📚 Key Vocabulary — All Terms You Need

📚 The Solar System

Our solar system consists of one star (the Sun), eight planets (plus dwarf planet Pluto), and their moons and other objects. It is a small part of the Milky Way galaxy.

Planet Order — Mnemonic

Planet Types

📚 Nebulae and Nuclear Fusion

A nebula is a cloud of dust and gas (mainly hydrogen and helium). Gravity pulls it together. As it collapses, friction heats the gases. Once the core temperature exceeds 10 million degrees, nuclear fusion begins.

Nuclear fusion is the combining of lighter nuclei to form heavier ones, releasing energy. In a star like the Sun, hydrogen nuclei fuse to form helium. Each helium nucleus has slightly less mass than the four hydrogen nuclei that formed it — this lost mass is converted to energy (E = mc²).

Why the Main Sequence Star is Stable

Gravity acts inward trying to compress the star. Radiation pressure from fusion pushes outward. These balance — the star is stable until hydrogen runs out.

Temperature Anomalies

Venus is hotter than Mercury despite being further from the Sun. Its thick CO₂ atmosphere traps heat via a strong greenhouse effect. Neptune is coldest — furthest from the Sun, receiving least solar radiation.

📚 Planetary Data — Patterns

The data table shows that as distance from the Sun increases:

Exception — Venus: much hotter than expected due to its greenhouse atmosphere. The student's conclusion ("temperature decreases with distance") is not completely correct because it doesn't account for Venus.

📚 From Nebula to Main Sequence Star

A nebula (cloud of dust, hydrogen and helium) is pulled together by gravity. Friction heats the gases as the cloud compresses. When the core exceeds ~10 million °C, nuclear fusion begins and a protostar forms, then a main sequence star.

The star is stable because gravity (inward) is exactly balanced by radiation pressure from fusion (outward). This balance holds for billions of years.

📚 Sun-Sized Star — Life Cycle After Main Sequence

When hydrogen runs out, fusion decreases → gravity wins → star contracts → core temperature rises → helium begins to fuse → radiation pressure increases greatly → star expands and cools on the outside, forming a red giant.

The outer layers then drift away gently into space as a planetary nebula. The exposed hot, dense core is called a white dwarf. Over a very long time it cools to become a black dwarf.

📚 Massive Star — Life Cycle After Main Sequence

Stars more massive than the Sun fuse heavier and heavier elements in their cores. This continues until an iron core forms. Iron is the heaviest element that can be produced by fusion — it actually absorbs energy rather than releasing it. Fusion stops, the core collapses catastrophically and the star explodes in a supernova.

Elements heavier than iron (gold, uranium etc.) are only created during the supernova explosion itself, when enormous energy is available. The explosion scatters all these elements throughout the universe.

📚 Q1 — Key Points for Stars and Life Cycles Questions

For "describe how stars are formed": include nebula → gravity → friction → fusion. For "why stable during main sequence": gravity balanced by radiation pressure. For "Sun-sized life cycle after main sequence": red giant → white dwarf → black dwarf in order.

📚 Elements and the Early Universe

The early universe contained only hydrogen (the lightest element). As stars formed and evolved, heavier elements were built up:

When a massive star explodes, it scatters all these elements across space. They eventually form new stars, planets — and everything on Earth, including us.

📚 What is a Satellite?

A satellite is any object in orbit around a planet.

What Keeps a Satellite in Orbit?

Gravity acts as the centripetal force, pulling the satellite continuously toward the centre of its orbit. Without gravity, the satellite would fly off in a straight line. The satellite doesn't fall because it is moving fast enough sideways that the Earth's surface curves away at the same rate.

Orbital Height and Speed

📚 Geostationary and Monitoring Satellites

Speed vs Velocity

A satellite in circular orbit moves at constant speed but its velocity is always changing — because direction is continuously changing. Velocity is a vector (it has direction); even if the magnitude stays the same, a change in direction means the velocity has changed.

Elliptical Orbits

Most satellites orbit in slightly elliptical (oval) paths. Comets have very extreme elliptical orbits around the Sun. In an elliptical orbit the speed is not constant:

📚 Centripetal Force and Historical Models

The centripetal force is the resultant force directed towards the centre of a circular orbit. For a satellite this is provided by gravity.

The size of the centripetal force depends on: (1) the mass of the satellite, and (2) the orbital speed. It does NOT depend on the height directly — though higher orbits have weaker gravity, which is why satellites there orbit more slowly.

Isaac Newton proposed the idea of artificial satellites with a thought experiment: if fired fast enough from a mountaintop, a cannonball would orbit the Earth. People accepted this because Newton was a well-respected scientist who had already made important discoveries (e.g. laws of motion and gravitation).

📚 Calculating Orbital Speed — Step by Step

Distance travelled in one orbit = circumference of the circular orbit:

Speed is then distance divided by time:

Unit Conversions — Essential

Worked Example

The Moon — Given Values

The Moon has an average distance from Earth of 391,400 km and a speed of 1,017 m/s with a period of 28 days. This is already completed in the table — use it to check your method.

📚 The Big Bang Theory

The Big Bang theory proposes that the universe began from an extremely small, extremely hot and dense point approximately 13.8 billion years ago. It then exploded outward and has been expanding ever since.

Because the universe is expanding, all galaxies are moving away from each other. The further away a galaxy is, the faster it appears to be receding.

📚 Red Shift — Evidence for the Big Bang

When a galaxy moves away from us, the light it emits is stretched — its wavelength increases and frequency decreases. This shift towards longer wavelengths is called red shift. The greater the red shift, the faster the galaxy is receding.

The Doppler Effect

This is the change in observed frequency/wavelength caused by relative motion between source and observer. A police siren sounds higher pitched as it approaches (waves compressed = shorter wavelength = higher frequency) and lower pitched as it moves away (waves stretched = longer wavelength = lower frequency). The same principle applies to light.

Cosmic Microwave Background Radiation (CMBR)

CMBR is faint microwave radiation detected uniformly from all directions in space. It is the residual heat energy from the original Big Bang fireball, now cooled to just 2.7 K. Its existence and uniformity is the second major piece of evidence for the Big Bang.

📚 Hubble's Law and Calculating the Age of the Universe

By measuring the red shift of many galaxies, Hubble found a directly proportional relationship between a galaxy's distance from Earth and its recession speed:

The Hubble constant is the gradient of the speed-distance graph (approximately 500 km/s per megaparsec using Hubble's original 1929 data). Dividing distance by speed for any galaxy gives an estimate of how long it has been travelling — approximately 13.8 billion years — the age of the universe.

📚 Hard Questions — Calculating Time from Galactic Data

The hard question gives distance (in zettametres, Zm) and recession speed (in Zm per billion years) for several galaxies. One zettametre = 10²¹ metres.

To find how long a galaxy has been moving away, use:

If all galaxies give roughly the same time value, this supports the Big Bang — all matter began moving from the same point at the same moment in time. The calculated times should all be close to ~13 billion years.

Example

Abell 963: distance = 25,000 Zm, speed = 1,950 Zm per billion years. Time = 25,000 ÷ 1,950 ≈ 12.8 billion years.

📚 Reading Stellar Spectra

Every element absorbs light at specific wavelengths, creating dark absorption lines at those exact positions in the spectrum. Each star has a unique pattern of dark lines. When we compare a star's spectrum to a reference (e.g. the Sun):