For a quick dive into all the data obtained, see
https://docs.google.com/spreadsheets/d/1ZBHiK6EDUbIbYOH8RJSuY1CsfLG7TMRB5gpkYqIBU7o/edit#gid=1028853181
INTRO
Understanding how the Stellar Forge works to build the galaxy, this astonishing mimicry of the real milky way, is an undertaking as old as the game, with many CMDRs working on that subject, examining a multitude of properties like star class distribution, mass distribution, stellar age or stellar density and stellar metallicity during DW2. The most recent effort to measure stellar density has been the AX15 Expedition science project by CMDR Satsuma which I took part in and which led me to start my own little density mapping project. It also reinvigorated a long-held fascination of mine: The structure of the galaxy. It is one of the most interesting aspects of the representation of real-life cosmology in-game, and the most prominent features of that structure undoubtedly are its magnificient spiral arms, the formation of which is widely explained with the density wave theory. Which begs the question: Does the Stellar Forge act in general accordance with the density wave theory? The AX15 mission confirmed the expected correlation between stellar density and location relative to the spiral arms, but stellar density is but one measurable property. So I decided to include two additional ones in this examination which might shed some more light on wether the Forge takes into account the DWT: star type and stellar age. Thus, the Tremolino Project was born, named after my exploration ship and it's route toward the core (and the probably overly shaky results), to collect data about stellar density, stellar age and star type and to find out if density waves are just a myth or indeed alive and well in the ED galaxy…
battered, but not beaten - the "Tremolino" close to it's destination
GOALS
1) Testing the density wave theory regarding expected distributions of stellar types and stellar age and differences in stellar density among arm and interarm regions
2) Examining variations in average stellar density and age between the upper (z>=0) and lower (z<0) layer of the galactic disk and bulge
3) Examining average stellar density as a possible predictor of average stellar age
HYPOTHESES
1) The front and midst of stellar arms contain more blue, young stars, the backside and the arm gaps more reddish, older ones (frontside is defined here as the side of an arm where stars orbiting the galactic center enter it, while the backside is the opposite side where they exit the arm if they live long enough to do so. For simplification, when looking at the map below, the frontside is the "upper" side of the arm, the backside the "lower side").
2) As a result, average stellar age is lower in the front, gradually rising as one progresses through the arm towards the backside, with the highest value outside the arms.
3) Average stellar age and stellar density are connected, showing antiproportionality - the lower the stellar density in a region, the higher the average age and vice versa.
4) This connection breaks down together with all density wave patterns at the radius of the inner Lindblad resonance (ILR), approx. 9.700 ly from the galactic center, at the edge of the galactic bulge.
5) The abundance of Population II stars is higher in the bulge and as a result of that, average stellar age is higher in the bulge than inside the disk.
METHODOLOGY
I plotted a direct course from the southern edge of the Scutum-Centaurus Arm (System: Prieleau HX-W c1-6) to Sagittarius A* using Spansh and made density measurements approx. every 500 ly on the way, using the center-weighted (CW) method incorporated by CMDR Satsuma during the AX15 Expedition science project (see more on that and the techniques used here). To minimze selection bias when choosing star systems inside the columns, I set the galmap mode to visited/not visited stars. When picking a star system, I tried to prioritize systems closest to the z-sampling-levels (heights) over those nearest the x/y coordinates of the uppermost system, where each column started. I tried my best to make the columns straight, keeping x/y coordinate aberations from the uppermost system at 10 ly or less when possible.
The result were 39 density columns 500 ly high (from z=250 to z=-250), each consisting of 25 systems and density values for a sphere with a 20 ly radius around them. In addition, I recorded star class and age of those systems' primary stars.
All data was gathered in Horizons.
I counted the number of dim, lower mass and bright, higher mass main stars in the columns, defining lower mass stars as Y, L, M and K class stars and higher mass stars as O, B and A class stars. To have an additional data point regarding star class encapsulating more systems, I used EDSM's API to find the primary stars of systems in a 50 ly radius around the z=0 system of the corresponding column (for example: https://www.edsm.net/api-v1/sphere-systems?systemName=Oephaik PW-G b10-219&radius=50&showPrimaryStar=1) and again counted the lower/higher mass stars. Since this was mainly done to determine wether the star classes corresponded with the actual dimming/brightening of the arms while moving through them and that connection should break down together with the Lindblad resonance @ approx. 9,700 ly from Sgr A*, this additional EDSM data was only taken for columns 1-20 (9,000 ly from the center).
