P.7.1-4 - Studying the Universe

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Cambridge IGCSE Physics Flashcards on P.7.1-4 - Studying the Universe, created by franimal on 06/20/2014.

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Question Answer
What does the word 'phases' mean in astronomy? The changing appearance due to the relative positions of the Earth, Sun and Moon.
What are groups of stars that make pictures called and how is this word spelt? constellations
What is it called when you measure a day's time by the Sun and when you measure a day's time by the stars? A solar day is the sun, a sidereal day is the stars.
What two words mean 'an imaginary object of which the observer is the centre and on which all celestial objects are considered to lie'. celestial sphere
What two angles are used to measure where a star is, no matter where you are? right ascension and declination
What is the system called for working out where a star is in the sky and what is the reference point called? The equatorial coordinate system using a reference point called a vernal equinox point.
What is it called when the planets slow down and appear to go in reverse to their usual movements? retrograde motion
What modern-day explanation is there for retrograde motion? Mars is seen from Earth against a backdrop of fixed stars. As Earth orbits the sun faster than Mars, it approaches then passes Mars, and this causes a change in the angle at which we look at it.
What two kinds of eclipses are there? A solar eclipse where the Moon blocks the Sun, and a lunar eclipse where the Moon moves into the Earth's shadow.
What is almost exactly the same for the Moon and the Sun? Their angular size.
What two kinds of shadow are there for the Moon and the Sun? The umbra (total darkness) and the penumbra (partial darkness).
Why don't we see an eclipse twice a month or so? The Moon is on a (5 degrees) tilted plane so the Moon, Earth and Sun very rarely line up correctly.
What is the Sun's hot atmosphere called? The solar corona.
What does a telescope do? It makes an image that is clearer and more detailed than what we can see with our eyes.
What differences are there between a real image and a virtual one? A real image is formed where the light rays focus, whilst a virtual image is one where the light rays appear to focus there but don't actually - such as in a mirror.
How does a ray diagram explain how a real image is made in a pinhole camera? Light from an object travels in a straight line and some passes through the pinhole, hitting the screen to cause an upside down image. Light reflected off the object is directly used to make the image.
What is it called when light bends as it passes through a material and what kind of lens does this in telescopes? In a converging lens, light refracts.
What is the horizontal line that passes through a converging lens called? The principal axis.
What is the point at which all the light rays meet called? The focus.
What is the distance between the center of the converging lens and the focus called? The focal length.
What equation shows the power of a lens, and what is this measured in? The power (dioptres, D) = 1/focal length in meters.
Why does light refract moving through a lens? Visible light contains all the colours of the rainbow, which in turn have different frequencies, so refract at different angles. They refract once at entering the lens and once at exiting.
In a telescope, there are two lenses. Which is near your eye and which is on the other end, and what job does each do? The eyepiece lens is next to your eye and magnifies an image, the objective lens is further away and collects light.
What is the other way of saying an object appears upside down? It's image is inverted.
What is it called when a lens has a large diameter and why does this matter? The lens has a large aperture, which is the light-gathering area of a telescope. The bigger the aperture the more light is gathered, so fainter objects can be seen.
If a telescope makes the Moon seem 50x bigger, what does it have? An angular magnification of 50.
What is an object made up of many points of light called? An extended object.
What is the equation for magnification? focal length of objective lens/focal length of eyepiece lens.
What is the rainbow of light that comes from a prism called and what is the splitting of light called? A spectrum of light appears when dispersion occurs.
How are spectrometers used in astronomy? They're attached to telescopes to break the light from the object into it's spectra, which can tell astronomers what elements are present.
Early telescopes were refractors, what kind of telescope are modern day telescopes and how do they work? Reflectors use curved mirrors to focus light.
What must a mirror be to focus light? It must be parabolic, or cone-shaped concave.
What is the law of reflection? angle of incidence = angle of reflection
What is used in a reflecting telescope so you don't have to sit inside the telescope? A plane mirror, just before the focal point is reflects light out of the telescope into an eyepiece at the side.
What is needed in a telescope to get a clear image? Good resolving power.
What happens when waves hit the edge of a barrier or pass through a gap in a barrier and what is this called? The waves bend and spread out after the barrier, which is called diffraction.
