P7 - new cards Flashcards
What is a year?
The time it takes the Earth to complete one orbit of the sun.
The Earth takes 365.25 days to complete one orbit
What is the tilt of the moons orbit round the earth compared to the orbit of the earth around the sun?
About 5 degrees.
What is the position of the moon during the various phases?
What does the earth spin around?
An imaginary line called the axis.
What is the angle of the axis of the earth to the plane of the planetary disk?
Why is the sidereal day shorter than 24hrs?
Why do we see different constellations in summer and in winter?
The earth moves around the sun, so we are facing in a different direction at night.
Which way does the moon APPEAR to move, seen from earth?
East to West
Which way does the moon ACTUALLY orbit the earth?
From west to east, it actually moves the opposite way to what we see.
What is the effect of the moons orbit from west to east on the length of time that it takes for the moon to be in the same place overhead?
It is longer than a day, it is 24h 49mins.
What do we see happening to the position of the moon over a month?
It appears to move “back” slightly every night.
What is meant by retrograde motion?
A planet going the “wrong” way, moving west to east rather than the normal east to west.
Draw a picture to explain retrograde motion
Explain retrograde motion in words.
- The nearer to the star a planet is, the faster it moves.
- Earth therefore moves faster than eg mars.
- We are therefore catching up with mars, and leaving it behind.
- As we do so our angle of view changes against the background of the stars are the planet therefore appears to move backwards.
Phases of the moon - draw a picture to explain the phases of the moon.
Draw a picture of a solar eclipse. Explain the umbra and the penumbra.
Draw a picture of a lunar eclipse. What phase of the moon does an eclipse happen in?
Why are lunar eclipses more common than solar eclipses?
The earth’s shadow is bigger than the moons shadow.
Why are eclipses rare? Draw a picture to demonstrate.
How are the coordinates of a star given?
The angle along the direction of the equator, and the angle along the line connecting the two poles.
What does light travel as?
Waves
What is the substance that light travels through called?
The medium
What does the speed of light depend on?
The medium
What happens to the frequency of a wave when it changes medium?
It remains constant
What happens to the speed of a wave as it changes medium?
It changes
If the speed of the wave changes, and the frequency remains constant, what happens to the wavelength?
It has to change.
What do we call a change of wave direction as it goes from one medium to another?
Refraction
Draw a picture of refraction
What type of lens does a refracting telescope use?
Convex lenses.
What happens to parallel rays of lights that enter a convex lens?
They converge and come to a point at the focus.
What is meant by the focal length of a convex lens?
The distance between the centre of the lens and the point at which all the light comes to a point, the focus.
What are the rules for drwing ray diagrams?
- Use arrows to show the direction in which light is travelling.
- A ray through the centre of the lens does not change direction.
- A ray through the focus emerges parallel to the principal axis.
What is the implication of the great distance of the stars on the rays of light reaching earth?
They are parallel. (Near enough. Stop arguing).
Draw a ray diagram that shows the light from a star passing through a convex lens.
What are light rays from objects inside the solar system like?
The objects are closer, so the light rays are not parallel.
What happens to an image when light rays hit a lens at an angle?
The image is inverted.
What is the focal length of a lens?
The distance from the lens to the focal point (the image)
What shape convex lens has a shorter focal length?
A fat one - it is therefore more powerful
How do you measure the power of a lens?
- Measured in dioptres
Power (dioptres) = 1/focal length (meters-1)
What is inside a refracting telescope?
An objective lens - large apparture, long focal length
An eyepiece lens - small apparture, short focal length
Draw a ray diagram of a telescope.
Why do we use telescopes?
They magnify the image; the large apparture collects more light, so we can see dimmer stars than with the naked eye.
How do you calculate the magnification of a telescope?
Focal length of objective lens/ focal length of eyepiece lens.
How does a reflecting telescope work?
It uses a concave mirror to bring light to a focus. This is then magnified by an eyepiece lens.
What are the advantages of reflecting telescopes?
- It is easier to make a large mirror.
- It is hard to make a large lens with no imperfections.
- Big convex lenses are very fat in the middle, so they absorb light, making faint objects even more faint.
