It's surprising just how far some deep sky objects stretch across the sky. This composite shows a deeper exposure aligned over a wide-angle image of the Caradon Observatory site. The outer Milky Way runs from upper left to lower right, from Gemini to Cassiopeia. Visible above the trees is the red smear of NGC 1499, the California Nebula, to scale with the foreground image. It covers some 3 degrees of the sky, six full Moons across. While invisible to the naked eye it's not that far out of sight, making a challenging binocular target. However, in long-exposure photographs it's one of the brightest nebulae in the sky.
A closer view shows its structure, glowing red from ionised hydrogen with hints of blue from glowing oxygen giving it a pink hue. Dark features are concentrations of dust, often sites of star formation. It's a vista sculpted by natural processes - stellar winds and radiation pressure from bright stars, and gravity.
Xi Persei is a runaway star, ejected from its birthplace either by a close encounter with another star or the supernova of a companion. A chance encounter has brought it close to a molecular cloud, making the California Nebula visible. The plane of the Milky Way is littered with these concentrations of gas and dust, largely invisible in the visible spectrum but detectable by radio and infra-red telescopes. The nebula is just an optically bright feature of a much larger complex. If Xi Persei had been launched in another direction it might well have painted, and to a degree sculpted, an entirely different deep sky object.
The monochrome image above was taken using a short telephoto lens on the 26th March and blended into a colour image taken previously. Imaging it with a telescope usually requires shooting separate panes to stitch together in a mosaic, as it will not fit within the field of view. This spectacular 103 hour effort by Sara Wager shows the nebula in intricate detail and is best viewed on a large high-resolution monitor.
Here's a view of the Kit Hill chimney stack taken from almost 6 miles away, taken with a cheap telescope and an old webcam. On the whole it's a fair representation of what can be seen through the eyepiece. Colours are a little washed out because Kit Hill was in shadow but it's just possible to make out people walking around the stack. However, the static image doesn't show the 'heat haze' effect caused by moving currents of air, the eyepiece view isn't as sharp.
Jupiter has been pasted in at the same image scale as the close up. Jupiter's apparent size varies depending on where it and Earth are in their orbits around the Sun. Even at its closest approach (628 million kilometres) Jupiter's angular size is only 50 arc-seconds or 0.014 degrees. Photographing Jupiter is like taking a picture of a 5 pence piece from 320 feet away. (But while high magnification is required to see the planets, some nebulae cover surprisingly large patches of sky.)
The full-size image of Jupiter shows more detail. The major cloud bands are visible and the great red spot, a giant storm at least 300 years old, is at lower right. The dark mark is the shadow of one of Jupiter's moons.
This image was taken using a different telescope, a 5" refractor, and a 4x magnifying Barlow lens. This combination gives about 3.6 times the magnification of the telescope in the top image, when used with the same camera. To image the planets and Moon amateurs use a technique called 'lucky imaging', to overcome distortion caused by atmospheric turbulence. A short video is taken of the target and the clearest frames are selected and combined using free stacking software, such as Registax. For an example of amateur planetary imaging at its best I'd recommend taking a look at Damien Peach's website.
Professional planetary imagers such as NASA either use the Hubble Space Telescope or simply park a spacecraft in orbit around their target, options beyond the budget of most amateurs. The Juno probe is currently at Jupiter and returning spectacular images.
Going back to the telescope in the first image, a Celestron Astromaster 114, is not one I'd recommend. The telescope itself is useful but it's let down by the mount and finder, making it a frustrating business to find objects in the sky. Small Dobsonian telescopes are often recommend for people starting out in astronomy due to their low cost, ease of use and light grasp. They are a type of Newtonian telescope which use a mirror to gather and focus light. Compared to a pair of binoculars they are more stable and produce a brighter image. There are even cardboard self-assembly versions available.
40,000 years ago a star exploded. This is the result.
This is the supernova remnant Simeis 147, sometimes known as the Spaghetti Nebula. It spans about 3 degrees of sky on the Auriga/Taurus border, a diameter of six full Moons. The bright star Elnath is at the bottom of the frame, which is easily visible to the naked eye.
These star charts, generated using World Wide Telescope, show the location of Simeis 147 in the sky.
Orion is sinking out of sight from the UK at this time of year but Auriga is still visible to the west as it gets dark, just above Venus.
The star chart above was generated using the free software Stellarium.
The bright red filaments of Simeis 147 are largely composed of glowing hydrogen, heated as material expelled by the explosion crashes into its surroundings. Over the next few thousand years it will continue to expand and dim, eventually fading into the background. This inverted monochrome image, taken through a hydrogen-alpha filter, shows the structure of the nebula more clearly.
(No telescope was used to take these images, just a short telephoto lens, in order to fit it all in the field of view. The lens and camera ride on a lightweight motorised star tracker powered by AA batteries. Total exposure time was about 35 minutes for the colour starfield and 50 minutes for the hydrogen-alpha. The hydrogen alpha data was then blended into the red channel of the colour image to produce the final version.)
(Amateurs have taken far better images of Simeis 147 than the one above. This is the best I could find, a spectacular 47 hour telescope mosaic by Franco Sgueglia & Francesco Sferlazza. The blue filaments are composed of glowing ionised oxygen.)
40,000 years ago, when light from the supernova first reached Earth, our ancestors would have certainly noticed. For a few months it would have been the brightest thing in the sky, after the Sun and Moon and perhaps Venus. In the dark skies of the neolithic, darker than all but the most remote locations today, it's sudden appearance would have been dramatic and spectacular. Unfortunately we'll never know what they made of it as writing would not be developed for another 30,000 years.
But we do have accounts of more recent supernova events. Kepler’s Nova of 1604 was clearly visible to the naked eye, and in 1987 a star exploded in the Large Magellanic Cloud, a nearby satellite galaxy of the Milky Way. The Hubble Space Telescope tracked its evolution over several years.
