Pedro Castle, Sunday, 6th July 2008

We will aim to meet 7.30. p.m. on Sunday 6th July at Pedro Castle.
However, it is the rainy season. If the sky is more than 50% cloud covered we cancel. Given the changeableness of the weather, you can appreciate that this can sometimes be a difficult decision to make.
In the west Saturn, Mars, the bright star Regulus and the Moon will all be close to each other in the sky when darkness falls. Jupiter will be rising in the east.
Close to the zenith, Arcturus is the most luminous object in that part of the night sky.
The brightest star in the constellation Boötes, it is the third brightest star in the night sky, with a visual magnitude of −0.05, after Sirius and Canopus. It is the second brightest star visible from northern latitudes and the brightest star in the northern celestial hemisphere.
Researching Arcturus led me to this wonderful piece of astronomical lore, quite new to me.
The Local Interstellar Cloud, casually called the Local Fluff, is the interstellar cloud (roughly 30 light years across) through which our solar system is currently moving. The cloud has a temperature of 6000° C, about the same temperature as the surface of the Sun. It is very thin, with 0.26 atoms per cubic centimeter; approximately one-fifth the density of the galactic interstellar medium and twice that of the gas in the Local Bubble. In comparison, Earth's atmosphere at STP has 2.7 × 1019 molecules per cubic centimeter.
The Sun, with a few other local stars, is embedded in the Local Fluff. Notable nearby planetary systems include those of Alpha Centauri, Vega, Arcturus, and Fomalhaut.
Our Sun may exit the Local Interstellar Cloud during the next 10,000 years. Much remains unknown about the local ISM, including details of its distribution, its origin, and how it affects the Sun and the Earth, although it is thought that the Local Interstellar Cloud's effects on Earth are effectively cancelled by the solar wind and the Sun's magnetic field.
Arcturus is a type K1.5 IIIpe red giant star — the letters "pe" stand for "peculiar emission," which indicates that the spectrum of light given off by the star is unusual and full of emission lines. This is not too uncommon in red giants, but Arcturus has a particularly strong case of the phenomenon. It is at least 110 times more luminous than the Sun, but this underestimates its strength as much of the "light" it gives off is in the infrared; total power output is about 180 times that of the Sun. The lower output in visible light is due to a lower efficacy as the star has a lower surface temperature than the Sun.
Its mass is hard to exactly determine, but may be about the same as the Sun, and is no more than 1.5 solar masses. Arcturus is likely to be considerably older than the Sun, and much like what the Sun will be in its red giant phase.
According to the Hipparcos satellite, Arcturus is 36.7 light years (11.3 parsecs) from Earth, relatively close in astronomical terms. Hipparcos also suggested that Arcturus is a binary star, with the companion about twenty times dimmer than the primary and orbiting close enough to be at the very limits of our current ability to make it out. Recent results remain inconclusive, but do support the marginal Hipparcos detection of a binary companion.

Pedro Castle, Thursday June 5th, 7.30 p.m.

