M42 - The Great Orion Nebula

I've been meaning to try out Astro Pixel Processor (APP) for a while now with a view to buying it as my main astro processing software, but I've not had any image data which I regarded as being worthy of extensive processing until now.

Following my successful Team Viewer based remote setup using a long weatherproof network cable from my house into to my garden, I can now control my laptop for finding targets, controlling my mount and capturing image data from the warmth and comfort of being indoors.  I use Carte du Ciel to locate the target and Astro Photography Tool (APT) to plate-solve the image and centralise it in the field of view.

The first target I chose was M42, the Orion Nebula, my thoughts being that if I can't get decent images of this then I might as well give up.  I setup an exposure plan in APT to capture a range of exposures from 10 seconds to 120 seconds, in order to have correctly exposed shots of both the core and the outer nebulosity.  I also took some dark frames with the same exposure and corresponding bias frames, but no flats.  Then I loaded them all into APP.

I'm not going to go into the processing details here as I'm still on the steep learning curve of this sophisticated software, but APP calibrates the main images (lights) with darks and bias frames, before analysing the stars it finds, aligning or registering the images, normalising them and finally stacking or integrating them into a single image file.  All this is done automatically with no intervention if the default values are used.

Following integration, the image can be stretched to widen the dynamic range and reveal details unseen in the initial images and various other adjustments made before being saved.  Although the image APP produces is pretty good, it is normal to process it further in a graphics package such as PhotoShop or GIMP to tease out even more subtle detail. And this is what I have done here.  

So these images all stem from the same set of data captured in APT and calibrated and stacked in APP.  You can play around with the colours and fine detail for ever and it's difficult to know when to stop.  Different techniques produce different colours and effects, it's up to the viewer to decide which they like best.  To some extent it's impossible to know what the true colours are as we can't actually see them with the naked eye.

The Main Sequence

In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar colour versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams (see below) after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. Stars on this band are known as main-sequence stars or dwarf stars. These are the most numerous true stars in the universe, and include the Earth's Sun.

After condensation and ignition of a star, it generates thermal energy in its dense core region through nuclear fusion of hydrogen into helium. During this stage of the star's lifetime, it is located on the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and age. The cores of main-sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward pressure of gravitational collapse from the overlying layers. The strong dependence of the rate of energy generation on temperature and pressure helps to sustain this balance. Energy generated at the core makes its way to the surface and is radiated away at the photosphere. The energy is carried by either radiation or convection, with the latter occurring in regions with steeper temperature gradients, higher opacity or both.

The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. Stars below about 1.5 times the mass of the Sun (1.5 M☉) primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton–proton chain. Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms. Main-sequence stars with more than two solar masses undergo convection in their core regions, which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur. Below this mass, stars have cores that are entirely radiative with convective zones near the surface. With decreasing stellar mass, the proportion of the star forming a convective envelope steadily increases. Main-sequence stars below 0.4 M☉ undergo convection throughout their mass. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen.

In general, the more massive a star is, the shorter its lifespan on the main sequence. After the hydrogen fuel at the core has been consumed, the star evolves away from the main sequence on the HR diagram, into a supergiant, red giant, or directly to a white dwarf.