How to make an Helix: Part #2

Gravity, as we know it, has the nasty habit of being attractive. This means material left unattended in the vacuum of space tends to fall towards somewhere. Naturally, this presents very inconvenient implications in the long run for our goal of making a Planetary Nebula. It will not work to simply scatter atoms and molecules of Hydrogen, Oxygen, and Nitrogen, 200 pc away from a White Dwarf, expecting them to stay there forever shining like an over-sized gas-discharge light bulb..
Planetary Nebulae are dynamic structures, in constant expansion. Their birth may be far less explosive than Type Ia supernovae, though we can imagine some symptoms of stellar indigestion that culminate in large-scale pulsations or mass ejections. Perhaps an extreme exaggeration of how our sun often spits particles in solar storms might help illustrate how the atmosphere ends up leaving the star behind, in one big, stellar-sized "cough". We could imagine a Red Giant star pulsating through two or three such seizures before the last of them strips the core of the star naked, thus revealing the White Dwarf within.. The material then sails the stellar wind, away from the star, glowing in atomic colours..

If we would build our Planetary Nebula by leaving material lying around with no initial velocity it would all eventually fall back onto the star, causing some sort of forced refuelling of the moody White Dwarf. That could make the star angry, and likely result in heavier elements being created in an atomic fusion process.. Hmm, I just had a dangerously Ironic (Fe) thought, but its not our goal now, as fusing Iron is usually "super-novic" and would destroy our nebular creation..

Focusing back on our goals again, for creating our Planetary Nebula we what to mimic NGC 7293, which has some really interesting concepts and features! All that ejecta mentioned will have gone out towards space. Some faster, some slower, but all away from the central star. Again spectroscopy can lend a hand here, this time in "High Resolution" (dispersion: wavelength per pixel). Dispersion high-enough to make visible the Doppler shift of the expansion fronts.
If you imagine a semi-transparent baloon that is being inflated, in the middle of the balloon you will see the central star, and aside it you will see the surface of the balloon that is moving towards you, and the other surface moving away. So, for the colour that is being emitted on the "balloon walls" we should see half of it shifted to the blue, and another half shifted to the red. Seen in the image below, this particular shift of roughly 1.7 angstroms between the two walls translated to a velocity difference of, give or take, 49 km/s along the direction of sight, making the expansion rate of the "ionization front" of half that: 24,5 km/s, just like astrophysicists had previously measured! The spectrum also lets us know which parts of the nebula are moving toward us or away, as illustrated on the right.

NGC7293 Spectrum on Flickr. Spectrograph slit oriented nearly with East-West direction.

This particular colour wavelength we are looking at corresponds to a certain amount of ionization, as mentioned in the previous post. Radiation from the star reaches this area with enough intensity to ionize gas. It is thought that this ionization does not occur further outside because the density of this gas and dust prevents it. Some portion of the gas/dust may be "in the shade" from receiving UV radiation in the needed quantity to get ionized, thus we are not able to see that area in this wavelength. If we were to look at this object in CO wavelength (radio) we would find leftovers from previous mass ejections, and measuring their speeds we would find the values wikipedia states (32 and 40 km/s).

For our Nebula, we can further decorate the interior with the sort of detail we see on NGC 7293, arising from the various ejections at different velocities and their interaction with stellar wind! As result of the various ejections of gas and dust in the past, the space around the nebula has become crowded with chunks of material moving away from the star. A speedier ejection actually met up with this material left behind the now-stronger stellar wind is currently eroding its way through it. This is visible as cusps of matter that leave a cometary tail behind, as they get "eroded" by the radiation streaming out of the star.

So all this are good ideas for how to make and decorate a Planetary Nebula realistically. As a final tip, always keep in mind that a radiation-bounded nebula (where portions of if are hauntingly concealed from sight, shaded from the illuminating star) will keep people interested in it for longer time. Always keep your creations mysterious to the human eye; they always captivate new minds, and work as an excellent advertising tool for the wonders of space! I wish you the best of luck in creating your first Planetary Nebulae!

How to make an Helix: Part #1

Haven't you ever wanted to copy something you saw? ..And to author a portion of beauty through imitation?
Imagine we wanted to replicate one of the most charismatic planetary nebulas in the sky, NGC7293 - The Helix Nebula. What would we need for that?
This object's charisma depends greatly in its size! It looks big; it is close to us; it is actually the closest Planetary Nebula to us. Something we build, to be equally amazing would need to be located near the observer, also.

