Getting the Most from a CCD Spectrograph.
Contents
1. from 10 INCH 4 5 549 e Extremely portable e Tube 45 x 12 7 inches e Diffraction limited optics e Includes wide field eyepiece e Improved mount system 13 1 INCH f 4 5 899 e A BIG scope for less money e Easy 1 person setup e Tube 57 x 15 7 inches e Diffraction limited optics e Includes wide field eyepiece 8 INCH f 7 e Lunar planetary scope e Tube 56 x 10 5 inches e Diffraction limited optics e Includes high magnification eyepiece e Extra stable mount HOW TO ORDER Send no money A deposit is NEVER required to process your order Pay just before shipment by check money order Visa or MasterCard For orders or information Call 1 561 795 2201 Web www murni com Fax 1 561 795 9889 tno New ae Coulter Optical Div MURNAGHAN Instruments 1781 Primrose Ln West Palm Beach FL 33414 Bigger Scopes Better Optics Less Money July 2000 Sky amp Telescope The observatory s Meade 16 inch telescope was used for the spectra with this article be done with CCD spectrographs We also review the basic concepts of spectroscopy and the steps involved in obtaining and calibrating spectroscopic data Amateurs are already demonstrating a remarkable proficiency with CCD spec trographs One of the most impressive examples we have seen is Maurice Gav in s galaxy and quasar redshift measure ments S amp T June 1999 page 14 The box on page 127 illustrates an easier and equa2. sion lines in deep sky objects such as the Orion Nebula and the spiral galaxy M77 While enterprising amateurs can build their own spectrographs both Sivo Scien tific www sivo com and Santa Barbara Instrument Group www sbig com offer commercial units designed for astronomi cal work In this article we present some interesting spectroscopy projects that can All the spectra with this article were recorded at the rooftop observatory on Harvard University s Science Cen ter under some of North America s most light polluted skies Sheila Kannappan center built the observatory s spectrograph as a graduate student project with advice from coauthor Daniel Fabricant left and expert machin ing assistance from Charles Hughes right Sky amp Telescope July 2000 S amp T CRAIG MICHAEL UTTER 126 ODYSSEY TELESCOPES Seeing the rea Universe PRESS RELEASE MURNAGHAN Instruments Buys Coulter Optical Assets Production of Odyssey Telescopes will continue New product benefits from financial stability and management of MURNAGHAN Instru ments Corporation THE ODYSSEY ADVANTAGE e No assembly required e Improved Dob mount and focuser e Double strength Sonotube e Eyepiece included e Many options available e Lowest prices anywhere 8 INCH f 4 5 399 e Deep space scope e Tube 36 x 10 5 inches e Diffraction limited optics e Includes wide field eyepiece e Reengineered mount system from 399
3. Hot O and B stars glow brightest at blue wavelengths while our Sun which is cooler emits the most light at yellow and green wave lengths The coolest late type stars ap pear redder Many spectra show dark absorption lines and emission lines which tell of other physical processes The dark lines arise when molecules and atoms in a star s atmosphere absorb continuum light at specific wavelengths Late type stars show spectra rich in metal heavy ele ment absorption lines and sometimes broad bands of molecular absorption In hotter stars molecules are disrupted and metals are ionized so most of the ab sorption lines disappear at visible wave lengths On the other hand these early type stars show the hydrogen Balmer series of absorption lines prominently These hydrogen lines can also appear as emission features bright lines in the spectrum under different physical conditions For example radiation from hot O and B stars can ionize surround ing gas clouds and produce line emit ting H II regions astronomer s short hand for ionized hydrogen clouds such as the Orion Nebula Hydrogen emission lines along with a host of others from el ements such as oxygen nitrogen and sulfur are common in the spectra of gaseous nebulae Measuring Velocities When a light source is moving along our line of sight we can measure its velocity using the Doppler shift Just as the pitch of a train whistle
4. Nebula or the galaxy M77 by com bining a pair of 5 to 10 minute expo sures Since the characteristics of most amateur spectrographs will be roughly comparable to ours these exposure times will approximately scale by telescope aperture for point sources For example an 8 inch telescope has one quarter the light collecting area of our 16 inch so it would need a 40 second exposure for a 2nd magnitude star For extended sources the exposure time will also de pend on what fraction of the object is imaged onto the spectrograph s entrance aperture decreasing a telescope s focal length or increasing the entrance aper ture will decrease the exposure time Just as with conventional CCD imaging ex posures longer than about 10 minutes should be assembled from a series of shorter exposures to make it easy to re move things like cosmic ray artifacts from the final image Calibration Unlike a raw pictorial image a raw spec trograph image does not always resemble the final calibrated data that will be ex tracted from it Without calibration the raw frame yields a spectrum that is high ly distorted by the way the telescope and spectrograph transmit and detect differ ent wavelengths of light Fortunately most of this distortion can be removed Flux Calibrating CCD Spectra he table on page 131 lists some bright stars for which reference spectra sam pled every 1 6 nanometers 16 ang stroms between 330 and 755 nan
