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Northern Rockies Skies for April


March 29, 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.

Say adios to the conspicuous winter constellations: Orion the hunter, Taurus the bull, and Canis Major, the big dog.

Sirius, the brightest star in the sky in Canis Major, is as bright as Jupiter, which can be seen right after sunset in Aries. The brightest object in the sky right after sunset is Venus, which is located in Taurus this month.

April heralds some of our summer constellations and the night skies will now be ruled by two familiar constellations -- Ursa Major, the Big Bear, and Leo the lion. Both are visible, high and to the northeast, and easily identifiable.

The Big Bear, known to most as the Big Dipper, allows us to spot the star Polaris by following northward a straight line from the two stars at the end of the dipper. Polaris, the North Star, is in the constellation Ursa Minor, which means Little Bear, and is known as the Little Dipper.

Note that Mars is in Leo and will rival the brightness of the star Regulus. Toward the zenith, you can see the constellation Auriga and its bright star, Capella, as well as the two Gemini Twins, Castor and Pollux.

For our planetary watchers this April, you can see Jupiter, Venus, Mars and Saturn, which rises right after sunset in the constellation Virgo.

Meteor shower alert for April 21-22: Look in Lyra after midnight. The Lynids will average around 20 meteors per hour.

April Interest:  Stellar Death: 5 -- Gamma-Ray Bursts
(best URL:  http://en.wikipedia.org/wiki/Gamma-ray_burst)

Infrequently, a star that goes supernova also will have very unusual characteristics that result in the emission of a gamma-ray burst at nearly the same time as the supernova itself. The required stellar prerequisites appear to be very rapid rotation; prior ejection of the outer layers of the star; and very low ratio of metals to hydrogen in the star's composition.

These conditions generate extremely intense magnetic and electric fields. This causes the acceleration of charged particles that emit gamma rays. Photons are just like ordinary light, except that the gamma-ray photons are at least 100,000 times more energetic than visible photons. Such photons also are generated, for instance, in energy transitions that occur within a nucleus. Satellites, designed to monitor the Earth's atmosphere for violations of the nuclear test ban treaty created in the late 1960s, instead detected gamma-ray bursts coming from outer space.

Gamma-ray bursts look like randomly placed pulses, with total durations lasting from a fraction of a second to hundreds of seconds. This wide variety of burst appearances, combined with the fairly low angular resolution of the satellites' gamma-ray detectors, kept astronomers guessing for 30 years as to the nature of the bursts. Hypotheses ranged from the burst sources being near our solar system, to somewhere within our Milky Way galaxy, to the limiting distance of the universe -- billions of light years distant. The luminosity of a burst, at such cosmological distances, would be, very briefly, around 1 billion times the luminosity of a galaxy.

The cosmological hypothesis turned out to be true, as revealed when ground-based telescopes and orbiting X-ray telescopes detected long-lived afterglows (lasting hours to months in the optical part of the spectrum) associated with the bursts, and localized their sources to lie within small young galaxies. These host galaxies lived in the universe about 1-3 billion years after the Big Bang, or 10-13 billion years ago.

Approximately one gamma-ray burst per day can be seen from Earth. The bursts that we get to see are "beamed" in our direction, like a narrow-cone search light. For each burst we see, about 500 are beamed in other directions.

Evidence points to two kinds of bursts: Short ones, possibly associated with coalescing binary white dwarfs or neutron stars, are Type Ia supernovae. Long ones, associated with extreme core-collapse, are Type Ic supernovae.

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

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