The best way to begin is to let an astronomer take you for your first spin, while you enjoy the ride. The dome in the Hayden Planetarium, or in any other large planetarium, is an ideal theater for traveling along the simulated starways. But if you want to drive, to pilot your own spaceflight and change course whenever you like, you can download the free software and catalog of celestial objects at the Hayden Planetarium's Digital Universe Atlas. Ride or drive, either way the look and feel of our stellar and galactic neighborhoods have become accessible not only to the professional astronomer, but to virtually everyone.
The Digital Universe atlas has grown out of a convergence of two great streams of technical achievement: celestial mapmaking, the product of centuries of observation and scientific breakthrough, combined with hardware and software engineering, which enables sophisticated data visualization. In principle, the merging of those two streams is simple; in practice it is laborious but brilliantly synergistic. Together they have drawn back the curtain on the universe in all its three-dimensional glory.
Knowing the positions of celestial objects makes it straightforward to calculate their apparent relative positions from any fixed perspective. Recalculating positions from a series of perspectives along a smooth trajectory, and displaying them rapidly in sequence, creates the illusion of smooth, animated motion through space. Thus, an abstract collection of data becomes a visceral experience.
Cosmic cartography begins with astronomical measurements. Astronomers share those measurements through a network of catalogs and publications, and computers have combined and seamlessly integrated the network into the atlas. In that way, hundreds of thousands of celestial objects from numerous catalogs have found their way into the Digital Universe.
At its base, the Digital Universe atlas is built on highly precise astrometry—the “latitude” and “longitude” of objects in the sky—combined with the best available estimates of the distances of those objects from Earth. The work of mapping the former two coordinates, at ever-increasing precision, constitutes thousands of years of effort. But surprisingly, it was only 166 years ago that astronomers made the first relatively accurate distance measurement to an object outside our solar system. Until that time, nothing definite was known except that the stars were very far away.
In 1838 Friedrich Bessel, then director of the Königsberg Observatory in Berlin, calculated the distance to the star 61 Cygni. Bessel measured how the star appeared to shift relative to the surrounding stars, a result of viewing it from one side of the Earth’s orbit around the Sun, then observing it again from the other side of the orbit six months later. This shift in perspective is called parallax, and from its magnitude Bessel calculated the approximate distance to 61 Cygni with simple geometry.
The parallax method remains the most accurate technique for measuring distances to objects outside our solar system. But the diameter of the Earth’s orbit, 186 million miles, limits the use of the method to the nearest stars, within about 500 light-years of Earth. A light-year is about 6 trillion miles, and so 500 light-years seems quite a substantial distance. Yet it constitutes only a small “bubble” of observable space, centered on Earth.
How can distances to objects be surveyed beyond our neighborhood bubble? Within our Milky Way, a star’s spectrum reveals its luminosity class—hence, its intrinsic brightness. By comparing a star’s intrinsic brightness with its apparent brightness (as seen from Earth), its distance can be estimated. Unfortunately, the method is not as accurate as parallax, but it is the best method available for most stars that are too far away for parallax measurements.
Another method relies on the knowledge of variable stars, whose brightness varies periodically. The rate at which the apparent brightness of certain variable stars changes is directly related to their intrinsic brightness. If you measure the period over which a star varies, as well as its brightness as seen from Earth, you can estimate its distance. In 1918 the astronomer Harlow Shapley applied that method to find the distance to many globular star clusters in the Milky Way. In fact, he was ultimately able to locate the center of our galaxy—a giant leap forward in the spatial understanding of the universe. With the variable-star method, the bubble of known distances extends outward into extragalactic space, about 50 million light-years from Earth.
Beyond that distance, other objects can serve as “standard candles”—objects whose brightness at their source is always the same, much like lightbulbs of equal wattage. For example, if one lightbulb is ten feet from an observer, and a second lightbulb is a hundred feet away, the farther lightbulb would appear to be the dimmer (a hundred times dimmer, to be precise). Similarly, astronomers infer distance by assuming, on sound independent grounds, that certain astronomical objects all have the same “wattage,” and so they can all serve as mutually corroborating standard candles. One standard candle, detectable from as far away as about 5 billion light-years, is the explosion of a certain kind of massive dying star known as a type-Ia supernova. All type-Ia explosions are assumed to be of similar luminosity.