5. for IDL are available on the Web http idlastro gsfc nasa gov homepage html WaveMetrics IGOR www wavemetrics com is much less expensive than IDL and it offers extensive built in graphing and analysis features as well as program ming capabilities IGOR was not de signed specifically for astronomy but it is flexible and the user s manual and technical support are excellent Some other programs that may be use ful include Maxim DL www cyanogen com PDL the Perl Data Language available free from http www aao gov au local www kgb pdl and DS9 a quick look program we use to check ex posure levels of the data obtained at the telescope available free from http hea www harvard edu RD ds9 DS9 lets you Raw Data CCD counts Extracted Spectrum After Bias Subtraction 0 100 200 300 Pixel Number E 3 QO Wavelength D Calibrated 500 550 600 650 Wavelength nanometers E Pa Flux Calibrated Vv w 2 ic oD fas 500 Wavelength nanometers 550 600 650 run a mouse over a CCD image and in stantly obtain an intensity plot for the row of pixels under the cursor Exposure Times Spectroscopy in general and high reso lution spectroscopy in particular re quires longer exposure times than con ventional imaging This is because light from a star or other source is spread into a spectrum that covers many pixels on the CCD rather than being concentrated onto just a few as
6. rises and falls as the train approaches and then recedes light waves are blueshifted to higher frequen cies when they come from an approach ing source and are redshifted to lower frequencies for a receding source The formula for this shift is v c AA A where is the wavelength of the spectral feature for a source at rest AA is the change in the wavelength c is the speed of light and v is the velocity of the source relative to the observer The ex The Life of a Star Birth Star forming regions such as the Orion Nebula glow with the light of young hot stars as well as light from the ionized gas bubbles these stars carve out of the cold gas clouds around them As the bub bles expand their shock fronts can compress the rest of the cloud driv ing additional waves of star formation The strong lines in the Orion Nebula s spectrum include the hydrogen Balmer series of emission lines starting with hydrogen alpha 656 3 nanometers and hydrogen beta 486 1 as well as emission lines of ionized nitrogen N II at 654 8 and 658 4 and twice ionized oxygen O Ill at 495 9 and 500 7 Orion Nebula Hydrogen alpha Hydrogen beta Doubly ionized oxygen Relative intensity gt 500 550 600 650 Wavelength nanometers Midlife Most stars fall into the familiar OBAFGKM spectral classifica tion system The earliest types O and B represent the hottest brightest and most massive stars while the later types may be either d
7. 600 650 Wavelength nanometers Planets Planets shine by the reflected light of the Sun but their at mospheres may reprocess that light in interesting ways Above the spectra of Jupiter and Saturn reveal a broad methane absorption fea ture at about 620 nanometers superposed on the reflected G type solar spectrum Jupiter s atmosphere also creates an ammonia absorp tion feature from 640 to 650 nanometers Both spectra are somewhat redder than the Sun s because the planets atmospheres preferentially reflect red light Sky amp Telescope July 2000 127 128 Spectroscopy Project Ideas Project Required resolution Spectral range Magnitude range Notes nanometers nanometers Stellar spectral types 1 470 670 0 5 350 900 better Solar system objects 1 470 670 0 10 350 900 better Outer planets hard to track Variable stars 1 470 670 3 15 350 900 better Supernova identification 1 350 700 Limited only by 0 5 nanometer resolution your ambition better Smaller range okay Stellar radial velocities 0 3 470 670 0 5 Requires careful wavelength calibration Nebular emission lines ice 470 670 14 Lower resolution a help for faint targets Galaxy redshifts IE 650 700 14 Choose range based on for example redshift and desired lines Emission lines in active galactic 0 3 1 470 780 14 Choose subrange based on nuclei AGNs including quasars or any subrange redshift and desired lines pansion of the universe also produces a cosmic redshif
