Fichas
STELLAR RADIATION
Fichas del conjunto (42)
- 1Classifying stars by temperature
- 2LuminosityThe luminosity of a star is the total power (energy per second) it radiates. The power radiated per square meter depends on surface temperature. The luminosity depends on power per square meter and the surface area of the star.
- 3Apparent and absolute magnitudeThe apparent magnitude is how bright a star looks to an observer. The absolute magnitude is how bright a star look from a distance of 10.0 parsecs.
- 4Continuous spectrumContinuous spectrum: comes from hot (incandescent) bodies such as hot or molten iron. By determining where the peak of the energy is in the spectrum you can find the temperature of the star.
- 5—Note:
The temperature scale runs from high to low
The luminosity scale uses the run as a reference
Because of the large range in scales of temperatures and luminosities the scales are not linear
- 6Main sequence starsOrdinary dwarf stars that produce energy from the fusion of hydrogen and other light nuclei such as helium and carbon. The vast proportion (80% +) of stars fit in this category
- 7Red giantsCooler than the sun and emit less energy per square meter of surface, however, they have a higher luminosity. They have much larger areas than sun like stars.
- 8White dwarfsWhat remains of old stars. They are very hot when they stop producing energy but they have a relatively low luminosity suggesting a small surface area. They take billions of years to cool down.
- 9SupergiantsEnormous and very bright.
- 10Formation of a starGravitational attraction of interstellar hydrogen nuclei. The loss of potential energy leads to an increase in gas temperature. Gas becomes denser and when temperature becomes high enough the nuclear fusion process begins. This creates helium nuclei which can also fuse as temperature rises, liberating more energy.
- 11Line spectrumThe result of excitation of atoms. The spectra only contain certain well-defined wavelengths. The wavelengths present are characteristic of the element that produces the light so comparison with a laboratory source reveals the elements in the star
- 12Band spectrumProduced in the same way as line spectrum but is produced by molecules. It consists of bands of light produced by a range of wavelengths separated by gaps
- 13The Inverse Square law
- 14Black body radiationA body that is capable of absorbing all the radiation that falls on it is called a black body. Since it does not reflect energy from any other source all the energy radiated is thermal energy due to its temperature. When analysed the resulting radiation forms a continuous spectrum. The color of light that is emitted depends on the temperature of the star.
- 15Lives of Sun-like starsStar collapses as all hydrogen nuclei in core are used up. Core temperature rises as helium nuclei fuse. Hydrogen in the outer layer starts to fuse. Temperature of outer layer rises and it expands. Temperature falls as star expands and becomes a red giant. Fusion of helium nuclei raises temperature of core, further and heavier elements form. Star collapses to become a white dwarf.
- 16Wein's lawAs the temperature rises, the peak of the black body radiation graph moves to a shorter wavelength. Wein's law related the wavelength at which most radiation is emitted to the absolute temperature of the body that emits the radiation.
maxT = 0.0029 mK
The equation can be used to find the temperature of the surface of a star provided that the peak wavelength is known. - 17Effect of temperature on radiated powerThe power radiated by a black body increases rapidly with temperature. Double the temperature of a surface (in K) means that the power radiated by each square meter of the surface is 16 times greater.
- 18Calculating the luminosity of stars1. Obtain a record of how the intensity varies with wavelength
2. Determine the wavelength at which the maximum intensity occurs
3. Use Wein's law to determine the temperature
4. Determine the power emitted per square meter from the surface - 19Emission spectraEmission line spectra can be seen when light from a gas discharge tube is analysed using a spectrometer. Each line is due to the presence of light of a well-defined wavelength in the emitted light. The set of the lines that form is unique to the element of the gas in the discharge tube. The presence of a particular element in a light source can be shown by identifying some of the wavelengths that are characteristic of the element.
- 20Atomic energy levelsLight is emitted when atomic electrons that have moved into excited states give up some of their energy. Electrons can only exist in discrete energies which are referred to as electron energy levels. In a given atom, electrons can only have certain energy levels. They can only move between the energy levels by absorbing or giving up energy in amounts equal to the difference in energies between the levels. Each element has a different set of energy levels which give rise to the unique set of spectral lines.
- 21Energy of bound electronsElectrons are bound for atoms. Atoms that are ionized have had one or more electrons removed and energy is needed to do this. An electron that is just free on an atom is defined as having zero energy. For a bound electron, energy has to be added to release it so within the atom the electron has negative energy. The lowest possible energy that an electron can occupy is called the ground state.
- 22Electron voltsOne electron volt is the energy gained by an electron when it is accelerated by a potential difference of 1V. The energy gained is the potential difference multiplied by the charge so 1 eV is equivalent to So 1 eV is equivalent to 1.6x10(-19)J
- 23Production of emission line spectrumHeating the element in a Bunsen flame or passing through a current through a gas in a discharge tube provides energy to cause excitation of the atomic electrons. They are raised into a higher energy level. Different atoms will gain different amount of energy, in a gas atoms will exist in all the possible energy levels. The electrons are in excited states, which are not stable. Electrons remain in these states for a very short time before releasing energy and returning to lower energy states. This is called relaxation.
