Emission and Reflection Nebulae

Emission nebulae are clouds of interstellar gas that primarily emit purple and bluish light. These nebulae are ionised by the UV light of hot stars, in which one or more electrons are removed from the electron shell around the atomic nucleus. Upon recombination (in which an electron returns to the shell) photons of a particular wavelength (= colour) are emitted, which gives rise to the characteristic glow of the gas clouds. Emission nebulae are mainly featured by purple light (from ionised hydrogen) and bluish light (from ionised oxygen). In case of reflection nebulae the nearby stars are not hot enough to ionise them so that such nebulae do not shine themselves. Instead the stellar light is scattered from the microscopic particels, which renders the nebula visible. Reflection nebulae are generally featured by their blue colour, since blue light is more strongly scattered than red light (the colour of the sky results from a similar process).

IC 434 (Horsehead Nebula + Flame Nebula)
Maksutov-Newton (top) and ED-Refractor (bottom), Canon 1100Da, 3.2 hours

The Horsehead Nebula in the constellation of Orion is ca. 1500 ly away from us. The above image shows the glow of purple light from ionized hydrogen behind a 'curtain' of dense molecular gas and dust. The bulk of the UV-emitting stars is located behind that dust curtain so that the purple light is seen indirectly via reflection. A mixture of absorbing dust and emitting gas is visible in the lower left.

Rosette Nebula with star cluster NGC 2244
Evostar 72, Canon 1100Da, 4.5 Stunden

The term 'Strömgren Sphere' goes back to the Danish astrophysicist Bengt Strömgren (1908 - 1987). This is a geaseous nebula, in which stars induce radiation from the gas (mainly hydrogen). Stars with very high surface temperatures ionise the ambient gas via their intense UV radiation. The Strömgren Sphere is featured by a rather clear-cut outer edge: it is here where the ionising radiation dies away. The size of the region allows to estimate the number of stars with certain surface temperatures required as 'culprits'. When the ejected electrons recombine with the atomic nucleus a characteristic radiation is emitted, observable at specific wavelengths. Most intense is the so-called Hα radiation at a wavelength of 6563 Å (656 nanometers). Its characteristic colour is purple.

This is well visible in the above image. The Rosette Nebula is a classsical Strömgren Sphere with a diameter of ca. 130 ly, which we observe at a distance of about 5000 ly in the constellation of Monoceros. The very bright stars in the central region are the 'culprits' here.

Rosette Nebula with star cluster NGC 2244
Maksutov-Newton, Canon 1100Da, 7.3 hours

The Rosette Nebula has an angular extent too large for my Maksutov-Newton telescope and camera, it is somewhat larger than the available field of view! Nevertheless, one can see that the immediate surrounding of the ionizing stars in the centre is darker - the radtiation pressure of the newly formed stars has pushed away the gas.

Orion Nebula M 42 (NGC 1976)

Maksutov-Newton (top) and ED-Refractor (below), Canon 1100Da, 3.2 hours

The Orion nebula, located at a distance of 1350 ly, is the prototype of an emission nebula. The nebula in the homonymous constellation is visible throughout the whole winter period. One can see it already with the naked eye, and in a good binocular it provides a marvellous view. In its centre, overexposed here, the so-called trapezium stars are located. They are arranged in a trapezium and ionize the surrounding gas over a large distance.

Eagle Nebula M 16 (IC 4703)

Maksutov-Newton, 78 minutes

The eagle nebula is also a large star-forming region, located at a distance of 7000 ly. A picture taken by the Hubble Space Telescope brought this nebula to high publicity: its centre is marked by the so-called Pillars of Creation, which also well visible in this image here - are 'hanging down'.

Bubble Nebula NGC 7635
Maksutov-Newton, Canon 1100Da, 2.4 hours

The Bubble Nebula is ca. 7000 ly distant. Here, a gas bubble is being created by a stellar wind. The star ejects a large amount of gas, which moves into the surroundings at a speed of 28 km/s, whereby it impinges on the ambient molecular cloud. This produces a shockwave, which forms the outer envelope of the bubble.

