An hour or so after sunset in June. Twilight is draining from the sky. The first star to appear is Arcturus - a brilliant orange star high in the south. As the sky darkens further - look to the left and a little higher in the sky. You should make out a small semi-circle of stars. This is Corona Borealis - the Northern Crown.
The brightest star in the Northern Crown is the second magnitude Alphecca. To the eye it appears to be a single star. In reality Alphecca is a binary system consisting of two main sequence stars. One is about twice the mass of the Sun - and correspondingly hotter and more luminous. The other component a little less than the mass of the Sun. The two stars eclipse each other as they orbit their centre of mass. Alphecca is an eclipsing binary (like the more famous Algol in Perseus) with eclipses occurring every 17 days. However, the variation in brightness is only a few tenths of a magnitude - too little to discern with the naked eye. Alphecca is about 75 light-years away.
The Northern Crown is lacking in bright deep sky objects like star clusters or galaxies but it makes up for it with a couple of intriguing variable stars.
The Northern Crown is well known to astronomers for a couple of its odd stars!
The first is known as T Corona Borealis, (T CrB for short) or more informally, The Blaze Star. It is normally invisible to the unaided eye but it erupted dramatically in 1866 and 1946, becoming the brightest star in the constellation. T CrB is a nova and one of the few recurrent novae in the Milky Way that we know of. Novae consist of white dwarf (a compact, inert stellar remnant) which is accreting gaseous material on its surface from an evolved companion star (in this case, a red giant). A critical point is aperiodically reached and the accreted material ignites in a thermonuclear runaway and material is ejected at high speed from the surface of the white dwarf. During previous eruptions the nova brightened to magnitude +2: rivalling Alphecca, the brightest of the stars the Northern Crown. Eventually the nuclear runaway shuts down and the nova returns to its usual dim self and the process repeats. In recent years there have been signs that T CrB is brightening as it did in the run up to the 1946 eruption. We may not have to wait long to see this star blaze again.
The second odd star is called R Corona Borealis (R CrB, nicknamed the "reverse nova" for reasons we'll get into). Binoculars or a telescope are needed to see the star because it usually hovers on the border of naked eye visibility. At irregular intervals of months or years this star dramatically fades become perhaps 4000 times dimmer – requiring a very big telescope to see. This behaviour is almost the opposite of how the Blaze Star behaves. What's going on here?
R CrB is a yellow giant star which has ended its hydrogen burning and evolved away from the main sequence. The core is now burning helium to make carbon, oxygen and nitrogen. Convection dredges up these nuclear products from the core and transports them to the cooler outer layers of the star. The carbon is thought to condense out at suitably low temperatures and densities to form a dark soot. This builds up in the outer atmosphere preventing visible light getting out and causing it the star to fade. As time passes the radiation can't escape the interior and the pressure rises. Eventually the carbon is expelled from the star causing it to return, albeit temporarily, to normal brightness. Despite the nickname there is nothing nova-like about this mechanism. Also, this change in brightness is only seen at visible wavelengths; the star remains at fairly constant brightness in the infrared.
The constellation chart at the top shows the locations of T CrB and R CrB. More accurate finder charts are shown below courtesy of the AAVSO.
You can hardly call the period between sunset and sunrise "night" in Northumberland. Between early May and early August it never gets completely dark. But still. There are things to see despite the encroaching twilight. Here is the sky for this month.
Venus and Jupiter are the brightest planets in the sky at the moment. Venus is over in the northwest just after sunset and is visible for a couple of hours before it sets. Jupiter is in the southern sky in the constellation Libra. It too is visible as soon as the sky darkens after sunset. Saturn can be found very low in the southeast after midnight. Telescopes will show the rings easily enough but at such low altitude it will be difficult to make out detail (such as the Cassini ring division) or markings on the disk. Titan and Rhea - the largest and brightest moons - should be visible without a problem. The planet Mars rises a couple of hours after Saturn. Although very low in the sky - it is bright and getting brighter. Mars will be at opposition at the end of July and at its closest to Earth since 2003.
Noctilucent clouds (NLCs) are the highest clouds in our atmosphere and they appear during the summer months around poles. Northern NLC season has started already; the AIM satellite has detected the first clouds of the summer during late May and ground sightings are now coming in. You can read more about NLCs in a blog article at NASTRO I wrote a couple of years ago.
During the month I'll focus on some of the stars and constellations visible during these short summer evenings.
Those night-shining summer clouds are back again!
I generally don't care for clouds but I make an exception for these! Noctilucent clouds are seeded by meteor smoke and are the highest clouds in the atmosphere. They form about 80 km (50 miles) above the ground and only appear around the poles during the summer months. Northumberland strikes a nice balance between being far enough north but not so far north that it doesn't get dark enough to see them.
Noctilucent clouds appear between June and August every year but the frequency is related the solar cycle. We expect to see more during solar minimum years when the upper atmosphere is colder. Hopefully there will be many more opportunities over the next month or so to see them.
Go outside a couple of hours after sunset and look for these delicate blue-white clouds over the northern horizon.
