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c Swinburne University of Technology, 2010 29 OF 1 AGE P from their : Carl Hergenrother, 1997 Credit HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning Colours and Spectral Types:spectra Learning about c Swinburne University of Technology, 2010 29 OF 2 AGE P of spectra shape; the spectral line shift; line broadening; and spectral line strength. HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning – – – – what spectra can tell us about the properties of stars by studying: the three types of specra - continuous, absorption and emission; and • • In this Activity we will learn about the enormous amount of information contained in the Summary stars. In particular, we will discuss: c Swinburne University of Technology, 2010 29 OF 3 AGE P of a , provide additional against ) of the . spectral lines flux (or intensity stellar spectrum spectra HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning we can plot the information about the properties of the star. Studying stellar spectracomposition can and tell motion us ofthe three about stars. different type the of But spectra. , first, let’s learn about to obtain the Stellar When we break starlight up into its component We can glean athe lot overall of shape of information its about spectrum,the including the the spectrum star colour by at reaches which studying narrowness or its broadness of maximum the spectrum. height Further,of the the position and spectrum, the whetherand it overall the is relative shifted toward strength the of red or blue, c Swinburne University of Technology, 2010 29 OF 4 AGE P emitted by a black body. : astronomers electromagnetic radiation HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning 1. This is the overall hill-shaped spectrum of There are three types of spectra of interest to Three types of spectra c Swinburne University of Technology, 2010 29 OF 5 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning 2. Absorption spectrum The absorption spectrum is abecause continuous something spectrum, absorbed but them with between the the flux source of and certain Earth. reduced c Swinburne University of Technology, 2010 29 OF 6 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning 3. An emission spectrum looks veryspecific different: wavelengths. Rather than a continuousYou spectrum, will we see see the emission spectrum at shown below in red. Why ? c Swinburne University of Technology, 2010 29 OF 7 AGE P comprising the gas. HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning An emission spectrum isabsorbing made it. by an object such asIt a can cloud only of emit those gasthe same emitting wavelengths radiation that rather it than can absorb, and those wavelengths will depend on Emission spectra explained c Swinburne University of Technology, 2010 29 OF 8 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning The type of spectrum youarranged. see will depend on how the source, the cloud of gas and how you are all Emission versus absorption spectra c Swinburne University of Technology, 2010 29 OF 9 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning In the real world, we always see a mixture of all three! 3. Emission spectrum 2. Absorption spectrum Here are simplified versions of the three spectra1. as Continuous they spectrum might be observed on photographic film. The three spectra c Swinburne University of Technology, 2010 29 OF 10 AGE P methods) and we have the spectrum of the parallax to the star (by distance HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning star, we can detremine the motion of theHowever, we star, need the to size make of two the important star, and assumptions. the its temperature. Stellar spectra can beexample, extremely if powerful, we know providing the a great deal of information about stars. For Two important assumptions c Swinburne University of Technology, 2010 29 OF 11 AGE P and nearby stars. , it is assumed that a star behaves like click here. , radiation Magnitudes and Colours of Stars blackbody radiation Blackbody and emits a spectrum that is well described by various equations and laws of thermal 1 HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning blackbody To learn more about 1 As we saw in the Activity a This assumption is made sobe that compared the quantitatively, overall and shapes is of based spectra on and what the we position know of about their the maxima can . Assumption 1: c Swinburne University of Technology, 2010 29 OF 12 AGE P (H), some composition: Mostly initial (He), and traces of other elements (mostly light elements). HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning As far as we can tell, this is true; stars seem to be made up of much the same kind of stuff. It is assumed that almost all stars have the same Assumption 2: Same composition c Swinburne University of Technology, 2010 29 OF 13 AGE P absorption spectrum. However continuous - and the results is an spectral lines absorb particular wavelengths as the radiation passes through spectra. emission photosphere HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning If the photospheres of stars have prettyin much the the absorption same in spectra composition, for then all these stars. linesThe should same occur should apply to it. As a resultspectrum. there are gaps in the spectrum - Why assume the stars have the same initialBecause composition? radiation leaving the inner regions of a star forms a different particles in the c Swinburne University of Technology, 2010 29 OF 14 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning These two assumptions are made sostrength that of the absorption overall or shape emission of lines spectrum, within andIn the the position, spectra reality, width there of and different are stars differences canabsorption both be and in compared. emission the lines. overall continuous spectrumIt and is in these the differences fine that help detail astronomers of to the find out more about stars. c Swinburne University of Technology, 2010 29 OF 15 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning We will start offspectrum. with the differences that you can see in the overall shape and position of the Let’s have a look at somecan of be the deduced. differences between the spectra of stars, and see what information Comparing stellar spectra c Swinburne University of