Section 8 - Astrophysics P1 Flashcards
What is the universe a collection of?
billions of galaxies
What is a galaxy?
a large collection of stars
What galaxy is earth part of?
the milky way galaxy
What is our sun within the milky way galaxy?
- our sun is just one of many billions of stars which form the milky way galaxy
- its about halfway along one of the spiral arms of the milky way
What is the force which keeps stars together within a galaxy?
gravity
-galaxies also rotate within the universe
What is most of the universe?
empty space
What makes up our solar system?
Our solar system has one star, the sun, and everything that orbits it makes up the solar system
- planets
- dwarf-planets
- moons
- artificial satellites
- asteroids
- comets
What is a satellite?
A satellite is an object that orbits a second, more massive object
What is a planet?
Large objects that orbit a star
How many planets do we have in our solar system and what are they called? (order from the sun outwards)
eight planets
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
What is a dwarf-planet?
They are planet-like object that aren’t big enough to be planets (e.g. Pluto)
What is a moon?
They orbit planets with almost circular orbit
-they’re a type of natural satellite
What is an artificial satellite?
man-made that usually orbit Earth in a fairly circular orbit
What is an asteroid?
Lumps of rock and metal that orbit the sun
-usually found in an asteroid belt
What is a comet?
Lumps of ice and dust that orbit the sun
-their orbits are often highly elliptical(stretched circle)
What is the orbit of planets around the sun described as?
Almost circular
What does a circular orbit mean?
-it is constantly changing direction (and so constantly accelerating), which means there must be a force acting on it
What the force which causes a circular orbit called?
Centripetal force
-it acts toward the centre of a circle
If the centripetal force acts towards the centre of a circle, how come planets remain in orbit?
-since the object is already moving, this force towards the centre of a circle just causes it to change its direction instead of just falling towards the centre of a circle
How do orbits occur circular?
it keeps accelerating towards what it’s orbiting but the instantaneous velocity keeps it travelling in circle
- the gravitational force makes this happen
- the gravitational attraction of the sun keeps planets in orbit, satellites are kept in orbit by a planet in the same way
What does gravitational field strength depend on?
- the mass of the body creating the field
- the larger the mass of the body, the stronger the gravitational field (e.g. earth’s GFS is greater than the moons)
- the distance you are from a planet or star, the closer you are the greater the force
What are orbits of moons and planets normally?
slightly elliptical
What are the orbits of comets around the sun?
very elliptical
- orbit with Sun at one focus (near one end of the orbit)
- have very long orbital periods because of the distance they travel
- comet travels quicker closer to the Sun than further away (because of the gravitational pull)
What are geostationary satellites?
artificial Earth satellites that have an orbital period of exactly one day
-useful in communications
What is the equation linking orbital radius, time period and orbital speed?
orbital speed = (2π x orbital radius) / time period
What is the path of stellar evolution?
Nebula - Protostar - Main Sequence Star - Red Giant - White Dwarf
OR
Nebula - Protostar - Main Sequence Star - Red Supergiant - Supernova - Neutron Star OR Black Hole
What is the Nebula stage of stellar evolution?
Stars initially form from a cloud of dust and gas called a Nebula
What is the Protostar stage of stellar evolution?
Force of gravity pulls the dust and gas together to form a Protostar
the temperature rises as the star becomes more dense and their are more collisions
when the temperature is high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei
this gives out lost of energy keeping the core of the star hot (a star is born)
What is the Main Sequence Star stage of stellar evolution?
star enters a long, stable period
outward pressure from thermal expansion(the energy produced from nuclear fusion tries to expand the star) balances the force of gravity pulling everything in
-lasts for several billion years (the heavier the star the shorter the time)
What is the Red Giant stage of stellar evolution?
the hydrogen in the core of the star begins to run out, the force due to gravity is larger than the pressure of thermal expansion
the star is compressed until it is dense and hot enough that the energy created causes the outer layers of the star to expand
small stars become red giants (red because the surface cools)
What is the White Dwarf stage of stellar evolution?
the red giant then becomes unstable and ejects its outer layer of dust and gas
leaving behind a hot, dense solid core
What is the Red Supergiant stage of stellar evolution?
the hydrogen in the core of the star begins to run out, the force due to gravity is larger than the pressure of thermal expansion
the star is compressed until it is dense and hot enough that the energy created causes the outer layers of the star to expand
small stars become red supergiants (red because the surface cools)
What is the Supernova stage of stellar evolution?
The Red Supergiant then begins to glow brightly again, as it undergoes more fusion to make heavier elements
they expand and contract several times as the balance shifts between thermal expansion and gravity
they eventually explode in a supernova
What is the Neutron Star OR Black Hole stage of stellar evolution?
the exploding supernova, throws outer layers of dust and gas into space, leaving a dense care called a Neutron Star
-if the star is big enough it will collapse and become a black hole
What is a black hole?
A super dense point in space that not even light can escape from
What can stars be classified by?
Their colours
What does the colour of a star depend on?
The visible light it emits
- all stars emit visible light, how much it emits of each frequency depends on the surface temperature
- stars of a similar colour will have a similar surface temperature
What colours do we use to classify stars?
red orange yellow white blue
What visible light will a cooler star emit?
-visible light at the lowest frequency of visible light, so red
What visible light will a hotter star emit?
The hotter a star is, the more light of higher frequencies it will emit
-they appear blue
What is the order of visible light according to the surface temperature of stars? (starting from the coolest)
Red Orange Yellow White Blue
