Recall from last week that we are discussing the history of our understanding of that streak of hazy light we have called the Milky Way since ancient times.
With the publication of the Principia in 1687, Isaac Newton gave astronomers many of the tools they needed to measure the shape and size of the Milky Way in what is perhaps the greatest leap forward of science at any time in history. He recognized, as Kepler before him, that part of the light from a spherical object was lost depending on its distance as their light spread out from their spherical shape.
Most importantly, his laws of gravity and motion made it possible to fix objects in space without the need for solid spheres, which had plagued astronomy since its dim beginning. The problem now was to determine the Milky Way’s shape.
The celestial sphere slowly melted away, but the pace was agonizingly slow by our contemporary standards. Borrowing an idea from Newton, Edmond Halley of Halley’s Comet fame speculated that the stars might be arranged in concentric “shells” of varying distances revolving at varying velocities.
Neither Halley nor Newton could adequately explain why the stars were clumped together in sections like the Milky Way. It took the speculations of a mystic, gardener, and amateur astronomer named Thomas Wright to make the breakthrough.
In 1750, he described the Milky Way as a flattened ring of which the sun and earth are constituent parts. The appearance of the Milky Way was explained as “an optical effect due to our immersion in what locally approximates to a flat layer of stars.” As we look through the thickest part of the ring, we see the densest collection of stars, the Milky Way.
Immanuel Kant, one of the greatest minds to ever inhabit our planet, saw a review of Wright’s work that misinterpreted Wright’s ring as a spinning, flattened disk. In 1755, Kant proposed that the Milky Way was such a disk held in place by a harmonious equilibrium between the motion of its spin, which made it want to flatten and fly apart, and the mutual gravity of the stars and planets which made them want to fall into the center. Newton’s laws of gravity and motion had finally borne Milky-Way fruit.
Even more significantly, Kant borrowed and expanded upon another idea from Wright. He proposed that some of the faint objects we see in the sky, called nebulae, also looked like tiny disks tilted at various angles. Those nebulae might in fact be “island universes” much like our Milky Way. His words seem prescient to us now:
It is far more natural and conceivable to regard them as being not such enormous single stars but systems of many, whose distance presents them in such a narrow space that the light, which is individually imperceptible from each of them, reaches us on account of their immense multitude in a uniform pale glimmer. Their analogy with the stellar system in which we find ourselves, their shape, which is just what it ought to be according to our theory, the feebleness of their light which demands a presupposed infinite distance: all this is in perfect harmony with the view that these elliptical figures are just universes and, so to speak, Milky Ways, like those whose constitution we have just unfolded.
Seen in retrospect, those words are stunningly correct. But in 1755, there was little evidence to support his hypothesis. Still, his words sparked a long, sometimes-bitter debate that was eventually to lead to our modern certitude.
The problem was that there was no reliable way of determining the distance to the stars in our Milky Way or the fuzzy ovals that we have come to call galaxies but which were during the 19th century called “spiral nebulae.”
William Herschel, perhaps the greatest of all telescope makers and observers, was undaunted by the distance problem. He simply made the critical (if erroneous) assumption that all stars were the same brightness to start with. Thus, fainter stars were increasingly farther away than brighter stars according to a strict set of Newtonian calculations.
He then observed thousands of stars and placed them in the Milky Way based on Newton’s brightness-distance law.
The drawing he produced in 1785, shows a flattened disk with “crooked branches.” That’s pretty close to our modern depiction of the Milky Way. Herschel also believed that the Milky Way was a “detached nebula,” which opened up the possibility that other Milky Ways existed, as Kant had proposed over 100 years before.
By the early 20th century, astronomers were still assuming the same distance-brightness relationship. Jacobus Kapteyn used star images from astronomers all over the world to produce a flattened disk similar to — but more accurate than — the one Herschel had produced over a century before.
Both images were flawed because intervening gas and dust among the stars absorbed some of the light they produced and because stars are not equal in their brightness to start with.
As a result, Kapteyn’s proposed Milky Way was much smaller than the Milky Way we know today. He also believed that when he mapped the Milky Way, he was mapping the entire universe. To Kapteyn, the spiral nebulae were part of the Milky Way.
Over time, astronomers refined their ability to gauge the distance to the stars, and the familiar spiral shape of the Milky Way emerged. As telescopes got bigger, the “spiral nebulae” also began to reveal their spiral structure. The basic similarity of structure seems obvious to us today, but it was hardly obvious to 19th-century astronomers.
Thus, the question of the Milky Way’s size and extent remained. Did the Milky Way represent the whole universe? Did it include the spiral nebulae, which came to be considered by many astronomers to be part of the Milky Way? They were perhaps individual stars in the process of formation as gasses streamed into their central bulge.
That point of view came to be called the Kapteyn universe. Its early 20th century its major proponent was Harlow Shapley of Mount Wilson Solar Observatory.
Still, the theory that the spiral nebulae were in fact separate milky ways had its proponents as well. Notable among them was Heber Curtis, of Lick Observatory.
That debate raged throughout the early 20th century. It culminated in the Great Debate between Shapley and Curtis at the Smithsonian on April 26, 1920.
Just four years later, the issue was decided once and for all. Two critical advancements spelled doom to the Kapteyn/Shapley universe.
Telescope technology advanced to the state that a few bright stars could be seen in the spiral nebulae in very large telescopes. More importantly, some of those stars, called Cepheid variables, pulsated. They varied in brightness in a regular way.
Between 1908 and 1912, astronomer Henrietta Leavitt discovered a relationship in Cepheid variables between the period of their pulsations and their brightnesses. Brighter Cepheids have longer periods of pulsation. That period-luminosity relationship can be used to measure distances to nearby galaxies.
The time it takes for a star to brighten and fade can be calculated to find the star’s actual, or “intrinsic,” brightness. Astronomers could then compare its intrinsic brightness with the star’s measured brightness to find the distance.
It was easy to measure the distance to Cepheids in the Milky Way, but telescopes were not yet big enough in 1912 to see individual stars in the spiral nebulae, if in fact those stars existed at all.
In 1917, just five years after Leavitt’s remarkable discovery, a new telescope was put into operation on Mt. Wilson. The Hooker Telescope, then the largest telescope in the world, was finally capable of resolving some of the brighter stars in the spiral nebulae.
Between 1923 and 1924, astronomer Edwin Hubble measured the distance to Cepheid variables in the Andromeda Nebula, also called M31, which turns out to be the closest of the spiral nebulae to our Milky Way.
He determined that M31 was 700,000 light years away, which is more than three times farther than the largest contemporary estimate of the size of the Milky Way. The Andromeda Nebula became the Andromeda Galaxy.
The Andromeda Galaxy turns out to be more than three times farther away than Hubble estimated, but no matter. He showed us finally what the Milky Way really is: a spinning whirlpool of 300 billion stars, one of perhaps trillions of galaxies in a universe of unimaginable size.
That’s a far cry from Aristotle’s belief that the Milky Way was vapor rising from a crack in the earth. A more-or-less steady interest in the Milky Way over 2,500 years had finally caused our galaxy to reveal its secrets. Science had proven once again that a careful study of the natural world will over time yield a deepening understanding of the universe and our tiny place in it.
Tom Burns is the former director of the Perkins Observatory in Delaware.