1 Astrophysics – Glossary • gives information of speed relative to Earth • gives information as to rotational speed This question is about the solar system. • the approaching star produces blue shifted lines; the receding star produces red shifted lines in the █ State the name of the planet in the solar system that spectrum. (4) has the greatest mass. ► intensity - wavelength distribution (Plank’s+Wien’s laws) ► Jupiter (1) gives information on surface temperature; █ an orbit around the Sun between that of Saturn and of █ Define the luminosity (of a star). Neptune. ► The total power (total energy per unit time) ► Uranus (1) radiated by an object (star). (1) ► █ State where, in the solar system, the asteroid belt is If we regard stars to be black body radiators, then luminosity L of a star is given by the expression. found. 2 4 4 ► between orbits of Mars and Jupiter / 4πR σT = A σT 2AU → 3 ½ AU from Sun (1) where R is the radius of the star, A it’s surface area, T it’s surface temperature, █ State two features of the orbits of comets. and σ is the Stefan-Boltzmann constant. ► highly elliptical; large orbits - most of orbit outside orbits of furthest planets (1) █ State one factor that determines the luminosity of a star ► mass / temperature / surface area / radius / type of █ Distinguish between a constellation, a stellar cluster and star / composition (1) galaxy. ► constellation: Pattern of stars/ A group of stars that █ Describe what is meant by apparent brightness. form a pattern as seen from the Earth/ ► The power from the star received (incident) per An arbitrary formation of stars perceived as a figure square metre of the Earth’s surface or design, not close to one another in space / not ► If the energy radiated by a star is emitted uniformly bound by gravitation. in all directions, then apparent brightness is ► L stellar cluster: Groups of stars held together by b gravitation in same region of space, created 4d 2 roughly at the same time; where L is luminosity (power radiated) of the star ► galaxy: A huge group of stars, dust, and gas held and d its distance from the Earth. together by gravity, often containing billions of stars, measuring many light years across. █ Magnitude Scale (e.g. ≈1010 stars, diameter≈105 ly) (4) ►Magnitudes are a way of assigning a number to a star so we know how bright it is █ State why the Hipparcos satellite which orbits Earth is █ Define apparent magnitude (Greeks). able to measure stellar parallaxes for stars at considerably ► greater distances than 130 pc. apparent magnitude is a measure of how bright an object appears from Earth; (1) ► no atmospheric turbulence / no irregular refraction; (1) ( Every one step in magnitude corresponds to a factor of 2.51 █ Briefly describe the nature of a star. 3 change in brightness. Ex: m = 6 and m = 9, then b = (2.51) b ) ► A star is a massive body of gas held together by 1 2 1 2 gravity, with fusion going on at its center, giving off █ Define absolute magnitude. electromagnetic radiation. (2) ► is the apparent magnitude that a star would have if it were at distance of 10 pc from Earth. (2) █ Stars are very massive. State why stable stars are not crushed inwards under gravitational pressure. • It is the true measurement of a star’s brightness seen ► there is an equilibrium between radiation pressure from a set distance. d and gravitational pressure (2) m – M = 5 log ( ) █ Explain the term black-body radiation m – apparent magnitude M – absolute magnitude of the star ► the radiation emitted by a black body (perfect d – its distance from the Earth measured in parsecs. emitter/perfect absorber) at temperature T • If two stars have the same absolute magnitude but different in accordance with the Planck’s law ; (1) apparent magnitude they would have the same brightness if they █ Discuss two different ways in which atomic spectra can were both at distance of 10 pc from Earth, so we conclude they be used to deduce physical data for stars. have the same luminosity , but are at different distances from Earth !!!!!!!!!!!!!! ► line absorption spectra: gives information on • Every one step in absolute magnitude corresponds to a composition of outer layers factor of 2.51 change in luminosity. Ex: M1 = – 2 and M2 = 5, ► Doppler Shift / red shift / blue shift: 7 then L1 / L2 = (2.51) 2 █ Explain the importance of Cepheid variables for █ The diagram shows the (intensity) spectrum of the estimating distances to galaxies. radiation emitted by a black-body at a temperature T. ► There is a relationship between period and luminosity Sketch the spectrum of radiation emitted by the black for Cepheid variables (discovered by Henrietta Swan Leavitt body at a higher/lower T. nearly a century ago), so measuring the period gives the ► higher temperature : lower T: luminosity and hence the distance (b = L/4πd2). peak higher; peak lower; Distances to galaxies are then known