Our solar system

About half way along the Orion arm of the Milky Way galaxy, 26 thousand light years from its centre, lies a pretty average star, once called "Sol", and a system of planets.
Occupying a tiny space within one part of a truly enormous galaxy this "Sol-ar system" consisting of a central star and orbiting planets is possibly the most important place in the known universe !!

It is, of course, our Solar system.

What exactly does our solar system consist of?

The solar system consists of

• the Sun (once known as "Sol" to the Romans) and the objects that orbit it, that is,
• the 8 major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune,
• a few dwarf planets (Pluto, Eris, Ceres and possibly others),
• lots of minor planets or asteroids, and finally
• all the moons or natural satellites of the planets.

In the diagram below the planet sizes are to scale but not the distances.

You probably already know a lot about the main or major planets such as the order of their distance from the Sun, but if not it is the order shown above.
You might also know that the first 4 are "small" solid, rocky planets, known as terrestial planets and the other 4 are, by comparison, giant planets but not solid; Jupiter and Saturn are made almost entirely of gases whilst Uranus and Neptune are made of various ices.
You probably also know that all 4 of the giant planets have rings though none are as spectacular or visible as those around Saturn.
All 8 planets have orbits that lie in the same plane known as the ecliptic (that's why we can draw them along the same line as in the diagram above).

You are less likely to know much about the so called dwarf planets. Even this title is a controversial one; many astronomers refuse to accept it. Anyway, most agree that Pluto, which was once ranked alongside the other 8 "major" planets is now just a dwarf planet.
In simple terms a dwarf planet is a planet of small mass but one that has a spheroidal shape (as opposed to the irregular shapes of the minor planets). So, Pluto is a dwarf planet because it has a mass that is only 68% of that of our moon, and our moon is not massive by any planetary measure. Eris is about the same mass as Pluto, 67% of that of our moon, whilst Ceres is much smaller at only 27% of the mass of our moon. There are other objects that are proposed to be included in the list of dwarf planets but you shouldn't be expected to know all their names.
Although dwarf planets have a sufficient gravitational field to pull themselves into their spheroidal shape, the strength of the field is not sufficient to "clear the neighbourhood" of their orbit, meaning that there can be other objects within their orbit path. This doesn't occur with the more massive major planets whose gravitational field is strong enough to pull all close objects into itself during its formation.

The minor planets (or asteroids) are generally small, solid, irregular shape objects that orbit the Sun somewhere between the orbits of Mars and Jupiter.
Occasionally one can be deflected from this region and be pulled into an orbit around the Sun that might make it pass close to or even impact the Earth!
Many have passed safely by the Earth but a few have struck us. One arrived unexpectedly a few years ago and crashed in Russia, but did relatively little damage since it impacted in a forest. More seriously, it is thought that the extinction of the dinosaurs and lots of other species a long time ago, was due to the effect of a large asteroid strike.
The largest asteroid is Vesta at about 530 km in diameter, but most are only metres in diameter; the total mass of all asteroids combined is less than that of our moon.

A natural satellite is a natural object (ie not a made object) that orbits another natural object. We call the Earth's natural satellite, the Moon, and so we tend to call all the natural satellites of the other planets "moons".
At one time a book on astronomy could easily list all the moons of the solar system on less than one page; now there is no chance that that could be done; we have discovered more than 200 moons in the Solar System. Of the major planets, all except Mercury and Venus have moons; even Pluto and other dwarf planets have moons and even more surprising, so do some asteroids.

You don't need to learn everything about all the above objects; a lot of what you have to do is to make logical/sensible decisions. For example, if asked - "which major planet is most likely to take the greatest time to orbit the Sun?" - you would surely answer "Neptune" because it is furthest from the Sun. Similarly if asked - "which planet, Saturn or Mercury, is likely to have the highest average temperature?" - you would answer Mercury because it is closer to the Sun.

A quick note regarding scale: the diagram above shows the sizes of the major planets to scale.
The diagram below also shows the sizes to scale; it reveals just how large the Sun is, doesn't it; it is so much bigger than all of the planets put together.

But neither of the two diagrams show the distances within the solar system to scale.
To get an idea of the size of the Solar System consider this: if the Sun was 3cm in diameter, the gas giants would be about 3mm in diameter, and Earth would be only 0.3mm in diameter (smaller than a flea) and the distance from the Sun to the most distant planet, Neptune would be a huge 100m ! This shows us just how vast are the distances within the Solar System.
Here is another indication of the vastness of the Solar System - it took Voyager2 12 years, travelling at 19,000 m/s (or 42,000 miles an hour) to pass Neptune.

And yet, this "vast" Solar System occupies a tiny part of the unbelievably large Milky Way Galaxy.

The life cycle of a star

Like lots of things stars are born, they have a lifetime and then they die. They have a life cycle. But unlike most things, their life cycle lasts for millions or billions of years.

Birth
A star is born when a cloud of dust and hydrogen gas, called a nebula, is pulled together by gravity over millions of years, squeezing the gases into a smaller and smaller volume until the temperature within the dense gas begins to rise sufficiently to set off nuclear fusion reactions. At this point the new star, known as a protostar, can be said to be born and it begins to emit light.

Eventually when the temperature is sufficiently large the inward pull of gravity meets an outward push caused by the nuclear fusion reactions (called radiation pressure). When the inward pull of gravity equals the outward push of the radiation pressure, the protostar becomes a true Star and it begins its long stable period.

Note: once the protostar forms, gravity will begin to pull material together into orbit around the protostar; this material will form over billions of years and become planets. This is how our Sun became the centre of a solar system.

Stable period

During this main period of its life (also known as main sequence), a star remains in a stable state - it does not shrink or expand.