- 24The electrons lose energy as they move to lower energy states by emitting a photon of electromagnetic radiation. The photon energy hf is equal to the energy difference between the levelshf = E(1) – E(2) or hc/ = E(1) – E(2) since c=f
- 25—Groups of wavelengths have been identified in the line spectrum of hydrogen. The Lyman series is the result of transitions into the n=1 level. This gives rise to high energy photons in the ultraviolet part of the spectrum. The visible Balmer series comes from transitions into the n=2 level. The Paschen series is the result of transitions into the n=3 level and gives infrared radiation
- 26Absorption spectraStars behave like black bodies at very high temperatures the spectrum observed from them is not continuous. Dark lines cross the spectra showing that some wavelengths are missing or have a reduced intensity. The wavelengths that are missing are characteristic of the elements present in the outer regions of the star through which the light has to pass before reaching earth.
- 27Formation of an absorption spectrumIf star is viewed through interstellar gas electrons in gas atoms are excited into higher energy levels. Only photons with energy equal to the difference in energy between two levels can be absorbed by gas. Only certain well-defined frequencies are removed from spectrum.
- 28Formation of an absorption spectrum 2Once the electrons have been excited they are in an unstable state. As they relax into lower states they radiate energy. The intensity of a given wavelength in a particular direction is reduced because the light that is re-radiated from the gas cloud travels in all directions so less goes straight on. The electrons relax in stages emitting lower energy photons that are in a different part of the electromagnetic spectrum.
- 29Absorption spectra in the laboratory
- 30Absorption spectra in the laboratory - the theoryWithout bright light source in place, emission spectrum of the sodium flare consists of a yellow line corresponding to a wavelength of 590nm. When a diffraction grating is used, the single line can be seen as two yellow lines very close together.If a lamp emitting very bright right white is viewed through sodium flame, two dark lines cross the lamp's continuous spectrum in exactly the same place as the lines that are seen in the sodium emission spectrum
- 31Absorption spectra in the laboratory - alternative methodWhite light is passed through iodine vapour produced by gently heating a few iodine crystals in a sealed boiling tube. The continuous spectrum consists of equally spaced dark bands. The spectrum is caused by the quantised vibration states that the iodine molecules can exist in. Iodine molecules can only take in well defined chunks of energy.
- 32Red shiftThis is the main evidence for the expansion of the Universe. The wavelength of radiation from stars has a longer wavelength than radiation from a similar source in a laboratory on earth. The Doppler effect predicts such a change when a source of radiation is moving away from an observer. This leads to the conclusion that stars and galaxies are all tending to move apart, so the Universe is expanding
- 33Red shift 2Optical wavelengths are shifted towards the red end of the spectrum when the source is moving away from the observer. The faster the motion, the greater the change in wavelength towards the red end of the spectrum. All waves in the spectrum will be affected, even the absorption lines.
- 34The Doppler effectIn terms of sound waves the pitch of a sound increases as it moves towards a listener and decreases as it moves away. The effect occurs whenever a source and observer are moving relative to one another.
- 35The Doppler effect explainedAs the source moves, the waves are being bunched into a smaller distance in the direction in which it is moving and stretched in the opposite direction. This means that wave front pass an observer ahead of the source more often so that the sound has a higher frequency.
- 36Calculating velocity using the doppler effectThe change in wavelength over the original wavelength is equal to the change in frequency over the original frequency
- 37Binary stars, rotating stars and galaxiesSometimes two stars rotate about their centre of mass. As they rotate, one star moves away from the earth so the spectral lines are red shifted, the other star moves towards the earth producing a blue shifted spectrum. The velocities that give rise to the motions can be found from the shifts and the rate of rotation determined. Similarly the spectral lines of light that is emitted from opposite sides of a spinning star are blue and red shifted so that its rotational speed can be found
- 38Evidence for dark matterThe orbital speed of starts in a galaxy can also be found using the doppler effect. Such measurements have shown that the orbital periods of stars can be far higher than expected for the estimated mass of the material in a galaxy. This means that some of the mass of the galaxy is unaccounted for. The missing mass is called dark matter and scientists are still unsure of what it is.
- 39Hubble LawHubble suggested that the recessional speeds of galaxies at different distances from the Earth shows that they are moving at different speeds. The further away they are the faster they are moving. This gave rise to the Hubble Law v=Hd where H is the Hubble constant.
- 40Age of the UniverseThe age of the Universe can be estimated from the Hubble constant. Assuming that it holds for all stars, the maximum speed of a star is the speed of light. The most distant observable galaxy would be at a speed of c/H. This is 4600 Mpc away. The distance is the speed c multiplied by the time it has take t to get there following the big band. This leads to an age of 1.5x10(10 years)
- 41QuasarsA massive and extremely remote celestial object, emitting exceptionally large amounts of energy, which typically has a starlike image in a telescope. It has been suggested that quasars contain massive black holes.
- 42Lives of sun like stars after the White Dwarf phaseStars with mass like the sun will fade away, Stars with mass 1.4 times greater than sun can either explode into smaller white dwarfs or collapse suddenly and become a supernova
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- Descripción
- PHYSICS AQA B Fichas sobre STELLAR RADIATION, creado por Heather Mansfield el 25/04/2014.
- Creado por
- Heather Mansfield
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