- Mirrors reflect all the colours the same. Lenses refract blue light more than red, distorting the image
Draw a picture of a lens distorting the image due to differences of refraction
What is white light made up of?
A mixture of colours
Which of the visible colours has the highest frequency?
Violet
What happens in a prism?
Light is split due to the different amounts of refraction, this is called dispersal.
Draw a picture of a prism.
What happens to light at a diffraction grating?
It is dispersed, and forms spectra
Why would astronomers want to split the light from a star and look at the spectrum?
It is possible to identify elements from the spectrum.
What is a diffraction grating?
Very close, very thin scratches on glass.
When is the effect of diffraction greatest?
When the size of the gap is similar to, or smaller, than the wavelength.
What is a light year?
The distance light travels in a year.
What is meant by parallax?
As the earth orbits the sun the closer stars appear to move against the background of the very distant ‘fixed’ stars.
This is caused by the earths movement, and is not a real movement of the star.
Picture to illustrate parallax?
What is the parallax angle?
It is HALF the angle that the star has apparently moved in 6 months.
They are tiny, measured in seconds. A second is 1/3600 of a degree
What is a parsec?
A way of measuring distance.
1/parallax angle (in seconds) gives the distance in parsecs.
Picture to illustrate parallax?
What is a parsec in light years?
It is the distance to a star where the parallax angle is 1 second - roughly about a light year.
Our closest star is 4 light years away, so the parallax angle would be 1/4 of a second of arc.
What is luminosity?
The amount of light emitted in a second.
What determines luminosity?
- Star size
- Star temperature
How much more light would a star with twice the radius give out?
The area of the disc that is visible is a function of the square of the radius, so twice the radius = four times the luminosity.
How does the temperature of the star affect the luminosity?
A star that is twice as hot will give out 16 times the light.
(Remember - temperature has to be in degrees K)
What is the observed intensity, or brightness, of a star?
How much light we can see from earth
What does brightness depend on?
The luminosity of the star; the distance from earth.
How does distance affect brightness?
A star half the distance will be 4 times brighter.
Why does brightness go down with distance, even when there is nothing in the way?
The light spreads out and covers a greater area.
What is a cepheid variable star?
A star whose brightness varies over time. This is believed to be because the star expands and contracts.
How do cepheid variable stars help us?
There is a correlation between the period of variation and the luminosity. It is therefore possible to know the luminosity by looking on a graph and reading off the period.
What are nebulae?
Fuzzy patches of the sky.
What was Shapeley v Curtis?
Shapeley:
- Thought that the Milky Way was the entire universe
- Thought nebulae were clouds of gas within Milky Way
Curtis:
- Thought that spiral nebulae were huge distant clusters of stars
Who was right, Shapeley or Curtis?
Curtis was right on multiple galaxies, he found a cepheid variable in Andromeda and proved the distance was much further than anything within our galaxy.
What is redshift?
When we compare spectra from stars in distant galaxies the spectra are shifted towards the red end of the spectrum, proving that the distant glaxies are moving away very quickly.
(Doppler effect)
How could you calculate the distance of a galaxy from the speed of recession?
Speed of recession = Hubble constant x distance
How does redshift support the big bang theory?
Everything is moving apart, space itself is expanding, so it could have all started in a singularity.
What do all hot objects give off?
Radiation.
What happens to the colour of a star as temperature goes up? (Colour exaggerated)
How does the spectrum of a star tell us the elements that are present?
As the white light travels through the stars atmosphere the atoms of elements absorb the light of a specific wavelength.
What, other than the elements present, can the spectrum of a star tell us?
The peak frequency of the spectrum gives an accurate value for the temperature of the star - as the temperature goes up the radiation emitted becomes more energetic.
What would a spectrum from a star look like?
What are stars made up of?
Hot gas
Why do gases exert pressure?
The particles move very quickly in random directions, when they hit something they change direction, thereby exerting a force on the thing they hit.
How are pressure and volume related in gases?
If you reduce the volume of the gas the number of collisions of the atoms with the container increases, so they pressure goes up,
How are pressure and volume related in a gas? (Formula)
Pressure is inversely proportional to volume.
