The stars we see at night have positions that are fixed, relative to one another, right? Throughout history, a belief in the “fixed” and changeless stars was held by many cultures. But at the beginning of the 19th Century, Italian astronomer Giuseppe Piazzi repeatedly measured the position of a star called 61 Cygni. Piazzi’s work overturned such long-held beliefs and led to research that ultimately allowed astronomers to accurately measure the scale of the Milky Way Galaxy and to learn of our location within it.
61 Cygni is a binary star that is visible to the unaided eye and can be resolved as a pair of orange stars in a small telescope. Piazzi’s measurements showed that both stars in the 61 Cygni system are moving relative to background stars at about 5.3 arc seconds per year. At that rate, 61 Cygni moves half a degree, the apparent diameter of the full Moon, in about 340 years. That’s 1/8 of the apparent diameter of the Moon in a human lifetime. Piazzi called 61 Cygni his “flying star.”
This was a revelation. Piazzi concluded that such a high apparent speed, or “proper motion”, as it’s called by astronomers, indicated that 61 Cygni is likely one of the nearest star systems. As telescopes improved through the 19th Century, astronomers attempted to measure stellar distances by the parallax method. You can visualize parallax by holding a finger at arm’s length and then closing one eye, then your other eye. The position of your finger shifts relative to background objects. By measuring the angle of the shift and the distance between your eyes, it’s possible to calculate the distance of your finger from your eyes using basic trigonometry. The “baseline” for this experiment is the distance between your eyes. Applying the parallax method by using Earth’s 186-million-mile-wide orbit as a baseline, German astronomer Friedrich Bessel in 1837 measured 61 Cygni’s distance at about 11 light years, or 65 trillion miles. This was the first reliable distance measurement for any star beyond the Sun.
In recent decades, parallax techniques have vastly improved. Now astronomers use parallaxes from radio telescopes and space probes to measure distances across our galaxy. They’ve learned that our Solar System resides about halfway between the center and edge of the Milky Way Galaxy, a large spiral galaxy, having a diameter exceeding 100,000 light years.
Music written and produced by Kenny Mihelich (ma-HELL-itch). Western Slope Skies is produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and voiced by Art Trevena.
Web links:
https://earthsky.org/brightest-stars/61-cygni-suns-near-neighbor/
https://en.wikipedia.org/wiki/61_Cygni#/media/File:61_Cygni_Proper_Motion.gif
https://www.space.com/30417-parallax.html
https://public.nrao.edu/gallery/the-parallax-technique/
https://www.esa.int/Science_Exploration/Space_Science/Gaia
