What are comets
frozen rocks that move around the Sun in often very elliptical orbits
Where is the asteroid belt
between Mars and Jupiter
How many stars are in the milky way
100 billion
How was the sun formed
The Sun formed about 4.5 billion years ago when a cloud of gas and dust (a nebula) was pulled together by gravity
The particles in the cloud got faster and collided more. The gas warmed up and eventually became hot enough to glow. This was a protostar.
As the protostar gets more dense, more collisions take place and it gets hotter. Eventually the cloud got hot enough for hydrogen atoms to fuse, forming helium. We call a star in this state a main sequence star.
How are main sequence stars stable
The outward force of pressure caused by fusion is balanced with the inwards force of gravity
This is called equilibrium
How are elements heavier than iron formed
In a supernova
What is fusion
light nuclei collide at high speed and join to create a larger, heavier nucleus. This gives out a large amount of energy in the form of light and heat
Stages of the formation of stars
- Nebula
Giant cloud of hydrogen gas and dust - Protostar
Gravity within a nebula pulls the particles closer together until it forms a hot ball of gas.
As the particles are pulled closer together the density of the protostar will increase - more frequent collisions = temperature increase - Main Sequence Star
Once the protostar becomes hot enough, nuclear fusion reactions occur within its core
The hydrogen nuclei will fuse to form helium nuclei
Every fusion reaction releases heat (and light) energy which keeps the core hot
Life cycle of low-mass stars
- Red Giant
After several billion years the hydrogen causing the fusion reactions in the star will begin to run out - fusion begins to stop
The core collapses because the inward force due to gravity will become greater than the outward force
A new series of reactions will then occur around the core - the outer part of the star to expand
It is red because the outer surface starts to cool - White Dwarf
Fusion will stop eventually and the star will collapse and it turns white. - Black Dwarf
It will eventually fade out
Life cycle of high-mass stars
- Become red supergiants then they collapse. The collapse is more violent as there is more mass.
- A supernova explosion occurs. This is where elements heavier than iron can form from fusion.
- The explosion compresses the core into a neutron star, an extremely dense object made only of neutrons.
- If the original star was massive enough, it becomes a black hole instead of a neutron star. The
gravitational field of a black hole is so strong nothing can escape from it, not even light.
How do we decide if something is a planet, moon or satellite?
A planet orbits a star, enough gravity to make it spherical and sweeps out its own orbital path of other smaller objects.
A moon orbits a planet.
A satellite is something that orbits something else. The Moon is a natural satellite. There are many artificial (man-made) satellites.
Describe a circular orbit
An object orbits another at a constant speed. Its direction is constantly changing so its velocity is changing (speed in a given direction). Therefore we say the object is accelerating towards the centre of the circle. The acceleration is a change of velocity per second.
What happens to an orbit when the speed changes
If an object in orbit slows down, it will fall into a lower orbit (closer to the Earth). If it gets too slow, it will crash to the surface.
If an object in orbit speeds up, it will move to a higher orbit (further from the Earth). If it gets sufficiently fast it will escape the gravitational pull of the object it is orbiting and fly off into space.
At the correct speed an object will orbit at a constant height and speed.
Explain red-shift
Light waves are stretched out if a star or galaxy is moving away from an observer. The wavelength of the light increases.This is called red-shift because the light is shifted towards the longer wavelength red end of the visible spectrum.
If a star or galaxy was moving towards the observer, the light waves would be compressed making the wavelength shorter and we would say the light has been blue-shifted.
What is CMBR
Microwaves coming from every direction in space which were red-shifted 13.5 billion years ago. This suggests there was a cosmically massive event.
Steady state theory
Suggests universe was expanding, but it had always been there and
matter was created into the spaces that the universe expanded into.
Big yawn
If the universe is less dense than a certain amount. Expand forever, the stars will gradually die out
and the universe will cool
Big crunch
If the universe is more dense than a certain amount. Stop expanding and collapse - Some scientists currently think the universe has gone through cycles of this.
