Questions to accompany “Putting Stars In Their Place” Astronomy Magazine, November 2000

1. Human lifetimes are measured in ______but most stars live for ______of years.

2. Astronomers in the 1800s could identify the chemical ______making up a star by looking at its spectra. (A star’s spectrum results from the light from the star being broken up into its component wavelengths with lines resulting indicating what the star is made of and not made of…)

3. The first recorded person to study stars was an Italian Astronomer… he wanted to know if the chemical compositions of stars were as varied as the number of stars out there… the answer was no since all stars could be classified into ____ separate classes ranging from bright, white stars with lots of ______to faint, reddish stars with lots of ______.

4. This Italian Astronomer decided that the differences in the chemical composition of stars were due to their ______.

5. Some astronomers suspected that the changes in stellar spectra could be not the result of temperature…. Instead they suspected that spectra could change because stars had different ______.

6. What was the major improvement to attempting to index stellar spectra that allowed the compilation of the Draper Catalog?

7. Stellar classification has the distinction of being the first major advance in astronomy in which ______played a leading role.

8. Pickering and Fleming classified stars ______A through Q based upon the strength of the hydrogen lines in the star’s spectrum.

9. What did Antonia Maury notice about the spectra of stars that led her to expand the classification system of Pickering and Fleming?

10. Cannon built upon the work of Pickering, Fleming and Maury by rearranging their stellar sequence listings by concentrating on things other than Hydrogen… she ordered them like this: ______

11. Cannon’s classes rank stars according to color with __ blue stars and __ red stars.

12. Cannon’s classes also correspond to different stellar temperatures… __ stars are the hottest while __ are the coolest (note- the article doesn’t specifically tell you what the coolest classification is but use your head!)

13. Hertzsprung analyzed stars within Cannon’s classes and discovered that they moved at ______and were at ______from the Earth.

14. If stars moved rapidly they were ( closer to / farther away from ) the Earth.

15. If stars moved slowly they were ( closer to / farther away from ) the Earth.

16. Whaddaya think will happen to the Earth in about 6 billion years when the sun swells up to become a red giant star?

17. Russell agreed with Hertzsprung and concluded that there were two kinds of stars, _____ and ______.

18. When looking at the Hertzsprung-Russell (HR) diagram, what is plotted along the x (horizontal) axis?

19. When looking at the Hertzsprung-Russell (HR) diagram, what is plotted along the y (vertical) axis?

20. Write a definition (not found in this article) of the word that you wrote as an answer to number 19.

21. Where can hot and bright stars be found on the HR diagram?

22. Where can cool and dim stars be found on the HR diagram?

23. Whaddaya call that wide strip of stars plotted from upper left to lower right that includes our own sun?

24. What are stars located in the upper right hand corner called?

25. What are stars located in the lower left hand corner called?

26. What makes a white dwarf star shine?

I THEIR PLACE

Tbecreation oJ tbe Hertzsprung-Russell diagram wasa landmarkadoance in ourunderstandlng of tbe stars,

by rlames Trefil

Imagine you are being given an important assignmentby a taskmasterwith a strange senseof humor. Your job is to study a forest, but you can only work accordingto a rather bizarre set of rules. For five minutes you can take all the picturesyou want. You can photographthousands of squaremiles of treesin any part of the spectrum.You can look north, east,south, and west.But you are not allowed to touch a tree or leaf, and after your five minutes are up' you can collectno further data. Even with theseconstraints, you could do a pretty good job of figurinq out how the forest worked or reconstructing the life cycle of a pine tree. You would seeseedlings popping up out of the forest floor, mature trees,and dead logs. By seeingtrees in all stagesof growth and death, you could ascer-. tain their life cycles.You would also realize that there are many different kinds of trees - oak, pine, and spruce - and understand how their life cyclesdiffered from one another. It wouldn't be easy.You might find yourself engagedin debateswith other scientistsabout whether oaks and pines were separatespecies or different stagesin the life of a singletree. But in the end, you probably would succeedin your task. While no one would ever try to study a forest this way, astronomers are forced to study starsunder a similar set of rules. The reasonis simple: human lifetimes are measuredin decades,science in centuries,but most starslive for

HenryNorris Russell and Ejnar Hertzsprung studied stellar spectra to developthe HR Diagram.