One could start off with a dying Sun-like star! The dying star would evolve from the "Red Giant" to the "White Dwarf" stage of stellar life, and in the process eject material from its atmosphere. When in the White Dwarf stage, strong stellar winds would blow the gas further away, and intense ultraviolet radiation should make it glow ionizedly pretty to our eyes.

Perhaps we could use our own Sun for this purpose; the trouble being the need to wait quite some time for our star to construct something similar!
Maybe we could fake all this, and spread the material in space around an existing white dwarf, giving it positions and speeds identical to those in the Helix Nebula. This would be a faster way to "copy" the Helix, at least within our imaginative brain... However, we still need to know what material to use for the gas, so it glows in the same manner as in the Helix.
For this, spectroscopy is handy, as it allows us to peek at what colours the Nebula is shining off!
Here is a spectrum, in visible light, of the nebula across its diameter! Each colour is separated horizontally, and the the (Esat-West) diameter is represented vertically. The faint horizontal lines are actually stars, that shine in all colours like very hot bodies do.

NGC7293 spectrum on Flickr.

Those colours are the result of ionizing the gas via intense radiation by the central star. visible from left to right are wavelengths of light corresponding to Hydrogen, Oxygen, then more Hydrogen and Nitrogen.

As for the quantities for each element, that is trickier to know. But we can try to match the intensity of colours, adding more or less atoms to get the right intensity of each colour. We should keep in mind, though, that what we see of this nebula is radiation-bounded; this means we only see the portion of nebula that is made visible by the radiation of the central star, and then the gas and dust of the nebula itself can cast shadow over more "invisible" and not yet ionized material.

So we now know what to put in a "New Helix" recipe; next we need to know how fast all of it should be moving in a bubble shape, to make it really realistic! For that, stay tuned for Part Two of "How to make an Helix"!

I hope this shed some light on how to properly replicate a planetary nebula, in case you ever stumble across the need for such endeavour..

Eppure Egli Ruota

Shortly after remaining alive -- or immediately before that -- Galileo Galilei allegedly muttered "Eppur si muove", in a legendary reference to a certain rocky planet that moves around a solar-type star in the habitable zone of our own solar system!.. Legend says he would say such a thing in presence of some irrefutable evidence that the given earthy planet would, in fact, show the aforementioned movement...
Well, I now say "Eppure Egli Ruota" inspired by this...
I took a picture of Jupiter; not a normal picture, though!.. In between the telescope and the sensor there was a weird light-wasting gadget capable of spreading wavelengths of light along one direction of the focal plane.. Science guys refer to it as a "Spectroscope"!..
The thing is it has enough resolution in picking out different wavelengths to actually reveal the Doppler effect on "stuff" moving at sufficiently fast speeds and in adequate directions. Here is what you get when pointing the slit spectrograph to Jupiter:

The horizontal "line" has a thickness identical to the diameter of the planet. The vertical dark lines are particular colors that our atmosphere absorbs. The oblique darks lines are absorption lines from the solar light itself, being reflected by the moving (turning) "surface" of Jupiter!
The inclination of the lines is intrinsically related to the speed at which the planet is turning. Calculating it only roughly, gives me a value between 8 km/s and 10 km/s that is a bit below Wikipedia's mentioned speed of 12.6 km/s. I did not have a special care in aligning the slit with Jupiter' equator, so this is well expected!
So, this Ziggy-zaggy picture is, actually, one other way of "seeing" Jupiter from Earth.. Here, Jupiter is not round-looking, it seems quite still.. Yet, "he" turns!

A Galaxy through another one

NGC 6946 RGB, a photo by Filipe Dias on Flickr.

After a complex day, last night I managed to escape to a reasonably dark place, and take my mind away from some of the worries that living people tend to have while alive..
Right through most of the stars that make our own galaxy, we occasionally get a glimpse of other galaxies lying around.. This particular one, NGC6946, is "face on", easily displaying its spiral arms, and is one of those that are most "near-by" our own Milky Way galaxy. In the same picture there is an Open Cluster of stars, NGC6939, that resides in our galaxy. Lots of scattered stars are equally visible, belonging to a region just 11 degrees away from the plane of our galaxy, in the South-West corner of constellation Cepheus.

