Stars and the Universe
Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)
The Sun as a Star
- The Sun is the closest star to the Earth
- The Sun is a star of medium size, consisting mostly of hydrogen (~75%) and helium (~24%)
- It radiates most of its energy in the infrared, visible light and ultraviolet regions of the electromagnetic spectrum
Nuclear Fusion in Stars
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- Stars are powered by nuclear reactions that release energy
- In stable stars like the Sun, these reactions involve the fusion of hydrogen into helium
- hydrogen nuclei combine at extremely high temperatures and pressures to form helium, releasing large amounts of energy - this is what makes stars shine
Astronomical Distances: Light-Years
- Astronomical distances are measured in light-years
- A light-year is the distance that light travels through a vacuum in one year - it is a unit of distance, not time
Calculating Light Travel Time
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- The time taken for light to travel a given distance can be found by rearranging the wave speed equation:
- t = (d/c)
- Where c = 3 × 10^8 m/s is the speed of light in a vacuum
Question: Muchindu wants to know how long light takes to travel from the Sun to the Earth, a distance of 1.5 × 10^11 m.
Answert = (d/c) = 1.5 × 10^113 × 10^8 = 500 s ≈ 8.3 minutes
Why Planets Orbit the Sun
- The Sun contains most of the mass of the Solar System
- this explains why its gravitational field dominates the Solar System, and why the planets orbit it
- The force that keeps an object in orbit around the Sun is the gravitational attraction of the Sun
Orbital Speed
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- Formula:
- v = (2π r/T)
- Where:
- v = orbital speed (m/s)
- r = radius of the orbit (m)
- T = orbital period (s)
- This comes from the fact that, in one full orbit, an object travels a distance equal to the circumference of its (circular) orbit, 2pi r, in a time equal to one orbital period, T
Question: Museli estimates the Earth's orbital speed around the Sun, using an orbital radius of 1.5 × 10^11 m and an orbital period of 1 year (3.15 × 10^7 s).
write down the equation. v = (2π r/T) Answerv = 2π × 1.5 × 10^113.15 × 10^7 ≈ 3.0 × 10^4 m/s
Orbital Speed and Distance from the Sun
- The Sun's gravitational field strength decreases as distance from the Sun increases
- Because of this, the orbital speeds of the planets decrease as their distance from the Sun increases
- planets close to the Sun, such as Mercury, orbit much faster than planets far from the Sun, such as Neptune
Life Cycle of a Star
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All stars form from clouds of gas and dust in space, then evolve over millions or billions of years. The eventual fate of a star depends primarily on its initial mass.
Protostar to Stable Star
- A star is formed as a protostar from an interstellar cloud of gas and dust, which collapses inward due to its own gravitational attraction
- When the core becomes hot enough, nuclear fusion begins, and the protostar becomes a stable star
- a stable star can last billions of years, but it eventually runs out of hydrogen fuel - what happens next depends on the star's mass
Life Cycle: Small Mass Stars (like the Sun)
- A star of about the same mass as the Sun expands to become a red giant
- It then sheds its outer layers, forming a planetary nebula, leaving a white dwarf behind at its centre
The life cycle of a small mass star, such as the Sun: red giant → white dwarf + planetary nebula
Life Cycle: Large Mass Stars
- A large mass star expands much further than a small mass star, forming a red supergiant
- The red supergiant explodes as a supernova, forming a nebula and leaving a collapsed core behind:
- if the remaining core is moderately massive, it forms a neutron star
- if the remaining core is very massive, it forms a black hole
- The nebula left behind by a supernova may eventually form new stars with orbiting planets
The life cycle of a large or very large mass star ends in a supernova, leaving a neutron star or a black hole
Summary: Life Cycle Comparison
| Stage | Small Mass Star (like the Sun) | Large / Very Large Mass Star |
|---|---|---|
| Formation | Interstellar cloud → Protostar → Stable Star | |
| Expands to become | Red Giant | Red Supergiant |
| End-of-life event | Outer layers gently blown away | Supernova (violent explosion) |
| Ejected material | Planetary Nebula | Nebula (may form new stars with orbiting planets) |
| Remnant core | White Dwarf | Neutron Star (large mass) or Black Hole (very large mass) |
Galaxies
- A galaxy is a massive system made up of many billions of stars
- The Sun is a star in a galaxy called the Milky Way
- Other stars that make up the Milky Way are much further away from the Earth than the Sun is from the Earth
- the Sun is by far the closest star to us
The Universe
- The Milky Way is one of many billions of galaxies that make up the Universe
- the Milky Way's diameter is approximately 100 000 light-years
The Big Bang Theory
The Big Bang Theory is supported by many astronomical observations, including the fact that light from distant galaxies is shifted to longer wavelengths, showing that galaxies are moving apart.
- The Big Bang Theory states that:
- the Universe expanded from a single point of high density and temperature
- the Universe is still expanding
- the Universe is approximately 13.8 billion years old
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