Sky Guide / June and July
The weeks around the northern summer solstice (June 21st) are the shortest and lightest of the year. In Northumberland we don't see astronomical darkness at all during this period!
Stars and constellations
Sunset doesn't happen until very late evening at this time of year and the stars only begin to appear after 10pm, or later in the weeks around the June solstice.
The first stars to appear are Arcturus (in Bootes) and Vega (in Lyra). You should contrast the colours of those two stars. Arcturus, high in the western sky, is the brightest star in the northern hemisphere of the sky. It is an old, orange giant star. On the other hand Vega is a young, main sequence star and it shines with a pure white light. More about both of these stars below.
Take a look towards the southern horizon. With a clear view you might make out Antares, the leading star of Scorpius, hovering just above the horizon. Antares is a first magnitude star and is easy to recognise as it's flanked by two fainter stars. The low altitude of Antares means its light is dimmed and may not be as easy on hazy evenings. Antares is a red supergiant and one of the largest stars known. If placed in the centre of our solar system the outer layers would lie beyond the orbit of Mars.
Antares belongs to the constellation Scorpius (the Scorpion) and from more southerly latitudes it is a prominent zodiac constellation with many deep sky objects. The densest starfields of the Milky Way are towards this direction and the picture above shows something of how rich this region is; with interstellar dust, stars and more distance globular clusters.
Notable stars and deep sky objects
Remember - summer twilight is going to make it tough to see all but the brightest of the star clusters, nebulae and galaxies! Objects in the southern part of the sky will be less affected. Here is my pick of the best objects during this period.
α Lyrae / Vega
Vega is one of the brightest stars in the northern hemisphere of the sky and is almost overhead on summer evenings in the UK. It is a notable star in many respects!
To astronomers Vega is important because it calibrates one of the standard systems of measuring magnitudes (brightness) of other stars in the sky. It is the zero point of the scale (similar to the role 0 degrees Celsius plays in the temperature scale!)
During the 1980s observations from the IRAS satellite picked up an excess of infrared radiation coming from Vega. This was attributed to a disk of material surrounding the star. This is disk of material may resemble the Kuiper Belt of our own solar system. To date no planets have been found orbiting Vega.
Vega is occasionally the northern Pole Star. The precession of the Earth's rotation axis points towards Vega during each 26,000 year cycle. The last occasion was around 12,000 BCE and the next will be around 14,000 AD.
M 13 / Hercules Globular Cluster
A showpiece globular cluster well placed in the evening sky from Spring until Autumn. Even in the twilight of summer this object will impress!
M 13 is a ball of approximately 300,000 stars at a distance of about 22,000 light-years. With magnitude of +5.8 it is visible easily with binoculars and to the naked-eye on from dark-sky locations. Moderate sized telescopes will resolve stars all the way to the core.
Another fine globular cluster in Hercules - often overlooked in favour of its more famous neighbour M 13. The cluster is probably beyond the naked eye limit of most people but binoculars can pick it up as a fuzzy ball of light.
M 92 is one of the oldest globular clusters of the Milky Way; astronomers can tell because there is little 'contamination' from heavy elements in the spectra of its stars. Just the pristine lines of hydrogen and helium from the early days of the universe. M 92 lies at a distance of nearly 27,000 light-years.
α Herculis / Rasalgethi
A nice double star for modest sized telescopes. This system comprises of an evolved red giant (actually termed an asymptotic giant branch star) and a slightly less massive yellow giant.
The stars are separated by 4.6 arcseconds and are fairly easy to split on telescopes of 4 inches or more at moderate magnification. The pair orbit one another over a period of 3,600 years.
More to follow...