Crux, the Southern Cross, will be visible for the rest of this month. On the night of the 5th, the constellation will be hanging above the southern horizon.
Crux is the smallest constellation in the sky, but one of the most celebrated. The early Portuguese navigators saw it as a symbol of their faith, and the mystery of the unknown lent it an additional charm in the minds of those from whom the southern skies were hidden. There are other cross patterns formed by stars, but the distinguishing feature of the two bright pointers alpha and beta Centauri make Crux unmistakable.
Crux lies in a dense and brilliant part of the Milky Way, which makes the famous dark nebula known as the Coalsack striking in silhouette against the star background.
This is the head of the Emu to the native aborigines of Australia. The rest of the Emu is made from the dark lanes in the Milky Way.
Alpha Crucis was too far south to have been given an ancient name so Acrux is simply a combination of the A in alpha and Crux. Being of first magnitude (0.83), it is the 12th brightest star in the sky and 321 light years distant from us. Under high power, a telescope reveals it to be a binary system with two very similar B type stars having magnitudes of 1.33 and 1.73 separated by 4 arc seconds. With surface temperatures of around 27,000K they are highly luminous, the brightest 25,000 time more luminous than our Sun. In fact, the brighter star is itself a double, the two component stars orbit each other every 76 days but are too close to split with a telescope. So Acrux is a triple star system. A further, type B4, star lies 90 arc seconds away and may belong to the system as well. It has a similar proper motion - that is, its direction of motion through space - but is probably a more distant star that just happens to lie in the same direction.
NGC 4755 (k, kappa Crucis) is an open cluster of stars, that contains about 100 stars and is about 10 million years old. It lies some 7500 light years away and spans a 10 arc minutes field of view so filling a volume of space about 20 light years across. Lying close to Beta Crucis, it is easy to find and best seen with binoculars or a telescope at low power. It contains many highly luminous blue-white stars along with a central red supergiant that makes a beautiful colour contrast. It was named the Jewel box by Sir John Herschel who called it a "casket of variously coloured precious stones". It can easily be seen by the unaided eye as a star, and indeed was originally catalogued as such in pre-telescope times.
Saturn and Mars are visible in a line (called the ecliptic) with the three day old crescent Moon.
The ecliptic is the apparent path that the Sun traces out in the sky during the year. As it appears to move in the sky in relation to the stars, the apparent path aligns with the planets throughout the course of the year. The orbit of the Moon is inclined by about 5° on the ecliptic. The moon crosses the ecliptic about twice per month. If this happens during new moon a solar eclipse occurs, during full moon a lunar eclipse. This was the way the ancients could trace the ecliptic along the sky; they marked the places where eclipses could occur and the name ecliptic is derived from being the place where eclipses occur.

Pedro Castle,Thursday, 8th May

The familiar constellation of the Great Bear, Ursa Major, is visible in the north, although it may appear “upside down” to anyone from the higher latitudes.
The seven brightest stars, located in the Bear's hindquarters and tail, form the well-known Big Dipper asterism. (In Britain, this asterism is known as the Plough.) The stars Merak (β Ursae Majoris) and Dubhe are known as the "pointer stars" because they are helpful for finding Polaris, also known as the North Star. By visually tracing a line from Merak through Dubhe and continuing, one's eye will land on Polaris, accurately indicating true north.
Mizar, a star in the Big Dipper, forms the famous optical double star with Alcor.
Several bright galaxies are found in Ursa Major, including the pair Messier 81 (one of the brightest galaxies in the sky) and Messier 82 above the bear's head, and Pinwheel Galaxy (M101), a beautiful spiral northwest of η Ursae Majoris.
Polaris, sometimes known as the Lodestar, is the brightest star in the constellation Ursa Minor. It is very close to the north celestial pole (42′ away as of 2006), making it the current northern pole star.
Polaris has the common reputation of being the brightest star in the sky, whereas near dead-on second magnitude (2.02) it comes in at number 48. Its lower rank, however, is largely determined by its great distance of 430 light years. The star is actually an evolved class F (F7) yellow supergiant 2500 times more luminous than our Sun with a temperature of about 6000 Kelvin, which leads to a radius 45 times that of the Sun and a mass of six times solar. Hydrogen fusion has stopped in the star's core, and it is now passing through a phase of instability wherein it pulsates over a period of about four days, almost invisibly changing its brightness as the brightest "Cepheid" variable star in the sky.
Polaris has the distinction of having a pair of companions, one near, the other far. About 18 seconds of arc away lies an eighth magnitude F3 1.4 (or so) solar mass dwarf, which at the distance of Polaris must be at least 2400 Astronomical Units away and take at least 42,000 years to orbit. Much closer is an F7 dwarf (noted from spectroscopy and resolved by the Hubble Space Telescope, giving us three F stars in one pot), that at a measured average distance of only 17 AU takes but 29.6 years to go around, a high eccentricity running it between 27 and 6.7 AU.

