NGC 1579: Trifid of the North (2009 Jan 30)

[bystander]

http://apod.nasa.gov/apod/ap090130.html

So, let me see if I've got this right. NGC 1579 is not a true emission nebula? Instead it scatters the light from an embedded massive star, similar to the way the dust in Mar's atmosphere scatters light giving a red sky? Or is it just the Hα emission from the star shining through?

[neufer]

http://apod.nasa.gov/apod/ap090130.html

So, let me see if I've got this right. NGC 1579 is not a true emission nebula? Instead it scatters the light from an embedded massive star, similar to the way the dust in Mar's atmosphere scatters light giving a red sky? Or is it just the Hα emission from the star shining through?

It is scattering/reflecting the intense Hα emission of an embedded massive star.

<<The majority of American astro photographers mislabel this nebula as an emission nebula. All peer-reviewed scientific studies support the conclusion that NGC 1579 is a cold cloud of dust reflecting the intense h-alpha light of 'LkH-alpha 101'.

Redman, et.al. (1986) conducted polarization studies to determine the illuminating source. The abstract from that study follows;

"The linear polarization of the reflection nebula NGC 1579 and the CO (1 to 0) emission from the associated molecular gas have been mapped for several minutes of arc around the exciting star LkH-alpha 101. These maps show conclusively that LkH-alpha 101 is the sole significant source of illumination in the region. The dust in the reflection nebula appears to be uniform over the illuminated region and is uniformly illuminated by LkH-alpha 101. Despite the patchy obscuration, the dark cloud which obscures LkH-alpha 101 does not
surround the star. LkH-alpha 101 may have formed out of a placental cloud whose remnants now include four molecular cloud fragments, two in front of and two behind the reflection nebula, as well as an H I cloud previously detected in the region."

Herbirg, et.al. (2004) reports;

"The central region of the dark cloud L1482 is illuminated by LkHalpha 101, a heavily reddened (AV~10 mag) high-luminosity (>=8×103 Lsolar) star having an unusual emission-line spectrum plus a featureless continuum. About 35 much fainter (mostly between R=16 and >21) Halpha emitters have been found in the cloud. Their color-magnitude distribution suggests a median age of about 0.5 Myr, with considerable dispersion. There are also at least five bright B-type stars in the cloud, presumably of about the same age; none show the peculiarities expected of HAeBe stars. Dereddened, their apparent V magnitudes lead to a distance of about 700 pc. Radio observations suggest that the optical object LkHalpha 101 is in fact a hot star surrounded by a small H II region, both inside an optically thick dust shell. The level of ionization inferred from the shape of the radio continuum corresponds to a Lyman continuum luminosity appropriate for an early B-type zero-age main-sequence star. The V-I color is consistent with a heavily reddened star of that type. However, the optical spectrum does not conform to this expectation: the absorption lines of an OB star are not detected. Also, the [O III] lines of an H II region are absent, possibly because those upper levels are collisionally deexcited at high densities. There are several distinct contributors to the optical spectrum of LkHalpha 101. The Halpha emission line is very strong, with wings extending to about +/-1700 km s-1, which could be produced by a thin overlying layer of hot electron scatterers. There is no sign of P Cygni type mass ejection. Lines of Si II are narrower, while the many Fe II lines are still narrower and are double with a splitting of about 20 km s-1. Lines of [Fe II], [O I], and are similarly sharp but are single, at the same velocity as the Fe II average. Work by Tuthill et al. allowed the inference, from K-band interferometry, that the central source is actually a small horseshoe-shaped arc about 0.05" (35 AU) across. A tipped annulus of that size in rotation about a 15 Msolar star would produce double spectrum lines having about the splitting observed for Fe II. The totality of observational evidence encourages the belief that LkHalpha 101 is a massive star caught in an early evolutionary state."

With all the evidence supporting a reflection nebula, there are still those that insist that if it can be imaged in H-alpha, it must be an emission nebula (one can image the moon and planets in h-alpha so using that logic, they also must be emission nebulae).>>

[bystander]

<<With all the evidence supporting a reflection nebula, there are still those that insist that if it can be imaged in H-alpha, it must be an emission nebula (one can image the moon and planets in h-alpha so using that logic, they also must be emission nebulae).>>

Wow, so we have an emission nebula orbiting the earth? A true planetary nebula? Or would that be lunatary? 8)

[neufer]

<<With all the evidence supporting a reflection nebula, there are still those that insist that if it can be imaged in H-alpha, it must be an emission nebula (one can image the moon and planets in h-alpha so using that logic, they also must be emission nebulae).>>

Wow, so we have an emission nebula orbiting the earth? A true planetary nebula? Or would that be lunatary? 8)

Get Sirius , bystander!

<<Sirius is the brightest star in the night sky with a visual apparent magnitude of −1.46, almost twice as bright as Canopus, the next brightest star. What the naked eye perceives as a single star is actually a binary star system, consisting of a white main sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. Class A stars are amongst the more common naked eye stars, and are white or bluish-white. They have strong hydrogen lines, at a maximum by A0, and also lines of ionized metals (Fe II, Mg II, Si II) at a maximum at A5. The presence of Ca II lines is notably strengthening by this point. About 1 in 160 of the main sequence stars in the solar neighborhood are Class A stars.>>