Astrography and Accuracy

[Reban]

I have a question for COTI members more knowledgeable than me about astrography, astronomy and the Milky Way Galaxy.

Recently I rediscovered this great illustration fro Security Leak Magazine

It shows the Local Arm of the Milky Way Galaxy and the situation and general layout of Charted Space. The illustrations from that issue were based on an article "Our Galaxy: The Milky Way by Astronomy Magazine" from about 1988.

My question is: How accurate is this map?

Since we are located in the middle of the map and our only observations are from there we only have an educated guess at what our galaxy looks like. In big picture terms thats fine.

But when it gets down to this level of detail how certain can we be what it looks like?

Specifically i'd like to know:

1. Does the Great Rift exist? Or is it a figment of the Traveller universe?

2. Is the general shape of the Local Arm correct? Now I believe that since this illustration was made observations suggest that the Local Arm is wider than originally thought, but are the edges right, or at least plausible.

3. Is there a better Real World illustration of what the Local Arm looks like? Not the pretty pictures I can Google that are artists impressions but a map showing the general shape and distances.


Now I'm well aware that Traveller is only a game and not always hard science, but I'm thinking about the advent of Hop and Skip drives and the way this area opens up for exploration. I'm also interested in this for an ATU in which the speed of jump is measured in parsecs per day rather than parsecs per week.

 

[whulorigan]

Specifically i'd like to know:

1. Does the Great Rift exist? Or is it a figment of the Traveller universe?

There actually is a Great Rift located at about this distance and direction from Earth:

https://en.wikipedia.org/wiki/Great_Rift_(astronomy)

However, it is composed of overlapping absorption clouds at varying distances which block visible light, and is not a void as it is in Traveller, but rather likely are regions of star formation. It merely appears to be a continuous band from the vantage point of Earth.

2. Is the general shape of the Local Arm correct? Now I believe that since this illustration was made observations suggest that the Local Arm is wider than originally thought, but are the edges right, or at least plausible.

Keep in mind that the galactic arms are largely a visual phenomenon due to the fact that they are regions of higher gas/dust density and thus are regions of current star formation. The reason why they appear prominent is that they contain many young hot blue and white stars as a result. The region between the arms has plenty of stars as well, but they tend to be older and dim red stars (the young bright ones having already lived out their lives and become white dwarfs or neutron stars).

 

[kilemall]

Keep in mind that the galactic arms are largely a visual phenomenon due to the fact that they are regions of higher gas/dust density and thus are regions of current star formation. The reason why they appear prominent is that they contain many young hot blue and white stars as a result. The region between the arms has plenty of stars as well, but they tend to be older and dim red stars (the young bright ones having already lived out their lives and become white dwarfs or neutron stars).

So, kind of an interstellar Fort Lauderdale then.

 

[whulorigan]

So, kind of an interstellar Fort Lauderdale then.

Intergalactic Arms Stellar Retirement Community . . .

 

[Reban]

There actually is a Great Rift located at about this distance and direction from Earth:

https://en.wikipedia.org/wiki/Great_Rift_(astronomy)

However, it is composed of overlapping absorption clouds at varying distances which block visible light, and is not a void as it is in Traveller, but rather likely are regions of star formation. It merely appears to be a continuous band from the vantage point of Earth.

Well that solves that: not a void. That's okay by me as Hop Skip and IMTU drives make the idea of a void or sparse region thats difficult to cross less important.

Keep in mind that the galactic arms are largely a visual phenomenon due to the fact that they are regions of higher gas/dust density and thus are regions of current star formation. The reason why they appear prominent is that they contain many young hot blue and white stars as a result. The region between the arms has plenty of stars as well, but they tend to be older and dim red stars (the young bright ones having already lived out their lives and become white dwarfs or neutron stars).

Yes I think what I'm looking for when I refer to "edge" is the region where the kind of star systems Travellers visit (i.e. the kind Traveller generates) falls off.

Or another way of putting my query:

If I draw a map of this area of the Local Arm based on our current observations and understanding, how will it differ to the 70s and 80s science based and pure fiction maps.

 

[whulorigan]

Yes I think what I'm looking for when I refer to "edge" is the region where the kind of star systems Travellers visit (i.e. the kind Traveller generates) falls off.

Or another way of putting my query:

If I draw a map of this area of the Local Arm based on our current observations and understanding, how will it differ to the 70s and 80s science based and pure fiction maps.

I am not aware of a good illustration that specifies what you are looking for. The write-up I did on the TravellerWiki for the Milky Way may be of some help to you, but will not specifically answer the question you asked.

 

[Reban]

Thanks Whulorigan. That's a good write-up and I think you're right; no one has made the kind of map I'm looking for.

However I think there's a way to model what I'm talking about using T5 star generation.

The megasectors (200pc x 320pc) within the density wave of the arm can be designated as normal density i.e. 50% chance of a stellar presence. While those outside the arm use the 3% chance of a stellar presence found in Rifts.

The result is blocky, but it allows me to draw in "edges" to the arm.

Actually in GT The Best of JTAS Vol 1 there's a similar method used to map this area but it uses hex maps at 1000pc and 100pc scales.

I think I'm lacking two bits of information; how wide is our bit of the Orion arm/spur, and are there any other major bits of 'terrain' that needs to be included?

 

[whulorigan]

The megasectors (200pc x 320pc) within the density wave of the arm can be designated as normal density i.e. 50% chance of a stellar presence. While those outside the arm use the 3% chance of a stellar presence found in Rifts.

The result is blocky, but it allows me to draw in "edges" to the arm.

I would also suggest biasing rift-stars in favor of K and M-type dwarfs and/or white dwarfs. You should have very few blue, white, or yellow "stragglers", as they would have lived out the course of their lives long ago. Those that are there would be "passing thru" the region by chance on their galactic orbit, or might have formed as the result of a localized random gas/dust density concentration.

Perhaps you could use an additional FLUX roll tailored to produce the results you want.


BTW, there is this from the Wikipedia article on the Milky Way that may be of some use:

The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, and in the Gould Belt, at a distance of 8.33 ± 0.35 kiloparsecs from the Galactic Center. The Sun is currently 5–30 parsecs [North of] the central plane of the Galactic disk. The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 parsecs.

 

[Lycanorukke]

I think I'm lacking two bits of information; how wide is our bit of the Orion arm/spur, and are there any other major bits of 'terrain' that needs to be included?

5000 ly with major stars (zoomable)

Orion Arm - 2000ly from Sol (including clusters and nebulae)

Perseus, Orion and Sagittarius arms (10,000 ly)
It shows the Orion spur is about 2000ly across at our point.

 

[Reban]

Thank you both.

I was aware of the Atlas of the Universe site but forgot to get the the measurements from it.

I suppose I should note at this point that this is "big picture thinking". By that I mean if you have access to Hop and Skip drives or alternate drives that open up more space, you should have an idea of what that space looks like.

When it comes to generating Sectors, Sub-sectors and systems each Traveller version has the necessary tools.

Within one of these MegaSectors with its 200pc x 320pc area there will be sectors and subsectors that are Rift, