Nebula size
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Nebula Size: A Comprehensive Overview
Dark Nebulae in the Small Magellanic Cloud
Dark nebulae in the Small Magellanic Cloud (SMC) have been cataloged extensively, revealing 45 objects with sizes ranging from 1' to 6' in major dimension. The average size of these nebulae is approximately 42 parsecs (pc), with an average extinction of a few tenths of a magnitude. These nebulae are generally about half the size of those found in the Large Magellanic Cloud (LMC) .
Diameter of the Andromeda Nebula
The Andromeda Nebula, also known as M31, has been studied using photoelectric photometry, which has shown that its diameter is larger than initially observed in photographs. The nebula fades gradually from a bright center outward, making its true size difficult to determine without advanced techniques. Measurements using a photoelectric amplifier have provided more accurate dimensions, highlighting the importance of such methods in understanding nebular sizes .
Formation and Size of the Solar Nebula
Models of the formation of the solar nebula suggest that the size of the nebula produced depends on various factors, including the total angular momentum, mass, and sound speed of the protostellar material. These models indicate that low-mass nebulas are produced under certain conditions, while more massive nebulas result under others. The size of the solar nebula is thus influenced by these parameters, with gravitational instabilities playing a significant role in its evolution .
Relative Sizes of Planetary Nebulae
The sizes of planetary nebulae (PNe) are challenging to determine due to uncertainties in their distances. However, by selecting nebulae with specific ranges of galactic latitude and surface brightness, researchers have established mean trends in nebular diameter. Circular and elliptical PNe, as well as the nuclei of bipolar nebulae (BPNe), have similar mean dimensions. Halos are typically about three times larger than circular and elliptical PNe, while the lobes of BPNe are about five times as large, making them the largest nebular structures .
Angular Dimensions of Planetary Nebulae
A study measuring the angular dimensions of 312 planetary nebulae using Hα (or Hα + [NII]) images applied three different measurement methods: direct measurements at the 10% level of peak surface brightness, Gaussian deconvolution, and second-moment deconvolution. The results showed that Gaussian deconvolution provides consistent but slightly underestimated diameters compared to the 10% measurements. Second-moment deconvolution, however, gave results that were less reliable, especially for poorly resolved nebulae .
Transitional Planetary Nebula IRAS 21282+5050
The young planetary nebula IRAS 21282+5050 has been imaged at 2 and 6 cm wavelengths, revealing dimensions of about 4" × 3". The nebula's low flux for its angular size and assumed distance suggests an average electron density of 2000-10000 cm⁻³, which is relatively low for compact planetary nebulae. At 2.2 μm (K-band), the nebula has a diameter of approximately 6" .
Dense Globules in the Dumbbell Nebula
In the Dumbbell Nebula (NGC 6853, M27), CCD images have revealed dusty globules of molecular gas silhouetted against central emissions. These globules have typical diameters of about 1.5 × 10¹⁶ cm, masses around 10⁻⁴ solar masses, and mean gas densities of approximately 5 × 10⁴ cm⁻³. These measurements provide insights into the structure and composition of nebular regions .
Revised Distance Scale of Planetary Nebulae
A new distance scale for planetary nebulae has been proposed, based on the flux density at 8 μm and the nebular apparent size. This method, calibrated using PNe in the LMC, offers a nearly extinction-free way to determine distances, making it particularly useful for PNe affected by interstellar extinction. Accurate determination of nebular size is crucial for this method .
Nebulae Around LBVs in the LMC
The morphology and kinematics of nebulae around Luminous Blue Variables (LBVs) in the LMC have been analyzed, showing that these nebulae are comparable in size to those in the Milky Way. However, the expansion velocities of LMC nebulae are significantly lower. Both LMC and Milky Way nebulae exhibit a diversity of morphologies, including bipolarity and outflow features .
Conclusion
The study of nebula sizes across different types and regions reveals a complex and varied picture. From dark nebulae in the SMC to planetary nebulae and LBV nebulae in the LMC, each type exhibits unique characteristics influenced by their formation processes and environmental conditions. Advanced measurement techniques and models continue to enhance our understanding of these fascinating cosmic structures.
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