How does diffraction connect to telescopes? The aperture makes waves distract, with longer wavelengths such as radio diffracting more.
What electromagnetic radiation is absorbed by the atmosphere and what is refracted? Absorbed: X-rays, gamma rays, much infrared UV. Transmitted: visible light, microwaves, radio waves, some infrared.
What is the cause of scintillation? Currents moving around dense patches of atmosphere make light refract.
Why does street lights in a nearby town bother astronomers and what is this effect called? Light is scattered by the atmosphere, causing light pollution.
How is a parallax angle worked out? They compare the angle the star is at in December and in June, to make an isosceles triangle. The parallax angle is half the angle moved by the star in this time.
What are parallax angles usually measured in? fractions of a second of arc
How many degrees is a second of arc? 1/3600 of a degree
What is the equation for working out a parsec? Distance (parsec) = 1/parallax angle (sec)
What is the power output of a star called? It's luminosity.
What are the stars that don't emit light at a steady rate called? They're Cepheid variables.
How does a Cepheid vary and what is changing? They vary very regularly in luminosity, and this is caused by them expanding and contracting.
What is a million parsecs called? A megaparsec.
How are Cepheid variables used to measure distance? The period of variation for a Cepheid is directly related to it's luminosity - so if two Cepheids have the same period then they have the same luminosity, and if one appears fainter this can be used as observed brightness.
What is 'a cluster of hundreds of thousands of old stars' called? A globular cluster.
What are the clouds sometimes seen by astronomers called? nebula/nebulae
What can supernova be used to measure? Distances, as one kind of supernova always has the same luminosity peak and period etc, so the brightness can be measured and compared.
There's a linear relationship between galaxies in redshift. The further away the star, the... The further away the star, the greater the speed of recession.
How is speed of recession worked out? speed of recession = Hubble constant x distance
What are the huge lumps that sometimes explode out of the Sun called? coronal mass ejections (CME)
What two signs are there of a hot star on a graph with the electromagnetic spectrum and intensity of radiation? The area under the graph will be large, and the 'peak frequency' will be greater with a high proportion of high energy radiation.
What is the definition of an emission spectrum? The electromagnetic frequencies emitted by an excited atom as it drops an energy level.
What is the source of sunlight called? The photosphere.
What does an absorption spectrum show? Which elements are present and so have absorbed light from a spectrum.
What is the energy in a photon equal to? The difference between the two energy levels the electron has dropped from.
Why do you get absorption lines? The electrons in a particular element can only absorb the photons that bring it exactly to the next level, so it only takes these to leave black lines or spaces on an absorption spectrum.
What makes two nuclei combine to make a new nucleus and what holds them back from doing this? The electrostatic force repels the two positive nuclei, but if nuclei get close enough the strong nuclear force takes over and binds them together.
When a proton decays to a neutron, what two things are emitted? A positron and gamma rays.
What is the line running through the middle of the H-R diagram called? The main sequence. (This is where our Sun is).
What is space filled with? interstellar medium (ISM) a low density gas
What relation is there between temperature and density in ISM? The more dense a cloud is, the smaller the temperature.
What four measurable quantities are there of a gas? Volume, mass, pressure, temperature.
For a fixed mass of gas at a constant temperature, what equation explains the relationship between pressure and volume? pressure x volume = constant
What model can be used to explain why pressure increases when volume decreases? The kinetic model.
What happens to volume and pressure when temperature decreases? They also decrease.
What scale has absolute zero as it's actual zero? The Kelvin scale.
What is a star called before it actually becomes a fully fledged fusing-star? A protostar.
Why does a supernova begin? Iron needs energy to fuse, rather than giving it out - so it can't. There's no heat so no pressure in the core, so the outer shells implode, only to bounce off the core and fly off in a huge explosion.
In supernova, what happens in the core? The temperature rises to 10 billion K, and large elements fuse, making the heavy elements in our world.
What is a neutron star? The collapsed remnant of a star, that is made almost entirely of neutrons and very, very dense.
What happens to a star like the Sun when it stops being a red giant? It throws off it's planetary nebula and becomes a white giant.
What are planets orbiting other stars called? exoplanets
What organisation actively looks for alien life? SETI or the Search for Extra-Terrestrial Intelligence
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