Astronomy notes:
Geocentric Model:
• Earth at centre of Solar System. Sun, Moon, other planets & stars orbiting it in perfect circles.
• Believed from ancient Greeks to 1500s as there weren’t any telescopes. People saw the Sun & Moon travelling across the
sky in a constant cycle every day.
• Epicycles were added to this model (a circle on a circle) to explain why everything didn’t orbit the Earth in a perfect circle
or in the same direction.
Heliocentric Model:
• Sun at centre of the Solar System. Initially, orbits were said to be circular, but the model was edited – they are elliptical.
• Galileo made high power telescopes & using them saw moons orbiting Jupiter, showing not everything orbited the Earth.
• Also, the phases of Venus showed that it orbited the Sun instead of Earth.
What’s in the Solar System?
1. Planets (My Very Educated Mother Just Showed Us Nine Planets)
a. Large objects orbiting a star – must be large enough to have a large gravity so that they can pull in any nearby objects
(apart from their natural satellites)
b. Smaller planets are made from rock, larger ones made from gas & have rings (of debris) attracted by their strong
gravitational field strength.
c. All planets rotate, but at different speeds.
i. Some rotate in the opposite direction/on a skewed axis due to collisions throwing its axis off balance.
d. All planets orbit the Sun on the same plane.
2. Dwarf planets – e.g., Pluto – to small to meet the requirements of being a planet, but they still orbit stars.
3. Satellites: Natural – e.g., Moons (not man-made & orbit planets). Man-made (some orbit Earth/Moon/Sun/other planets)
4. Asteroids - Lumps of rock & metals orbiting the Sun, found in the asteroid belt between Mars & Jupiter
5. Comets – e.g., Halley’s comet – lumps of ice & dust orbiting the Sun.
• Orbits – these objects are kept in orbit due to gravity:
o Moons, artificial satellites & planets have an almost circular orbit. Comets have a highly elliptical orbit.
Birth of a star
• Nebula (cloud of dust & gas) collapses due to gravity, forming a protostar.
• Gravity causes its density to increase. GPE decreases, KE increases, so the temperature rises.
• With a high enough temperature, hydrogen nuclei undergo nuclear fusion to form helium nuclei in the core. This gives
out energy, keeping the star hot.
• The star is now on the main sequence (long, stable period) because radiation pressure given out by fusion is balanced by
the force of gravity.
Main sequence (details)
• Fusion in the core releases energy, so the radiation pressure causes the core to push out. This is balanced with the force
of gravity acting on the star.
• The Sun is in the middle of the main sequence (at 7.5 billion years, 15 billion years total)
• Stars are on the main sequence for billions of years.
• More mass = shorter time on main sequence – uses up fuel quicker
Death of a star (similar mass to Sun)
• Runs out of hydrogen, so radiation pressure < gravity.
• Star compresses & becomes dense & very hot, so the energy released makes the outer layers expand into a Red Giant
(red = surface cools)
• It becomes unstable & ejects its outer layers, leaving behind a White Dwarf (hot, dense core)
• This cools, emits less & less energy and eventually disappears from sight
Death of a star (larger mass than Sun)
• Runs out of hydrogen, expands into Red Supergiant (bigger & brighter than Red Giants)
• This expands & contracts multiple times, forming elements like iron in nuclear reactions.
• Runs out of elements to fuse & becomes unstable.
• Explodes into a supernova, forming elements heavier than iron & ejecting them into the Universe to form new planets &
stars
• This exploding supernova throws the outer layers of dust & gas into space, leaving a neutron star (dense core)
• If the star is big enough, it will become a Black hole instead (dense point in space that nothing can escape from)
Geocentric Model:
• Earth at centre of Solar System. Sun, Moon, other planets & stars orbiting it in perfect circles.
• Believed from ancient Greeks to 1500s as there weren’t any telescopes. People saw the Sun & Moon travelling across the
sky in a constant cycle every day.
• Epicycles were added to this model (a circle on a circle) to explain why everything didn’t orbit the Earth in a perfect circle
or in the same direction.
Heliocentric Model:
• Sun at centre of the Solar System. Initially, orbits were said to be circular, but the model was edited – they are elliptical.
• Galileo made high power telescopes & using them saw moons orbiting Jupiter, showing not everything orbited the Earth.
• Also, the phases of Venus showed that it orbited the Sun instead of Earth.
What’s in the Solar System?
1. Planets (My Very Educated Mother Just Showed Us Nine Planets)
a. Large objects orbiting a star – must be large enough to have a large gravity so that they can pull in any nearby objects
(apart from their natural satellites)
b. Smaller planets are made from rock, larger ones made from gas & have rings (of debris) attracted by their strong
gravitational field strength.
c. All planets rotate, but at different speeds.
i. Some rotate in the opposite direction/on a skewed axis due to collisions throwing its axis off balance.
d. All planets orbit the Sun on the same plane.
2. Dwarf planets – e.g., Pluto – to small to meet the requirements of being a planet, but they still orbit stars.
3. Satellites: Natural – e.g., Moons (not man-made & orbit planets). Man-made (some orbit Earth/Moon/Sun/other planets)
4. Asteroids - Lumps of rock & metals orbiting the Sun, found in the asteroid belt between Mars & Jupiter
5. Comets – e.g., Halley’s comet – lumps of ice & dust orbiting the Sun.
• Orbits – these objects are kept in orbit due to gravity:
o Moons, artificial satellites & planets have an almost circular orbit. Comets have a highly elliptical orbit.
Birth of a star
• Nebula (cloud of dust & gas) collapses due to gravity, forming a protostar.
• Gravity causes its density to increase. GPE decreases, KE increases, so the temperature rises.
• With a high enough temperature, hydrogen nuclei undergo nuclear fusion to form helium nuclei in the core. This gives
out energy, keeping the star hot.
• The star is now on the main sequence (long, stable period) because radiation pressure given out by fusion is balanced by
the force of gravity.
Main sequence (details)
• Fusion in the core releases energy, so the radiation pressure causes the core to push out. This is balanced with the force
of gravity acting on the star.
• The Sun is in the middle of the main sequence (at 7.5 billion years, 15 billion years total)
• Stars are on the main sequence for billions of years.
• More mass = shorter time on main sequence – uses up fuel quicker
Death of a star (similar mass to Sun)
• Runs out of hydrogen, so radiation pressure < gravity.
• Star compresses & becomes dense & very hot, so the energy released makes the outer layers expand into a Red Giant
(red = surface cools)
• It becomes unstable & ejects its outer layers, leaving behind a White Dwarf (hot, dense core)
• This cools, emits less & less energy and eventually disappears from sight
Death of a star (larger mass than Sun)
• Runs out of hydrogen, expands into Red Supergiant (bigger & brighter than Red Giants)
• This expands & contracts multiple times, forming elements like iron in nuclear reactions.
• Runs out of elements to fuse & becomes unstable.
• Explodes into a supernova, forming elements heavier than iron & ejecting them into the Universe to form new planets &
stars
• This exploding supernova throws the outer layers of dust & gas into space, leaving a neutron star (dense core)
• If the star is big enough, it will become a Black hole instead (dense point in space that nothing can escape from)
