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Northern Rockies Skies for March: The Bright Stars of Winter

February 26, 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.

Winter is a great time to view some of the brightest stars in the night skies. Of the top 10 brightest stars, six can be seen in the early evening this month. Moreover, several in the top 20 also are visible. Here is the list (brightness rank, name, constellation) of bright stars in the March winter sky: 1, Sirius, Canis Major; 4, Arcturus, Bootes; 5, Capella, Aurigae; 6, Rigel, Orion; 7, Procyon, Canis Minoris; 9, Betelgeuse, Orion; 13, Aldebaran, Taurus; 16, Pollux, Gemini; 20, Regulus, Leo; and 22, Castor, Gemini.

So, check your sky map for March and locate these bright wonders. Don’t get them confused with Jupiter, which lies in Gemini this month. See if you can pick out this giant planet.

Planet Alert: Jupiter is on the meridian right after sunset in the constellation Gemini. Venus is the morning star. Mars rises around 11 p.m. and can be seen throughout the entire night.

The big news, the first day of spring is March 20. Let’s hope spring weather arrives.

March Interest: Great Debates: The Gamma-Ray Burst Distance Scale

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From their discovery in 1967, until the mid-1990s, the distance scale to gamma-ray bursts (GRBs) was nearly unconstrained. Upwards of 100 theories had been proposed, placing the burst sources anywhere from the cometary Oort Cloud (assumed to exist beyond but attending our solar system) up to cosmological distances of billions of light-years.

The primary reason for this uncertainty was that gamma-ray telescopes had very poor angular resolution and, therefore, no connections had been made to objects detected in other parts of the electromagnetic spectrum, such as the visible, where astronomers understood the physics and, therefore, the objects’ distance scales.

To commemorate the 75th anniversary of the great debate on the distance scale of the universe (last month's column), a debate on the GRB distance scale was held at the same venue as the 1920 debate, the Natural History Museum. By that time (1995), the distribution of GRB detections on the sky, being uniform, had effectively eliminated all possible scales except our Milky Way Galaxy's halo and the much more immense scale, the universe at large.

The proponents were Donald Lamb (University of Chicago) supporting the former, and the late Bohdan Paczynski (Princeton) arguing for the cosmological distance scale of the universe.

Lamb's position used physics, including several lines of evidence, now known to be spurious, that high-velocity neutron stars in the galaxy's halo emitted GRBs during magnetic starquakes. Specifically, such magnetic neutron stars could emit repeated GRBs over time without destroying the star, and such repetitions were erroneously thought to have been detected.

Paczynski's argument was entirely geometrical and refrained from employing any specific physical model. Rather, his fundamental position was that all other uniform distributions known in astronomy were for objects at cosmological distances, and that any reasonable halo distribution would exhibit some degree of nonuniformity on the sky.

Two years after this 1995 great debate, the first visual counterpart for a GRB was discovered and real physics brought to bear on the issue. As foreseen by Paczynski, the cosmological distance scale was correct. A new satellite, Swift, dedicated to detecting GRBs and their X-ray and visible wavelength counterparts, was flown.

All of the more than 300 Swift GRBs detected with redshifts measured by ground-based instruments are found at cosmological distances, their progenitors being massive, rapidly rotating worn-out stellar cores that explode (only once) and are briefly, for seconds, the most luminous objects in the universe.

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

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