Photosol Russelland Hsdzsprung:Aslronomical Society ol ths Pacific/ Backgroundimage:The Observatory ot th€ LJniv€rsltyol lrichigan

| ""rRoNoMY | "" ASTRONOMY billions of years.Compared to the lifetimes of stars,the entire record of systematicobservations of the heavensis little more than the blink of an eye. Like the would-be forester,astronomers are armed only with snapshotsof a single moment in the life of the universe. But evenwith theserestrictions, astronomers have pieced togetherthe life cyclesof stars.One of the crucial stepson this road to understanding was the developmentof the Hertzsprung-Russelldiagram. FirstSteps Astronomers at the end of the 1800sknew that starshad a finite lifetime, but they had no idea where stars'energy came from. They could identify some of the chemical elementsin starsby looking at their spectra.They could even measurethe distancesto nearbystars. But they were confronted with a problem - there are many types of stars in the sky. Some stars are white-hot, othersburn a cool red. Some are much biggerthan the sun, othersare much smaller.Like all scientistsconfronting com- plex natural phenomena,their first job was to try to bring someorder to the part of the universethey wereobserving. When scientistsare confronted with a collection of objects,they try to find similarities that allow them to define groupsand differentiateone group from others.In the 1700s, Swedishbiologist Carolus Linnaeus noted that squirrels were more like rabbits than they were like snakes,and that snakes, rabbits,and squirrelswere more like one another than they were like palm'trees.This led him to our presentscheme for classiftingliving thingsbased on genusand species. Harvardastronomers employed the 8-inch Bache refractor to classify The first person to classifr starswas Italian astronomer stars and assemblethe Draper Catalogue.Harvard corese observarory and fesuit priest Angelo Secchi.In the 1860s,he set out "to seeif the compositionof the starsis asvaried asthe starsare until the center liquefied and could contract no more, at innumerable."Working with spectraof 4,000stars, he found which point the star would start to cool off and die. Each star that they could be categorizedinto four separateclasses. would passthrough a given temperature twice - once in the Theseranged from bright, white starswith lots of hydrogen warming phase,and againduring cooling.A given spectrum to faint, reddishstars with lots of carbon. would appear twice in a star's lifetime, corresponding to the Secchifelt (correctly,as it turned out) that thesediffer- two times it reacheda particular temperature. encesarose because his difFerentclasses of stars had different Determining distance,while possiblefor nearby stars,took temperatures.That isn't too surprising.A piece of metal a long time and hadn't been done for many. Similarly, taking placedin a flame first glows dull red, then becomeswhite-hot stellarspectra required that light from an individual star be asits temperaturerises. But astronomersknew the differences isolated,run through a prism,"and allor,Jedto strike a photo- in stellarspectra could also have arisenbecause of different graphic plate. A single exposurecould take anywherefrom chemicalcompositions. This quandary provided one of the minutes to hours, depending on the star's brightness.Again, main battlegroundsfor the remainderof the l9th century. the processwas too laborious to be usedin large-scalesurveys. On one sidestood astronomerswho believedthat all stars havemore or lessthe sameinitial chemicalcomposition, but WomenTake the Lead that their spectrachange as they go through their life cycles. But all this changedin 1886.Harvard College Observa- On the other side were those who argued that different tory, under the direction of Edward Pickering, initiated a groups of starswere made from different collectionsof survey of the spectra of bright stars in the Northern Hemi- chemicals.Astronomers had little understandingof how sphere.Called the Draper Catalogue(after Henry DrapeS a atoms emit light and how conditions in a star might affect pioneer of American astronomy), it was assembledusing a that emission,so resolving this issuewas not a simple matter. new technique.Instead ofisolating the light from eachofthe In the words of American astronomer Walter SydneyAdams, stars in a field to obtain one spectrum per photographic "We can hardly hope to understand the behavior of matter exposure,a prism dispersedthe total light from the fibld. in distant stars when the mechanism of light given out by a Consequently,photos contained multiple bands, with each candle flame is still quite unknown to us." band representingthe spectrumof one star.Spectra of many The debatewas hamperedby other kinds of ignoranceas starscould finally be taken simultaneously.For the first time, well. For example, astronomers who argued that differences stellar classifierscould work with a large database."The in starswere due to evolutionaryprocesses didn't know just Americans," retorted one English astronomer, "have always how that evolution might take place. One popular theory been appreciativeof massproduction techniques." held that starsderive energy from heat releasedas gases Stellar classification has the distinction of being the first contracted under the force of gravity. A star would contract major advancein astronomy in which women played a