8. 87 00 01 50 03 01 7 5 1 B7 III IV E Ceti HR 718 022 28 10 08 27 6 4 3 B9 Ill T Orionis HR 1544 04 50 37 08 54 0 4 4 A1V n Hydrae HR 3454 08 43 14 03 23 9 4 3 B3 V O Crateris HR 4468 11736741 09 48 1 4 7 B9 5 V O Virginis HR 4963 13109 57 05 323 4 4 All 108 Virginis HR 5501 14 45 30 00 43 0 Sm B9 5 V 58 Aquili HR 7596 19 54 45 00 16 4 5 6 AO Ill g Aquarii HR 7950 20 47 41 09 29 8 3 8 A1V C Pegasi HR 8634 PEA 23 10 49 9 3 4 B8 V any artifacts identified in a search for small bright spots that appear in one image but not the others These can be removed when the spectra are added to gether The goal of wavelength calibration is to determine the wavelength at each pixel in the spectrum Fortunately a spectrograph that uses a diffraction grat ing to disperse light has to a first ap proximation a linear relationship be tween wavelength and pixel position Thus a few recognizable emission or ab sorption lines in the image will provide enough information to determine the entire wavelength scale The simplest so lution is to observe an astronomical source with strong and easily identifiable lines such as the Orion Nebula or Gam ma Cassiopeiae In this case you may wish to compensate for any Doppler shift in the reference spectra due to the ob jects radial velocity A more accurate wavelength calibration requires the spec trum of a calibration lamp The stability of this wavelength cal
9. Getting the Most from a CCD Spectrograph astro imaging BS Amateurs with backyard telescopes are taking their own spectra of stars nebulae and galaxies But there s more to it than just recording a spectrum s image By Sheila Kannappan and Daniel Fabricant URING THE LATTER HALF OF the 19th century amateur astronomers such as Henry Draper Lewis Rutherfurd and William and Margaret Huggins took the lead in developing new spectroscopic techniques for astronomy that helped es tablish the emerging field of astrophys ics Dissecting starlight by wavelength allowed astronomers to determine the physical nature of the Sun and stars re vealing their chemical compositions ra dial velocities and internal motions mi A century later high efficiency CCD spectrographs allow today s amateurs to obtain high quality spectra from their backyards using telescopes no larger than the ubiquitous 8 inch Schmidt Casse grain Even under light polluted skies such equipment reveals a fascinating va riety of features in the spectra of bright stars We recently built a fiber fed CCD spectrograph for use by students in Har vard University s astronomy courses At tached to a 16 inch telescope it captured the spectra accompanying this article from the roof of Harvard s Science Cen ter right in the heart of downtown Cam bridge Massachusetts We use it to observe bright stars and planets as well as emis
10. c tion consists of three essential parts background subtraction wavelength calibration and flux calibration Background subtraction removes un wanted signal in the spectra due to the CCD s bias and dark current as well as light pollution from the sky A simple way to accomplish this is to take an identical exposure of the blank sky next to the object and subtract it from the spectrum image Unfortunately this method wastes precious observing time so we usually use a different procedure Since bias subtraction is a necessary step for any further calibration we re move the bias and the dark current si multaneously by subtracting the average pixel value obtained from pixels on the CCD adjacent to those recording the spectrum While this method is not per fect using an average value does not take into account unusually hot or served spectrum The result will be a ratio of the star s official spectrum to the spectrum your equipment recorded at the various wavelengths It is now a simple matter to multiply all your spectra by this correction Doing this re moves all flux distortions introduced by your equipment If you don t care about the de tails of atmospheric absorption or about get ting absolute flux levels then this procedure is all you need In fact the spectra accompa nying this article were reduced by this method Furthermore you can reuse the same standard star data over and over if your setup d
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12. ger carry bag Comes in two configurations 12 volt 290 00 18 volt 390 00 18 volt version is for LX 200 owners One plug has a built in inverter for 18 volt output We accept all major credit cards KENDRICK ASTRO INSTRUMENTS 2920 Dundas St West Toronto Ont M6P 1Y8 Canada Tel 416 762 7946 Fax 416 762 2765 kendrick al com email kendrick kendrick ai com Sky amp Telescope July 2000 132 mounted spectrograph we can use a sin gle wavelength calibration for an entire night and indeed for weeks Flux calibration involves observing a standard star dividing its spectrum as recorded by the spectrograph by its offi cial spectrum as determined from a ref erence database and using the result to correct the flux levels of the rest of your data see the box on page 130 Because there are only a few bright standard stars there will not always be one conveniently placed for calibration As an alternative if you don t need an ac curate flux calibration but just want to remove gross instrumental distortions you may prefer to record the spectrum of a bright star with the same spectral type and luminosity class as one of the stan dard stars and then compare the spectra Stellar classification information is avail able on the Web at http simbad harvard edu This strategy has the advantage of giving you a wider choice of calibration stars at the expense of introducing small errors due to diffe
