell (H-R) Diagram is a fundamental tool in astrophysics. It is a scatter plot of stars showing the relationship between their luminosity (or absolute magnitude) on the y-axis and their surface temperature (or spectral class) on the x-axis. Note that the temperature axis is reversed, running from high to low.

Key regions on the H-R diagram include:

* Main Sequence: A diagonal band from the upper-left (hot, luminous) to the lower-right (cool, dim). About 90% of stars, including our Sun, are in this stage, fusing hydrogen into helium in their cores.

* Giants and Supergiants: Found in the upper-right. These stars have exhausted the hydrogen in their cores and are very large and luminous, despite having cool surfaces.

* White Dwarfs: Found in the lower-left. They are the hot, dense, and dim remnants of low-mass stars.

### Stellar Evolution: The Life Cycle of Stars

The life of a star is a battle between gravity, which tries to crush it, and the outward pressure from nuclear fusion in its core. A star's evolution is determined almost entirely by its initial mass.

1. Birth: Stars are born from vast, cold clouds of gas and dust called **nebulae**. Gravity causes a region of the nebula to contract and heat up, forming a **protostar**.

2. Main Sequence: When the core becomes hot and dense enough (around 10 million Kelvin), nuclear fusion begins. The protostar becomes a stable **main sequence star**, where it will spend most of its life.

3. Later Stages (Low-Mass Stars like the Sun):

After exhausting its core hydrogen, a low-mass star's core contracts and the outer layers expand and cool, turning it into a Red Giant. The outer layers are eventually ejected, forming a planetary nebula, leaving behind the extremely dense core: a White Dwarf. This remnant slowly cools over billions of years.

4. Later Stages (High-Mass Stars):

A star more than eight times the mass of the Sun becomes a Red Supergiant. Its life ends in a spectacular explosion called a supernova, which briefly outshines an entire galaxy. The supernova creates heavy elements and scatters them into space. The remnant core collapses into either a Neutron Star (an incredibly dense object) or, if the original star was massive enough, a Black Hole, an object with gravity so strong that not even light can escape.

### Cosmology: The Big Bang and the Expanding Universe

The study of the universe as a whole is cosmology. The cornerstone of modern cosmology is the observation that the universe is expanding. We know this through the cosmological redshift of light from distant galaxies. As space itself expands, the wavelength of light travelling through it is stretched, shifting it towards the red end of the spectrum. The amount of redshift is proportional to the galaxy's distance.

This observation is formalised in Hubble's Law:

Formula: v = H₀d

Where 'v' is the galaxy's recessional velocity, 'd' is its distance, and H₀ is the Hubble constant. This law is the primary evidence for the Big Bang Theory, which posits that the universe originated from an extremely hot, dense point approximately 13.8 billion years ago and has been expanding and cooling ever since. Another key piece of evidence is the Cosmic Microwave Background (CMB) radiation, which is the faint afterglow of the Big Bang, detected uniformly across the entire sky.