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ertainly never hearda shot fired in anger。）in 1919； now aged thirty； he moved to california and took up a position at the mountwilson observatory near los angeles。 swiftly； and more than a little unexpectedly； hebecame the most outstanding astronomer of the twentieth century。
it is worth pausing for a moment to consider just how little was known of the cosmos at thistime。 astronomers today believe there are perhaps 140 billion galaxies in the visible universe。
that鈥檚 a huge number； much bigger than merely saying it would lead you to suppose。 ifgalaxies were frozen peas； it would be enough to fill a large auditorium鈥攖he old bostongarden； say； or the royal albert hall。 （an astrophysicist named bruce gregory has actuallyputed this。） in 1919； when hubble first put his head to the eyepiece； the number of thesegalaxies that were known to us was exactly one： the milky way。 everything else was thoughtto be either part of the milky way itself or one of many distant； peripheral puffs of gas。
hubble quickly demonstrated how wrong that belief was。
over the next decade； hubble tackled two of the most fundamental questions of theuniverse： how old is it； and how big？ to answer both it is necessary to know two things鈥攈owfar away certain galaxies are and how fast they are flying away from us （what is known astheir recessional velocity）。 the red shift gives the speed at which galaxies are retiring； butdoesn鈥檛 tell us how far away they are to begin with。 for that you need what are known as鈥渟tandard candles鈥濃�攕tars whose brightness can be reliably calculated and used asbenchmarks to measure the brightness （and hence relative distance） of other stars。
hubble鈥檚 luck was to e along soon after an ingenious woman named henrietta swanleavitt had figured out a way to do so。 leavitt worked at the harvard college observatory asa puter； as they were known。 puters spent their lives studying photographic plates ofstars and making putations鈥攈ence the name。 it was little more than drudgery by another name； but it was as close as women could get to real astronomy at harvard鈥攐r indeed prettymuch anywhere鈥攊n those days。 the system； however unfair； did have certain unexpectedbenefits： it meant that half the finest minds available were directed to work that wouldotherwise have attracted little reflective attention； and it ensured that women ended up with anappreciation of the fine structure of the cosmos that often eluded their male counterparts。
one harvard puter； annie jump cannon； used her repetitive acquaintance with thestars to devise a system of stellar classifications so practical that it is still in use today。
leavitt鈥檚 contribution was even more profound。 she noticed that a type of star known as acepheid variable （after the constellation cepheus； where it first was identified） pulsated witha regular rhythm鈥攁 kind of stellar heartbeat。 cepheids are quite rare； but at least one of themis well known to most of us。 polaris； the pole star； is a cepheid。
we now know that cepheids throb as they do because they are elderly stars that havemoved past their 鈥渕ain sequence phase；鈥潯n the parlance of astronomers； and bee redgiants。 the chemistry of red giants is a little weighty for our purposes here （it requires anappreciation for the properties of singly ionized helium atoms； among quite a lot else）； but putsimply it means that they burn their remaining fuel in a way that produces a very rhythmic；very reliable brightening and dimming。 leavitt鈥檚 genius was to realize that by paring therelative magnitudes of cepheids at different points in the sky you could work out where theywere in relation to each other。 they could be used as 鈥渟tandard candles鈥濃�攁 term she coinedand still in universal use。 the method provided only relative distances； not absolute distances；but even so it was the first time that anyone had e up with a usable way to measure thelarge…scale universe。
（just to put these insights into perspective； it is perhaps worth noting that at the time leavittand cannon were inferring fundamental properties of the cosmos from dim smudges onphotographic plates； the harvard astronomer william h。 pickering； who could of course peerinto a first…class telescope as often as he wanted； was developing his seminal theory that darkpatches on the moon were caused by swarms of seasonally migrating insects。）bining leavitt鈥檚 cosmic yardstick with vesto slipher鈥檚 handy red shifts； edwin hubblenow began to measure selected points in space with a fresh eye。 in 1923 he showed that a puffof distant gossamer in the andromeda constellation known as m31 wasn鈥檛 a gas cloud at allbut a blaze of stars； a galaxy in its own right； a hundred thousand light…years across and atleast nine hundred thousand light…years away。 the universe was vaster鈥攙astly vaster鈥攖hananyone had ever supposed。 in 1924 he produced a landmark paper； 鈥渃epheids in spiralnebulae鈥潯。╪ebulae；from the latin for 鈥渃louds；鈥潯as his word for galaxies）； showing that theuniverse consisted not just of the milky way but of lots of independent galaxies鈥斺�渋slanduniverses鈥濃�攎any of them bigger than the milky way and much more distant。
this finding alone would have ensured hubble鈥檚 reputation； but he now turned to thequestion of working out just how much vaster the universe was； and made an even morestriking discovery。 hubble began to measure the spectra of distant galaxies鈥攖he business thatslipher had begun in arizona。 using mount wilson鈥檚 new hundred…inch hooker telescopeand some clever inferences； he worked out that all the galaxies in the sky （except for our ownlocal cluster） are moving away from us。 moreover； their speed and distance were neatlyproportional： the further away the galaxy； the faster it was moving。
this was truly startling。 the universe was expanding； swiftly and evenly in all directions。 itdidn鈥檛 take a huge amount of imagination to read backwards from this and realize that it must therefore have started from some central point。 far from being the stable； fixed； eternal voidthat everyone had always assumed； this was a universe that had a beginning。 it mighttherefore also have an end。
the wonder； as stephen hawking has noted； is that no one had hit on the idea of theexpanding universe before。 a static universe； as should have been obvious to newton andevery thinking astronomer since； would collapse in upon itself。 there was also the problemthat if stars had been burning indefinitely in a static universe they鈥檇 have made the wholeintolerably hot鈥攃ertainly much too hot for the likes of us。 an expanding universe resolvedmuch of this at a stroke。
hubble was a much better observer than a thinker and didn鈥檛 immediately appreciate thefull implications of what he had found。 partly this was because he was woefully ignorant ofeinstein鈥檚 general theory of relativity。 this was quite remarkable because； for one thing；einstein and his theory were world famous by now。 moreover； in 1929 albert michelson鈥攏ow in his twilight years but still one of the world鈥檚 most alert and esteemed scientists鈥攁ccepted a position at mount wilson to measure the velocity of light with his trustyinterferometer； and must surely have at least mentioned to him the applicability of einstein鈥檚theory to his own findings。
at all events； hubble failed to make theoretical hay when the chance was there。 instead； itwas left to a belgian priest…scholar （with a ph。d。 from mit） named georges lema？tre tobring together the two strands in his own 鈥渇ireworks theory；鈥潯hich suggested that theuniverse began as a geometrical point； a 鈥減rimeval atom；鈥潯hich burst into glory and hadbeen moving apart ever since。 it was an idea that very neatly anticipated the modernconception of the big bang but was so far ahead of its time that lema？tre seldom gets morethan the sentence or two that we have given him here。 the world would need additionaldecades； and the inadvertent discovery of cosmic background radiation by penzias and wilsonat their hissing antenna in new jersey； before the big bang would begin to move frominteresting idea to established theory。
neither hubble nor einstein would be much of a part of that big story。 though no onewould have guessed it at