PETE LAWRENCE
We remain close to the Northern Hemisphere’s summer solstice at the start of July, so nights are short. But by the end of the month true darkness returns just in time to take in some of the amazing sights on offer.
The Summer Triangle is a large asterism – an unofficial pattern – formed from three bright stars; Deneb in Cygnus, Vega in Lyra and Altair in Aquila. It dominates the view high to the south with the Milky Way passing down through the triangle, following its arrowed direction to the southern horizon.
To explain what the Milky Way is, imagine putting two fried eggs back-to-back. The yolks represent our galaxy’s core and the whites the plane in which the spiral arms, rich with star formation, are located. We now know the core is elongated in one direction, forming a bar, so the Milky Way is described as a barred spiral galaxy. Our Sun is one of several hundred billion stars gravitationally bound in this system, located roughly two-thirds out from the core. All the individual stars we see are ‘local’ Milky Way neighbours in a ‘bubble’ about 10,000 light years in diameter. The entire galaxy is at least 100,000 light years across and the billions of other suns merge to form the misty path known as the Milky Way.
Looking south leads your view into the galaxy’s core. From more southerly climes this is a spectacular sight, even able to cast shadows from dark sky locations. From the UK, its brightness is dimmed by the thick layer of atmosphere close to the horizon and its grandeur is diminished.
The trick to navigating this region of sky is to identify another asterism known as the Teapot, part of Sagittarius the Archer. Once found, look at the region northwest of the spout (up and right from the UK). A supermassive black hole is believed to lie here, its presence betrayed by a compact radio source known as Sagittarius A* (pronounced ‘Sagittarius A star’).
Let your gaze wander into the teapot’s steam where you’ll find the beautiful Lagoon Nebula, M8. Shining with the characteristic red-pink light of excited hydrogen atoms, this is a place of star birth. It would rival the Orion Nebula if it weren’t for the fact that it is so far south and low from the UK.
A short hop north locates the Trifid Nebula, M20, so called because of the three dark dust lanes crossing its brightest region. This appears smaller than M8 but just as stunning in photos. Part of its light is red-pink from hydrogen emission, and blue from reflected starlight.
The Milky Way can be seen under some light pollution but is greatly diminished. From a city it’s typically lost from view. From the UK, the easiest portion to see is that which passes down the vertical of the Northern Cross asterism in Cygnus. An interesting effect appears to make the Milky Way split in two. This is caused by a dark region of intervening dust obscuring more distant starlight. It is so obvious from a dark sky location that it even has a name – the Cygnus Rift.
The star at the bottom of the Northern Cross is Albireo. Located 8 degrees (16x the apparent diameter of the full Moon) east of Albireo is the Dumbbell Nebula, M27, a superb example of a planetary nebula just visible through steady binoculars. This class of nebula is formed when the outer layers of an intermediate or low-mass star eject into space as the star’s life ends, producing an envelope of gas that typically appears thinner towards the poles. The Ring Nebula, M57, in Lyra, is another example of this type.