Motion in the Universe and Stellar Evolution Flashcards
What is the universe?
- A large collection of billions of galaxies
- It is the entierety of space
What is a galaxy?
A large collection of billions of stars
What is a solar system?
A collection of planets revolving around a star
What is our galaxy known as?
The Milky Way
Why does the g of different celestial bodies vary?
- The masses and radii of different celestial bodies vary
- For example, the mass of the moon is around 1/81 of the mass of the earth, and the g of the moon is around 1/6 of the g of the earth
- Since the moon has a smaller radius you are closer to the centre when on the surface which is why it does not have 1/81 of the earth’s g
While gravitational field strength will decrease as you move away from the planet’s centre, it is fairly constant on the surface
What do planets, moons, artificial satellites and comets orbit respectively and why?
- Planets - stars
- Moons - planets
- Artificial satellites - the earth or any celestial body
- Comets - stars
- This is because what they are orbiting has a larger mass than them, so while they exert the same force on eachother the lighter one will be pulled towards the heavier one
- This graviational force causes them to maintain circular path around the celestial body
What are the orbits of planets like and how do they differ from planet to planet?
- They all orbit stars in a circular (though slightly elliptical) path in the same plane and direction
- However they have different orbital radii and travel at different speeds
- They therefore take different amounts of time to orbit the sun
Generally, the further away a planet is from the star, the slower it moves and the longer it takes to orbit due to the weaker gravitational force
What are the orbits of moons like?
- All moons orbit planets in a circular path
- Some planets have more than one moon
- The closer the moon is to the planet, the shorter its orbital radius will be, the faster it will move and the less time it will take to orbit the planet
What are the orbits of comets like?
- They orbit stars in a highly elliptical path, so their speed changes greatly depending on their distance from the star
- They do not all orbit in the same plane or direction
What is the relationship between orbital speed, orbital radius and time period?
Time period is the time taken for a celestial body to make a complete revolution around another celestial body
Orbital speed = 2 x π x orbital radius/time period
- This is literally just working out the circumference of the circle for the distance and then doing speed = distance/time
- This only works for circular orbits, so not comets
How can stars be classified according to their colour?
- The colour of stars relates to their temperature
- Blue stars are the hottest, while red stars are the coolest
- This is because blue wavelengths require the most energy to emit
As a star expands, it cools, which is why the larger stars are cooler
What is the life cycle of solar mass stars up to and including the point of it being a main sequence star?
Solar mass stars are stars with the approximate mass of our sun
- Nebula - All stars form from a cloud of hydrogen gas and dust
- Protostar - The force of gravity within the nebula pulls the particles together until it forms a hot ball of gas known as a protostar
- The temperature in a protostar will gradually increase as gravity causes the density to increase which causes more collisions
- Main sequence star - Once the protostar becomes hot enough, nuclear fusion will begin to occur in its core where hydrogen nuclei become helium nuclei, releasing large amounts of heat and light
- At this point a main sequence star is born, with the outward force of thermal expansion from fusion and the force of gravity being in equilibrium keeping the star stable
Main sequence is when stars tend to be the hottest, here they could be blue
What is the life cycle of solar mass stars from the point that they become a red giant up to and including them being a white dwarf?
- Red giant - After several billion years the hydrogen will run out and the fusion reactions begin to die down
- This causes a reduction in the outwards expansion force, causing the star to shrink and heat up
- A series of other reactions will begin to occur, such as helium fusing to form beryllium - these other reactions cause the outer part of the star to expand until a red giant is formed
- White dwarf - Eventually the red giant will become unstable and eject the outer layer of dust and gas (known as a planetary nebula)
- The core which is left will collapse completely due to the pull of gravity and a white dwarf will form
- The white dwarf will then start to cool down (as it has no source of energy) and emit less energy, eventually becoming a black dwarf
- A red giant is red because it has expanded and the outer surface has started to cool
- White dwarfs are hotter than red giants (evidenced by their colour) due to their smaller size and higher density
What is the life cycle of stars with a mass much larger than the sun?
These stars live for only hundreds of millions of years compared to other stars which live for hundreds of billions
- It occurs in an identical way up to and including it being a main sequence star
- Red supergiant - Here, after the hydrogen begins to run out and it shrinks and heats up, the new series of reactions such as helium fusing to form beryllium will be on a greater scale and create a much larger red super giant
- Supernova - Once all of the fusion reactions in the red supergiant stop, the core will collapse suddenly, causing a gigantic explosion called a supernova
- Neutron star or black hole - At the centre of the supernova, a dense body called a neutron star will form with the outer remnants being ejected into space
- In the cases of the largest stars, a black hole may form due to the neutron star continuously collapsing in on itself until something so dense and with just a high g forms that not even light can escape
How is the brightness of a star measured?
- Using the absolute magnitude scale
- This is because a dim star nearby may appear as luminescent as a bright star far away
- The absolute magnitude of stars measures how bright they would be if they were all the same distance from earth
- This distance is 10 parsecs (32.6 light years) away
This is in contrast to apparent magnitude which changes depending on the distance of the star from earth
