Discovered by William Herschel in 1790, NGC 1491 is a bright emission nebula in Perseus about 12,000 light years distant and about 21 light years wide. Despite its large size, its distance from Earth means that it appears visually small.
The HII region displays a great deal of structure. The 11th magnitude white/blue type O5 star BD+50 866, located near to the centre of the image below, illuminates the nearby hydrogen gas with high intensity ultra violet radiation, ionising the gas. Material ejected by the star has also created curtain fold and half bubble effects in the surrounding HII region. The bubble, which is closer to the star, is probably the result of high speed, gas outflows that push nearby gas and dust outward, creating a shock front and forming a layer around the star with variable density. A similar effect also seen in the bubble nebula, NGC 7635. Further from the star, the ionised ejected gas gradually slows down as it expands into non-ionised hydrogen. The boundary between the ionised non-ionised hydrogen is called an ionisation front which may have created the curtain fold.
I decided to try to extract as much detail as possible by acquiring a significant amount of Ha data which was then blended into the Lum and Red channels. The structure within the brightest regions was better revealed after application of PI’s HDRMT.
The image below represents 21 hours integration time and was taken with my Esprit 150.