Stars and the Universe

The Solar System consists of our star, the Sun, and everything gravitationally bound to it. This includes eight planets, their moons, dwarf planets like Pluto, asteroids, and comets. The Sun contains over 99% of the Solar System's total mass, and its immense gravity holds everything in orbit. The planets are divided into two groups: the four inner planets (Mercury, Venus, Earth, Mars) are small, rocky, and dense. The four outer planets (Jupiter, Saturn, Uranus, Neptune) are known as gas giants, being much larger and composed mainly of hydrogen, helium, and other gases.

An orbit is the curved path of a celestial object around a star, planet, or moon. This motion is caused by gravity. Gravity provides the constant pulling force, called the centripetal force, that keeps a smaller body moving in a circle or an ellipse around a larger one. For a stable, near-circular orbit like Earth's, the planet moves at a roughly constant speed. However, its velocity is always changing because its direction is always changing. The speed of an orbiting object can be calculated if you know its orbital radius and the time it takes to complete one orbit (the period). Comets have highly elliptical (oval-shaped) orbits. They speed up as they get closer to the Sun (where gravity is strongest) and slow down as they move further away.

Stars are born within vast, cold clouds of gas and dust known as nebulae. Gravity slowly begins to pull clumps of this material together. As a clump contracts, it spins faster and heats up, forming a hot, dense core called a protostar. This protostar continues to accrete (gather) more gas and dust from a surrounding disc. The temperature and pressure in the core of the protostar increase over millions of years. Eventually, the core becomes so hot and dense (around 15 million °C) that nuclear fusion begins. In this process, hydrogen nuclei are forced together to form helium, releasing an enormous amount of energy. This outward pressure from fusion balances the inward pull of gravity, creating a stable star. The star has now entered its long 'main sequence' phase.

A star's fate is determined by its initial mass. When a star runs out of hydrogen fuel in its core, it leaves the main sequence.

Low-Mass Stars (like the Sun): The core contracts and heats up, causing the outer layers to expand and cool, turning the star into a red giant. The core continues to fuse helium into carbon. Eventually, the outer layers drift away into space, creating a beautiful shell of gas called a planetary nebula. The hot, dense carbon core left behind is a white dwarf, which slowly cools and fades over billions of years.

High-Mass Stars (8+ times the Sun's mass): These stars evolve into red supergiants. They fuse heavier and heavier elements in their core. When the core becomes iron, fusion stops, and the star collapses catastrophically under its own gravity. This triggers a massive explosion called a supernova. The explosion is incredibly bright and scatters heavy elements formed inside the star across space. The remnant left behind depends on the mass: it will either be an ultra-dense neutron star or, if the original star was massive enough, a black hole, an object with gravity so strong that not even light can escape.

A galaxy is a vast system of billions of stars, along with gas, dust, and dark matter, all held together by gravity. Our home galaxy is called the Milky Way, a spiral galaxy containing an estimated 100-400 billion stars, including our Sun. The Universe is filled with billions of other galaxies of various shapes and sizes. The distances in space are so enormous that using kilometres or miles is impractical. Instead, astronomers use the light-year. A light-year is the distance light travels in one year. Since light is the fastest thing in the universe (travelling at 300,000 km/s), this is a very large distance: about 9.46 trillion kilometres. When we look at a star that is 10 light-years away, we are seeing the light that left it 10 years ago.

The Big Bang theory is the leading scientific model for how the Universe began. It states that the Universe started from an extremely hot, dense single point and has been expanding and cooling ever since. There are two key pieces of evidence for this theory:

1. Redshift of Distant Galaxies: When we observe light from distant galaxies, the spectral lines in their light are shifted towards the red end of the spectrum. This is called redshift. It is an example of the Doppler effect, indicating that the galaxies are moving away from us. Crucially, the further away a galaxy is, the greater its redshift, meaning the faster it is moving away. This observation that all distant galaxies are receding from us supports the idea of a uniformly expanding Universe.

1. Cosmic Microwave Background (CMB) Radiation: This is faint microwave radiation that can be detected coming from all directions in space. Scientists interpret the CMB as the 'afterglow' of the Big Bang. It is the leftover thermal energy from the initial, incredibly hot state of the Universe, which has been stretched to longer microwave wavelengths as the Universe expanded and cooled.

Hubble's Law describes the relationship found by Edwin Hubble between a galaxy's distance and its speed. It states that the recessional velocity(v) of a galaxy is directly proportional to its distance(d) from us. This can be written as an equation: v = H₀d. The constant of proportionality, H₀, is called the Hubble constant. It represents the rate of expansion of the Universe. By measuring the speed and distance of many galaxies, we can determine a value for H₀. This law is powerful because it allows us to estimate the age of the Universe. If we assume the expansion has been constant, the time since the Big Bang (the age of the Universe) is simply the reciprocal of the Hubble constant: Age ≈ 1/H₀. This is because if a galaxy at distance 'd' is moving at speed 'v', it must have taken a time t = d/v to get there. From Hubble's law, d/v = 1/H₀.