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The night sky for September 2016

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during September 2016.

The Stars

To the south and moving westward as night progresses you may see the Summer Triangle: the bright stars Deneb (in Cygnus), Vega (in Lyra) and below them Altair (in Aquila). Towards the south later in the evening you may spot the great square of Pegasus - adjacent to Andromeda and M31, the Andromeda Nebula. To the north lies "w" shaped Cassiopeia and Perseus. Between the two, close to the Milky Way, try to spot the Perseus Double Cluster with a goods pair of binoculars. You might also spot M33 may also be visible on a transparent night with a good pair of binoculars.

The Planets

The Moon

On September 4th and 21st you may spot The Alpine Valley, a cleft across the Appenine mountain chain. It is about 7 miles wide and 79 miles long and a thin rill runs along its length which is quite challenging to observe.

Highlights

Southern Hemisphere

Claire Bretherton from the Space Place at Carter Observatory in New Zealand speaks about the southern hemisphere night sky during September 2016.

Kia Ora and welcome to the September Jodcast from Space Place at Carter Observatory, here in Wellington , New Zealand.

It's hard to believe that it's now a year since the birth of my daughter, Seren (whose name means star in Welsh, by the way) so after taking some time out to spend with her I'm back to pick up the reigns of the southern night sky jodcast. Huge thanks to Haritina for doing such an amazing job in my absence.

September marks the start of spring here in the southern hemisphere, and as we head towards the equinox on the 23 rd , our days begin to lengthen. Sunset and sunrise times change rapidly at this time of the year, with the Sun dipping below the horizon an hour and a half later at the end of the month than at the beginning. By the end of September the Sun won't be setting until nearly half past 7. Whilst we're looking forward to some better weather, this will also mean fewer hours of darkness to get out and observe our Southern skies.

All five naked eye planets continue their planetary dance this month. Jupiter, Mercury and Venus begin the month close together above the Western horizon after dark. Venus is the brightest and the highest of the three, easy to spot, even in the twilight sky. Jupiter, with its golden glow is below with fainter Mercury to the left. Mercury slips quickly into the twilight sky, with Jupiter following by mid-month. After passing between us and the Sun, Mercury will make a morning appearance during late September, but won't be rising until twilight is well underway, making it very difficult to spot.

Venus moves quickly up away from the other two, passing up to the right of 0.9 Magnitude Spica around the 19 th of the month. By the end of the month it will be setting over 2 and a half hours after the Sun.

Mars and Saturn still form a triangle with Antares, high in the northern sky after dark. Saturn is almost directly below Antares, and the two remain close together throughout the month gradually dropping lower in our evening skies and setting by around midnight at the end of September. Mars, the brightest of the three, begins the month just to the right of Saturn and Antares, but holds its position in the sky as it moves eastwards against the background stars, drifting up and to the right away from Saturn as the month wears on.

The bright stars Canopus and Vega mark north-south after dark this month. Canopus is the second brightest star in the night time sky, and the brightest in the southern constellation of Carina, the keel. Carina was once part of the great ship Argo Navis, which sails across the southern skies. In 1752, the French astronomer Nicolas Louis de Lacaille split Argo Navis into three smaller constellations of more manageable size: Carina (the keel), Puppis (the poop deck, or stern), and Vela (the sails).

To Maori Canopus is known as Atutahi or Autahi, meaning "stand alone" because of its isolated position outside the band of the Milky Way. Canopus represents the ariki, or high chief of the heavens, and is circumpolar here in New Zealand, always visible in our night time skies.

Vega, in the constellation of Lyra, is the fifth brightest star in the sky, and at just 25 light years away, one of the brightest in our local neighbourhood. It is also one of the best studied, and was the first star outside our Sun to be photographed in 1850. Vega is also extensively used by astronomers for photometric calibration. It is used as a zero point to define the UBVRI photometric system first introduced in the 1950s, and extended in the 1970s, to classify stars according to their colours.

