Draw a diagram to show the stages that stars follow in their life cycles.
How are protostars formed?
• A cloud of dust and gas, a nebula, is drawn inwards under the force of its gravity.
• It begins to rotate increasingly rapidly.
• The centre becomes hotter.
• A hot, dense protostar is formed.
How do protostars develop into main sequence stars?
• Protostar's increasing force of gravity pulls in matter.
• As the star's volume increases, its density does too.
• Pressure therefore increases, causing more collisions.
• When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei.
What event causes protostars to become main sequence stars?
When nuclear fusion starts to happen.
Main sequence stars typically last several million years, because they are stable. How are they stable?
There is an equilibrium between the gravitational collapse of a star and its expansion due to the outward pressure of fusion energy.
What eventually causes a main sequence star to become unstable and turn into a red [super]giant?
The hydrogen supply dwindles to less that 50%, causing the star to swell.
Small main sequence stars swell into ___ ___ and larger ones turn into ___ ___.
1) Red giants 2) Red supergiants
The colour of a star indicates what?
List the colours of the hottest to coolest stars.
What happens to stars in the red giant stage?
• Fusion of elements beyond helium produces less energy each time.
• Eventually, the core will not have a high enough temperature or pressure to fuse the next element.
• The star's outer layers drift away (to form a new nebula)
• The core remains very hot and dense, becoming a white dwarf.
What happens to a white dwarf after some time?
Eventually, it will emit less and less energy, and cool to a brown or black dwarf.
What do stars do in the red supergiant stage?
• They glow brightly as they fuse elements up to iron.
• Fusion of iron doesn't produce any energy, removing the outward pressure.
• The core collapses; protons and electrons fuse to produce neutrons.
• Causes a shockwave through the outer layers of the star.
• The star explodes into a supernova, forming all elements heavier than iron and ejecting them into the universe.
Where do all naturally occurring elements beyond iron come from?
What happens once a red supergiant has become a supernova?
• The supernova ejects its outer layers of dust and gas into space
• Leaves a very dense core - a neutron star.
• If this star is dense enough, it becomes a black hole.
What is a black hole?
An object with such a strong gravitational field that it can even act on photons - i.e. no light can escape.
How many planets are in our solar system?
What do objects have to do in order to be classed as planets?
• Orbit a star
• Clear their neighbourhoods, i.e. have gravity strong enough to pull in nearby objects.
What is the name given to an object orbiting a star which can't clear its neighbourhood?
A dwarf planet.
What are natural satellites?
Moons that orbit planets.
Give an example of an artificial satellite.
What are asteroids?
Objects made of metal and rock-like material, which orbit stars in highly elliptical orbits.
What are comets?
Objects made of rocky material, dust and ice which vapourise as they approach stars.
What galaxy is our solar system part of?
The Milky Way.
Planets orbit stars in ___ orbits.
Circular (acually elliptical - but say circular).
If an object moves in an [almost] circular orbit, it is constantly changing ___, and therefore also ___.
1) Direction 2) Velocity
If an object moves in an [almost] circular orbit, it is constantly changing direction, and therefore also velocity. For this to happen while the object is at a certain speed, it must be ___. There must therefore be a __ __ acting on it, generated by __.
1) Accelerating. 2) Centripetal force. 3) Gravity.
Explain how, for circular orbits, the force of gravity leads to changing velocity, but not changing speed.
• If an object moves in a circular orbit, it is constantly changing direction, and therefore also velocity.
• For this to happen while the object is at a certain speed, it must be accelerating.
• There must therefore be a centripetal force acting on it, generated by gravity.
• The object keeps accelerating to what it's orbiting, but its instantaneous velocity (at a right angle to acceleration) keeps it in its elliptical orbit.
Why do objects in closer orbits to a star/planet need to orbit it faster?
The closer an object is to a star/planet, the stronger the gravitational force it experiences, the faster the object needs to travel to remain in orbit (and not crash into what it's orbiting).
For an object in a stable orbit, if its __ changes, the __ of its orbit must do as well.
1) speed 2) radius
What evidence do we have that the universe is expanding?
When we record light from distant galaxies, the wavelengths are longer than previously recorded; they are shifting towards the red end of the spectrum - called red-shift. Suggests the source of the light is moving away from us -the galaxies are receding quickly, from every direction.