P7 Flashcards
What direction to stars and sun appear to move?
East to west so the Earth moves from west to east
What is a sidereal day?
It’s the time taken for a star to return to the same position in the sky - 23 hours 56 minutes
What is a solar day?
Time taken for the sun to return to the same position in the sky - 24 hours.
Why does a solar day longer than a sidereal day?
Because the Earth orbits the sun in the same direction as it spins so it needs to make an extra rotation for it to appear in the same place
How long does it take for the Moon to appear in the same position in the sky?
25 hours because the moon is orbiting the Earth in the same direction the Earth is rotating
What is a lunar eclipse?
When the moon passes into the Earths shadow so the Earth blocks light coming from the sun so almost no light is reflected from the moon
What is a solar eclipse?
It’s when the moon is directly between the Sun and the Earth so the moon blocks the light from the sun
Why are eclipses rare?
Because the moons orbit is at a different plane so the Earth, Sun and Moon rarely line up
What is the celestial equator?
It’s an imaginary plane extended out from the Earths equator
What is right ascension?
This shows which direction to look across and is measured in degrees and time
What is declination?
Shows how high or below to look from the celestial equator and is measured in degrees
What is retrograde motion?
A planet appears to change direction and go the other way for a bit in relation to fixed stars making a loop in its track
How does retrograde motion work with Mars and Earth?
From Earth Mars appears to move left compared to fixed stars but then Earth overtakes Mars so it appears to change direction but as Earth moves vertically down Mars continues to appear going left
What is refraction?
When a wave hits a boundary between two materials, a part of the wave hits the boundary first so it slows while the other part carries on at normal speed an then hits the boundary which causes the wave to change direction
What happens if the speed changes?
As the frequency remains the same, the wavelength must change. If the speed decreases then wavelength gets shorter
What do converging/convex lenses do?
They use refraction to focus light waves and form an image. Firstly when it hits the light deviates towards to normal and when it leaves it deviates away from the normal
How does light disperse in in a triangular prism?
White light disperses into different colours that have different wavelengths and as the boundaries aren’t parallel the different lights form a spectrum that refracts different amounts, red refracts least
Where is the focal point?
Where Ray’s initially parallel to the the principle axis meet
How is a lens more powerful?
If it has a strongly curved surface so it strongly converges the rays of light so the focal length is shorter
What are extended objects?
They are stars far away they form a point image that are larger than a point
What does a simple refracting telescope use?
It uses 2 converging lenses, the objective lens to collect the light and form an image and the eyepiece magnifies it as it’s more powerful
Why do astronomical telescopes use mirrors instead of lenses?
Mirrors can be made larger so can capture more light. They are cheaper and easier to manufacture. They donor cause chromatic aberration
What is chromatic aberration?
When light passes through lens it refracts and different colour refract by different amounts so the image will appear blurry and coloured
What is diffraction?
When waves all spread out at the edges when they pass trough a gap or past an object
When is their little diffraction?
When the gap is wider than the wavelength
When is there only diffraction at the edges?
When the gap is a little wider than the wavelength
When is there maximum diffraction?
When the gap is the same as the wavelength
What is the aperture?
The diameter of the lens
How do you collect enough radiation from distant and faint objects?
Use telescopes with huge objective lens with a big aperture so it collects more radiation and a better image is formed
How can telescopes produce sharper images?
Have a larger aperture than the wavelength so little diffraction occurs so the image is not blurry
What is diffraction grating?
Series of narrow slits so when white light passes the different wavelengths are diffracted by different amounts which creates a spectrum and can to be used to analyse light coming from stars
What is parallax?
It’s the apparent change in position of a star against a distant fixed background and makes closer stars appear to move more than distant ones
What is the parallax angle?
Half the angle moved against background in 6 months. Measured in arcseconds and the smaller the angle, the further away the star
What are parsecs?
Unit of measuring distance for stars. The distance to a star is 1 parsec if the parallax angle is 1 arcesecond
What does the luminosity of a star depend on?
The size and temperature and the observed brightness depends on luminosity of how far away it is
What are cepheid variable stars?
They are stars that pulsate in brightness
What was shapelys argument?
He believed that the universe was one big Galaxy, he thought our sun & solar system were far from the centre and that nebulae were huge clouds of dust and gas that were part of the milkyway
What was Curtis’ argument?
That the universe had many galaxies, our galaxy was smaller and we were at the centre, spiral nebulae were very distant galaxies
What were nebulae?
They were faint, fuzzy objects seen through telescopes
How did Hubble help solve the debate?
He made observations of the andromeda nebula. Found that spiral shaped nebula had stars and cepheid variables which was used to find the distance which was further than any stars in our galaxy
What is redshift?
When a galaxy is moving away from us the wavelength of the light changes and becomes redder, the greater the red shift the greater the recession velocity
What is nuclear fusion?
When hydrogen nuclei fuse inside the sun into helium and the mass that’s lost is converted into energy
What is the peak frequency?
The wavelength where more of 1 frequency is emitted, the higher the temperature the higher the peak frequency
What is the absorption spectra?
At high temperatures electrons jump into higher energy levels by absorbing radiation. If at continuous spectrum of light shines through gas then elections in gas absorb certain frequencies making gaps in continuous spectrum so dark lines
What is the emission spectra?
When electrons fall to a lower energy level, losing energy by emitting radiation of a particular frequency which gives a series of bright lines
How do each element have its own like spectrum?
Because the energy levels in each atom are different for each element
How do stars produce absorption and emission spectras?
Because the photosphere of a star emits a continuous spectrum of a light which passes the through the gases in the atmosphere
Why are spectras useful?
Because they can work out what stars are made of
How do stars form?
Gravity causes denser regions of clouds of dust and gas to contract and form clumps which breaks into protostars. They continue to collapse which reduces volume, increased temperature and pressure. Allow fusion to take place which releases energy to stop gravitational collapse
What happens in the core?
Most of the fusion and the pressure makes it hotter and denser
What is the photosphere?
It’s the outer region of a star where energy is radiated into space. Energy released from fusion transported to surface by convection currents