Technology, 2010 29 OF 16 AGE P of the peak with respect to position . steepness HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning One of the most common differences between spectra is the 1. ”Themoved...” whole spectrum has stretched, and the peak has wavelength, combined with an overall change in c Swinburne University of Technology, 2010 29 OF 17 AGE P varies area star will have a hotter end of the spectrum. blue closer to the HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning A cooler star will have a lower, flatter(Strictly peak speaking, closer every to blackbody the spectrum red has end the of same the functional spectrum. form, but the total with size and temperature andincreasing temperature.) the peak obey’s Wien’s law, peaking at a shorter wavelengths with According to the ruleshigher, sharper governing peak blackbody radiation, the spectrum of a c Swinburne University of Technology, 2010 29 OF 18 AGE P area under the graph multiplied by the radiated is proportional to the . HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning total For the diagram below, we can clearly see that a hot star must radiate more energy than a cool star. of the light The c Swinburne University of Technology, 2010 29 OF 19 AGE P is moved towards the red end or the blue end. HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning This effect is due to stellarblue end, motions and - those stars moving moving away towards will us have will their have spectrum their shifted spectrum towards shifted the to red the end. Sometimes the spectrum of aentire star spectrum is like that of our Sun (or another known star)... except that the 2. ”The entire spectrum is shifted” There isn’t much change in “steepness” or intensity, just in position. c Swinburne University of Technology, 2010 29 OF 20 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning If the spectrum (emission or absorption) wereobject compared which to is a not known, calibrated moving spectrum relativeshifted for to towards a the Earth, similar blue then end the of same the features spectrum. would be there but would all be Blue shift c Swinburne University of Technology, 2010 29 OF 21 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning Similarly, if an object emits lightfeatures as of it the moves spectrum rapidly are away shifted from towards us, the then red its end light of is the “stretched” spectrum. and all Red shift c Swinburne University of Technology, 2010 29 OF 22 AGE P , that speed or other object is zooming towards the Earth at any significant galaxy HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning Exactly the same sort of equationEarth. is In used that to case work the out light the will speed be of red an shifted. object if it is racing away from speed can easily be calculated by checking theThe spectrum entire of spectrum the will star. beunshifted shifted wavelength towards are the related by: blue end. The shift in wavelength and the “original” So if a star or Red/blue shift c Swinburne University of Technology, 2010 29 OF 23 AGE P ! That is, you can study the individual lines astronomy HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning within a spectrum and gain information from them. We will now look at some of the variations that are found. You can do more thanblue examine shift. the peak position and heightIt of is a also spectrum, possible to and read check ”between it the lines” for in red or Reading between the lines c Swinburne University of Technology, 2010 29 OF 24 AGE P the red end and the both HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning Sometimes the lines within a spectrum are spread out, moved towards 3. ”Why aren’t my lines sharp?” blue end of the spectrum at the same time. What does this mean? c Swinburne University of Technology, 2010 29 OF 25 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning Gas particles, when in an interstellarthey cloud emit or light. in the photosphere of aThe star, hotter are the moving gas, at the randomus more as blue- likely or it red-shifted. is that some fast-moving particles will emit light which reaches c Swinburne University of Technology, 2010 29 OF 26 AGE P ”. broadening HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning So a spectral line that wouldis be in expected to fact be spread very acrossThis thin neighbouring is (confined called to wavelengths “ just as one well particular due wavelength) to the random motion in a hot gas. c Swinburne University of Technology, 2010 29 OF 27 AGE P HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning What if the strength of the lines differs between the spectrum of a star and that of the Sun? 4. ”Why are the lines different strengths?” c Swinburne University of Technology, 2010 29 OF 28 AGE P Balmer of the star and relative elemental abundances can also affect the chemical composition . HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning line strengths. The more Carbon presentwill at possess. the surface of the star, the moreIn strength the its carbon next lines Activity, we will study the most important of these processes: in particular, the series This is yet another indicationwhich of allow temperature. light If to be temperature emitted in also a vary:Different gas line some varies, strengths processes the can get sorts also stronger be ofstars. and due processes others to The get changes weaker. in the relative chemical abundance between two c Swinburne University of Technology, 2010 29 OF 29 AGE P the entire spectrum is shifted . of absorption or emission lines can also indicate the temperature spectral type rapidly towards us (or away from us), then line strength will have a low, flat spectrum peaked towards the red end; moving will have a high spectrum peaked towards the blue end; HET603-M05A01: Coloursabout stars and from their Spectral spectra Types: Learning cool stars if a star is that very hot stars will have broadened spectralthat lines; the and relative of the star. hot stars towards the blue (or the red) end of the spectrum; • • • • • In the next Activity,classifed we according will to their learn more about line strength, the , and how stars are In this Activity, weemission. have We learnt also learnt about how three a different star’s properties types can of affect its spectra spectrum - and in continous, particular absorption that and Summary