Pedro Castle,Tuesday, 8th April

Bright, ruddy Mars will be overhead in the middle of the constellation Gemini, approximately by Castor’s hand.
Castor is a visual double star made up of two blue-white stars, A and B, of 1.9 and 2.9 magnitudes respectively. The pair orbit each other every 400 years and are now as close as they ever get, making them somewhat of a challenge to split and requiring good seeing. In fact their spectra show that each is itself a double star. Castor A is made up of two identical 2 solar mass stars orbiting each other every 9.2 days while the stars that make up Castor B orbit even faster, every 2.9 days.
Pollux is one of few stars visible to the naked eye which features an extrasolar planet.

Saturn will be in the east as darkness falls, very close to the star Regulus, the heart of the Lion, Leo.
Regulus is the brightest star in the constellation Leo and one of the brightest stars in the nighttime sky. Regulus is approximately 77.5 light years from Earth.
Regulus has about 3.5 times the Sun’s mass and is a young star of only a few hundred million years. It is spinning extremely rapidly, with a rotation period of only 15.9 hours, which causes it to have a highly oblate shape. This results in so-called gravity darkening: the photosphere at Regulus' poles is considerably hotter, and five times brighter per unit surface area, than its equatorial region. If it were rotating only 16% faster, centripetal force would overcome gravity and the star would tear itself apart.
Regulus is a multiple star system composed of a hot, bright, bluish-white star with a pair of small, faint companions. The pair orbits the much-larger Regulus A with a period of over 130,000 years at a distance of some 4,200 AU. The companion pair have an orbital period of 2,000 years and are separated by about 100 AU.

Membership Form (print form below)

Pedro Castle, Monday March 10th, 7.00 p.m.

High overhead at viewing time lies the constellation Auriga.
The Charioteer (possibly Erechthonius, son of Vulcan) is among the brightest northern constellations, it lies midway between Perseus and Ursa Major in a region crossed by the Milky Way, and is the site of the galactic anticenter (the point in the sky diametrically opposite the center of the Galaxy). Gamma Aur, or El Nath, is shared with Taurus and is universally referred nowadays as Beta Tau.
Its brightest star is Capella, which is associated with the mythological she-goat Amalthea. The three stars ε Aurigae, ζ Aurigae and η Aurigae are called Haedi (the Kids). Aurigae and Zeta Aurigae are remarkable eclipsing binaries.
ε Aurigae and ζ Aurigae are peculiar binary stars. The orbital period of ε Aurigae is approximately 27 years, with an eclipse duration of about 18 months. The visible companion is a yellowish F-class supergiant. The type of the other star is not known. ζ Aurigae has a period of 970 days, the primary is a K-class supergiant and the secondary is a B-class main sequence star. Both these systems present a rare stage of binary evolution, as the components are in a short and active evolutionary stage.
Auriga has many open clusters and other objects because the Milky Way runs through it. The three brightest open clusters are M36, M37 and M38, all of which are visible in binoculars or a small telescope in suburban skies. A larger telescope resolves individual stars. The clusters are about 4100, 4400, and 4200 light years distant, respectively. Their apparent visual magnitudes are 6.3, 6.2, and 7.4, respectively.
Capella (α Aur / α Aurigae / Alpha Aurigae) is the brightest star in the constellation, and eleventh brightest star in the sky. Although it appears as a single point to the naked eye, Capella is actually a bright close binary pair of yellow giant stars along side a second, fainter binary.
A yellow star, it traditionally marks the left shoulder of the constellation's eponymous charioteer, or sometimes the goat that the charioteer is carrying. It is closer to the north celestial pole than any other first magnitude star (Polaris is only second magnitude) and as a result has played a significant role in many mythological writings. A tablet dating back to 2000 BC refers to Capella.
Astronomically, Capella's interest lies in the fact that it is an easily-studied non-eclipsing spectroscopic binary star. These two giant G-class stars have luminosities of around 50 and 80 times that of the Sun and lie less than 100 million km apart with an orbital period of 104.02 days. The stars were probably of spectral class A during their main sequence, similar to Sirius, and are in the process of becoming red giants in a few million more years as they continue to expand, cool, and brighten. Capella is a source of X-rays, probably due to surface magnetic activity on one of the pair. The Capella binary was the first star system to be imaged using a long baseline optical astronomical interferometer in observations by COAST in 1995.
The central stars also have a faint companion that is itself a double star, consisting of two M-class red dwarf stars, that orbit at around a light year away from the main pair.
In Australian Aboriginal mythology, Capella was Purra, the kangaroo, pursued and killed by the nearby Gemini twins or by the Orion hunter, or near the Orion belt twins canoe.
To the Bedouin people of the Negev and Sinai, Capella is known as al-'Ayūg, 'the dandy', in its role as pointing out the position of the Pleiades -its name more fully al-'Ayūg ath-Thurayyā 'dandy of the Pleiades'.