It’s difficult to convey just how violent and extreme an event a supernova is. Perhaps the easiest way is to talk about about Death Stars.
Alderaan appears to be a similar size to the Earth. Certainly it was inhabited by suspiciously human-like inhabitants, who appear to be comfortable in Earth gravity.
To completely destroy a planet it’s necessary to overcome the force of gravity holding it together. Earth’s gravitational binding energy can be calculated, it comes to 2.49 x 1032 Joules in scientific notation, or 249,000,000,000,000,000,000,000,000,000,000 Joules in longhand. That’s a lot of energy, as much as our Sun puts out in a week. (A single Joule is enough energy to lift a tomato a metre.)
The energy output of a typical type II supernova, caused by the collapse of a massive star, is 1.00 x 1046 Joules. The same as 40 trillion Death Stars firing at once. 40 trillion is a big number, if you counted one Death Star a second it would take you over a million years to tally them all. As the man said...
‘Bang’ doesn’t really do it justice. Fortunately there are no supernova candidates close enough to Earth to cause us any harm. But they can reach out and touch our atmosphere from enormous distances, evidence of past supernovae have been found in Antarctic ice cores, in the form of nitrate deposits.
Important safety tips from Lord Vader there. Although to be honest, given his obvious breathing difficulties he’s probably more worried about stormtroopers coughing on him than our well-being.
Kit lenses, commonly bundled with DSLR cameras, offer a natural entry point into astrophotography due to availability and their wide field of view, allowing reasonable exposure times from a fixed tripod. The major difficulties for first time users are finding the correct camera settings and focusing on infinity. This guide offers a few tips based on my own experiences and blunders.
This was one of my first attempts at shooting the Milky Way, taken on holiday in Menorca. I didn’t have a tripod with me so I simply laid the camera down on the patio, pointing straight up. It’s a 30 second exposure at 18mm focal length, giving the widest field of view. To capture as much light as possible the camera iris was opened fully, giving a focal ratio of f3.5.
Camera and Lens Settings
It’s worth experimenting with these settings before heading out, to help avoid the frustration of fighting the camera in the dark.
When shooting from a fixed tripod it’s important to gather as much light as possible. Typically this means shooting with the lens wide open by selecting the lowest aperture setting available.
Video Credit: Sam Morell of XRT-C
On 7th July, it was officially announced that Bodmin Moor and the peripheral areas had been designated as an International Dark Skies Landscape. This was the culmination of three years’ arduous work collecting vast amounts of data which led to a comprehensive application, jointly submitted by Cornwall Council and Caradon Observatory to the International Dark Skies Association, in Tucson. Significantly, this was the first time that an Area of Outstanding National Beauty (AONB) had been successful.
Here's a quick animation from the night of 24th/25th March showing the movement of Comet 41P against the background stars.
Each frame is a 2 minute exposure taken with a 135mm lens, the time-lapse is two hours long. On the 24th March it was visible in the bowl of The Big Dipper, or business end of The Plough if you prefer. The bright stars of this famous asterism are just out of frame. Over the next few days it moved into the neighbouring constellation of Draco.
Comet 41P goes by the name of Tuttle–Giacobini–Kresák which is a bit of a mouthful. This is because comets are typically named after their discoverers, 41P was independently found in 1858, 1907 and 1951. It is a short period comet which approaches the Sun every 50 years or so and is thought to have a nucleus about a mile across. The comet has a circular appearance rather than showing the classic cometary tail because of the viewing angle.
Rising to the East at this time of year, the three stars of the Summer Triangle are bright enough to punch through twilight and even a little wispy cloud. To the naked eye they appear as three white/blue points of similar brightness, but this is deceptive.
The two stars on the right of the image - Vega and Altair - are indeed quite similar. They are both A-class stars, more massive and more luminous than the Sun. As residents of our neighbourhood they are among the brightest in our sky.
The most surprising thing about Vega and Altair is: they are not round. They aren't even close to spherical.
Full astronomical darkness is now returning to the southern UK. Here's a chart from FLO's Clear Outside showing annual darkness for Bodmin Moor.
In the blue portion of the chart, centered around Midsummer, the sky never becomes fully dark. At that time of year the Sun never drops very far below the horizon, so the upper layers of the atmosphere are still lit up in an extended twilight. This isn't easily noticeable from brightly-lit towns and cities but is quite obvious from a dark, rural location. Astronomical images taken outside of astro-dark have a blue tint and are washed out to some degree by the reflected sunlight.
The 15 second exposure above was taken on the 18th June in SE Herts, at about 11:30 PM. The yellow colour at the bottom right is caused by light pollution, probably from the town of Bishop Stortford.
Practical astronomy with an almost-seven year old?
Spent some time with nephew-nearly-seven over Christmas and - despite the lure of lego ninjas and talking dinosaurs - he was quite happy to join me outside on a clear and frosty night. I started off by showing him a few landmarks of the sky - the Milky Way, most prominent stars and recognisable constellations - using a laser pointer to trace out their shapes. (After first explaining the importance of checking for aircraft, to avoid dazzling them. He certainly made a conscientious air traffic controller, for the rest of the evening I couldn't go near the the laser without a warning, including for planes fifty miles away and in the other direction.) Resting on the Pole star I tried to explain how the stars appear to rotate around it, as shown in the time-lapse video below.
Video by James Castelli, Cherry Springs, Pennsylvania - Aug 2008
Next we had a go at astrophotography, I set up a motorised tracking mount and attached a camera with a wide lens. I pointed the camera in roughly the right direction and disengaged the clutches so he could gently nudge it on to target. After a couple of attempts he got this shot of the Milky Way with a 2 minute exposure.