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{ Practical astronomy | Astronomy | The Milky Way and its nebulae }

The Milky Way and its nebulae

HII Regions

red emission nebula
M42 Orion nebula, on 1982-01-13.

emission nebula shaped like North America
North America nebula, on 2001-12-09.

In "HII" the H stands for the chemical element hydrogen. In spectroscopy, spectral lines from neutral atoms are denoted by the element followed by the roman number I. Singly ionised atoms have the the number II, and so on. "FeXIV" then stands for iron atoms that have lost 13 electrons due to ionisation. Hydrogen atoms have only one electron to begin with, so "HII" is fully ionised hydrogen. The strongest visible spectral line of HII is the Hα line with a deep red colour. Hβ, Hγ and Hδ are also visible as cyan, blue and violet lines, but they are not as intense as Hα.

An HII region is a region of luminous interstellar gas (where hydrogen is the most common element). The gas is ionised by ultraviolet light from nearby, young stars. Actually, these are star forming regions, where we find dark clouds of interstellar gas and dust, young stars that have formed from those clouds, and luminous hot gas that has been ionised by the light from the young stars.

The apparent shape of HII regions is determined by the distribution of the stars and of the ionisable gas, but also by the presence of any dark clouds in front of the ionised gas. The Orion nebula has a prominent dark "bay" stretching from the left into the brightest part of the nebula. For the North America nebula name obviously is derived from the shape as determined by the stellar radiation as well as the dark nebulae in front. Note how the density of stars is lower in the "oceans" both sides of the North American "continent". These areas are dark not because of a lack of interstellar matter, but because of there is more of it: more dust obstructing the stars behind it.

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Physical parameters (North America nebula):

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Image parameters (North America nebula):

Reflection nebulae

blue reflection nebulae
NGC1435 Merope nebula, on 2011-12-29.

Stars form from interstellar gas. The light of young stars, in this case the bright blue stars of the Pleiades cluster, can still affect the nearby interstellar medium. In the case, 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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Dark nebulae

bright and dark nebulae in Orion
Sword of Orion, on 2011-12-29.

Most of the area of the constellation of Orion is covered by a large star formation region. This large field of view contains several HII regions, each with associated dark nebulae. Toward the bottom is the well-known Orion nebula, with another nebula about 1° north. Near the top, left of the bright star is the Flame nebula. This HII region appears split into several parts by intervening lanes of dense interstellar gas and dust. A very famous - but faint - dark nebula is the Horsehead nebula a little distance south of the bright star.

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When comparing the Orion nebula in this image to the image above, observe the different colour. The slide emulsion used in 1982 has a good sensitivity to the deeply red Hα spectral line, but the digital camera used in 2011 has an infrated blocking filter that removes almost all the Hα light. This nebula has significant radiation in other hydrogen lines, and so is still well detected in the modern cameras. The infrared blocker handicap is more evident in the much redder HII regions near the top of the field.

For certain dSLR models, it is possible to have the infrared block filter removed or replaced to allow much more, or even all, Hα light through to the detector. If you use a CCD camera built for astronomy, there is no infrared block filter to begin with, and you would use red, green and blue filters to obtain colour images. In that case, you get the full Hα light without camera modification.

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Supernova remnants

bright nebula
M1 Crab nebula, on 2012-01-15.

When a massive star runs out of fuel to generate energy, it explodes in a supernova. In this process, a nebula is created that expands into interstellar space. The Crab nebula is about 1000 years old, created by a supernova that was observed by Chinese and Japanese astronomers in 1054.

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Supernova remnants are small compared to interstellar nebulae or star clusters. A longer focal length is required to image these objects. This usually makes for a slower f ratio and hence a longer exposure time. The combination of long exposure and large image scale requires the exposure to be "guided" rather than merely "tracked". In a tracked exposure, the equatorial mount and its motor drive are left to themselves. However, tracking errors of up to an arc minute occur over periods of about 10 min. To guide and exposure, a human observer or a fast camera will observe a nearby star and keep its position constant in the field of view. To do so, the mount will take correction signals on an ST-4 port. The control observation is often made through a second set of optics - the guide scope - mounted roughly parallel to the imaging optics. In this instance, a 400 mm lens was mounted piggyback on the telescope to feed an image into a webcam. A laptop recorded images from the webcam, stacked a number of them every 8 s, determined the drift of the guide star, and through a parallel port adapter and an RJ-12 cable sent analogue signals to the ST-4 port of the mount to make corrections.

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