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.
A long time since my last blog post! Busy at work, at home and studying. To calm my nerves I've been processing pictures of little bits of the heavens. Not my own images - the summer night sky is getting too bright - but data collected by various sky surveys and available online through the ESO Online Digitized Sky Survey.
A number of surveys are available and offer various degrees of sky coverage; from 45% of the sky (in blue filtered light) to 99% (in red and infrared). The web interface allows selection of a target or specific coordinates. The images can be displayed as a GIF or downloaded as GIF or FITS. The FITS file format contains a wider range of pixel intensities than GIF. I use FITS Liberator to select the best "window" to view the FITS data.
The images are scans of photographic plates taken by some very big telescopes. Many of them were obtained with the 1.2 metre UK Schmidt Telescope in Australia during the early 90s. Being scanned from the original plates means there are some interesting blemishes in some of the plates. Hairs, fingerprints and other defects can be found if you look carefully enough :-) They can, of course be photoshopped out these days!
Here is what typical GIF images look like:
There are subtle differences to the images; the blue filtered image shows the structure in the spiral arms more easily because of the hot (bluish), young stars there. The red filtered image shows the background glow of the disk - containing many more low mass, cooler (redder) stars.
These are grayscale images. To make a natural colour image we'd need an image taken with a green filter! However, no images in green were taken.
Just for fun...I wanted to see some colour images so I synthesised my own green image. This was done by averaging the pixel intensities in the red and blue images. For example, if a pixel is very bright in blue (say a value of 3000) and dimmer in red (say, 1000) then the I'd interpolate the green value to be (3000+1000)/2 = 2000. Doing this for every pixel generates a synthetic green image.
Armed with the red-green-blue (RGB) images it's a simple matter to blend the images to get a colour image:
The full res version can be viewed here. And you really should look at it! The amount of detail in the images is breathtaking.
So this has become my most recent astronomical diversion. You can see some of my other results on Flickr.
An interesting planetary conjunction for telescope users on January 12th. Brilliant Venus passes within half a degree of the distant ice giant Neptune.
The southwestern aspect of the sky, just before 7pm, looks like this:
This Stellarium rendition of the sky shows that Mars is not too far away from Venus in the sky as well. And the planet Uranus (needing at least binoculars) is high in the sky in the constellation Pisces.
Zooming in a bit on Venus to simulate the telescopic view:
Venus is 51% illuminated (very nearly half-full) and shining at magnitude -4.4. The distance to Venus is 102.1 million km. By contrast, Neptune is 45 times further away (4.6 billion km) and shining at magnitude +7.9.
It's an interesting scene to visualise on software (like Stellarium, above) but it might much more difficult to get a real picture. Venus is around 80,000 times brighter than Neptune. Exposing to capture the phase of Venus won't be long enough to capture a glimpse of Neptune. However, a greatly overexposed Venus should allow Neptune to be picked up in the same view.
The best view - if you can do it - will be with your own eye at a telescope eyepiece.
Another day, another diagram. This is from a presentation about the Sun which I'll shortly be giving at Berwick Educational Association.
The diagram is fairly simple. A series of overlaid wedges. A nice thing about PSTricks is that there's a package for almost everything. In this case the wedges can be colour shaded (using pst-slope) to give the impression of a decreasing temperature gradient.
The lines in the radiation zone should ideally be random-walk zig-zags to represent the path of a typical photon! I haven't worked out how to do that so for now they're wavy lines (using \ncsin). The convection lines are done using \pccurve and specifying the angles leaving and entering the nodes at the end points.
The other useful feature of LaTeX is the \multido command; it's necessary to just specify one command (for say, one convection loop) and let \multido put the shape at regular angle increments at a fixed radius.
Here's the code.
Solar Structure Diagram
Comet Catalina is currently well placed for UK observers wishing to see it. The comet is tracking north in the sky and over the next week it will bypass the familiar seven stars of The Plough.
An interesting photographic opportunity occurs on the night of January 16th (going into the early hours of the 17th) when the comet will be close to the celebrated double star Mizar (and Alcor) and a bright-ish galaxy called M101 (the Pinwheel).
The picture above is a Stellarium rendition of Mizar, Comet Catalina and the Pinwheel Galaxy. This is the late evening of January 16th. But given that the moon is above the horizon until just after midnight, best views (and pictures) will be obtained during the early hours of the 17th. This is a wide field of view, so I'll try to capture this with the Nikon D80 mounted directly onto my HEQ5 Pro mount; I think shooting at 200mm will frame the region nicely!
