8.1-The Life Cycle of Stars

Stars go through many traumatic stages in their lives

• Stars initially form from a cloud of dust and gas called a nebula.

• The force of gravity pulls the dust and gas together to form a protostar.

  • The temperature rises as the star gets denser and more particles collide with each other.

  • When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei.

  • This gives out huge amounts of energy, which keeps the core of the star hot.

  • A star is born.

• The star enters a long stable period where the outward pressure caused by the nuclear fusion that tries to expand the star balances the force of gravity pulling everything inwards.

  • In this stable period, it’s called a main sequence star and it typically lasts several billion years. (The sun is in the middle of this period.)

• Eventually the hydrogen begins to run out. The star then swells into a red giant (if it is a small star) or a red super giant (if it is a larger star).

• It becomes red because the surface cools.

  • Fusion of helium (and other elements) occurs.

  • Heavier elements (up to iron) are created in the core of the star.

• A small-to-medium-sized star like the sun then becomes unstable and ejects its outer layer of dust and gas.

• This leaves behind a hot, dense solid core-a white dwarf.

• As a white dwarf cools down, it emits less and less energy.

  • When it no longer emits a significant amount, it is called a black dwarf.

• Big stars start to glow brightly again as they undergo more fusion and expand and contract several times, forming elements as heavy as iron in various nuclear reactions.

  • Eventually they’ll explode in a supernova, forming elements heavier than iron and ejecting them into the universe to form new planets and stars.

• Stars and their life cycles produce and distribute all naturally occurring elements.

• The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a neutron star.

  • If the star is massive enough, it will become a black hole-a super dense point on space that not even light can escape from

8.2-The Solar System and Orbits

The sun is the centre of our solar system. It’s orbited by eight planets, along with a bunch of other objects.

Our Solar System has One Star-The Sun

The solar system is all the stuff that orbits our Sun. This includes things like:

• Planets-these are large objects that orbit a star.

• There are eight planets in our solar system.

• They must be large enough to have ‘cleared their neighbourhoods’.

• This means that their gravity is strong enough to have pulled in any nearby objects apart from their natural satellites.

• The planets are:

  • Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, Earth

• Dwarf planets, like Pluto.

  • These are planet-like objects that orbit stars, but do not meet all the rules for being a planet.

• Moons-these orbit planets.

  • They are a type of natural satellite

• Artificial satellites are satellites that humans have built.

  • They generally orbit the Earth.

Our solar system is a tiny part of the Milky Way galaxy. This is a massive collection of billions of stars that are held together by gravity.

Gravity provides the force that create orbits

• The planets move around the Sun in almost circular orbits (the same goes for the moon orbiting the earth)

• If an object is travelling in a circle, it is constantly changing direction, which means it is constantly accelerating

• This also means it is constantly changing velocity (but NOT changing speed).

• For an object to accelerate, there must be a force acting on it. This force is directed towards the centre of the circle.

• This force would cause the object to just fall towards whatever it was orbiting, but as the object is already moving, it just causes it to change its direction.

• The object keeps accelerating towards what it’s orbiting but the instantaneous velocity (which is at a right angle to the acceleration) keeps it travelling in a circle.

• The force that makes this happens is provided but the gravitational force between the planet and the sun (or the planet and its satellites)

The size of the orbit depends on the object’s speed

• The closer you get to a star or planet, the stronger the gravitational force is.

• The stronger the force, the faster the orbiting object needs to travel to remain in orbit (to not crash into the object it’s orbiting).

• For an object in a stable orbit, if the speed of the object changes, the size(radius) of its orbit must do so too. Faster moving objects will move in a stable orbit with a smaller radius that slower moving ones

8.3-Red-Shift and the Big Bang

The universe seems to be expanding

As big as the universe already is, it looks like it’s getting even bigger. All it galaxies seem to be moving away from each other. There’s good evidence for this.

• When we look at light from most distant galaxies, we find that the wavelength has increased

• The wavelengths are all longer than they should be-they are shifted towards the red end of the spectrum. This is called red shift.

  • This suggest the source of the light is moving away from us.

• Measurements of the red-shift indicate that these distance galaxies are moving away from us(receding) very quickly-and it’s the same result whichever direction you look in.

• More distant galaxies have greater red shifts than nearer ones. This means that more distant galaxies are moving away faster than nearer ones.

  • The inescapable conclusion appears to be that the whole universe (space itself) is expanding.

This evidence suggests the universe started with a bang

So all the galaxies are moving from each other at great speed-suggesting something must have got them going. That ‘something’ was probably a big explosion-the Big Bang. Here’s the theory:

• Initially, all the matter in the universe occupied a very small space.

• This tiny space was very dense and so was very hot.

  • Then it ‘exploded’-space started expanding, and the expansion is still going on.

New evidence might change out theories·

Something important to remember is that the Big Band theory is the best guess we have so far. Whenever scientists discover new evidence, they must either make a new theory or change a current one to explain what they have observed

• There are still lots we do not know about the universe.

• Observation of supernovas from 1998 to the present day appear to show the distant galaxies are moving away from us faster and faster.

• Currently, scientists think the universe is mostly made up of dark matter and dark energy.

• Dark matter is the name given to an unknown substance which holds galaxies together but does not emit any electromagnetic radiation.

• Dark energy is thought to be responsible for the accelerated expansion of the universe. But no-one knows what these things are, so there are lots of different theories about it.

• These theories get tested and are either accepted or rejected.