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Far Out: A Space-Time Chronicle; Abrams; 2009
Nebulae
The measurement of stellar parallax was just one part of astronomy’s maturation during the middle of the 19th century; the outlines of additional rungs in the cosmic distance ladder were also taking shape. By this time, increasingly sensitive telescopes were discovering mysterious nebulae scattered among the stars in droves. Some of the nebulae looked like clouds, others looked like spirals. But what were they? Some astronomers thought they were planetary systems in various stages of formation or destruction; others thought they might even be island universes, huge accretions of stars reduced to faint smudges by their vast distance.
As the mystery of the nebulae deepened, the techniques that would eventually help solve it emerged. In 1814, the German optician Joseph von Frauenhofer fashioned a spectroscope (a telescope mated to a prism that can split starlight into its constituent colors) for viewing the Sun. He noticed, in looking at the Sun’s spectrum, thin lines where colors were absent. These spectral shadows were shown to be caused by a luminous object’s chemical composition and thus could be used to measure the material of the stars. By 1848, the French physicist Armand Fizeau proposed the dark lines could also be used to measure a star’s motion by exploiting an effect first noticed by the Austrian physicist Christian Doppler. Doppler suggested that all waves, be they sound or light, would alter their wavelengths as their sources moved. Fizeau posited that shifts in stellar spectral lines toward the shorter, bluer wavelengths (blueshifts) indicated stellar motion toward the observer, and that shifts toward longer, redder wavelengths (redshifts) meant stellar motion away from an observer.
However, the key breakthrough that would help solve the nebulae mystery did not come until the first decade of the 20th century. While performing the menial task of counting stars on photographic plates for her employer, Edward Pickering, the American astronomer Henrietta Leavitt noticed something curious in the Small and Large Magellanic Clouds, two huge groupings of stars. In the Clouds, certain variable stars, stars that periodically brighten and dim, exhibited a cycle of brightening and dimming with metronomic regularity. More importantly, it appeared that the brighter these unique variable stars were, the longer the period of their cycle.
These stars were later named Cepheid variables, and their oscillatory behavior proved crucial to the cosmic distance ladder. By measuring the timing of a Cepheid’s pulsation, its intrinsic luminosity can be inferred. Cepheids are standard candles, objects whose true luminosity we know. By comparing a standard candle’s luminosity with its apparent brightness in the sky, its distance from Earth can be reliably estimated. Cepheids soon became key measures for distances in the Milky Way.
Galaxies
Beginning in 1912, the American astronomer Vesto Slipher began noting that nearly all the spiral nebulae he observed displayed redshifts—they all seemed to be moving away from the Earth at great speed. One notable exception was the first spiral nebulae he measured, which today we know as the Andromeda galaxy. Slipher’s observations showed that Andromeda was heavily blueshifted, and he estimated that it was hurtling toward us at hundreds of kilometers per second. In the early 1920s another American astronomer, Edwin Hubble, used what was then the largest telescope on Earth, the 2.5-meter Hooker telescope, to study Cepheids in Andromeda. Hubble’s data indicated that Andromeda was more than a million light-years away. At a stroke, our view of the universe transformed. Andromeda and the other spiral nebulae weren’t planetary systems mid-formation in our own galaxy; they were each galaxies themselves, in a universe far more immense and spacious than most had dared to dream.
But even this was but a prelude to a greater discovery. Using the galactic redshifts collected by Slipher and others, in 1929 Hubble and his colleague Milton Humason found that a galaxy’s redshift increased in proportion with the galaxy’s distance from Earth. This observation is now known as Hubble’s Law and is now used extensively to estimate distances to far-away galaxies. Hubble’s Law showed conclusively that the universe was expanding.
For decades, this remained the best observational evidence for the then-controversial big bang theory, which stated that the universe sprang into existence from an infinitesimal point billions of years ago. Today, we number galaxies in the hundreds of billions, but there may be innumerably more beyond where we can see, masked by space so vast that their galactic light has yet to reach us here.
The Universe
Beyond stellar parallax, which is useful only out to hundreds of light-years, or Cepheids, which can measure distances to nearby galaxies, additional techniques exist. Many of these also involve using celestial objects and phenomena as standard candles, linking their luminosity with some other aspect of their behavior, like rotation rate or size or color. The most notable additional technique uses exploding stars, particularly ones called Type Ia supernovae.
According to theory, a Type Ia supernova only occurs in a special kind of binary star system, where a giant star and a white dwarf star orbit one another. For the small, dense white dwarf, this is an all-you-can-eat buffet: It easily pulls gas off its larger, puffy companion, gradually building up layers of material on its surface. But once the growing white dwarf reaches a total mass about 1.4 times that of our Sun, it is doomed by its gluttony. At this point, like clockwork, the accumulated weight of all the extra material ignites a thermonuclear fusion reaction that engulfs the entire star. After a blinding flash of light, all that’s left of the white dwarf is a slowly cooling, expanding shell of radioactive debris. By assuming that Type Ia’s always detonate the same amount of material and release the same amount of energy regardless of where they’re found in the universe, astronomers can estimate their intrinsic brightness and measure immense distances of hundreds of millions, even billions, of light-years.
In the late 1990s, two teams, the Supernova Cosmology Project and the High-z Supernova Search Team, were studying Type Ia supernovae to sharpen estimates of the universe’s expansion rate. But instead of simply refining the established measurements, both teams independently discovered something astonishing: The supernovae they observed at the edge of the visible universe were dimmer, and thus further away, than they should have been according to Hubble’s Law. It appeared that the universe was not only expanding, but also accelerating in its expansion. The rate of acceleration suggested that whatever powered it accounted for nearly 75 percent of the total energy in the entire universe. This strange force, now labeled dark energy, is one of the deepest mysteries in cosmology.
Much of the work that remains for pinning down interstellar, intergalactic, and extragalactic distances could hold great importance for our future. Gaia, a spacecraft launching in 2011 or 2012 that is designed to use parallax and other techniques to measure the motions of the Milky Way’s stars, will construct a three-dimensional star map of the galaxy and potentially find hundreds of new extrasolar planets.
Fundamental refinements in parallax measurement will gradually trickle up the rungs of the cosmic distance ladder, sharpening distance estimations for standard candles in nearby galaxies and galactic clusters. With better measurements, we will finally know whether Andromeda, hurtling in our general direction at hundreds of kilometers per second, will eventually collide with our Milky Way in some 3 billion years.
Improved measurements of the universe’s accelerating expansion could even help explain what exactly dark energy is and perhaps even forecast the ultimate fate of the entire universe. Will it accelerate in its expansion forever or could it perhaps slow or even reverse its course, eventually collapsing inward on itself in a “big crunch?” Could it then rise phoenix-like from the cosmic genesis of another big bang? The truth is, we don’t really know—not yet—but the answers may be somewhere out there, hidden in the depths of the unbounded sky.
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