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Solar System | Galaxy | Universe | Star Life Cycle | Times and seasons | Moon
Our solar system
Sun + nine planets + moons + asteroids + dust + gas + comets all held together by gravity.
The sun is an average star but it has over 99.9% of the mass of the solar system.
It is mostly composed of hydrogen and helium gases.
Nuclear fusion of hydrogen to helium is the sun’s energy source.
The sun is about a million times larger than the Earth.
Core temperature of the sun: 15 million degrees centigrade. Surface temperature: 6,000˚C.
Planet order from the sun:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.
Planet mass order
Jupiter, Saturn, Neptune, Uranus, Earth, Venus, Mars, Mercury, Pluto.
Venus and Mars are the planets closest to the Earth.
Mercury and Venus are the only planets without a moon.
The asteroid belt lies between Mars and Jupiter.
A planet is a large natural satellite that only orbits a star.
A moon is a natural satellite of a planet – so moons orbit a planet and a star.
Planets (“wandering stars”) are visible because they reflect the light of the sun.
The sun is visible because it produces its own light – it is luminous.
More than a hundred planets have been detected in orbit around other stars.
Galaxy
A galaxy is a huge collection of solar systems held together by gravity.
Our solar system is part of the Milky Way galaxy.
The Milky Way is a spiral galaxy, contains about 200 billion stars, and is 100,000 light years in diameter and 10,000 light years deep at its centre.
A light year is a measure of distance: the distance light travels in one year.
Speed of light: 300,000 km s-1.
A light year is about 9.5 x 1012 km (9,500,000,000,000 km).
Universe
All the space, matter and energy in existence.
Approximately 28 billion light years in diameter, 16 billion years old, holds more than a billion galaxies and is expanding.
Suggested origin: “Big Bang” i.e. the massive explosion of a ‘singularity’ or ‘primordial particle’.
Star life cycle
- Began as a nebula - a large cloud of dust and gas.
- The nebula contracts due to gravity, causing the density, pressure and temperature to rise producing a glowing infrared protostar.
- When the core temperature reaches 10 million ˚C, nuclear fusion of hydrogen to helium begins forming a ‘main sequence star’.
- Star now remains at a constant size and energy output for a long time – 10 billion years for our sun, which is now 5 billion years old.
- Red Giant Stage: greatly increased energy output due to complex changes in the star causes it to expand – our sun’s diameter is expected to increase four times. Expansion of the outer gases causes cooling.
- White Dwarf Stage: the core collapses due to gravity, forming a very hot white dwarf star about earth size – a million times smaller than the sun is now.
- Black Dwarf Stage: nuclear fusion gradually decreases and finally stops.
Days, years, seasons
Earth spins from west to east on own axis every 24 hours.
A day is the length of time it takes the earth to complete one rotation on its axis.
The side of Earth facing the Sun has day; the side turned away has night.
Each 24-hour rotation gives each place on earth about 12 hours of light and 12 hours of darkness.
365.25 days for one orbit of the sun – this is the astronomical year.
A civil calendar year is 365 days. Make up for the extra quarter day by adding one day every fourth (leap) year.
The earth’s velocity around the sun is 30 km s-1.
Seasons
Unequal heating of the earth surface by the sun causes the seasons.
In summer there is longer daytime and the light concentration is greater – therefore each square metre of surface is receiving more energy from the sun.
In winter there is shorter daytime and the light concentration is less – therefore each square metre of surface is receiving less energy from the sun.
The difference in day length and light concentration is because the earth’s axis of rotation is not vertical to the plane of its orbit around the sun — it is permanently tilted at an angle of 23.5˚ from the vertical.
Moon
The moon is a satellite of Earth – about 380,000 km from the earth.
The moon orbits the earth from a west to east direction.
27.3 days to complete one orbit of the earth at a velocity of about 1,000 m s-1.
The same side of the moon always faces the earth – the moon completes one rotation on it axis in the same time as it takes to complete one orbit of the earth.
Phases of the moon
The moon appears to undergo a regular change in shape as it orbits the earth.
These phases are due to the change in the lighting of the side of moon that permanently faces the earth as the moon orbits the earth – New Moon, Crescent, First Quarter, Gibbous, Full Moon, Gibbous, Last Quarter, Crescent (C-shaped).
- Lunar month: the time to complete one cycle of lunar phases – 29.5 days.
- Lunar eclipse: Sun, Earth, Moon in a straight line – Full Moon phase.
- Solar Eclipse: Sun, Moon and Earth in a straight line – New Moon phase.
The sun and the moon appear to be the same size: the diameter of the moon is about 400 times less than the sun’s but the sun is about 400 times further away than the moon.
Moon much less mass than the Sun but it is closer to Earth so it has a greater gravitational pull on oceans. This causes tides twice a day.
Very high and low tides (Spring Tides): New Moon and Full Moon phases.
Sun and moon in the same straight line: sun moon earth or sun earth moon.
Smallest tide rise and fall (Neap Tides) First Quarter and Last Quarter phases.
Sun, earth and moon form a right angle.
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