13. i bration throughout the night depends on whether the spectrograph optics flex as the telescope moves or the tempera ture changes With our fiber fed bench ZIP OUT ROOF a EASY SETUP t DURABLE PORTABLE RAINFLY ROOMY ME TENT l Ohi SHE a ER D tad 4 ZIP OUT SLOTS LIGHTPROOF FAST SETUP PORTABLE DURABLE ROOMY Optional rain fly 95 00 m IRS T Es ULTIMATE Il 6160 00 JESERVING CHA ULTIMATE 125 00 Ultimate Il has a folding accessory tray for eyepieces filters charts etc Made from Baltic Birch Plywood Seat is adjustable from 9 to32 Buy Direct or from your favourite dealer 30 00 C5 ETX 125 30 00 AP 105mm 30 00 AP 130mm 30 00 9 25 SCT LASER COL LIN Collimators for Schmidt Cassegraines and Newtonians 1 25 100 00 AP 155mm 35 00 7 Mak 8 SCT SCT NGF s 175 00 Face of collimator angled at 45 for easy reading Lj E m pr i a Fi A a PHA i FIL i We are the exclusive North American distributors for Coronado Instruments H Alpha filters and telescopes Dealer inquiries welcome ASP 60 2625 00 ASP 60 BF30 3108 00 DEW REMOVER SYSTEM DEWCAPS Flexible Felt lined Durable Save up to 85 vs Brand Name 12V 17 AMP HOUR Comes with float charger carry bag built inj J fused cigarette lighter power socket 125 00 33 AMP HOUR BATTERY 4 fused power plugs LCD for state of charge 1 amp float char
14. in direct imaging The more widely the light is spread out into a spectrum the more sharply we can re solve closely spaced spectral features But this higher resolution comes at the ex pense of diluting the light and thus re quires longer exposures Many other fac tors also affect exposure time telescope aperture spectrograph efficien cy noise characteristics of the CCD cam era and sky brightness to name a few Rather than modeling all these factors we have found the easiest way to make exposure calculations is by scaling from other spectroscopic observations of simi lar objects Once you have obtained suc Sky amp Telescope July 2000 129 130 cessful spectra for a few objects simply multiply those exposures by a factor of 2 5 for each magnitude fainter that you wish to observe This technique works only for stars since typically just a small fraction of the light from extended ob jects passes through a spectrograph slit On the other hand an emission nebula is sometimes much easier to observe with a spectrograph than its surface brightness would imply since almost all of its light will be concentrated in a few bright emis sion lines rather than spread across a con tinuum With our 16 inch telescope and the spectrograph operating at 0 6 nanometer resolution we obtain the spectrum of a 2nd magnitude star with a 10 second ex posure Switching to 1 6 nanometer res olution we can get a spectrum of the Ring
15. lly entertaining project observing the life cycle of stars We obtained most of the spectra shown with exposures of 30 seconds or less The table on page 128 summarizes the technical requirements for several spec troscopy projects suited to amateur equipment All are within the capabilities of an 8 inch telescope and a high quality spectrograph Typical projects fall into two main categories measuring spectral properties to determine chemical and physical processes in celestial objects and measuring the objects radial ve locities speeds along the observer s line of sight The following brief review of the basic concepts involved can be sup plemented with material in books such as James Kaler s Stars and Their Spectra W H Freeman 1997 and Lawrence Aller s Atoms Stars and Nebulae Cam bridge University Press 1991 Measuring Spectral Properties Generally speaking a spectrum consists of a continuum light spread over a broad continuous range of wavelengths as well S amp T CRAIG MICHAEL UTTER as some narrow spectral features or lines superposed on it The continuum alone often reveals something about the conditions under which the light was produced In the case of normal stars it represents light emit ted by the star s photosphere visible sur face This light is primarily thermal blackbody radiation and thus the wave length of its peak intensity indicates the photosphere s temperature
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17. nts made of exotic matter Massive stars may die gloriously in supernova explosions while less massive stars such as the Sun will form planetary nebulae Both processes enrich the interstel lar gas and provide raw materials for future generations of stars In our Harvard program we haven t yet captured the spectrum of a supernova but we have recorded the spectra of planetary nebulae Equally spectacular are the spectra of aging stars such as the well known variables Mira Omicron Ceti and Gamma Cassiopeiae Mira is surrounded by a patchy dusty envelope and its spec trum has a bizarrely saw toothed structure caused by titanium oxide absorption features Low mass stars such as the Sun are likely to go Mira Relative intensity gt 500 550 600 650 Wavelength nanometers through a similar stage late Gamma in their lives before finally Cassiopeiae ejecting their envelopes and Hydrogen i Iph settling down as white Apoa dwarfs Gamma Cassiopeiae is orbited by just such a rem nant companion either a white dwarf or a neutron star 500 550 600 650 The companion a bright X ray source sucks matter from Gamma Cassiopeiae and this gas produces Balmer series emission lines superposed on the bright star s B type stellar spectrum Relative intensity gt Wavelength nanometers Ammonia Jupiter absorption band 2 Planetary Spectra e9 lt Saturn i Methane g absorption band 500 550