During this period, the fusion reactions occurring within the star are quite simple ones. Hydrogen atoms fuse together to make helium atoms and at the same time energy is released in the form of light. But take note - in this process the star is using up its hydrogen fuel supply; it will eventually run out!

So there is a limit to the length of this stable period. It can be millions or billions of years (our star, the Sun, is already over 4 billion years old but is about half way through its stable period, so don't panic!)

The Death of a star

As the star's supply of hydrogen fuel begins to run out, the outward force due to the radiation pressure decreases so the delicate balance that kept the star stable for so long is disrupted; the inward force of gravity dominates causing the star to contract.
This, however, has the effect of increasing the temperature at the core which kick starts further more complex fusion reactions.
These more complex fusion reactions boost the outward radiation pressure, which wins the battle against gravity, causing the star to expand rapidly, becoming a Giant star.
However, since the star expands, its much larger outer layer cools; it cools so much that the star's colour changes from a typical orange/yellow to a red (Note: in light terms, red light is much cooler than orange or yellow or white). This old, dying star is now a Red Giant.

What happens next depends on the original size of the star. There are two sizes to consider; stars originally of similar mass to our Sun, and stars that were originally much more massive.

1. Stars similar to our Sun:
After becoming a Red Giant and beginning to cool the fusion reactions will weaken and so gravity will once again take control and cause the start to contract.
It contracts and contracts to form a compact star known as a White Dwarf but as its name implies, it glows "white" meaning its gone hot again due to the massive contraction of the remaining material. So another round of even more complex fusion reactions occcur but the White Dwarf due to these complex reactions, burns through its fusion material quickly and the star dulls eventually fizzling out into a Black Dwarf! And that's the end of it.

2. Stars much more massive than our Sun:
For stars of much greater original mass, the end is much more dramatic! First of all, when they become a Giant, they become a Red SuperGiant.
When they, like the smaller stars, collapse, they do so rapidly and with a BANG, blowing off their outer layers - this is a Supernova.
Following the Supernova one of two things can happen:
i) the remaining centre of the star will contract further to form an incredibly dense Neutron Star, or
ii) the remaining centre of the star will contract further and further until the gravitational forces within become so strong that no light can escape - this is a Black Hole.

Note:
Have you ever wondered where all of our abundance of elements come from eg carbon, iron etc ?
They are made within the dying stars when they are undergoing their "more complex fusion reactions" as described above.
For example, the Helium nuclei made during the "normal" simple fusion reactions, begin to fuse together to make Carbon;

4 2 He + 4 2 He + 4 2 He → 12 6 C + ENERGY

The Carbon nuclei then fuse with other Helium or Carbon nuclei to make even heavier elements, and so on, all the way up to the heaviest elements such as lead or uranium.

But, you might be thinking, how do these abundance of elements get from the inside of the dying star to a planet such as Earth?
Well, when a particularly massive star explodes in a supernova as described above, all of the elements formed within it get scattered throughout that region of space.
Now here is the really interesting part:
The "debris" of a supernova is the gas and dust that we called a nebula, the birth place for new stars and planets!

So we can deduce that our Sun and our planet Earth, and the rest of our Solar System, since it contains an abundance of complex elements, must have formed from a nebula that was the debris of a very large star going supernova many billions of years ago. Isn't that cool ?

Orbital motion

What keeps a planet orbiting the Sun,
or an artificial satellite orbiting the Earth?

The answer is: the force of Gravity.

In the diagrams above the letter v represent the velocity of the orbiting object at any instant of time. You can see that if it wasn't for Gravity constantly pulling on the object, it would simply keep moving in the direction of v. The force of Gravity literally keeps the object circling.

The rate or speed at which the object orbits depends on its distance from the central object.
The greater the distance, the lower the speed.

So, in the case of the 8 major planets, Mercury which is closest to the Sun, orbits at the greatest speed, and Neptune which is furthest from the Sun, orbits at the slowest speed.
Consequently since Mercury orbits at the fastest speed and has the smallest orbital path, it completes one orbit in 88 days;
Neptune which is slowest and has largest orbital path takes a huge 165 years !

In the diagram above, Mercury has the largest velocity arrow, Neptune has the smallest.

Some similarities and differences in the way the major planets orbit the Sun
Similarities

1. They all orbit in the same direction around the Sun.
2. Their orbits are not quite circular; they are ellipses with the Sun at one focus.
3. They all lie on the same plane (we mentioned this earlier; you can see it on the first picture at the start of this page).

Differences

1. They orbit at different distances from the Sun.
2. They orbit at different speeds .
3. The time they take to complete one orbit of the Sun increases from Mercury to Neptune.

Notes
1. When one object orbits another (eg a planet orbits the Sun) its velocity keeps changing even though its speed remains constant.

This might sound strange but you should know from your knowledge of Forces and Motion (AQA 4.5.6) that Velocity is defined as "Speed in a specified direction". So since the planet's direction keeps changing as its orbits the Sun, its Velocity keeps changing but its Speed, which does not depend on direction, remains fixed.

2. If an orbiting object changes its speed then the radius of its orbit must change if it is to remain in a stable orbit.

Now this can't apply to planets or natural moons whose speeds and orbital radius are fixed, but it does apply to man-made artificial satellites.
To put an artificial satellite at a precise distance in orbit above the Earth, it must be made to move at a very precise speed eg to orbit at a height of 300km above the Earth's surface, a speed of 7.8 km/s is required; such a satellite will complete one orbit in just 90 minutes. But if, for some reason, the satellite slowed down then it would naturally move further out to a higher orbit, taking longer to complete one orbit.