Sunday, October 25, 2015

planetary nebula primer III, morphology

Planetary nebula can come in a variety of shapes.  Here are more old images demonstrating morphology.  Although relatively rare, some are nearly perfect spheres, the undisturbed exhalation of a dying star:

Abell 39:
Abell 34 "oozes faintness":
note the tiny background galaxy superimposed at the bottom edge

Most have more complex shapes.  The reason for the unusual shapes is not well understood.  Theories include interactions between the ejected gas layers, magnetic fields, planetary systems, and binary stars. 

Minkowski's Butterfly is thought to have a binary star orbiting
the white dwarf causing the central constriction with jets above
and below:

Here are a few classic bright-round-things-about-the-size-of-a-planet:

Cat Eye
Ghost of Jupiter

Clown Nebula
Blinking Planetary

Often there are faint outer layers evident on longer exposure reflecting multiple episodes of the central star shedding outer layers:
Cat eye
Blinking Planetary

Many are bipolar structures with a ring-like central constriction.  M 76 and M 57  are an example of two sides of the same coin.  

M 76 the little dumbbell left               M 57  the ring nebula right
side view of a ring                                     face on view of a ring
M57 (right) shows a bright ring with a very faint outer shell.  
Rotate that 90 degrees and you've got M76 (left) with the side view of the bright central ring appearing as a rectangle and the faint outer ring revealed as expansions blowing out on either side of the ring. 

Some have pairs of jets, ejecting material, typified by the saturn nebula:
Saturn Nebula

The jets are classically called ansae (wings), either way they are flying things.

Some of the jets have low energy NII regions at the tip aptly known by the acronym FLIERS 
(Fast Low-Ionization Emission Regions) which are more evident when a red NII filter is used:
Saturn Nebula red NII FLIERS

Blinking Planetary red NII FLIERS
IC2149, the Easter Egg Nebula
FLIERS appear to be relatively young, moving outwards at supersonic speeds.  Though poorly understood, the colorful wingtips make for nice images.
IC 4593 White-Eyed-Pea
IC 4593 displays many of the properties above and then some:

-small bright planetary

-faint outer core

-bipolar jets


-BONUS: the outer core is deformed by interaction with interstellar medium generating a bow wave upper right

More planetary nebula images here

A more scientific account of the modern concepts in planetary
nebulae morphology:

Shape, structure, and morphology in planetary nebulae

Richard A. Shaw

Tuesday, October 20, 2015

astropics: planetary nebula primer II white dwarfs, helium, and diamonds in the sky

A planetary nebula results from a white dwarf illuminating the gas that was ejected by a dying star...

Basically, a star is big ball of gas, mostly hydrogen.
The star is so massive that gravity forces the material to collapse in on itself.
The compression/temperature at the core becomes so great that hydrogen ions fuse forming helium and releasing a tremendous burst of light/energy which, in turn, pushes the gas back out preventing further collapse.  When a medium sized star starts to run out of hydrogen to fuse, it begins to collapse again until helium fusion begins, causing the star to expand and, expel portions of its outer layers.  After several iterations of this, helium fusion ceases and there's nothing left but the very hot inner core surrounded by expelled gas. 
Since fusion has stopped, the central star can be though of as dead/inert, doing nothing but cooling down until the end of time.  While one might think of them as "dead", the massive compression makes white dwarfs among the hottest "stars" at 50,000-100,000 K; the cooling takes billions of years.  This white dwarf is very small and therefore faint, but gives off much higher energy photons than most stars.  For example, ionized helium (very high energy transition) is rarely seen except in planetary nebulae around white dwarfs. 

Here are older images of two of the most famous planetary nebulae, shot with narrow band filters showing high energy blue helium in the center, teal/green oxygen further out, with low energy red nitrogen in the outer ring, or mid-substance pillars (also note the small bluish central star/white dwarf):
M 57, the ring nebula:

M 27, the dumbbell nebula:

Fun Factoids:
In the late expansion phase, the star grows to 100 times it's original size and cools, becoming a red giant.  Since the red giant is emitting light from a much larger surface, it's much brighter than the original star (example Betelgeuse, Orion's shoulder).  Conversely, a tiny white dwarf is very faint, but hot, emitting much higher energy photons which can ionize helium while the brighter red giant cannot--the photoelectric effect on a cosmic scale. 

Fusion of helium etc can continue on up to oxygen and carbon.  When the planetary nebula sheds its outer layers, it sends carbon oxygen etc into outer space, allowing future solar systems to form with higher oxygen and carbon content--cosmic recycling.  

Rather than collapsing into a white dwarf, a larger star explodes forming a supernova. 

It is thought that some of the coolest white dwarfs crystallize, and since they are primarily made of carbon/oxygen...yup, you got it, planet-sized diamonds.  Which of course is great material for comic-sci-fi, pink floyd songs, etc.

Image details:
M 57 8" LX200R, SX AO, IDAS LPR, SX H9, H9c
astrodon 5nm Ha, 5nm OIII, 3nm NII, CS 10nm HeII, OO 5nm HeII filter, IDAS LPR
RGB 29x20 min
Ha 34x20 min, NII 23x20 min, OIII 22x20 min, HeII 31x20 min

M27 NII 34x20 min, OIII 12x20 min, HeII 30x20 min
8" LX200R, SX AO, SX H9
astrodon 3nm OIII, 3nm NII, CS 10nm HeII,
Los Alamitos, CA Bortle white skies

Sunday, October 11, 2015

little gem nebula NGC 6818, planetary primer I

Planetary nebulae are clouds of glowing gas surrounding a “dying” star.  Visually, many are relatively bright grey patches (by deep sky object standard, though very faint compared to an actual planet).  Spectrographically, they typically have very strong doubly ionized oxygen (OIII) emissions compared to other nebulae.  An OIII filter is a handy tool for the amateur astronomer, enhancing these nebulae and confirming their identity.  Though not detectable visually, they also tend to be relatively strong in singly ionized Nitrogen (NII) and singly ionized Helium (denoted HeII, which I may avoid going forward due to the unfortunate confusion). 

NGC 6818 aka the little gem is a small bright planetary nebula in sagittarius.  Roughly planet sized and near the ecliptic (path of the planets through the sky) one can see why it gets the moniker “planetary”, but at roughly 6,000 light years away it’s 1/2 light year across.  The blue/teal in this image is due to OIII emissions and red due to NII emissions. 

Here’s the OIII



8" LX200R, SX Trius 694 0.4"/px
astrodon 3nm NII, 3nm OIII
NII 19x20 min OIII 24x5 min


Wednesday, October 7, 2015

planetary nebula Abell 72

Abell 72 is a faint planetary nebula
It's weak signal is almost all (teal/blue) OIII (doubly ionized oxygen)
the blue central star shows nicely in this image:
the other interesting structure in the field is the small faint galaxy pair just below the nebula.
at 979 million light years distant, the pair is quite far in the background.  it took 7 hours of exposure with a narrow band oxygen III filter to bring out the faint planetary nebula. an additional 9 hours of broad band exposure were required to bring out the faint galaxies.  interestingly, the nebula was barely detectable in the broad band image, showing the power of narrow band filters in light polluted skies.  

8" LX200R, SX-AO, astrodon 5nm OIII filter, chroma LPR filter, SX H9c/H9, ASA DDM 60

luminance 56x5 min, RGB 14x20 min, OIII 22x20 min