Last week, we began a history of attempts to map the Milky Way by determining the distances to stars. We started with second century BCE astronomer Hipparchus, who successfully calculated the moon’s distance.
He invented the parallax method, a trigonometric measure of an object’s shift in position when observed from two separate locations. The technique uses that shift to determine the distance to the object.
In 1838, two millennia after Hipparchus, German astronomer Friedrich Bessel used the parallax method to measure the distance to some closer stars in the Milky Way.
The problem with parallax is that even the closest stars are very far away. We’d have to move our observing locations far apart to see any shift in a star against the background of even farther stars.
Bessel measured the position of close stars by observing them at six-month intervals — from either side of Earth’s orbit, giving us a baseline. That’s one heck of a baseline of the diameter of Earth’s orbit, about 186 million miles.
Bessel picked 61 Cygni because he guessed it might be nearby.
During the early 1800s, he repeatedly measured the star’s position against fainter background stars. He then did a parallel set of observations six months later when Earth was on the other side of its orbit.
The star had moved. Bessel became the first person to measure accurately a star’s parallax shift.
He calculated the distance at 11.4 light-years, which is impressively close considering Bessel’s use of early-19th-century telescope technology.
Soon thereafter, Scottish astronomer Charles Henderson announced the distance to the closest star to the sun, Alpha Centauri. His figure, 4.2 light-years, was right on the money.
He had made his calculations years earlier than Bessel but did not publish them because the calculated distance seemed unbelievably large.
Two problems hampered the parallax method. For starters, Earth’s atmosphere is turbulent. Our moving blanket of air makes it quite challenging to resolve two close objects.
As a result, Bessel’s parallax method was limited to a few dozen light-years at first. However, astronomers have developed optical systems in telescopes that adapt to turbulence in our atmosphere, increasing the distance that a ground-based telescope can measure to more than a thousand light years.
Still, the Milky Way disk is 100,000 light-years across. Astronomers needed a method to see and measure farther distances.
A second problem is that our Milky Way is very dusty. The images of far-away stars are obscured by intervening interstellar dust and gas.
However, light from the radio band of the electromagnetic spectrum passes more successfully through the dust clouds than visible light.
The rise of radio astronomy solved both problems.
Long radio waves pierce the galactic dust clouds more readily than the shorter wavelengths of visible light.
Radio telescopes are particularly effective at detecting clouds of hydrogen, called neutral hydrogen, that have not yet formed into stars.
Astronomers can map the overall structure of the Milky Way employing a particularly ingenious method.
They first determine the velocity of those clouds as they orbit the center of the Milky Way. They then compare those velocities with the relative speeds of the Milky Way’s rotation at points of varying distances from the center.
In general, astronomical objects near the center of a galaxy revolve very quickly around the center. At increasing distances from the center, they move more slowly.
By placing those hydrogen cloud velocities within that galactic revolutionary model, astronomers can estimate the distances to those clouds. To those data, astronomers added stellar distances determined by other methods previously discussed (notably the parallax technique and Levitt’s Cepheid-variable method).
In 1953, only one human lifetime ago, astronomers concluded that the Milky Way has spiral arms, much like those astronomers had detected in many external galaxies.
Parallax is still the most reliable method for measuring the distance to objects in our galaxy. But atmospheric turbulence and interstellar dust hinder the technique, at least if you want to measure millions or even billions of stars across the width and breadth of the Milky Way.
Astronomers needed a telescope outside Earth’s turbulent blanket of air. That telescope must examine stars in portions of the electromagnetic spectrum that can pass relatively unimpeded through galactic dust.
More of that next week.
Tom Burns is the former director of the Perkins Observatory in Delaware.