if the Cepheid peak sharper; peak blunter; can be ascertained to be within a specific galaxy; (3) peak to left; peak to right; (2) █ Cepheids are sometimes referred to as standard (In this graph it is assumed that candles. Explain what is meant by this. intensity is actually power/unit ► the phrase standard candle means having a source of surface of the star. Otherwise it light with known luminosity; might be confusing, because ► measuring the period of a Cepheid allows its luminosity bigger star at lower temperature can have greater luminosity, so to be estimated; other stars in the same galaxy can be the graph may be above the compared to this known luminosity (2) graph of the star with smaller (approximately the same distance) surface area but higher █ Describe the characteristic by which a Cepheid variable temperature.) may be identified from Earth. ► light output varies periodically; rapid brightening, (higher T: shorter wavelengths, higher frequencies; higher energy. The curve should be above the existing curve at all locations ) gradual dimming; (2) (lower T: longer wavelengths, lower frequencies, lower energy. █ Outline the cause of this characteristic. The curve should be below the existing curve at all locations ) ► Caused by expansion / contraction of surface; █ Label the x-axis of the graph brighter as it expands; (2) ► wavelength λ (1) █ State the mechanism for the variation in the luminosity of the Cepheid variable. █ Define a binary star ► outer surface expands and contracts periodically due ► A binary star is a stellar system consisting of two stars to interactions of matter and radiation in the stellar orbiting around their common center of mass. (1) atmosphere; (brighter as it expands); (Binary stars can either be distinguished optically (visual binaries) ► changing surface temperature (2) or by indirect techniques, such as spectroscopy.) █ State what can be deduced from knowing the period of █ Techniques for determining stellar distances include the the binary and the separation of the stars. use of stellar parallax, spectroscopic parallax and Cepheid ► the total mass of the binary system (1) variables. Outline all three: ► stellar parallax: █ What property, other than separation of the stars, of a two apparent positions of the star as seen by an observer visual binary system is measured in order to find the mass from two widely separated points are compared and of the system? recorded; the maximum ► the orbital period of the two stars (1) angular variation from the █ State the difference between a visual binary star, a mean, p, is recorded; the spectroscopic binary star and eclipsing binaries distance (in parsecs) can be ► visual binary: a system of stars that can be seen as calculated using geometry two separate stars with a telescope and sometimes with the unaided eye. d pc 1 ► spectroscopic binary: A binary-star system which from p sec Earth appears as a single star, but whose light spectrum (spectral lines) shows back-and-forth Doppler shifts as spectroscopic parallax: two stars orbit one another. • light from star analyzed (relative amplitudes of the ► eclipsing binary: (Rare) binary-star system in which the absorption spectrum lines) to give indication of stellar class two stars are too close to be seen separately but is • HR diagram used to estimate the luminosity aligned in such a way that from Earth we periodically • distance away calculated from apparent brightness observe changes in brightness as each star successively passes in front of the other, that is, eclipses the other. (3) Cepheid variables: • these stars’ brightness vary over time 3 • the time period of the variation is related to their █ In a Hertzsprung-Russell diagram, luminosity is plotted luminosity against temperature. Explain why the diagram alone does •thus measurements of the time period of one star can be not enable the luminosity of a particular star to be used to calculate its luminosity determined from its temperature. • its distance away is calculated from maximum apparent brightness ► luminosity depends on the surface temperature and the surface area (L = A σT4 ); A particular luminosity can not be █ A sketch of a Hertzsprung-Russell diagram is shown determined from the graph without knowing the surface below. area, but the graph does not include area. (3) On the diagram, identify the (i) main sequence █ State two characteristics of a white dwarf that are (label this M), different to the characteristics of a main sequence star. (ii) red giant region ► has smaller radius; (label this R), ► more dense; (iii) white dwarf region ► higher surface temperature; (label this W). ► energy not produced by nuclear fusion (2) Stars may be described in terms of their spectral classes. ► (2) Class Colour Class Colour O bluish F yellowish white B blue white G yellowish A white K orange M red █ Describe the colour of a B star.