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January 29, 2014 — A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.
This month, around 9 p.m. on the meridian (the imaginary line running from the north cardinal point through the zenith and down to the south cardinal point), will be two stellar objects that compete in brightness. The more northern object is Jupiter the planet, located in Gemini. The second bright star is Sirius, the brightest star in the constellation Canis Major, the big dog. Sirius also is the brightest star in all of our night skies.
North of Canis Major lies Canis Minor and its brightest star, Prycyon, which rivals the brightness of Regulus and Capella, Betelgeuse and Rigel. Canis Major in Greek mythology is associated with the fastest dog in the universe. It follows Orion the hunter.
Sirius, the “dog star,” takes its name from the Greek word meaning scorching or burning. Since Sirius would rise in the daytime during the hottest months of July and August, the Greeks thought this extra heat source was due to the presence of the glowing Sirius. Sirius also marked the annual flooding of the Nile for the Egyptians.
Today, we know that Sirius is a binary star a little less than nine light years away. Prycyon means “before the dog,” since it rises before Sirius. It is the seventh brightest star in the night sky and is about 11.5 light years from the sun.
Other planets for the month: Mars, near Spica, rises around midnight; Saturn rises two hours later; and Venus rises an hour before sunrise.
Great Debates: The Scale of the Universe
(Best URL: http://apod.nasa.gov/debate/debate20.html)
With construction of ever larger telescopes, mapping of distributions of stars and star clusters -- starting in the 1700s -- revealed two grand designs of nature: Our own Milky Way, a galaxy viewed from the inside with not much clarity; and numerous nebulae with obvious spiral structure.
Early on, the famous German philosopher Immanuel Kant (1724-1804) conjectured that these spiral nebulae were "island universes" in their own right, external to our galaxy and lying at great distances. Such fundamental questions -- the shape and size of the Milky Way, and the nature of the spiral nebulae -- were considered in the first astronomical "Great Debate" in 1920, between Harlow Shapley and Heber Curtis.
Both men were professional astronomers who presented at the Natural History Museum to both a general audience and to members of the National Academy of Sciences. Purportedly, Albert Einstein was in attendance. The younger of the two debaters, Shapley, gave a popular delivery geared to the general scientific audience.
Shapley's essential position was that our Milky Way galaxy constituted the entire universe, and that our sun was significantly offset from the center of the galaxy. He inferred the size of Milky Way from the period-luminosity relation of Cepheid variable stars, their periods indicating luminosities and, so, revealing their distances.
By mapping the distribution of globular clusters -- spheroidal assemblies of 100,000 to 1 million stars --- Shapley showed that our sun and its solar system lie far from the distribution's centroid, the apparent center of our Milky Way. Relying on pairs of faulty measurements of several spiral nebulae taken many years apart, Shapley believed the spirals to be internal to our galaxy. If instead external, the pairs of photographs would indicate very rapid rotation of their spiral arms -- nearly as fast as the speed of light -- considered highly unlikely by most astronomers.
Curtis' arguments were more detailed and data oriented. Although he used the best and most extensive collection of stellar positions and distances, he incorrectly gauged the position of the sun and the Milky Way's size because its distant reaches are shrouded by dark dust clouds populating the Galactic Plane (globular clusters can be seen to larger distances since their spatial distribution is spheroidal).
Also, Curtis thought the Cepheid period-luminosity relation to be contaminated, which it was -- two kinds of Cepheids were included in Shapley's data set.
Curtis' main valid point, however, relied on novae occurring in the spiral nebulae, just like the novae occurring in the Milky Way. By calibration of nova distances, he inferred that the spiral nebulae were galaxies external to, comparable to, and at vast distances from our Milky Way Galaxy. Seven years later, Edwin Hubble found Cepheid variables in our sister galaxy, Andromeda, confirming the spiral nebulae to be external galaxies. The scale of the universe, as we understood it at that point in time, increased enormously.
To view this month’s sky chart, click here.