On the night,Mars will be high overhead, with Saturn rising in the east, close to the brightest star in Leo, Regulus.
clear skies
Nick

Pedro Castle, Monday February 11th

As darkness falls on Monday the bright star Aldebaran can be seen directly overhead.
Aldebaran is the brightest star in the constellation Taurus and one of the brightest stars in the nighttime sky. It has the Bayer designation Alpha Tauri. Because of its location in the head of Taurus, it has historically been called the Bull's Eye. Aldebaran has the appearance of being the brightest member of the more scattered Hyades cluster, which is the closest star cluster to Earth. However, it is merely located in the line of sight between the Earth and the Hyades, and is actually an independent star.
In 1997, a possible substellar companion was reported, with a mass at least 11 times that of Jupiter with an orbital period of around 2 years, however this has not been confirmed.
Aldebaran is a K5 III star, which means it is orangish, large, and has moved off of the main sequence after exhausting the hydrogen fuel in its core. It has a minor companion (a dim M2 dwarf orbiting at several hundred AU). Now primarily fusing helium, the main star has expanded to a diameter of approximately 5.3 × 107 km, or about 38 times the diameter of the Sun.
The Hipparcos satellite has measured it as 65.1 light years away, and it shines with 150 times the Sun's luminosity. With an apparent magnitude of 0.87, it is the 14th brightest star.
The name Aldebaran comes from the Arabic (الدبران al-dabarān) meaning "the follower" and refers to the way the star follows the Pleiades star cluster in its nightly journey across the sky.
In Persia it was known as Satvis and Kugard.[4]
It is known as 畢宿五 (Bìxiùwŭ, the Fifth Star of the Net) in Chinese.
Aldebaran is identified as the lunar mansion Rohini in Hindu astronomy and as one of the twenty-seven daughters of Daksha and the wife of the god Chandra.
The Romans called it Palilicium.
For the Dakotas (a branch of the Native American Sioux tribe), Aldebaran took on a heroic aspect. The young star was the child of the sun and the lady Blue Star. One day, he desired to hunt the white buffalo (the Pleiades). After he pulled up a sapling to make a spear, a hole was made in the ground and he could see all the people of Earth down below. The white buffalo took this chance to push him through. He was found by an old woman and was to be known as Old Woman's Grandson. On Earth, he killed many strange monsters that had been troubling the Native Americans; one monster of which was a serpent that caused drought. The young hero killed the serpent, releasing a great stream of water that became the Mississippi River. In time, Old Woman's Grandson remembered the white buffalo and returned to hunting him in the sky to fulfill his destiny.
For the Seris of northwestern Mexico, this star is providing light for the seven women giving birth (Pleiades).
Noticeably redder, and brighter, is the nearby planet Mars.
Mars is close to the β (beta) star of Auriga on the illustration above.
Called Menkalinan, it is a double star consisting of two blue white giant A2 class stars. They have magnitudes of 1.9 and 2.7 and are 90 light years distant.