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{ Practical astronomy | Astronomy | Stars and star clusters }

Stars and star clusters


star trails of the constellation Orion
Star field Orion.

Stars give their name to astronomy, as they outnumber anything else visible at night with the naked eye. Humans have probably always made up patterns in the distribution of stars. These constellations do not change noticeably for generations. The most recognised modern constellation is Orion, the Hunter. Notice the different colours of Betelgeuse (left shoulder) and Rigel (right foot). Red stars have a relatively cool surface, while blue stars have a very hot surface. The brighter stars of Orion are between 0 and 3.5 mag, but their distances vary a lot more, from 10 to 500 pc. Some stars appear bright because they are bright, others because they are close.

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The equipment here is quite ordinary: An SLR with 50 mm lens, slow slide film, tripod. Then open the shutter for a few minutes (with a cable release). The only problem is location: at sea level near urban areas the sky background begins to register quite soon. This image was taken from the site of a professional observatory in southern Spain, 2100 m high and far from the cities.

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light curve of epsilon Aurigae
Light curve of ε Aurigae during its 2009/2011 eclipse.

The graph plots the brightness of the star ε Aurigae against time. Values along the bottom axis are Julian Days (minus 2,400,000 days) with the corresponding years and months indicated along the top axis. Values along the vertical axis are stellar magnitudes relative to one of the comparison stars, η Aurigae. The symbol in the bottom left corner indicates the typical precision of the measurements. The measurements themselves are indicated by red crosses. The grey curve is drawn by hand and eye and should be taken with a pinch of salt.

ε Aurigae is an eclipsing variable. About every 27 years, the main star - a white supergiant 135 times the size of the Sun - is eclipsed by a disc of dust and gas that revolves around a blue main sequence star. The blue star and its disc revolve around the white supergiant in 27 years. The disc only ever covers half the face of the supergiant, resulting in the drop of brightness of 0.8 mag. Even then, the remaining light from the supergiant dominates and the blue star remains invisible.

The light curve shows that in late August or early September 2009 the disc began to move in front of the star. By January or February 2010 it had covered as much of the star as it ever would. In March 2011, the disc began to move off the face of the star. This happened about 50% more rapidly and by early June 2011 the whole star was visible again. Both during ingress and egress, the light curve shows a delay around half way through. Finally, during the time between ingress and egress, the eclipse was not constantly deep. Rather, more light from the supergiant seems to make it to Earth around the centre of the eclipse. The shape of the light curve may tell us something about the structure and movement of the eclipsing disc.

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The brightness of ε Aurigae was measured in most cases from groups of 4 to 10 raw frames taken with a digital SLR (partly a Canon EOS 300D, partly a 400D) and an f = 135 mm f/4 lens; exposures were 5 s at 100 ISO. Only the green channel of the frames was used.

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light curve of Algol
Light curve of Algol, 2001-10-27.

This graph is a light curve of the perhaps best-known variable star - Algol. It varies rather more rapidly than ε Aur, a bit more often than every 3 days, with the minimum lasting about nine hours. The graph covers about five hours and shows the decline from its regular magnitude 2.1. The minimum would have been reached just beyond the end of the graph 1.4 mag down from the maximum. The observation had to end because of a lot of dew forming on the equipment.

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Regarding Algol, in 2012 I put together some materials to carry out and evaluate observations of this variable star.


galaxy M101 with supernova SN2011fe
Supernova in M101, on 2011-09-15.

The image shows a supernova that went off in late August 2011. The supernova - named 2011fe - is the star on the southern periphery of the extended "smudge" that is the galaxy Messier 101. A supernova occurs at the end of a massive star's life. A sudden burst of nuclear fission boosts the brightness to be comparable to that of a whole galaxy. These events are rare, but when humankind collectively observes many galaxies on a regular basis, the odds increase considerably. This supernova is quite bright at 10.3 mag. In a dark-sky location and armed with a finder chart, it should be visible in binoculars. The galaxy is 8.2 mag, about 10 times brighter. Because the light of the galaxy is spread over an area, it hardly shows in the image.

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The original image is dominated by light pollution and vignette (darkening in the corners of the field). After stacking, a curved background was fitted and subtracted to remove light pollution.

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Open star clusters

open star cluster
M45 Pleiades, on 2011-12-29.

Stars form from interstellar gas. Often, the collapsing cloud of interstellar gas is sufficient to form several or many stars. The image shows Messier 45, named the Pleiades or the Seven Sisters, which is an open cluster of young stars. In some open star clusters, one can still see evidence of the nearby interstellar medium. In the case of M45, the mostly blue light from the brightest cluster members is reflected or scattered by nearby insterstellar dust. This is what causes the blue nebulosity.

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The exposure is limited by the brightness of the sky due to light pollution. The individual frames are close to being dominated by sodium light reflected and scattered by the Earth's atmosphere. Direct light from a nearby streetlight and from a neighbour's security light are screened by placing a cylindrical shield toward the front of the lens, or better, by using separate screens on tripods to cast shadows on the lens aperture.

The short exposure of the frames limits how faint an object can be discerned over the noise. By aligning many frames on the stars seen in them and then stacking the frames into a single image, the noise is reduced relative to the signal from the sky. This is because the noise is random and changes from frame to frame, ultimately averaging out to nothing. The signal is the same in each frame and remains unaffected by averaging many frames.

The noise level is mostly due to taking the frame and not so much due to the length of the exposure. Taking 100 frames reduces the noise by a factor 10. But if we could take a single frame with 100 times longer exposure, we would have 100 times the signal with only slightly more noise. That would reduce the noise by a factor of 50 or more.

The exposure of the individual frames is limited by several factors: the sky background will at some point overexpose the frame; tracking inaccuracies will eventually smear out the image; ultimately, the exposure time might exceed the battery life or the length of the period of darkness. Light pollution poses an additional problem, because the background brightness it introduces into the image, at very high contrast, is not smooth and even enough to be removed properly. Using a dark-sky location or a light pollution filter to suppress sodium and mercury spectral lines would improve matters.

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