Venus has been a morning sky object for months. Saturn has been hidden behind the Sun until recently and has joined Venus in the sky before sunrise. They're moving across the sky a somewhat different speeds and in the second week of January the two planets pass each other.
For a few hours on the morning of January 9th the two planets will be separated by gap just 1/5th of the diameter of the moon. Unless your eyesight is fairly bad, they will still appear as two distinct points of light but it's still rare to see bright planets so close together.
Venus is much the brighter of the two planets; although smaller than Saturn it is much nearer to us and shrouded by highly reflective clouds.
Here is the simulated view through my 8-inch Meade LX10 with a 5mm eyepiece at around 6.30am.
Venus and Saturn look roughly the same size (the disk of Saturn, without the rings). Without any depth perception it might help if you know that the actual size of Saturn is nearly 10 times larger than Venus. Therefore, for them to look the same size, Saturn must be about 10 times further from us than Venus.
There are a number of other planetary conjunctions in 2016 - most will happen in the last quarter of the year.
The simulation below shows the Earth and Moon at the time of the eclipse. It takes several hours for the moon to pass through the shadow.
The video doesn't show what really makes these eclipses special: at mid-eclipse the full moon is dimmed to an orange/copper colour by sunlight which has been filtered through the Earth’s atmosphere. The exact appearance and degree to which the Moon is dimmed is somewhat unpredictable because it depends on the state of our atmosphere at the time.
Visibility of the eclipse
In the UK: Where to look?
It's a full moon: if the sky is clear you won't have a problem finding it! The middle of the eclipse is at 3.46am (BST) and the moon will be in the southwest sky among the stars of Pisces.
The Earth’s shadow consists of two regions: a dark central region (the umbra) and a lighter boundary (the penumbra). The umbra is a region where all direct sunlight is cut off and the penumbra is a region where only part of the sunlight is blocked. You can witness the sight of these two shadows on a smaller scale by examining the shadow of your hand or another object as cast by desktop lamp.
The times at which the Moon enters and leaves the penumbra and umbra are as follows:
How to observe
You don't need big telescopes to observe a lunar eclipse; the changing face of the moon as the Earth's shadow falls across it will be obvious to the naked eye. Binoculars will provide a fine view of the moon too. In many recent total lunar eclipses a band of turquoise light, caused by light scattering through our ozone layer has been widely observed: binoculars and small telescopes should be enough to show this.
The ideal place on Earth to see this eclipse would be the north coast of Brazil - near the border with French Guiyana; the moon will be overhead at mid-eclipse. Good views for my wife's family in Venezuela too!
Future lunar eclipses
There won’t be another total lunar eclipse visible in its entirety from Northumberland until 2019, so local amateur astronomers will be trying to observe this one despite the unsociable hour of it!
Venus is at inferior conjunction today. That means it positioned between the Earth and Sun.
When the alignment is exact Venus is seen to transit in front of the Sun. At most inferior conjunctions the alignment is not exact and Venus passes above or below the Sun. That was how things were today; Venus passed just under 8 degrees south of the Sun.
Here is the simulated view provided by Stellarium:
It's very difficult to observe Venus under these circumstances. The planet isn't visible to the naked eye. It's so close to the Sun that even when the telescope is aimed at Venus some sunlight can still enter the optical system and cook the inside of the telescope!
I was using a Celestron NexStar 102SLT to observe Venus. The telescope is tracking Venus on the HEQ5 Pro mount. Without being able to do a polar alignment in daylight it was fairly tricky to find Venus. My method was to get the 'scope pointed at the Sun (and then correctly focussed) with a solar filter in place. Then I offset the telescope by the required number of degrees in RA and Dec and hoped for the best after removing the solar filter.
Venus was relatively easy to see once it entered the field of view. A razor sharp white crescent against bright blue sky. The air was a little turbulent and that stopped me getting a good picture with the camera. I did get the crescent though:
Venus has been visible in the evening sky after sunset since late 2014. After today it is technically a morning sky object - visible before dawn. Venus will be shining in the morning sky before sunrise before the end of the month.
Welcome to my blog!
Dr Adrian Jannetta. Amateur astronomer, maths teacher and science enthusiast.