18. oesn t change We use one calibration for many nights Algol calibrated uncalibrated 600 Wavelength nanometers July 2000 Sky amp Telescope 550 600 Wavelength nanometers cold pixels it is adequate for bright star work where there is a lot of signal relative to the level of the bias and dark current For fainter targets it is important to subtract the sky background as well as the pixel by pixel bias and dark current because an errant pixel or emission from skyglow could easily be mistaken for a real feature in a weak spectrum With a slit type spectrograph the sky is present across the whole slit so its level can be estimated from pixels adjacent to the tar get s spectrum Our spectrograph uses optical fibers to transfer light from the telescope to the spectrograph where the fibers are arranged in a line to act like an entrance slit With this system a fiber at the telescope s focal plane can be posi tioned on blank sky to record the back ground while a matching fiber is posi tioned on the target To remove the sky background we wavelength calibrate the sky s spectrum as described below and subtract it from the target s wavelength calibrated spectrum With very long exposures cosmic ray artifacts become a problem In this case multiple exposures should be taken and Bright Standard Stars Star Designation R A 2000 0 Dec Magnitude Spectral type 29 Piscium HR 90
19. ometers are available on the Web http adc gsfc nasa gov adc cgi cat pl catalogs 2 2179 The fluxes in the reference spectra are given in monochromatic magnitudes which may be converted to absolute flux at any given wavelength using the equation ase OA oe where F is given in photons per square cen timeter per second per nanometer is the wavelength in nanometers and mag is the monochromatic magnitude Saturn Relative intensity gt 550 calibrated uncalibrated For example the reference star HR 8634 has a monochromatic magnitude of 3 4 at a wavelength of 555 6 nanometers Evaluating the above equation gives an absolute flux of about 430 photons per square centimeter per second per nanometer at this wavelength Because a CCD s pixel value is directly pro portional to the number of photons striking it as long as the pixel value remains well below its saturation level you can perform a first order flux calibration by dividing your observed spectrum of a standard star into the star s reference spectrum obtained from the Web site Before dividing you may need to manipulate the reference spectrum to match the range and resolution of the ob Methane absorption band Relative intensity gt with a basic calibration procedure that corrects for everything except atmos pheric absorption We applied this sort of minimal processing to calibrate the spectra with this article The data redu
20. rences in spectral shape and radial velocity A remaining concern is atmospheric absorption which takes its heaviest toll at the blue end of the spectrum The basic flux calibration described above partially corrects for this effect However a more thorough calibration is needed to refine the correction Absorption by the Earth s atmosphere depends on how high the object is in the sky At low altitudes the light path through the atmosphere is long and results in greater absorption To model this effect requires observing stan dard stars at different altitudes Alterna tively you can skip the modeling if you observe a standard star very close to the object of interest immediately before or after recording the object s spectrum so that the light path through the atmos phere is similar for both targets This method is preferable if there are thin clouds present Have Fun This article gives an overview of the tools and techniques you need to collect work with and understand spectra Amateur spectroscopy is entering a renaissance and the fun is just beginning D SHEILA KANNAPPAN and DANIEL FABRICANT are astronomers at the Harvard Smithsonian Center for Astrophysics with special interests in the evolution of galaxies and the large scale structure of the universe July 2000 Sky amp Telescope G a 1 000 NGT 6 telescope LIGHT _ WEIGHT 1 1 4 anodized with stainless steel beari