| ""rRoNoMY | "" leading role. The first stellar classification work reminiscent of the modern classification schemewas devised by Picker- ing and his assistantWilliamina Fleming. They classified stars alphabeticallyA through Q based upon the strength of hydrogenlines in the spectrum. In 1888,Antonia Maury the nieceof Henry Draper and one of the few women in the United Statestrained in physicsand astronomy at the end of the 1800s,entered the picture. Also working at Harvard, she noticed subtle differencesin stellar spectra.For example,some starshad wide spectrallines, while others had thin lines. This led her to propose expanding the systeminto 22 classes,including subdivisionsa, b, and c. After Maury left Harvard due to differenceswith Picker- ing, Pickering turned to Annie |ump Cannon. Cannon analyzedthe spectraof 1,100individual stars.Rather than focus solelyon hydrogenlines, she rearrangedthe sequence to representa smoother progressionof spectralfeatures due to the presenceof highly ionized elementssuch as helium, nitrogen, and silicon.Like Maury's before,Cannon's classifi- cation systembegan with O, with the final sequencereading O, B, A, F, G, K, and M. In addition, Cannon's scheme resultedin the ordering of starsaccording to color, ranging from blue O starsto red M stars. Astronomy professorshave traditionally taught their studentsthe mnemonic "Oh be a fine girl, kiss me" to help them remember the spectralsequence OBAFGKM. They often substitute"guy" for "girl" in our age,when we pay attention to matters of gender. Today,we understandthat Cannon'sclasses correspond to differentstellar temperatures. The hotteststars, with i.-p.r- aturesaround 40,000kelvins (70,000'F), are called O stars. The sun, with a surfacetemperature of 5800kelvins (10,400" F), is a G star.While Maury's systemwas far too complex to be useful, her discoveryof important distinctions not captured by the original classificationsystem was a crucial step forward, becauseit inspired the work of Danish astronomerEjnar Hertzsprung. Giantsand Dwarfs Hertzsprungwas born into a prosperousfamily in Den- mark. He studied chemical engineering and came to astronomyby working on the chemistry of photographic film. Looking closelyat red starsusing Maury's classification scheme,he found that starsin Maury's subdivisionsmoved at different rates and were therefore at different distances. The fainter stars moved rapidly, which meant that they were closeto Earth.These stars appear faint becausethey are faint. Another group of red stars- Maury's c stars- barely moved at all, which meant that they had to be far away. Thesestars actually emit lots of light but appear faint becauseof their distance. Hertzsprung realizedthat if a star has a low surface temperature,as was true for red stars,each square foot of surfacearea emits a relatively small amount of light. The only way such a star can emit a lot of light is if it has a large Surface area.Today we call thesestars red giants and supergiantsand recognizethem as a late stagein the life of stars.When the sun becomesa red giant 6 billion yearsfrom now' its outer atmos- phere will extend outside the present orbit of Earth.

Topto bottom:Three Harvard astronomers, Williamina Fleming, Annie JumpCannon, and Antonia Maury, played pivotal roles in developingthe modern Stellar classification scheme, Harvardconese observarory

