I haven't used my ZenithStar 61 for a few weeks and I didn't have the energy to bring out the AVX and the FLT 91.  So I settled on my smaller and much lighter rig and just spent time imaging Stars.  There are times I wish I had a reflector, maybe I would be able to get more detail.

Arcturus, Alpha Boötis (α Boo), is a giant star of the spectral type K0 III located in the northern constellation Boötes. It is the brightest star in Boötes and the 4th brightest star in the sky. With an apparent magnitude of -0.05, it is the brightest star in the northern celestial hemisphere, just outshining Vega and Capella. It is only fainter than the three exceptionally bright southern stars, Sirius, Canopus and Alpha Centauri. Arcturus lies at a distance of 36.7 light years from Earth.Arcturus is a giant star of the spectral type K0 III. It is the brightest class K giant in the sky, outshining Aldebaran (mag. 0.86) in the constellation Taurus and Pollux (mag. 1.14) in Gemini. It has an absolute magnitude of about -0.30.With an estimated age of 7.1 billion years, Arcturus is a bit older than the Sun (4.5 billion years). It is also more evolved, having exhausted the supply of hydrogen in its core, moved off the main sequence, and expanded into a giant. It has about the same mass as the Sun (1.08 solar masses), but has grown to 25.4 times its size. It has an estimated surface temperature of 4,286 K and is 170 times more luminous than the Sun.Arcturus does not have any known companions, which means that its mass cannot be measured directly. The estimated value of 1.08 ± 0.06 solar masses is based on evolutionary modelling and observed physical properties. A 2012 analysis of Arcturus and Aldebaran’s carbon and oxygen isotopic ratios yielded a slightly higher value of 1.2 solar masses.The star has also been the subject of asteroseismological measurements, which allow direct calculation of its mass, but the current modelling is not considered accurate. It is however, useful for comparison with other methods. The measurements yielded values of 0.8 ± 0.2 solar masses and 27.9 ± 3.4 solar radii.Arcturus is not massive enough to go out as a supernova. Instead, it will end its life by casting out its outer shell to form a planetary nebula, leaving behind a compact white dwarf. The Sun will share its fate billions of years from now.The estimated age of Arcturus is in the range from 6 to 8.5 billion years, but its evolutionary phase is uncertain. The colour index indicates that it has entered the red giant branch and the hydrogen shell burning stage after exhausting the supply of hydrogen in its core. As it keeps ascending the red giant branch, some of the helium left in its core will be compacted into degenerate matter, which will increase the core’s density and temperature. Once the temperature reaches about 100 million K, it will ignite the helium, starting helium fusion in the core. This will increase the temperature which, in turn, will increase the fusion rate, triggering a runaway reaction that will produce a helium flash, a very brief flash of intense helium fusion lasting only for a few minutes, but having an energy output comparable to that of our entire galaxy.However, a study by Charbonnel et al. published in 1998 suggested that the star has already moved past the helium flash stage and is above the horizontal branch (core helium phase burning stage in metal-poor stars).Arcturus only has a third of the Sun’s metallicity, but it has a higher ratio of alpha elements (oxygen, silicon, neon, etc.) relative to iron and is suspected to be a Population II star, formed from the gas that emerged right after the big bang, during an earlier time of the universe. These stars are very rare in the Sun’s neighbourhood, making up only 4% of all stars in the vicinity of the solar system.Arcturus has a high proper motion – about 2 arcseconds per year – because it is located only 36.7 light years away. The only first magnitude star with a higher proper motion is Alpha Centauri, the nearest star system to Earth at 4.37 light years.The proper motion of Arcturus was discovered by the English astronomer Sir Edmond Halley in 1718. Halley compared his astrometric measurements to those given by the Greco-Roman astronomer Ptolemy in the 2nd century CE and found that Arcturus, Sirius and Procyon had shifted in the sky since ancient times. This realization led to the discovery of proper motion in what were previously believed to be “fixed” stars.Arcturus has a parallax of 88.83 milliarcseconds per year with a margin of error of 0.54 milliarcseconds. This yields a distance margin of error of 0.23 light years.The Hipparcos satellite data suggests that Arcturus may be a binary star with a companion in a tight orbit. The proposed companion is about 20 times fainter than Arcturus and its being in a close orbit makes it exceptionally difficult to make the star out. A study published in 2005 did not confirm its existence, but stated that analysis of the data obtained from near-infrared interferometric measurements suggested the presence of a subgiant companion, supporting the Hipparcos data.Arcturus is one of the most luminous stars in our neighbourhood. In visible light, its energy output is 110 times that of the Sun, but much of the star’s output is in infrared wavelengths because it has a lower surface temperature than the Sun. Its total luminosity amounts to about 180 times solar.Arcturus has a J band magnitude of -2.2. In near-infrared wavelengths, it is fainter only than the red supergiant Betelgeuse (-2.9) in the constellation Orion and the red giant Mira variable star R Doradus (-2.6) in Dorado.

