There are no truly dark skies in Northumberland during July. By the end of the month the twilight is deep enough to see all but the faintest stars. For many of us it is also the start of a new season of observing!
This star chart below shows the sky visible from Northumberland during July 2019.
Ophiuchus and friends
The constellation Ophiuchus (the Serpent Bearer) occupies much of the southern sky. Although its stars are not particularly bright (none are greater than second magnitude) it forms a huge closed figure in the sky. During July this year Jupiter is among the southern reaches of the constellation low above the southern horizon. The Serpent Bearer is bearing, well, a serpent! Serpens is the only constellation in the entire sky comprising of two non-contiguous sections. Serpens Caput (the Head) lies to the west of Ophiuchus whilst the Serpens Cauda (the Tail) lies to the east. There's plenty to see if you have a telescope - lots and lots of globular clusters scattered throughout this part of the sky. The brightest is M 5 (located SW of the Serpent's head). M 10 and M 12 are found within the body of Ophiuchus. Another - M 9 - is not far from Jupiter but perhaps too far south to be seen well from Northumberland.
To the north of Ophiuchus are the brightest stars in the northern sky: Arcturus (high in the west) and Vega (almost overhead) show contrastic colours of orange and white respectively. Between them is a prominent semi-circle of faint stars marking Corona Borealis (Northern Crown) and a rather obscure pattern marking the hero Hercules. Within Hercules is a group of four faint stars (nicknamed The Keystone) which forms the centre of Hercules. The brightest globular cluster in the northern sky (M 13) is easy to find if you can locate the Keystone. A little further to the north is another globular cluster (M 92) which almost rivals M 13 when viewed through a telescope.
Why so many globulars?
There are lots of globular clusters scattered across the summer night sky. Six months from now the evening sky will contain very few. Why the asymmetry? It's all down to the fact that globular clusters are scattered randomly throughout the halo of the Milky Way which is centred on the middle of the Milky Way. If the solar system was located at the centre of the Milky Way then we'd probably see roughly equal numbers of globular clusters in the evening sky no matter what time of the year. But we're not in the middle of the Milky Way! We're located towards the edge and so we see more globulars on one side of the sky than we do on the other. In the early 20th century the astronomer Harlow Shapley was able to figure out how far from the centre of the Milky Way we must be using this asymmetry in globular cluster distribution in the sky. The modern estimate is 26,500 light-years.
Milky Way season begins
The Milky Way runs from roughly north to south this month. This is the beginning of a period in which the brightest stretches of the Milky Way are visible from Northumberland. The Galactic Core is situated in the constellation Sagittarius which unfortunately is always low on the southern horizon at best. However, the broad expanse of Milky Way to the north - running east of Ophuichus and extending north towards Cygnus - are one of the best things about the summer night sky in Northumberland. Look for the Cygnus Rift where the Milky Way branches into two streams just south of Deneb in Cygnus. Binoculars show countless stars and there are dozens of open star clusters and bright nebulae on view. The Eagle Nebula (M 16), Omega Nebula (M 17) and the stunning duo comprising the Lagoon (M 8) and Triffid Nebula (M 20) are well worth tracking down despite their low elevation.
Next month: the Summer Triangle.