P16 Flashcards
Whats the life cycle of stars ?
1) Stars initially form from a cloud of dust and gas called a nebula.
2) 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 enoush, hydrogen nuclei undergo nuclear fusion to form helium nuclei. This gives out huge amounts of energu, which keeps the core of the star hot. A star is born.
3) 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 stable period.)
4) 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.
5) A small-to-medium-sized star like the Sun then becomes unstable and ejects its outer laver of dust and gas. This leaves behind a hot, dense solid core - a white dwarf.
6) 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.
7) Big stars, however, start to slow 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.
8) 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 in space that not even light can escape from.
Whats the main sequence of a star ?
STARS LIKE OUR SUN RELEASE ENERGY AS HYDROGEN FUSES TOGETHER, DOING SO FOR MILLIONS OF YEARS. THIS ENERGY WILL BE RELEASED UNTIL THE STAR RUNS OUT OF HYDROGEN.
What happens at the end of a main sequence star that is the same size as our sun ?
-WHEN THE STAR RUNS OUT OF HYDROGEN THIS IS THE END OF ITS ‘MAIN SEQUENCE’ AND IT BEGINS TO SWELL OUT.
-STARS THE SAME SIZE AS OUR SUN (OR SMALLER) SWELL OUT AND COOL DOWN, TURNING RED – A ‘RED GIANT’.
-WHEN FUSION STOPS NO MORE RADIATION IS RELEASED AND THE STAR COLLAPSES IN ON ITSELF, THIS FORMS A ‘WHITE DWARF’, THIS IS SMALLER THAN IT WAS BEFORE. EVENTUALLY IT BECOMES COLD, FORMING A ‘BLACK DWARF’
What happens at the end of a main sequence star thats bigger than our sun ?
-IT SWELLS TO FORM A ‘SUPER- REDGIANT’ WHICH COLLAPSES
-THE STARS MATTER COMPRESSES THE CORE, THEN THIS COMPRESSION SUDDENLY REVERSES AND THE STARS EXPLODES, THIS IS KNOWN AS A ‘SUPERNOVA’.
What elements form naturally during the explosion of the supernova ?
Elements heavier than iron are formed naturally during the explosion of a supernova, therefore heavy elements like uranium could only be present from the remains of supernova.
What are planets ?
Large objects that orbit the sun (there are 8 in our solar system)
And they have to have gravity strong enough to have pulled in any nearby objects apart from their natural satelites (moon)
Whats a solar system ?
A solar system is all the stuff that orbits our sun
What are dwarf planets ?
These are planet-like objects that orbit stars, but they don’t meet all the requirements for being a planet
What are moons ?
They orbit planets and they’re a type of natural satelite
What are artificial satelites ?
They are satelites that humans have built and they generally orbit the earth
What is the milky way galaxy ?
Its a massive collection of billions of stars that are held together by gravity
What creates the orbits ?
Gravity provides the force that creates the orbits
What are the planets constantly changing when orbiting the sun ? Where does the force act on a planet ?
The planets are constantly changing direction when orbiting the sun therefore they are constantly accelerating
But for a object to accelerate there must be a force acting on it and for a planet this force is directed acting towards the centre of the circle. This force would usually make the planet just fall into the middle of the circle (sun) but as the planet is already moving, the force just causes it to change direction. The planet keeps on accelerating towards what it’s orbiting (the sun) but instanteneous velocity keeps it traveling in a circle
What force gets stronger as you get closer to a star or a planet?
Gravitational force
So the stronger the force, the faster the orbiting object needs to travel to remain in orbit
For an object in a stable orbit, if the speed of the object changes what must also change ?
The radius of the orbit
Faster moving objects will move in a stable orbit with a smaller radius than slower moving ones
Faster - smaller orbit
Slower - bigger orbit
