August Sky Notes
Written by John Stapleton
Image Credit: BBC Sky At Night Magazine
Starlight, Starbright
Currently both amateur and professional astronomers are studying a particular area of the night sky. This area lies between the “keystone” asterism in the constellation of Hercules and the constellation of Boötes marked by the bright red star Arcturus and contains a small circle of stars known as Corona Borealis (the Northern Crown). This small constellation contains a star designated T Coronae which is known to be a recurring nova. A nova is a star that explodes, throwing off its outer shell of material and becoming much brighter in the process. This brightening usually lasts for a few days and then the star slowly fades back to (even less than) its original magnitude over several weeks. A recurring nova is a star that undergoes this process more than once and at regular, predictable intervals.
The magnitude of a star is a measure of its brightness such that the higher the number quoted, the fainter a star is. The brightest star in the sky, Sirius in the constellation of Canis Major, shines at magnitude -1.46, whilst Venus at its brightest reaches magnitude -5 and the International Space Station can get as bright as -6. The Full Moon has a magnitude of -12.6 and the Sun, a magnitude of -27.6. The faintest stars you can see with the unaided eye, from a reasonably dark site would be of magnitude +6. An example of such a star is Polaris, the Pole Star.
The magnitude scale is a logarithmic scale defined as Magnitude 1 being 100 times brighter than magnitude 6 (this is what makes the decimals in a magnitude figure significant). This means that a magnitude 1 star is two-and-a-half times brighter than a magnitude 2 star and so on.
Once again there has been some media exaggeration over a forthcoming astronomical event. Some media reports have suggested that T Coronae will become a new star in the sky (which, after all, is what “nova stella” means) whilst others have suggested that this event will produce a “second Sun” in the sky. The truth is that T Coronae, which is a magnitude 10 star (well below naked eye visibility) will brighten by three or four magnitudes (becoming 10 times brighter than it is now) to reach magnitude 6 (the faintest star visible to the naked eye) and as bright as Polaris, which is not a bright star at all.
Nevertheless this is a rare astronomical event and of great interest to astronomers around the world. T Coronae has already undergone a phase of brightening though to be a precursor to the main event which is expected to happen any time from now to September 2024.
Sky Notes
Please note all times given in this article are in GMT so remember to add an hour to get the time in BST.
Sky Schedule
Sun: Astronomical darkness increases to 6 hours by the end of the month, occurring at about 9pm, so fainter objects are again becoming observable.
Mercury and Venus: Mercury is too close to the Sun to be visible this month. Venus, however, is an unmistakable object seen low in the Western evening sky just after sunset and setting by about 8pm (so still in daylight). Venus is becoming brighter as it moves in its orbit and approaches nearer to the Earth. Because Venus is nearer the Sun than the Earth, we see it showing phases which mimic those we see of the Moon at the same time. If you can hold a good pair of binoculars steady enough, you should be able to see this phase. Great care should be taken when observing both Mercury and Venus as you should NEVER LOOK DIRECTLY AT THE SUN.
Mars: The red planet is in the evening sky but has receded so far from the Earth that it is now barely visible. At this time, Mars is 20 times fainter than Jupiter.
Jupiter: The largest planet rises just before midnight and is seen against the background constellation of Taurus. The coloured bands and zones on the surface of the planet can be distinguished with small telescopes as can the Great Red Spot, a colossal hurricane, greater in diameter than the planet Earth. The Galilean moons can be seen with a good pair of binoculars or a small (bird-spotting) telescope.
Saturn: The ringed planet can be found in the evening sky rising around 8pm and remaining visible right through to dawn. It lies against the background stars of Aquarius. The rings are currently closing up, from our point of view, as Saturn tilts relative to the Earth. This will make the rings increasingly difficult to see. Saturn’s largest moon, Titan, can usually be seen with a good pair of binoculars or a small telescope, and, during this time four or five smaller moons of Saturn may also be seen through a good-sized telescope.
Uranus and Neptune: Uranus rises around 10.30pm in the constellation of Taurus where it lies close to Jupiter. It is becoming fainter so will need a telescope to find it. Look for a tiny greenish disc compared to the pinpoints which are the stars. Neptune rises about 8.30pm in the constellation of Pisces. It is even more distant than Uranus and is, consequently, even fainter.
Meteor Shower: The Perseid meteor shower peaks on 11th-12th this month. This is the best shower of the year with up to 60 meteors an hour including some very bright ones. This shower is often the brightest and most spectacular, of the annual meteor showers. This year there the Moon phase is favourable so many more of the shooting stars should be visible. The Perseids are so called because they appear to radiate from a point in the constellation of Perseus. The maximum of activity takes place on the night of 11th/12th when up to 60 meteors an hour may be seen in ideal conditions. This figure is standardized by assuming the radiant point will be directly overhead so that meteors travelling in all directions can be seen. However in actuality the radiant is relatively low in the North-Eastern sky until the early hours, and this is the best time for observation, although it is worth looking out from the moment it gets dark, after 22.30. Observing meteors could not be simpler. The only necessary bit of equipment is a garden recliner or deck chair so that you can lay back and look at the sky about 40 degrees above the horizon comfortably. If you want to make a record, simply count the number of meteors you see. You can also note the direction of travel, the duration, which will typically be seconds, and anything unusual such as colour, faint trails, or sound. This shower is known for producing occasional, bright fireballs. The minor shower known as the Kappa Cygnids also occurs during August, reaching a peak on 17th. This sparse shower often gets overlooked because of the high levels of interest in the Perseids earlier in the month.
Comet: There are no bright comets expected this month.
Moon: The New Moon occurs on August 4th with First Quarter on August 12th. Full Moon then follows on 19th with Third Quarter on August 26th.