Another clear night was forecast so I was set with my targets: a couple of hours each on M104 (the Sombrero Galaxy) and M64 (the Black Eye Galaxy). In the end, there were clear spells and lots of cloud. Rather than a couple of hours I got half an hour on each object!
These were taken with my usual setup: Nikon D90 at prime focus of the 8 inch (200mm) Meade LX10. ISO800 with 4 minute subs. Images stacked in Deepsky Stacker and tweaked in Photoshop.
Both galaxies have prominent dust lanes.
Before packing up I noticed Cygnus was pretty high in the eastern sky so I grabbed some subs of Albireo, the famous double star.
Albireo is a fantastic sight through any telescope: a bright golden primary with a fainter bluish secondary separated by around 35 arcseconds. The above picture was taken on the same setup as the earlier galaxy pictures. I decided to expose for a rich starfield which meant overexposing Albireo. The colour still present well so I'm happy with the image!
I was mostly facing south while taking my pictures. At around 1am I glanced towards the north and saw faint rays of light reaching 20 or 30 degrees above the horizon: an unexpected aurora! I went for my other camera and tripod but the activity had died down substantially. I took a 30 second exposure at ISO1600 and this is what I got:
The colours were not particularly visible to the eye; the overall impression was that the sky was bright in the north. Got to bed (after image processing!) at around 4am.
Last night provided a brief window of opportunity between the end of (nautical) twilight and moonrise to get some images. It was going to have to be bright target because of the time limit. In the end I chose the globular cluster M5 in the constellation Serpens.
M5 is one of the brighter globular clusters in the sky. The apparent magnitude is given as +5.7 which makes it a naked eye object from exceptionally dark skies. The angular size is around 20 arcminutes - around 2/3 of the apparent diameter of the full moon. I suspect it's often overlooked by my astronomical friends in favour of M13 (the Hercules Globular) which is a shade brighter, larger and higher in the sky.
M5 is thought to be around 24,000 light-years away. The number of stars in the cluster runs into the low hundreds of thousands. It is physically one of the larger Milky Way globular clusters with an estimated diameter of around 150 light-years.
It doesn't get dark enough to take pictures like this now until after 11pm. When will the next clear sky come? I've already chosen my next target!
This is a whole sky chart for the May night sky. The seven stars of The Plough (part of Ursa Major) are overhead. Follow the tail stars (Alioth, Mizar, Alkaid) and arc down to Arcturus, the leading star of Bootes (the Herdsman) and the brightest star above the horizon at this time. Arcturus is an evolved orange giant star and a bit of an interloper in the neighbourhood. It is around 37 light-years away but plunging through the disk of the Milky Way along a trajectory which brought it in from the halo surrounding our Galaxy.
Keep following the curve through Arcturus and you'll arrive at Spica, a bright white star nearly half way up the sky above the southern horizon. Spica is the brightest star in Virgo, the Maiden. Spica appears as a single star to the eye and even through large telescopes. However, spectroscopic analysis of the light from Spica reveals it to consist of a pair of stars - each slightly heavier than the Sun - in close orbit around each other with a period of 4 days. The average distance between the pair is just 2 million km!
The star Porrima is also a notable binary star. It consists of two stars slightly heavier and more luminous than our Sun orbiting each other with a period of 168 years. They were at their closest together in 2011 at which point only large observatory telescopes could resolve the two stars. In 2018 I’ve been able to split the two stars with my modest 8-inch telescope. They will continue to separate for the rest of my life!
Beyond the stars of Virgo and beyond the Milky Way is the Virgo Cluster of galaxies. You can see the brightest members with their Messier designations on the chart above. The centre of the Virgo Cluster lies around 50 million light-years away. There are about 1500 – 2000 galaxies in the cluster. The Virgo Cluster is heart of a much larger structure – the Virgo Super Cluster and our Milky Way is an outlying member of that! The Milky Way, the Andromeda Galaxy and a few dozen others in the neighbourhood are being drawn across the universe by the invisible gravitational pull of these distant Spring-time galaxies.
Over in the west the last of the traditional winter constellations are sinking below the horizon. Gemini, with the twin bright stars Castor and Pollux, can be seen for a short period after dusk. Venus is traversing Gemini this month. Cancer (the Crab) and Leo (the Lion) are a little higher. The Beehive cluster (M 44) in Cancer and the Leo Triplet of galaxies (whose brightest members are M 65 and M 66) can still be found with binoculars and telescopes respectively.
In the east the Milky Way and summer constellations are beginning to rise. Vega, Deneb and Altair - bright stars which form an asterism called The Summer Triangle - mark the most prominent section of the Milky Way. With each successive evening the Milky Way climbs higher in the sky a little earlier. Unfortunately, a period of all night twilight is beginning for the northern UK and the nights are getting shorter and lighter. Better opportunities to see the Milky Way will come in August and September.