66 2ooo | ""tRoNoMy | ""vEMBER Hertzsprung's work establishedthat there were two parallel (itl seriesof stars- dwarfs and giants. K H-flellcl 116 Oll I lcll I Hertzsprung's discovery meant that determining a star's rtll 'lt "i' 'P temperature doesn't tell astronomerseverything they need to R'ild I il 'tlFl t r Orionis I know about it. A classificationscheme based on temperature Ron i l*o's*., lr { l9'lhuri alone will mix together very different - faint red objects T---- I Sirius stars and red giants, applesand oranges.When he plotted ^'*ll ns P frianguli stellarluminosities against Maury's spectralclasses, he found f,rif | * (ieminorrnr that the starsfell into two main groups. Most of the stars,the 12ilUl,,. ,,'Jl'.'., , . I 6 dwarfs, fell in a band he called the main sequence.The others ,.Iill ll iiil I'rot-von were the giants. -[;;Iil,il|4wrT.'T'l]T', , l,;"r,"rr" Hertzsprung published thesefindings in 1905and 1907in (hl (;lly lcl tr:l f the journal Zeitschriftfiir WissenschaftIichePhotographie (Jour- ,'*rulfliiili'irt,lf,ir,',' 'lu "? nal of Scientific Photograplry).This was, to put it mildly, an $r<;cnrinorum obscure publication not likely to be read by other *'tlllll*rlil lrll lr .*1, ,'i,iArctlrus astronomers.In later years,British astronomer Sir Arthur .'IIlllfflffil r! I ffi era"ro,"n Eddington wrote to Hertzsprung,"One of the sins of your -lf[tllllllillilffill!ilfrsf'''' $B",",s"u." youth was to publish important resultsin inaccessibleplaces." ", .9 M4 At the same time, American astronomer Henry Norris lS' I er'*'"i .9 Russellbegan looking at the same kinds of problems at M5 ffiff]w<:vg"i Princeton But Hertzsprung had M7 University. whereas been Illilll .a motivated primarily by trying to straighten out the classifica- !"- tion of stellar spectra, Russellwas trying to determine how stars evolve.He helped set up one of the first stellar distance Astronomersclassify stars according to thestrength of dark measurementprograms by using photographicplates instead spectrallines that indicatethe presence of variouselements. of visual observations.So when he tackled the problem of red stars,he had direct data on their distances. F In 1909,Russell came to the sameconplusion reached by But the main point was clear.When entered on the HR Hertzsprung:There were two kinds of stars,dwarfs and giants. diagram,virtually all starsfall into one of thesethree classes, Like Hertzsprung,he also noted that for most stars,the hotter and within the main sequence,the hotter starsare more the surface,the more energy the star poured into space.Rus- luminous than cooler ones. Hertzsprung and Russellinde- sell didn't announcehis resultsuntil fune l9l3 at a meeting in pendently discoveredthis fact about the main sequence- London, and again in Decemberat a meeting of the Astro- the former by plotting starsin large clusters,the latter from nomical and AstrophysicalSociety in Atlanta. his distancemeasurement data. BecauseRussell was the first to publish the diagram in Not only can every star be representedby a point on the something close to its modern form, and becausehe pre- HR diagram, but the life cycle of every star can be repre- sentedhis work in mainstreampublications and meetings,he sentedby a trajectory. For example,the sun begins on the receivedthe lionb share of credit when the usefulnessof the right as a cool, contracting cloud of interstellar gas.As it diagram became apparent to other astronomers.For years it warms up, it moves left toward the main sequence.Finally, was known as the "Russelldiagram." In 1933,Swedish when the nuclear fires ignite and the star begins to fuse astronomerBengt Strcimgrenresurrected Hertzsprung's old hydrogen,it sits on the main sequencq,staying more or less papersand argued for his having made the discoveryinde- in one placeuntil all the hydrogenin the core is consumed. pendentlyof Russell.It's beenthe HR diagramever since. The sun will spend about I I billion yearson the main sequence,of which 4.6 billion yearshave alreadypassed. Workhorse of Astronomy When the hydrogenin the core is exhausted,more complex All the complexity of the different kinds of stars is nuclear reactionsbegin. Thesereactions will causethe sun's contained in this diagram. A star'stemperature lies on the surfaceto cool and swellup, at which point the sun movesto horizontal axis and its luminosity lies on the vertical. The HR the red giant part of the diagram. Finally,when all nuclear diagram revealsthree important groupings of stars.The most reactionsstop, the sun becomesa hot cinder in spaceand striking is the broad band running from the upper left (hot moves down to the white dwarf region. and bright) to lower right (cool and dim). This band, which Thus, the HR diagram solvedthe problem of classifring includesthe sun, is Hertzsprung'smain sequenceand includes stars.It ordersthe starsby placingthem as specificpoints on stars that generatetheir energy by fusing hydrogen into the diagram, and it tells astronomers about stellar life cycles helium (see"How StarsShine," ]anuary 1998).The starsin the becauseeach star's history and future can be represente{by a upper right hand corner (cool and bright) are the red giants. single curve. No wonder that the HR diagram has become The stars in the lower left (hot and dim) are called white the workhorseof modern stellarastronomv! E dwarfs. These stars are no longer running nuclear reactions and shine only becausethey are cooling off. Think of them as stellar cinders. Although a few white dwarfs can be seen in GeorgeMason University physicist lames Trefil is a member of both Hertzsprung'sand Russell'soriginal plots, neither one Astronomyi editorial advisory board. He has authored or coauthored felt he had enough information to comment on this classof 27 books,the most recent of which is Other Worlds. The American stars, and it was a decade later before astronomers came to Institute of Physicsawarded Trefil the 2000 Andrew W Gemant recognizetheir true nature. Award for linking physicsto the arts and humanities.

NOVEMBER 2OOO 67 | ^"TRONOMY