Astronomy – number of stars

Once again, Daf HaYomi is nearing the  astronomical sugyos of Maseches Rosh  Hashana – a good place to investigate the  wonders of the Cosmos.


In sefer Bereishis, Hashem promises  Avraham that his children will be as  numerous as the stars and the sand on the  seashores; the Gemora (Berachos 32b) cites  Hashem telling Klal Yisroel, “My daughter, I  created 12 constellations in the sky, and for  each constellation I created 30 rihaton, and  for each rihaton I created 30 karton, and  for every karton I created 30 ligyon, and  for every ligyon I created 30 gistara, and  for every gistara I suspended 365,000 times  10,000 stars corresponding to the days of the  solar year.”
That comes up to about a million, million,  million stars.

In old times when an eagle-eyed person  in the deepest canyon could detect perhaps  5,000 stars on the blackest night, people may  have wondered about these above statements.  Where were all the stars?      The Rambam (Yesodei HaTorah 3:8) hints  at an answer by talking about “all the visible  stars,” intimating that other stars exist but  cannot be seen. Though he seems to refer  to stars, they are invisible because of their  smallness and not because of their distance  from earth. (See Mefaresh.)      The Gemora’s massive numbers of stars  remained invisible until recently. Although  man has thrust out the frontiers of space for  millenia, marveling at each new revelation of  its immensity, the most momentous discovery  was only 82 years ago. This was when man’s  vision of the universe took a quantum leap in  one night.

“The history of astronomy is a history of  receding horizons.” (Edwin Hubble)  The story began with men standing outside  for thousands of dark nights, patiently  measuring, observing and calculating. Four  centuries before the common era, Aristotle  figured out that the world is spherical because  of the circular shadow it casts on the moon  during eclipses. Later in about 3511/250 BCE,  Libyan astronomer Eratosthenes calculated  the world’s circumference by measuring the  sun’s angle simultaneously in two distant  locations. His figure was only 2% off the  mark.

The Greek savant Aristarchus correctly  calculated the moon’s approximate size and  distance from earth through logic and math,  but his attempt to measure the sun was far off  the mark – he thought it was only 20 times  larger and further off than the moon. Far more  accurate, is the approximation cited by the  Rambam (ibid), that the sun’s diameter is 170  times larger than earth’s.

The Greeks did even worse with the stars  – Ptolemy’s calculation that the stars are only  about 6 million miles away was 100 million  times off the mark.

The first scientist to gain a true perspective  of the cosmos was Rav Levi ben Gershom  (4044/1288-4104/1344). Also known as the  Ralbag, he was not only a profound Tanach  commentator and philosopher, but also studied  astronomy intensively, explaining that this led  to greater appreciation of Hashem’s might.

He refused to rely blindly on the  astronomical data of the famous Greek  astronomer, Ptolemy, believing in checking  up things himself with the finest instruments  and techniques of his time. His findings are  recorded in his major philosophical work,  Milchamos Hashem.

In his chapters on astronomy, he multiplies  Ptolemy’s paltry estimation of the stars’  distance from earth by a factor of one billion,  stating that the stars twinkle about 100,000  light years (each light year about 6,000 billion  miles) from earth. This estimate is correct for  some stars.

Where were all the stars hiding?  Astronomers discovered that the vast majority  of them are invisible. Although Galileo (1564  – 1642) is not credited with inventing the  telescope, he was the first person to develop  telescopes for astronomical research, and  peering through his lenses he gave the star  census a jump start.

Take for example, the Pleiades star cluster,  known as the “Seven Sisters” because of its  seven stars that are visible with unaided eye.  With his primitive telescope, Galileo spotted  an extra smaller 42 stars glowing among  their big sisters, and nowadays an amateur’s  telescope can detect hundreds.

Using telescopes, people also discovered  that the Milky Way, always regarded as a  ghostly cloud stretching across the night  sky, was actually a gigantic conglomeration  of stars. Then, in the 18th century scientists  learnt that the Milky Way is actually the home  of our planet Earth. This was when William  Hershel of England built a giant telescope  (for his time) and figured out that our sun is  only one of the teeming stars that comprises  the vast celestial pancake known as the Milky  Way Galaxy.