Next, the number of Pop II stars was counted. I counted all stars older than 11,000 myr. Finally I added a table column indicating wether the density column was located in an interarm gap, the back, or the front of an arm according to its position on the EDSM map.
an overview of the data obtained inside the galactic disk
This was purely done by eyeballing and accuracy therefore is rather fuzzy, but it gives you a general idea of how areal brightness at the columns changes throughout the voyage to Sgr A*.
the route taken
RESULTS
1) Relative brightness as perceived on the EDSM map seems to be correlated with the number of bright stars in the column, especially at the front of the arms. This is the case for both the column-measured and the EDSM API obtained datapoints for the number of bright stars.
eyeballed back, midst and front of galactic arms and stellar age, as well as number of bright and dim stars
2) The hypothesized correlation for dimmer stars isn't found; Dimmer stars aren't more numerous in the back of the arms or the interarm gaps, suggesting a more uniform distribution across the galaxy's structures.
3) Stellar age changes significantly when moving through an arm. The proposed age diversity between front/midst and back/gap seems to exist. Average stellar age tends to be higher at the back, gradually decreasing while traversing the midst (where most bright stars reach the end their lifespan) and the front of the arms (where the most stars are born) which is consistent with RL-observations of stellar ages.
4) There is a correlation between the number of bright stars and their position in the arms. However it isn't as conclusive as expected (see the datapoint for the midst of the Norma Arm where there is a spike of dimmer stars and a notable absence of bright stars in the column). Futher examination is required. All in all, it is save to say that the further the distance from the front of an arm inside the ILR area (outside of the 9,700 ly radius from the center), the lower the concentration of bright stars, consistent with the eyeball-observation result in 1).
5) However, the proposed antiproportionality between average stellar age and stellar density isn't present as the range in density rises higher the nearer one comes to the galactic center, while the stellar age naturally remains in a fixed range. This shows that stellar density by itself was a poorly chosen parameter for measurement against stellar age. Instead, one could derive another measurement for density that stays in a fixed range (maybe something like "stellar density relative to local brightness"?), but that would go beyond this examination. What is clear is that the methodology is far from optimal and needs further development, so that's a win in itself, I guess?
6) Lastly, the expected higher abundance of Population II stars inside the bulge (everything in a 9,700 ly radius around the core) compared to the abundance in the disk couldn't be found. Instead, the exact opposite was the case. Average stellar age correlated with these findings, showing 5,131.99 myr for the bulge and 5,550.57 myr for the disk.
INTERPRETATION
Based on the results above, the ED-galaxy seems indeed to mimic the expected star distribution both in density and brightness one would expect from the Density Wave Model and the expected stellar age gradient across the arms was observed as well. So according to this observation, density waves are indeed a thing in Elite Dangerous! Kudos to Fdev!
The expected higher number of Pop II stars in the bulge isn't represented. But, as mentioned under "Constraints", this could easily be because of the very limited sample range provided in this examination.
In any case, further studies need to be made to confirm both of the findings above and make them less... jittery.
CONSTRAINTS
1) Lack of data, obviously. 975 star systems where observed, which, when stretched over 18,500 lightyears in length and 500 ly in height, really isn't much, although it may seem that way at first glance.
2) Methodology, especially for finding a link between stellar age and stellar density. A new parameter has to be chosen to adequatly compare the two and find a direct proportionality of any kind between them.
IDEAS FOR FURTHER EXAMINATIONS
It would be interesitng to see wether the Pop II star abundance is consistently higher in the disk when approaching Sgr A* from different points of the outer edge of the disk. Generally, more jigsaw routes toward Sagi like the one I used from different parts of the galaxy would help to determine the accuracy of the findings above. If anyone has comparable data, please submit it here and I will try to incorporate it into the sheet
As always - the more data, the better!
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