21. reserva day Scientific Expeditions Inc 227 West Miami Ave Suite 3 Venice FL 34285 Fax 941 485 0647 E mail scienex aol com July 2000 Sky amp Telescope il solar eclipse of the new millenni June 21 2001 in exotic MOUS For more information call toll free 800 344 6867 int your Web browser to http www skypuh jenx atrica2001 html __ lll Calibrating a Spectrum a EE E EE L E E Ree eo O ae a CCD counts Mercury Neon Calibration Lamp Wavelength nanometers 500 550 600 650 100 200 Pixel Number 300 Transforming images from a CCD spectrograph into useful spectra requires a series of cali bration steps The authors fiber fed spectrograph produces a discrete spectrum for each of its six fibers At top is the featureless looking spectrum image for the star Algol the light of which fell on a single fiber Plotting the brightness of rows of pixels spanning this spec trum produces the raw data graph Another spectrum image of light from a mercury neon calibration lamp illuminating all six fibers and its corresponding plot determines the wavelength recorded by each pixel in the row and is used to wavelength calibrate the as tronomical spectrum This spectrum however is still highly distorted because of instru mental effects in particular the varying spectral sensitivity of the CCD detector Only when flux calibrated see page 130 does
22. t defined as z AA A Cos mic redshifts are often converted into units of velocity according to v cz for small values of z In addition to displacing the wave lengths of spectral lines Doppler shifts can also broaden the lines if material is moving at a range of different velocities Some of the broadest lines result from material swirling around and falling into black holes at the centers of galaxies In such cases a velocity spread of about 1 000 kilometers per second is often observed The ability to measure line broadening is directly limited by the wavelength res olution of a spectrograph which can be converted into velocity units using the Doppler shift formula given above where Ad is the wavelength resolution A is the central wavelength being observed and the velocity resolution v is the result of the calculation However one can often measure overall redshifts and blueshifts with a precision much better than the resolution limit a ca Sky amp Telescope and xpeditions will be there invite you to join us We ll bus with the Moon s shadow id where weather prospects ent and totality lasts nearly 4 We ll also go on safari in South est game reserves visit majestic ils and view the spectacular sky with the Milky Way high lead and the Magellanic jds near the meridian ur 11 day expeditions degin June 15th Prices fart at 5 325 per erson based on louble occupancy pace is limited your
23. the plot accurately reveal Algol s spectral features Hardware and Software In addition to a spectrograph and tele scope a handy piece of auxiliary equip ment is a calibration lamp An ideal lamp has many narrow emission lines throughout the wavelength range of in terest These lines are useful for focusing the spectrograph and determining wave lengths within a spectrum With our stu dent spectrograph we use a mercury neon lamp sold by Oriel Instruments 150 Long Beach Blvd Stratford CT 06615 www oriel com Spectral analysis software is not as ma ture as the image processing programs widely available to today s amateur as tronomers Nevertheless you will need analysis software to perform essential calibration of your CCD spectra without which the spectra will appear distorted and difficult to interpret Commercial units such as the SBIG spectrograph usu ally come with basic software but you may need to supplement it Professional astronomers often proc ess their spectral data using IRAF the free image processing and data analysis software available from the National Optical Astronomy Observatory http iraf noao edu iraf web Unfortunately IRAF has a steep learning curve and runs only on Unix or Linux operating systems Another powerful analysis pro gram is Research Systems s IDL www rsinc com Although quite expensive it is user friendly and will run on any computer platform Free astronomy tools
24. warfs on the main sequence or giants that have left it To the eye early type stars like Vega appear pale bluish while late types like Betelgeuse appear reddish An intermediate star such as the Sun a G dwarf appears pale yellow The reason is clear from the shapes of the continuum spectra below the hotter the star the more energy it emits at shorter bluer wavelengths following the law of blackbody radiation Stellar absorption lines also vary by star type The Balmer series of lines including hydrogen alpha and beta are strongest for A stars while later type stars are rich in metal lines such as iron 527 0 nano meters the sodium D doublet about 589 5 the magnesium triplet about 517 5 and the calcium iron blend about 649 5 Although such metals are also present in early type stars at the hotter temperatures they are more highly ionized so the neutral metal lines disappear from the spectrum In the very latest type stars molecular gas forms in the star s cool outer atmosphere and causes broad absorption bands such Stellar Types Hydrogen alpha gt 2 Z B Cassiopeiae g VU Z o Aquarii Sodi odium cee T Magnesium ieee p Calcium ron KO URRA a Cassiopeiae K5 Aldebaran 500 550 600 650 Wavelength nanometers as the titanium oxide bands at about 590 and 625 nanometers and other wavelengths Old Age and Death As stars grow old they bloat oscillate and ex plode leaving tiny remna
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