Northern Rockies Skies for February

January 27, 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.

The next two months will be a great time to study and admire several spectacular planets: Venus, Jupiter, Mars, Saturn and the fleeting Mercury.

First the obvious: Right after sunset around 6:30 to 7 p.m. in the constellation Pisces you will see Venus and Jupiter, the brightest two objects in the night sky. Then two hours later Mars appears on the eastern horizon and around 11 p.m. Saturn starts it prominent march across the sky, visible for the remainder of the morning.

An added gift this February and March is an opportunity to see the fleeting and fast-moving planet Mercury. From Feb. 20 until mid March, Mercury will be bright enough and far enough away from the sun to see it immediately after sunset, even with the naked eye. But take your binoculars or small telescopes out to enjoy those amazing planets that have graced our solar system with spectacular views over millennia.

Oh, by the way, don't forget our winter constellations: Orion, the Hunter; Canis Major, the Big Dog; the angry Taurus the Bull; and the lovely whitish-blue stars of the Pleiades, often called the Seven Sisters. What a wonderful way to bring in the coming of spring next month. Enjoy.

February 2012 Interest: Stellar Death III - Supernovae Ia
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When a star goes supernova, and is briefly as luminous as an entire galaxy, this event marks "The End" -- the irreversible, catastrophic destruction of the stellar object.

Compared to the star's prior lifetime of millions to billions of years, the start-to-finish time for it to come apart in the ensuing explosion is very short, less than a minute. Left in its place is an expanding nebula -- the supernova remnant -- and possibly a compact object, black hole or neutron star, or nothing at the site of the explosion. The classic example of such a remnant is the Crab Nebula, the supernova of 1054 A.D.

Supernovae are divided into two broad categories, types I and II. The distinguishing characteristic is spectral: Type II's show evidence of hydrogen in their atmospheres, type I's do not.

Type Ia's occur by coalescence of two white dwarfs or by accretion of matter from a companion star in a binary orbit with a white dwarf. At the point of "going supernova," the mass of the object is sufficient to precipitate detonation of the star's carbon core -- most of the star then undergoes rapid fusion, thereby releasing enough energy to disrupt the star completely, leaving no compact object behind.

The dominant energy release during the subsequent expansion is due to radioactive decay of nickel-56 (half-life six days) into cobalt-56 (half-life 77 days), and then to iron. Other heavier elements are also formed by fusion during the explosion and ejected into interstellar space -- material which is then available for the next generation of stars.

The primary importance of type Ia supernovae as a measurement tool is that the radiant energy liberated during the explosion and the time profile of this energy release are fairly well calibrated. Type Ia's are thus "standard candles" whose distances can be inferred by observing the explosion's luminosity for roughly a month and comparing with templates.

Observations of type Ia's are among the primary methods used to measure the expansion of the universe. Since the late 1990s, measurement accuracy has been good enough to define a speeding up of the expansion during the last few billion years.

The inferred agent of this acceleration is referred to as "dark energy," essentially a pressure of unknown nature and origin, not associated with luminous or dark matter.

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

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