Stargazing in May

Looking south from Northern Ireland after sunset in the middle of May and you will see there is a parade of planets dominating the sky.

Looking south at 10.30pm on 15 May 2014. (Image credit: Colin Johnston/Stellarium)

On 15 May 2014 about 10.30pm, sweep westwards from the rising full Moon in the east to pale yellowish Saturn lying in the constellation of Libra, further to its west, in Virgo is Mars.  Moving further still in the same direction until you are looking due west will be giant Jupiter in Gemini.  Further still and very low in the sky is little Mercury in Taurus. A little above Mercury will be the bright star Capella (Alpha Aurigae). This quartet of arc of planets are arranged along a great arc in the sky called the Ecliptic, this marks the path the Sun seems to move across the sky. All the planets are found within a few degrees of the ecliptic.

Going back eastwards to Saturn and Mars, Saturn is at its brightest and best in May 2014. On 10 May 2014 Saturn is at opposition, meaning Earth was directly between the Sun and Saturn. To the unaided eye it is a bright star, with binoculars steadied on a tripod it will look distinctly elongated, a telescope will reveal the planet with its rings like a tiny three-dimensional jewel. If you can see the rings you surely also be able to pick out some members of Saturn’s huge family of moons, especially mighty Titan. If you have access to a telescope take a look at Mars too and try to spot its elusive surface features, including the dazzling white northern polar cap.

Above the planets Mars and Saturn, and making a nice triangle with them is the star Arcturus. Now to me Mars is the Beige Planet.  I say beige because, sorry, Mars has never looked blood red to me but Arcturus does indeed look a bright-orange red, I wonder how often casual observers of the night sky confuse them.

Arcturus (Alpha Boötis) is the second brightest star visible from Earth’s northern hemisphere. Why is Arcturus so bright? The answer is that it is both big and close. Arcturus, a K class orange giant, is about 25 times as wide as the Sun and is located about 36.7 light-years from Earth. This distance means that any observers on an Arcturian planet (none are known) are seeing the Sun as it was in 1977, any listeners to our radio and TV signals out there must be just wondering if this Star Wars movie is going to catch on or not.

Actually the distance between Arcturus and the Sun is getting smaller. Arcturus is one of the fastest moving stars known whizzing along at 120 km/s with respect to the Sun. . Right now it is almost as close as it will ever get to the Solar System, and it will eventually start to recede. It has given its name to a stellar stream of many stars which share similar characteristics and are moving at roughly the same speed and in the same general direction. Like other stellar streams, the Arcturus Stream may have an exotic origin; it is almost certainly the remains of an ancient dwarf satellite galaxy cannibalised by the Milky Way billions of years ago. It is is strange to think that our familiar night sky contains stars which formed in other galaxies.

Let’s go to near the centre of the Mars-Arcturus-Saturn triangle.  This is where you find Vesta, a large main belt asteroid. This is a fascinating body, essentially a protoplanet that has survived since the formation of the Solar System. It is the only asteroid that can be seen with the unaided eye. However this is very challenging, demanding very dark and clear skies and excellent eyesight. Binoculars will show it though, and in 2014 if you can see Vesta with binoculars or a telescope you will also see Ceres, the largest main belt asteroid and only dwarf planet inside the orbit of Uranus. There is a useful printable chart at Sky & Telescope’s website. Both objects will be unspectacular, appearing as little stars, which is appropriate as “asteroid” means “little star”. Between Vesta and Ceres (and invisible to any telescope) the Dawn spacecraft is buzzing along under the thrust of its three ion thrusters. Dawn orbited Vesta for 14 months before setting course for Ceres where it will arrive in February 2015. Watch out for this, as I think Ceres will turn out to have lots of surprises in stock for us!

Vesta seen from the Dawn spacecraft as it orbited the asteroid (Image credit: NASA)

Going back to Arcturus and following a line down from it takes us to blue-white Spica (Alpha Virginis). Spica is not a single star but rather a very close binary star system. The primary is a B class giant about ten times as massive as the Sun. Stars like this are short-lived: one day it will end its life in a spectacular supernova explosion.  Orbiting the primary is another only slightly smaller B class giant. This stellar duo is about 260 light years away from us.

Taking Arcturus, Spica and a third star together we can make a nice equilateral triangle in the sky. This third star is Denebola (Beta Leonis). It is, like Arcturus, about 36 light years away but the two stars are unrelated. A fast rotating A type star, Denobola’s high speed spin has distorted its shape from spherical to an oblate spheroid, essentially the shape of a fat discus. This may sound freakish but several bright stars such as the brightest star in Leo, Regulus, and Vega are similarly misshapen.  Denebola has no known planets but is surrounded by a disc of dusty debris about 39 AU from the star. There are similar discs around many stars including Vega and Beta Pictoris so presumably such features are common throughout space but how they relate to possible planetary systems is unknown at present.

The spring night sky is not as busy as the skies of other seasons but there are always some lovely and fascinating things to see up there.

(Article by Colin Johnston, Science Education Director)