Northern Rockies Skies for October

September 26, 2012

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.

We still see the triangle of bright stars (Vega, Deneb and Altair) dominating the early night sky. These stars connect the constellations of Lyra (the lyre, turtle or vulture), Cygnus (the swan) and Aquila (the eagle).

Near 9 p.m., we find the great square of Pegasus (the winged horse) and Andromeda (the chained lady) rising toward the zenith, accompanying Jupiter on the northeast horizon.

Within the confines of Andromeda, the great Andromeda Galaxy is the most distant object you will ever see with the unaided eye. It clearly is better viewed, though, with a small telescope or binoculars. The Andromeda Galaxy (M 31 in the sky chart) lies 2 million light years away. It is as large as six times the diameter of the full moon.

Venus is the morning star this month, rising at 5 a.m. Watch for the Orionid meteor showers peaking around Oct. 21. Look to the east after midnight and expect to see 20 or more “shooting stars” every hour.

October 2012 Interest: Famous Astronomers -- Walter Baade

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Much of astronomical endeavor is refinement of less accurate or undifferentiated results, and often takes great effort to achieve. The German-American astronomer Wilhelm Heinrich Walter Baade made such contributions in several areas.

While stationed at Mount Wilson Observatory near Los Angeles, he utilized the 100-inch Hooker telescope to make many of his observations. During World War II blackout periods, L.A. light pollution was reduced, affording good enough conditions to study particular stars in the Andromeda Nebula (distance of 2.5 million light-years), sister galaxy to our Milky Way.

His comparison of similar stars in Andromeda and the Milky Way yielded two fundamental advances. First, he recognized that stars that appear to have the same color, or temperature (as deduced from spectroscopy), can be divided into two groups, Populations I and II. The former are stars with ages in the millions of years. Population II are much older stars with ages up to billions of years, and these stars often can be giants, their atmospheres inflated to much larger radii.

Perhaps, of greater importance, Baade was able to reclassify a particular kind of periodic variable star, Cepheids (named after the canonical example, Delta Cephei), into two groups, classical and Type II Cepheids -- members of Populations I and II, respectively. For both groups, their brightness oscillates up and down with characteristic periods that are proportional to their luminosities. In 1929, Edwin Hubble had used an undifferentiated sample of Cepheids to calibrate the expansion and scale of the universe.

By weeding out Type II Cepheids from the sample, Baade's work -- published in 1952 -- showed the universe's size to be roughly a factor of two larger than Hubble had figured. With Fritz Zwicky, Baade invented the term "supernova" to describe dying stars that explode and leave behind a hypothesized "neutron star" (and, in some cases, as we now understand, a black hole). They pioneered observations of supernovae, using them like the Cepheids as distance indicators for more distant galaxies in the universe.

With Rudolph Minkowski, Baade studied some of the first known cosmic radio sources, such as Cygnus A. He also is credited with discovering several asteroids, including Hidalgo and the Apollo-class Icarus. He first associated Baade's Star -- a rotating neutron star, the Crab Pulsar -- with the Crab Nebula.

To view this month's sky chart, click here.

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