The Sky Tonight is a monthly update of the amazing things you can find when looking up from here in Western Australia. This section is written by Jacquie Milner, resident astronomer at the Scitech Planetarium.

The casual observer

The Equinox occurs on the 23rd of September this year. After this day southern hemisphere observers will have more daylight than night time for the next six months. Summer is on the way!

Did you miss out on seeing all five naked-eye planets in one go during July? Well, four of them are still there! Only Mercury is missing. Venus is lowest down in the west, with Jupiter getting closer to it. Saturn is probably the hardest to find as it’s not so bright, so the Moon might help on the night of the 17th, as seen in the diagram below. Then Mars is furthest to the east, slowly fading but still bright as well. They are all lined up along the ecliptic, the path the Sun takes through the sky during the year. But do make an effort during September, as you won’t get this view in October when Venus will dash away out of sight to the morning side of the Sun.

The sky looking to the northwest at 8pm on 17th September from southern Australia.

Dates of interest

1st – Venus next to Spica, the brightest star in Virgo

13th – Moon forms a triangle with Venus (to left) and Jupiter (above), evening sky

14th – Moon to the right of Jupiter, evening sky

17th – Moon next to Saturn, evening sky

20th – Moon under Mars, evening sky

23rd - Equinox

Moon phases

Planets to look for

Make the most of Venus during September, as it will quickly drop out of sight when October comes as it changes from the evening to the morning side of the Sun as we see it! This means it is going to pass us on the inside – or, if you could see it from overhead, it would be passing between the Sun and Earth – what is known as an inferior conjunction. It will all happen rather quickly, and Venus will pop up in the morning sky in a couple of weeks. But in the meantime, we get to enjoy Venus at its brightest.

Jupiter is not far away, the next planet along the ecliptic and also quite bright. The Moon will form a triangle with Venus and Jupiter on the 13th, then move higher up next to Jupiter on the 14th of September. Jupiter is now going forward again, or in “prograde” motion, towards the east, so you should start noticing it move away from the star Zubenelgenubi (alpha Librae), where it has been pausing the last few months.

Saturn has been the hard one to spot this year, drowned out by Venus, Jupiter and even Mars! It turns prograde this month as well, so it won’t appear to move far at all. If you’re not sure where it is near the Teapot asterism of Sagittarius, look for it near the Moon on the 17th.

Mars is still quite bright, near the top of Capricornus, even though it is a month past opposition, but it will start to fade now. It will also shrink in size as seen through a telescope as we here on Earth pull away from the red planet, leaving it behind in it’s slower orbit. Sadly a global dust storm ruined our view this opposition, and all we got to see was a big orange ball. We will just have to wait until the next opposition in September 2020 to try again.

If you are chasing a look at Mercury, this is not a good month for you. The fast moving planet spends most of the month lost in the Sun’s glare. Best to wait until November, when there is an unusually good evening apparition for that time of year.

Constellation of Month

Cygnus the Swan

Cygnus the Swan is a large constellation that stretches along the Milky Way in the northern part of the sky. From Australia we see the Swan flying upwards through Milky Way, his tail pointing down towards the horizon and two broad wings stretching to either side.

The brightest star in Cygnus is the star marking the tail, Deneb (which means ‘the tail', too). This is a massive blue-white supergiant that shines 54,000 times brighter than our sun. Fortunately it lies about 1500 light years away, otherwise our sky would be quite bright at night!

There are some interesting objects to be found within Cygnus, as it covers so much of the Milky Way, but not all are easy to see in a telescope. There are large areas of nebulosity (huge areas of gas and dust). The North America nebula is near Deneb and shows up well in photographs, but not so well in telescopes. New stars are being born here. But another large nebula, popularly known as the Veil Nebula, is the remains of a supernova, a star that exploded. The Veil Nebula covers an area of sky 5 times wider than the moon.

Cygnus also contains a black hole, known as Cygnus X-1. It is one of the strongest sources of x-rays in our sky and was found in 1964, when rockets with Geiger counters were sent up into space to scan the sky as an experiment (we are protected from x-rays from space by our atmosphere). It lies about 6000 light years away, so it is no threat to us.

Objects for the small telescope

Albireo

When we look up into the sky we see the stars as individual points of light. But many of those stars are actually double or multiple star systems – they are so far away that their light combines together to look as one.

Double stars can be fun to look at for their colours. It is not easy for us humans to see colour at night; we are made for bright daylight conditions and often objects in a telescope just appear in shades of grey, but the brighter stars sometimes show their colour to us. Double stars make the colour even easier to see, as you have two or more stars to compare to each other in the eyepiece.

Albireo, or beta Cygni, is an easy double star that marks the beak of Cygnus the Swan. The two components of the star are widely separated, so you only need a low magnification to split them apart. One is typically described as blue and the other yellow. Or you might like to call them aqua and topaz. Or ... what colours do you see?

Feature article

OSIRIS-REx at Bennu

The second half of 2018 is going to be all about asteroids. We’ve already taken a look at the Hayabusa2 mission at asteroid Ryugu, now we have OSIRIS-REx beginning its arrival at asteroid Bennu for another sample return mission.

OSIRIS-REx is an acronym for Origins-Spectral Interpretation-Resource Identification-Security-Regolith Explorer. “Security”? Bennu is what we call a NEO, or Near Earth Object, and there is a chance it could have a very close encounter with the Earth in a few hundred years. So OSIRIS-REx will study not only what the asteroid is made of but will also measure the Yarkovsky Effect that is being applied to it. This is the force applied by sunlight heating the asteroid which can cause it to not only rotate but very slowly change its orbit over time.

OSIRIS-REx is also going to attempt to collect a sample of rock and dust from the surface of the asteroid, put it in a special capsule, and drop it back to the surface of the Earth when the asteroid passes back by our planet in a couple of years’ time. This is a risky operation and there is no guarantee it will work, although the method was thoroughly tested here on Earth before it launched.

To find out more about the mission, and see the sampling arm in action, go to the official website. Check out the “Word of the Week” page as well to learn some commonly used technical terms for space missions.

An artist’s impression of OSIRIS-REx at Bennu. Credit: NASA Goddard Space Flight Center