SKY REPORTER: On to Arcturus

Chart showing Arcturus, Bootes, the Herdsman (with club), and neighboring constellations from a 1776 French version of John Flamsteed’s 1729 Celestial Atlas, the first such work based on telescopic positional measurements of stars.
Credit: Linda Hall Library of Science, Engineering & Technology

Halfway between the tip of the Big Dipper’s handle and the present location of Mars we see the very bright star Arcturus shining as vividly as the Red Planet. With its slightly yellow color Arcturus is the brightest nighttime star in the northern half of the sky and the fourth brightest overall. Due to the intensity of its glow and other noteworthy aspects this alpha star of the constellation Bootes invites our attention.

Arcturus is well placed—high in the southwestern sky, about two-thirds up from horizon to zenith at 10 p.m. by mid-month, and nine p.m. when twilight fades June 30^th. It’s a giant star about 25 times the size of the Sun, located 37 light years from the Solar System. During June evenings the brightness and fine placement of Arcturus beckon.

Mobile contrariness is another reason we take note of Arcturus. Along with a number of fainter, apparently related stars, Arcturus lags behind in pathways taken by most of its stellar neighbors orbiting the Galaxy’s center. Traveling faster than any other bright star in our sky, over the course of a century Arcturus spans an angular distance toward the south-southeast equal to one tenth the lunar disk’s diameter. 

Suggestions have been made that Arcturus might be a relic split from a globular star cluster. Such entities contain hundreds of thousands of stars orbiting the Milky Way’s center, these objects are usually located in the Galaxy’s spherical Halo and rarely intersect the Galactic disk. Arcturus’ relatively low iron content was said to indicate similarity with globular cluster stars.

An alternate hypothesis described Arcturus and its cohorts as disengaged remnants of an ensnared dwarf galaxy shredded and “eaten” by the Milky Way billions of years ago. However recent studies indicate ranges of chemical elements seen in the Arcturus moving group are very different from properties typical of stars in dwarf galaxies. This consideration, along with estimated ages of Arcturus and its stellar companions relate more to properties generally found in stars of the Milky Way’s disk.

The February 2014 issue of “Astronomy & Astrophysics” reports research of Thomas Bensby and Sofia Feltzing of Lund University, and Sally Oey of the University of Michigan and their conclusions that studies published to date provide no clear evidence Arcturus and its fellow travelers are former members of either a captured dwarf galaxy or a disrupted globular cluster. These astronomers conclude that anomalous paths traveled by Arcturus and its associated stars result from disruptive  encounters experienced during traverses within the Milky Way’s disk.

The Moon

Planets

The earliest sunrise of the year occurs June 14^th at 5:24 a.m.

Summer begins 6:51 a.m. Eastern Daylight Time on the morning of Saturday June 21. Then the Sun is at its furthest northerly position, directly above a point in the Sahara Desert of southern Libya.

If you’re up early on the morning of June 24 look toward the east-northeastern horizon after 3:45 to see a beautiful conjunction of the waning Moon and brilliant Venus, positioned three degrees to the left of the thin lunar crescent. If the sky is very clear before twilight intervenes around 4:10 a.m., you may also spot the lovely Pleiades star cluster six degrees to the upper left of Venus. 

Latest sunset of 2014 is on the evening of June 27 at 8:31 pm.

Looking toward the west-northwestern horizon about nine p.m. Sunday June 29^th we might catch a glimpse of Jupiter, barely a degree of arc above the horizon and 15 degrees to the right of the thin waxing crescent Moon.