How big is the Milky Way?  Scientists had no idea until Friedrich  Bessel (5544/1784-5606/1846) estimated  that the distance to the star 61 Cygni (“The  Flying Star”) was 11.4 light years. Using this  star as a benchmark to estimate the distance  of other stars (by comparing their brightness  to that of Cygni), astronomers of those days  estimated that our Milky Way is 10,000 light  years across and 1,000 light years thick. It is  actually ten times larger and so vast that it is  currently believed to house about 300 billion  stars with plenty of elbow room.

Now scientists asked the $68,000 question.  Was the Milky Way the full extent of our  universe, or was it only one of countless other  galaxies?

The clue lay in a handful of cloudy blobs in  our night skies called nebulae, the Latin word  for clouds. After the advent of telescopes  people discovered far more of them – William  Hershel detected 2,500. This sparked off  a great debate. While Hershel and his  proponents believed that nebulae were stars  within our Milky Way surrounded by dust,  German philosopher Immanuel Kant argued  that the elliptical appearance of many nebulae  suggested that they were pan-caked galaxies  like our own, and it was their sideways view  that gave them their elongated egg-shape.

One of Kant’s arguments was that it seemed  ridiculous to limit Hashem’s creation to our  “paltry” corner of the Universe.  The argument raged for over a century,  culminating in the 5680/1920 debate in the  National Academy of Sciences between  young Harlow Shapley of Mount Wilson  Observatory and veteran astronomer Heber  Curtis of Lick Observatory. Their arguments  were so inconclusive that one writer thought  the problem was irresolvable, arguing, “We  have already reached a point where man’s  intellect begins to fail to yield him any more  light.”

However, there was a solution – to measure  the distance to the nebulae and see whether  they were near or far from our galaxy. The  only problem was, how to do the measuring?  The solution was based on a 5544/1784  discovery of Cepheid variable stars. These  stars periodically stop shining when their gas  is sucked inwards by their giant gravity, until  they cannot hold in their stomachs any longer  and explode back into light. Early in the last  century, Henrietta Leavitt calculated that the  brightnness of a Cepheid depends on its cycle  – brighter Cepheids have longer cycles. She  also worked out  that all Cepheids  sharing the same  cycle are equally  bright. Measure  a Cephied’s  cycle and you  know its level  of brightness.  With this rule,  Cepheids became  a yardstick to  measure the  universe.

How? Imagine  you are driving  on a highway and  see a motorbike  approaching in  the distance. You  can estimate its  distance by the  brightness of  its headlamp.  Similarly, if you  sight a distant  Cepheid star with  a 24 hour cycle,  you can calculate  its distance by  comparing its  brightness to a  similar Cepheid in  our galaxy whose  distance is already  calculated.

The trouble was  that no one had  ever discovered  a Cepheid star  inside a nebula (singular of nebulae).  The breakthrough came on the night of  5th October 5683/1923, when Edwin Hubble  took a photograph of the Andromeda Nebula  through Mount Wilson’s 100-inch telescope.  Comparing the photo with earlier shots of  the Nebula, Hubble noticed that a certain  star had disappeared and realized that he had  discovered a variable Cepheid in the dim  phase of its cycle.

Because this particular Cepheid had a  cycle of 31.415 days, indicating that it was  7,000 times brighter than the sun, its extreme  dimness revealed that it lay 900,000 light  years away, far from the confines of our  100,000 light year Milky Way. This clinched  the argument. Scientists now agreed that most  nebulae, until now considered by many as  little more than dust surrounding stars, were  actually massive galaxies, many of them  vastly larger than our own.

The Council of the American Astronomical  Society exulted that this discovery “opens up  depths of space previously inaccessible to  investigation and gives promise of still greater  advances in the near future. Meanwhile, it has  already expanded one hundred fold the known  volume of the material universe.”

Indeed, this was only the beginning. As  Hubble put it, “We find them (nebulae)  smaller and fainter, in constantly increasing  numbers, and we know that we are reaching  into space, farther and farther, until, with the  faintest nebulae that can be detected with the  greatest telescopes, we arrive at the frontier  of the known universe.”

Nowadays the universe is estimated as  countless billions times larger than our Milky  Way and the latest estimate of visible stars in  the universe was announced three years ago  in Australia. Dr. Simon Driver of Australian  National University had estimated the number  as 70 sextillion (70,000 million, million,  million) stars.

Of course, even Dr. Driver doesn’t know  if he got it right because he couldn’t actually  count the teeming stars. He merely made  an estimation of a thin sliver of sky and  extrapolated from that to the rest.  According to this estimate, stars outnumber  every grain of sand in every desert and every  beach on our planet ten to one. That’s a lot  of Jews.

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