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.
FROM THE GROUND UP
“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.
ARE WE THE ONLY ONE?
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.