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.
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Dr Adrian Jannetta. Amateur astronomer, maths teacher and science enthusiast.