8.5 Target-Specific Problems
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8.5.4 Planetary, Emission, and Dark Nebulae
Planetary nebulae (PNe), named for their planet-like appearance in early telescopes,
are the shells of stars that have been discarded during the red giant phase. The hot
progenitor star, often a white dwarf, ionizes the expanding envelope, which leads to
the emission of forbidden lines. Planetary nebulae have complex structures that are
very poorly understood. Many exhibit an hourglass-like structure and often appear
circular or elliptical in small telescopes. Planetary nebulae are rare in the Milky Way
despite a large number of known possible progenitor stars, suggesting that they are
very short lived, perhaps only about 10,000 years.
Planetary nebulae can be challenging if their angular size is small, as many are,
since that makes resolving detail difficult. Although it is possible to image PNe in
broadband filters, their very nature lends them to narrowband imaging, as that is
where the bulk of the emission lies. The use of narrowband filters will also reduce
the sky contribution, improving the signal-to-noise ratio. However, for very faint
PNe, very long exposures are needed, which can be achieved by taking multiple
images and stacking them later. If they are very small, the introduction of a Barlow
or similar device into the optical pathway can increase the plate scale.
Emission nebulae, of which PNe are a subclass, are clouds of gas often starforming and with embedded stars. These stars ionise the gas, which then deionises,
emitting light. Just as in the case of PNe, emission nebulae can be very diffuse and
faint but can also be very bright in some regions, especially if there are massive stars
illuminating the cloud. One such example is M42, also known as the Orion nebula,
as illustrated by Fig. 8.12. M42 is also a diffuse nebula, having no well-defined
boundary, as can be seen. In astrophysics, diatomic hydrogen is known as hydrogen
and denoted by H, disassociated hydrogen is known as HI, whilst ionised hydrogen
is known as HII. Hence M42, which is mostly ionised, is an HII region, as well as a
diffuse and an emission nebula.
The core of M42 is illuminated by a number of very massive stars, which can cause
the saturation of the core. Only by stretching the image, reducing the dominance of
the core emission, do we see the fine structure. There are a number of computational
filters that can enhance detail, for example a logarithmic stretch, although this will
make any results nonphotometric. However, in many cases it is the morphology that
interests us, in which case sacrificing the linearity of the image is acceptable.
Dark nebulae are dense cold molecular clouds. Because of their nature, they
are visible only in the optical region when located in front of an emission nebula.
Although they often appear dark in front of the emission nebula. Dark Nebula can be
seen in Fig. 8.12, although perhaps the most famous example is IC434. Hence when
you are imaging dark nebulae, the integration time will depend on the brightness of
the background source.
121
8.5.4 Planetary, Emission, and Dark Nebulae
Planetary nebulae (PNe), named for their planet-like appearance in early telescopes,
are the shells of stars that have been discarded during the red giant phase. The hot
progenitor star, often a white dwarf, ionizes the expanding envelope, which leads to
the emission of forbidden lines. Planetary nebulae have complex structures that are
very poorly understood. Many exhibit an hourglass-like structure and often appear
circular or elliptical in small telescopes. Planetary nebulae are rare in the Milky Way
despite a large number of known possible progenitor stars, suggesting that they are
very short lived, perhaps only about 10,000 years.
Planetary nebulae can be challenging if their angular size is small, as many are,
since that makes resolving detail difficult. Although it is possible to image PNe in
broadband filters, their very nature lends them to narrowband imaging, as that is
where the bulk of the emission lies. The use of narrowband filters will also reduce
the sky contribution, improving the signal-to-noise ratio. However, for very faint
PNe, very long exposures are needed, which can be achieved by taking multiple
images and stacking them later. If they are very small, the introduction of a Barlow
or similar device into the optical pathway can increase the plate scale.
Emission nebulae, of which PNe are a subclass, are clouds of gas often starforming and with embedded stars. These stars ionise the gas, which then deionises,
emitting light. Just as in the case of PNe, emission nebulae can be very diffuse and
faint but can also be very bright in some regions, especially if there are massive stars
illuminating the cloud. One such example is M42, also known as the Orion nebula,
as illustrated by Fig. 8.12. M42 is also a diffuse nebula, having no well-defined
boundary, as can be seen. In astrophysics, diatomic hydrogen is known as hydrogen
and denoted by H, disassociated hydrogen is known as HI, whilst ionised hydrogen
is known as HII. Hence M42, which is mostly ionised, is an HII region, as well as a
diffuse and an emission nebula.
The core of M42 is illuminated by a number of very massive stars, which can cause
the saturation of the core. Only by stretching the image, reducing the dominance of
the core emission, do we see the fine structure. There are a number of computational
filters that can enhance detail, for example a logarithmic stretch, although this will
make any results nonphotometric. However, in many cases it is the morphology that
interests us, in which case sacrificing the linearity of the image is acceptable.
Dark nebulae are dense cold molecular clouds. Because of their nature, they
are visible only in the optical region when located in front of an emission nebula.
Although they often appear dark in front of the emission nebula. Dark Nebula can be
seen in Fig. 8.12, although perhaps the most famous example is IC434. Hence when
you are imaging dark nebulae, the integration time will depend on the brightness of
the background source.