How are pressure and temperature related in gases (explanation)
As temperature rises, the particles move more quickly, so the pressure goes up.
What happens to the pressure of a gas as you cool it to absolute zero (-273°C)?
The particles stop moving at absolute zero (0° K) and therefore there is no pressure exerted.
How do you convert °C to °K?
You add 273.
Absolute zero is 0°K, -273°C
As you cool a gas, keeping the pressure constant, the volume decreases. What volume would the gas take up at 0°K?
Theoretically - zero.
The gas laws break down when we get to absolute zero - and we have actually got to temperatures slightly under absolute zero… but don’t tell 21st Century.
A graph plotting luminosity v temperature of various types of stars - you need to be able to label each bit as this has come up.
Explain a protostar.
Gravity compresses a cloud of gas (mainly H and He).
Gas particles get closer, volume of cloud reduces.
As they get closer together they start moving faster.
Temperature and pressure increase.
This mass of gas is called a protostar.
When does a protostar become a star?
When H nuclei get close enough they can fuse to form He.
This is nuclear fusion.
This process gives out a lot of energy as the He atom is slightly lighter than the two H atoms.
What is the source of energy in all stars?
Nuclear fusion.
A picture of the protostar lifecycle
Protostar formation
What is the first stage of the fusion of 2 H atoms?
What is the second stage of the fusion of two H atoms?
The overall picture showing the nuclear fusion of two H to form one He
What is a positron?
Like an electron, but positive. Emitted by some nuclear reactions to preserve the charge.
On nuclear equations, what must balance?
Mass (top number)
Charge (bottom number)
What is the equation that links mass and energy?
E = MC2 where C is the velocity of light in a vacuum
Draw a cross section of the sun.
How are red giants and supergiants formed?
- Main sequence star fuses H to give He
- Eventually runs out of H
- Pressure decreases (as less energy)
- Core collapses
- Hydrogen from the outer layers of the star collapses inwards
- New reactions start
- The pressure goes up and the stars outer layers expand
- The outer layers (photosphere) cool and turn red.
What forms a white dwarf?
- A star like our sun, with low mass
- It will not be able to go further once the He has been used up
- It will shrink down as less energy in the core
- A white dwarf has no more fusion going on in the core, it is just cooling slowly
What forms a supernova?
A red supergiant when it runs out of fuel.
What is the Hertzsprung-Russell diagram?
Do you REALLY know the Hertzsprung-Russell diagram?
What is an exoplanet?
A planet going round another star.
Have we found life anywhere except earth?
No
How would you answer a question about sites of telescopes?
Compare the advantages and disadvantages of each
State, with reasons, which one you believe to be best.
Possibles: labour force, light pollution, weather, dust, cost, environmental, transport links, infrastructure (water, electricity).
How could you solve the problem of the atmosphere reflecting light and distorting images when building a telescope?
Put it on a high mountain.
How would you solve the issue of light being refracted more in a damp atmosphere when building a telescope?
Put your telescope somewhere dry and dust free - eg Arizona.
How could you solve the problem of cloud making observation of the sky impossible when building a telescope?
Choose somewhere with no clouds - eg Arizona
What are the advantages of computer controlled observatories?
Can be used by astronomers many miles away
Images are stored and saved in digital format
Computers can enhance the images
Very easily shared
Why would you put a telescope in space?
- No atmosphere, which absorbs most IR/UV/Gamma/X-ray
- No atmospheric refraction (twinkle)
- No light pollution
- No weather.
Why would you not put a telescope in space?
- High cost of setting up
- High cost of maintenance
- Uncertainty of future funding
Why is international collaboration a good idea for telescopes?
- Share cost
- Share expertise
EXAM TIP - Know 2 examples!
Name 2 international collaboration telescopes
ESO
- European Southern Observatory
- In Chile
- Multiple telescopes
- 14 countries
- 1000< astronomers every year use it
Gran Telescopio Canarias
- In the Canaries
- On a volcano
- Funded mainly by Spain, also USA and Mexico
- Planning involved 1000 people from 100 countries.
