Cosmic Background
By Marcia Bartusiak
1 One hundred fifty years ago, in 1862, the first hint arrived that the stellar universe was far stranger than anyone imagined—or could imagine. It came with the knowledge that a faint companion slowly circles Sirius, the brightest star in the nighttime sky.
2 Astronomers at the time didn’t recognize what they had uncovered. It would take decades—until the 1910s— for them to fully realize that Sirius B, as the tiny companion came to be known, was a star like no other seen before. Once its nature was revealed, though, it didn’t take long for theorists to conceive of other bizarre creatures that might be residing in the stellar zoo.
3 The story begins, not in 1862, but two decades earlier. For a number of years, the noted German astronomer Friedrich Wilhelm Bessel, director of the Königsberg Observatory, had been going through old stellar catalogs, as well as making his own measurements, to track how the stars Sirius and Procyon were moving across the celestial sky over time. By 1844 he had enough data to announce that Sirius and Procyon weren’t traveling smoothly, as expected; instead, each star displayed a slight but distinct wobble—up and down, up and down. With great cleverness, Bessel deduced that each star’s quivering walk meant it was being pulled on by a dark, invisible companion circling it. Sirius’s companion, he estimated, completed one orbit every fifty years.
4 Bessel was clearly excited by his find; in his communication to Great Britain’s Royal Astronomical Society he wrote, “The subject ... seems to me so important for the whole of practical astronomy, that I think it worthy of having your attention directed to it.” 5 Astronomers did take notice, and some tried to discern Sirius’s companion through their telescopes. Unfortunately, at the time Bessel reported his discovery, Sirius B was at its closest to gleaming Sirius, from the point of view of an observer on Earth, and thus lost in the glare. But even years later, no one was successful in spotting the companion.
6 That all changed on January 31, 1862. That night in Cambridgeport, Massachusetts, Alvan Clark, the best telescope manufacturer in the United States, and his younger son, Alvan Graham Clark, were testing the optics for a new refractor they had been building for the University of Mississippi. It was going to be the biggest refracting telescope in the world. Looking at notable stars to carry out a color test of their 18.5-inch lens, the son observed a faint star very close to Sirius. This was Sirius B, a type of star now known as a “white dwarf” because of its color and size.
Adapted from Natural History, February, 2012
According to the information in the article, Sirius B was finally seen because
a) more than two decades of research had enabled astronomers to calculate the trajectory of that star’s orbit around Sirius.
b) its orbit had taken it to a point very close to the luminosity of Sirius, where it could be easily identified.
c) Alvan Clark and his younger son, Alvan Graham Clark, were testing the hypothesis that “white dwarf” stars existed.
d) Alvan Clark and his younger son, Alvan Graham Clark, were trying to prove that an invisible companion star was affecting Sirius’s trajectory.
e) an important component of a new telescope was being tested.