Crescent Nebula (NGC 6888)
Maksutov-Newton, Canon 1100Da, 8.1 hours

The Crescent Nebula at a distance of 4700 ly is produced by an extremely strong stellar wind from the so-called Wolf-Rayet star WR 136, which is located close to the centre of the nebula. The extreme radiation pressure of this very massive star accelerates the expelled gas of the star to very high expansion velocities.

Heart Nebula (IC 1805)
Evostar 72, Canon 1100Da, 8.2 hours

IC 1805 in the constellation of Cassiopeia is a 7500 ly distant emission nebula, with a central star cluster ionizing the surrounding gas. A salient heart-shaped structure of purple light emitted by the ionized hyrogen is interspersed with dark regions, in which the interstellar dust aborbs the stellar light.

Heart Nebula (IC 1805)
Maksutov-Newton, Canon 1100Da, 5.2 hours

Soul Nebula (IC 1848)
Evostar 72, Canon 1100Da, 8.2 hours

IC 1848, in the constellation of Cassiopeia, is yet another 7500 ly distant emission nebula, with a star cluster ionizing the surrounding gas. Also here, the purple light of ionized hyrogen exhibits dark constrictions, in which the interstellar dust aborbs the stellar and nebular light.

Soul Nebula (IC 1848)
Evostar 72, Canon 1100Da, 3.0 hours

NGC 7380
Maksutov-Newton, Canon 1100Da, 7.8 hours

NGC 7380, now with more magnification. Close inspection shows the HII region to be much more extended than what appears at first glance. The interstellar dust, however, dims the Hα emission significantly over a large area.

NGC 2264
Maksutov-Newton, Canon 1100Da, 4.3 hours

Similar to the Horsehead Nebula, NGC 2264 is an HII region with a dark cloud, the so-called Cone Nebula, in front of it. The ionization is prodruced by the intense UV radiation of an open star cluster.

IC 5146
Maksutov-Newton, Canon 1100Da, 4.3 hrs.

IC 5146, the Cocoon Nebula in the constellation of Cygnus is about 3000 ly distant and has a diameter of 10 ly. A cluster of young, hot stars in its inner region has ionised the surrounding hydrogen gas, giving rise to the purple light, which is emitted upon re-capturing the electrons. The result is called HII region. The image also discloses a dark band, which commences at the western edge of the HII rgion and extends towards the north-west. This obscuring dust has been expelled into the interstellar medium by previous generations of stars and consists of silicates and graphites.

IC 5146
Evostar 72, Canon 1100Da, 4.3 hrs.

In this image, taken with the shorter focal length, one can trace the dark dust band pretty far out tto the west, where it bifurcates and ramifies.

Trifid-Nebel (M20 or NGC 6514
Maksutov-Newton, Canon 1100Da, 2.9 hrs.

With its low declination (-23°), the Trifid Nebula in the constellation of Sagittarius is a challenge at the observatory of Meuspath. However, after 2.9 hrs. exposure the main characteristics of this emission, absorption and reflection nebula (three in one!) become nicely visible. The emission nebula emits its purple light in Hα, whereas the reflection nebula presents itself in bluish colour (like the sea and the blue sky - blue light is more strongly scattered than red light). And in between the dark dust bands subdivide the emission nebula into (allegedly three) parts (I always see four of them ...).

A really pretty object at a distance of 5200 ly. In order to image the nebula, which at our latitude rises at most 16° aove the horizon, for nearly three hours, a pruning saw and a hedge trimmer had to be utilised.

Wizard Nebula (NGC 7830)
Maksutov-Newton, Canon 1100Da, 7.8 hours

The Wizard Nebula in the constellation of Cepheus is 7000 ly distant. It is characterised by the purple Hα light and by the 'truncs' of the absorbing dust.