THE SOUTHWORTH PLANETARIUM
70 Falmouth Street  Portland, Maine 04103
(207) 780-4249   www.usm.maine.edu/southworth-planetarium
43.67° N 70.27° W
Founded January 1970
2022-2023: CVI
Sunrise: 5:01 a.m.
Sunset: 8:27 p.m.
Civil twilight begins: 4:22 a.m.
Civil twilight ends: 9:03 p.m.
Sun's host constellation: Taurus the Bull
Lunar phase: First Quarter 
Moon rise: 1:07 p.m.
Moon set: 1:01 a.m. (6/27/23)
Julian date: 2460122.29
             "You can judge your age by the amount of pain you feel when you come in contact with a new idea."  Pearl S. Buck


THE DAILY ASTRONOMER
Monday, June 26, 2023
Omega Centauri

Today we'll embark on a simple field trip:  a journey to the stunningly beautiful and elegantly named Omega Centauri. 

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Omega Centauri    [Image: European Southern Observatory]

Spanning 150 light years in diameter, this spherical stellar metropolis harbors more than 10 million stars and is as massive as four million Suns.     Despite its estimated 17,090 ight year distance, Omega Centauri shines at apparent magnitude 3.7 and to the unaided eye appears as a somewhat faint star.*  However when viewed through binoculars, its non-stellar nature becomes stunningly obvious.   Spanning 36 arc-minutes -slightly larger than the full moon- Omega Centauri reveals its stellar richness  to anyone who observes it through even low-power optical equipment.           Frustratingly, however, Omega Centauri is just a bit too far south to be visible from northern New England.  The star chart below shows the  night sky from 40 degrees N, where it is just visible above the horizon.  At 43 degrees N, Omega Centauri is tantalizingly close, but remains unseen. 

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The largest and most massive of the Milky Way's 150 known globular clusters, Omega Centauri is so huge that many astronomers assume it was originally the core of a dwarf galaxy which the Milky Way cannibalized long  ago.  This exposed core then joined the retinue of other globulars that orbit around the halo, the  spherical region centered on the galactic nucleus.    The principal feature suggesting this unusual origin pertains to its stellar population.   Omega Centauri contains stars with a wide range of metallicities.     Some are metal-rich and others metal-poor.* 
While metal poor stars tend to be quite old as they formed when the gaseous medium contained smaller ratios of metals, metal rich stars are of more recent vintage.      Most globulars, being ancient structures even by astronomical standards, don't exhibit such a metallic diversity.  Secondly, it seems likely that the Fimbulthul  star stream originated in Omega Centauri.  Astronomers have both measured the velocities and conducted a spectroscopic analysis of some of its stars.    The star's speeds and chemical constituents were consistent with those of the globular cluster.  

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The above image, captured by the GAIA satellite, shows Omega Centauri (enclosed in the white square) close to the Fimbulthul stream, highlighted in bright yellow to the upper left.

 While stars in the Sun's vicinity are separated by 4 - 5 light years on average, the stars within the cluster are about 0.1 light years apart (the density increases as one moves toward the center.)   One can scarcely imagine observing the dazzling night sky from a planet within this cluster!     We don't know if this cluster harbors any planets, but what a sky one would see from the surface of any of them!

Larger than the full moon, crammed with millions of stars, almost 12 billion years old and just below our sights.         We might not be able to admire you from up here, Omega Centauri, but we're delighted you exist all the same.



*It was considered a star until Edmund Halley (1656-1742) first observed it through his telescope in 1677.   Johannes Bayer (1572-1625)  designated it 'Omega Centauri' as he believed it to be the 24th brightest star in the constellation Centaurus.  In the Bayer Nomenclature system, the brightest stars within a constellation are assigned Greek letters in accordance to their relative brightness.  The brightest is named 'alpha,' the second brightest 'beta,' and so forth.     Even though we now know that Omega Cenatauri is not a star, its stellar name persists.   

**If you're a chemist, discretion is advised.    An astronomer considers any element heavier than helium to be a metal.  

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