New bolts

So, yes, I am now into astro-spectroscopy! :)
One of the big secrets about spectroscopy with reflective gratings, that few people mention, is that it can get heavy! So heavy, in fact, that your telescope may find its way into impossible balancing situations!
In order to balance the weight, I had to improvise "stuff" attached to the telescope tube in order to balance it.. It worked, but was not a pretty sight.
Today I went to the historical "Casa dos Parafusos" (bolts house) and bought 4 screws to attach a larger accessory plate to my mount so that I can attach a counterweight more securely. This 65-year old shop had a really historic feel to it, a few years ago. Two years ago I found it closed. It went through a lot of difficulties, but the PECOL group managed to "save it" last year. I was very glad to finally see it open and working! The shop was completely transformed! It now has a modern space with stuff on display, it has two desks for attending costumers, and big warehouse with all sort of bolts in stock. The shop is no longer a tourist destination, but if you need an odd bolt or a common bolt, it is still the place to go!

A closer look at Deneb

How close can you get to Deneb, the brightest star in constellation Cygnus?

Well, it kind of depends on how you measure "closeness".. If you think of "distance", as in a single-, two- or three- dimensional spacial measure, you could imagine "closeness" to be a specific value for that distance that is small enough compared to other reference values.. Therefore, the closest all we earthlings can physically get to Deneb is likely to be around 1400 light-years, give or take..

If you base your assumptions for "closeness" on comfort and planet-habitability concerns, you could possibly say that an observer could inhabit a big rock at a distance of -- give or take -- 245 AU from it (square root of how more luminous it is compared to our Sun, multiplied by our nicely habitable distance to our star), and still be able to drink a glass of refreshing liquid Dihydrogen Monoxide potion at ambient temperature...

Well then, what if I were to define "closeness" as, not a ratio of spacial physical distances, but instead a ratio of lengths (wavelengths) of the radiation we observe? Imagine I talk about "colour", and how close I can distinguish one colour from a more distant one! One emission or absorption from a chemical element from the other element!... Or, for instance, from one stationary element, and one moving element. Or even how colour can change with density of the material, and thus, star size!.. How is this for "closeness"?


So here is a raw spectrum (uncalibrated) from Deneb in the vicinity of H-alpha! The absorption lines in the spectrum are "rather thin", and so the star is "not very dense", and in lacking mass that can hold down its size it got big (wikipedia tells me it can be way over 100-suns-big)!
The odd V-shaped absorption in the H-alpha, on the right side of the spectrum, comes from the atmosphere of the star or things above it, whose Hydrogen atoms are absorbing radiation in a wide region. The shape of this profile actually reflects some behaviour in action.. Stellar winds, storms, and mass ejections, for instance!
The thin emission lines present, and marked as "lamp" are from a lamp of known spectrum, placed in front of the telescope, to be used for spectral calibration.

So how close did I get to Deneb? Each pixel in this graph is only 0.33 Angstroms of Deneb's colour, and it only reflects visible colours going from the so-or-so-red (640 nm) to the slightly-more-red (660 nm). This was close enough to burn quite some neurons of my brain! Ouch!

Recurrent Novae

Re-boom!

Expecting this? Perhaps not! Why on Earth (or outside it) would yet another "bang" be heard? You may have noticed the metaphor behind the first of a series of posts in this blog.. The first post was the "Big Bang", understandably. But at the beginning of each burst of posts, other kaboom-ish sounds would be encoded in post titles as if stellar light were sound..
While to some this blogging effort could resemble an old engine's exhaust, thumping in need of maintenance and regular use, I prefer to see it as a Recurrent-type of Nova.. One with no discernible periodicity.. One that shines brighter when it decides to..

So, do expect new posts with possibly interesting stuff attached to them..

Meanwhile... Recurrent Novae are Cataclysmic-type Variables, formed by multiple-star systems, where one of the stellar members decides to pull a prank on its close-by neighbour stealing some matter, and unleashing some rather luminous fireworks display in the process. They can happen once in "some years", denoting the short distance between stars. The most recent recorded outburst of a Recurrent Nova was that of Tau Pyxids, earlier this year, in April. It is still rather bright at (V) 8~9 mag.