Venus is visible for a couple of hours after sunset where it blazes brilliantly above the north west horizon. Keen sighted observers can actually spot it before the sun goes down! The planet is still on the far side of its orbit and telescopes reveal a very small disk (around 12-13 arcseconds) with a distinctly gibbous phase. Don't expect to see to any detail on Venus! The atmosphere is highly reflective and contrast can usually only be seen with UV filters.
The planet Jupiter comes to opposition on May 9th and is visible from sunset until sunrise. The best time to view Jupiter is when it is highest in the sky - around local midnight (1am BST). At opposition a planet is at its nearest to Earth for the year and appears larger through a telescope than at other times of the year. Jupiter is about 658 million km away (4.4 times the Earth-Sun distance) and the Jovian disk measures 45 arcseconds across its equator. Binoculars and small telescopes easily reveal the four largest moons (Io, Europa, Ganymede and Callisto) as they change position from night to night. Larger telescopes show the equatorial bands (looking rather like a pair of stripes parallel to the equator) which are clouds whipped by fast moving jet streams in Jupiter's atmosphere. Unfortunately, Jupiter is in the southern constellation Libra and is never more than about 20 degrees above the horizon (from Northumberland). At low elevation we view the planet through much more of the atmosphere and we need a very steady atmosphere indeed to see more detail in the clouds. The situation will begin to improve in around 5-6 years when Jupiter begins to head back into northern skies...
Two clear nights in a row. Must be summer.
After the experiences of the previous night (90% fixing technical problems, 10% taking a picture) I had high hopes when I saw the forecast was good again.
After sunset I was setup with the telescope, autoguider and camera within 20 minutes. Everything was tracking perfectly. The majority of the session was spent getting images of M51 - the Whirlpool Galaxy in the constellation Canes Venatici. This is the result:
M51 is a face-on spriral galaxy about 23 million light-years away. Historically, M51 is significant for being one of the "spiral nebulae" which divided the opinions of astronomers in the 19th and early 20th centuries. Was it a local (to the Milky Way) cloud of gas and dust? Or a distant version of the Milky Way? We now know it is the latter. The Whirlpool is smaller than the Milky Way - about 35% of the MW diameter. The companion galaxy on the left is NGC 5195 (also known as M51b) and is interacting with M51 in a gravitational dance being played out across the aeons.
First clear night for some considerable time last night! As we get into late April, the days are getting much longer and nights shorter and lighter. Despite the early start at work in the morning I was determined to get out and do some astronomy. My plan was to get some decent pictures of the Whirlpool Galaxy. That meant getting long exposures (ideally a few minutes each, or more) which meant setting the autoguider up with the laptop.
The autoguider is a CCD camera attached to the finderscope and the telescope mount which corrects tracking errors in realtime. I hadn't set this system up in over a year; I've mostly been imaging with a small, widefield telescope so haven't needed really precise guiding until last night. I'd hoped to get around 3 hours of imaging done on the Whirlpool. What ended up happening was two hours trying to get the CCD camera talking to the laptop (a new one...with various incompatibilities and the discovery that my CCD camera is not behaving well under Windows 10!). I then managed about half and hour of great tracking...but chose the globular cluster M13 because I really needed to get some sleep by that point.
So here it is; not a disastrous end to a frustrating night. The great Hercules Globular Cluster (M13).
Technical details: Nikon D90, ISO1600 at prime focus of the Mead LX10 (200mm / 8 inch). 3x3 minutes + 1x5 minute stacked exposures.
About an hour after sunset the crescent moon and Venus were putting on a nice show over the western horizon:
The moon is 4% illuminated and is more or less between the Earth and Sun. By contrast Venus is almost on the far side of the Sun; the telescopic view shows Venus to have a gibbous (almost full) phase at the moment. Although Venus and the moon looked close together tonight - that was just a line of sight effect! The moon was just under 372 thousand km away when the picture was taken. Venus was 226 million km (more than 600 times further than the moon). Space is big :-)
This morning was the first period clear after what seems like endless days of rain, sleet and snow. I planned to get some pictures of Mars and Saturn so had to get up at 4.30am. I was set up with the telescope and camera in the garden by 5.
Here's my picture of Mars, Saturn and the globular cluster M 22 through the telescope.
Mars (lower left) and Saturn (upper middle) are the brightest objects in the picture. The little dot to the right of Saturn is actually Titan, the biggest moon. The globular cluster M22 is at lower right in the picture. M 22 is a ball of stars (about 70,000 in all) about 35,000 light-years away.
The picture was a stack of 7 x 60 second images taken with the Nikon D90, ISO800, at prime focus of the 80mm refractor. This field was only 9 degrees above the horizon so I'm pretty happy with the detail I captured.
Here's a wide field shot of the sky taken with the camera mounted on a tripod. Mars and Saturn are about 2 degrees apart and among the stars of Sagittarius.
Here's a Stellarium rendition of that part of the sky.
This is one of the best times of the year for me to capture images of Sagittarius and the Galactic Centre. When the moon gets out of the way in about a week I'm going to try to get some pictures from this region again.
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Dr Adrian Jannetta. Amateur astronomer, maths teacher and science enthusiast.