Arcturus has a radius 25.4 times that of the Sun. While it is by no means tiny, it is smaller than many other giant stars, including the fellow orange giant Aldebaran (44.13 R☉) and the red giant Gacrux (84 R☉). It is also larger than a number of stars with the luminosity class III, including the orange giants Pollux (8.8 R☉) and Unukalhai (12 R☉), the yellow giant Capella (11.98 R☉), and the blue-white giant Hadar (13 R☉).

In 1993, Arcturus, Pollux and Aldebaran were the subjects of a study that measured the radial velocities of the three orange giants. A team of scientists at University of Texas at Austin found long-period variations in the relative radial velocities of all three stars, indicating the presence of a substellar companion. The hypothetical companion would have almost 12 Jupiter masses and orbit Arcturus from a distance of 1.1 astronomical units, almost the same as the distance from the Earth to the Sun. However, a companion has not yet been confirmed for Arcturus. The authors of the study concluded that the radial velocity variations (with a period of 233 days) in Arcturus were likely intrinsic to the star (due to pulsations, rotational modulation or chromospheric heating) and not caused by the gravity of an orbiting companion.Arcturus is usually listed as the fourth brightest star in the sky because it is only outshined by Sirius, Canopus and Alpha Centauri. However, Alpha Centauri AB consists of two components – Rigil Kentaurus(Alpha Centauri A) and Toliman (Alpha Centauri B) – both of which are fainter than Arcturus, even though their combined magnitude is -0.27. Alpha Centauri A has an apparent magnitude of +0.01 and Alpha Centauri B, +1.33, while Arcturus shines at magnitude -0.05. This makes Arcturus the third individual brightest star in the sky.Arcturus is one of the 58 bright stars that have been given a special status in the field of celestial navigation. It is the only navigational star in the constellation Boötes.Arcturus was not visible from Earth until about half a million years ago. It has been moving in our direction and has now almost reached its closest point to the solar system, moving at 122 km/s relative to the Sun. It will make the closest approach in approximately 4,000 years, coming a few hundredths of a light-year closer than it is now. It will then start to move away in a southerly direction and, in another half million years years, it will not be visible to the unaided eye at all.Arcturus is a member of the Arcturus moving group, a group of stars that share similar proper motion and are believed to be physically associated. Other members of the group include the red giants Anser (Alpha Vulpeculae) and 27 Cancri, and the orange giant Kappa Gruis.  These are mostly old stars with significantly lower levels of heavy elements, located in the Milky Way’s thick disk. They do not share the same velocities with the stars in the Milky Way plane and are travelling perpendicularly to the Galactic disk. It has been proposed that these stars came from a smaller satellite galaxy that was devoured by the Milky Way a long time ago.Arcturus’ nearest known neighbour is Muphrid, Eta Boötis, a binary star located at almost the same distance from the Sun (37.2 light years), that appears close to Arcturus in the sky. The two stars are separated by only 3.3 light years. An observer on a hypothetical planet orbiting Arcturus would see Muphrid as a magnitude -2.5 star (roughly the brightness of Mercury), while Arcturus would appear as bright as Venus (mag. -4.92 to -2.98) to an observer on a planet orbiting Muphrid.

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