Along with the nearby bright stars of Deneb, in Cygnus the swan, and Altair, in Aquila the eagle, Vega forms part of the "winter triangle" as seen here in the southern hemisphere. Altair is easy to spot, lying along the band of the Milky Way, midway up the northern sky after dark. Deneb is harder to see, just skirting along the horizon from northern parts of the country.

Between Vega and Altair is Albireo, or Beta Cygni, the beak star, marking the head of the swan. Although it appears as a single star to the naked eye, Albireo is in fact a double star and a lovely sight in a small telescope because of the easily seen contrast in colour between blue and gold components. The two stars are 35 arcseconds apart, meaning they are separated by 60 times the diameter of our Solar System, and may take 100,000 years to orbit each other.

The brighter magnitude 3 yellow star has also been found to be a binary star in its own right, but a much larger telescope and excellent observing conditions, or complex image processing, would be needed to resolve it.

Albireo is best viewed using low magnification, as the colours stand our more clearly when the stars appear close together. You might also try de-focussing your telescope a little to spread out the stars' light, making the colours easier to see.

A similar distance to the other side of Altair is Alpha Capricorni. Whilst it has the alpha designation, it is actually the 3rd brightest star in the zodiac constellation of Capricorn, the goat, and is commonly known as Al Giedi, meaning the kid. Alpha Cap is another double star, but this time the effect is purely coincidence.

Although these stars appear close together, their proximity is just a line of site effect, with the two components positioned at 109 and 690 light years away. Alpha-2, or Secunda Giedi is the closer and brighter of the two, a giant star with a luminosity around 40 times that of the Sun, and an apparent magnitude of 3.58. Alpha-1 or Prima Giedi is a supergiant over 5 times more distant, but at 5 times the mass and over 1000 times the luminosity of the Sun, it is only slightly fainter in our skies, at magnitude 4.3. Both are evolved G class yellow stars, at a similar temperature to our Sun, and Alpha Giediis a multiple star in its own right, with at least 3 faint companions nearby.

Prima and Secunda are located 6.6 arcminutes apart, around one fifth the diameter of the full moon, and can be separated fairly easily even with the naked eye.

As Haritina mentioned last month, the Milky Way is spectacular in our evening skies at this time of year, passing from Vega and Altair towards the north, through Sagittarius and Scorpius overhead, and down to the southern horizon, just to the west of Canopus. Midway up the southern sky you'll find Crux, the Southern Cross, with the pointers Alpha and Beta Centauri above.

Alpha Cen is another multiple star system, and at a distance of 4.37 light years, is the closest star system to our Sun. The two main components Alpha Cen A and B are both similar in mass and luminosity to the Sun and are close enough that they would fit within the orbit of Pluto. They are too close together to be resolved with the naked eye, but a pair of binoculars or small telescope will easily separate them.

The third component, Alpha Cen C, is a small, faint red Dwarf star, also known as Proxima Centauri, which can be glimpsed even in a small telescope. Lying 0.2 parsecs, or 15,000 AU from the AB pair, that's around 500 times the orbit of Neptune, Proxima Centauri's current distance of around 4.25 light years makes it the closest star to our Sun.

On 24 August, a team of astronomers led by Guillem Anglada-Escude, announced that they have discovered a potentially Earth-like planet orbiting around Proxima Centauri. The planet was identified using radial velocity measurements of its parent star. As the planet orbits it causes the star to wobble, and this wobble can be detected by measuring the Doppler shift in the stars spectrum.

Named Proxima b, the newly discovered planet takes just 11.2 days to orbit its star, putting it at 1/20 th of the distance from the Earth to the Sun, but, because Proxima is a much smaller, fainter star, the energy it receives is around 2/3 s the energy that reaches Earth, so Proxima b lies firmly in the habitable zone, the region of space where any water on the planet could be liquid. With a minimum mass of 1.3 times the Earth, this opens up an exciting possibility that not only have we found a planet around our nearest star, but that this planet may be Earth-like, and potentially, able to support life.

Wishing you clear skies from the team here at Space Place at Carter Observatory.

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The Night Sky This Month is one part of the Jodcast. The full show contains the latest news, interviews with astronomers, answers to listener questions and more.

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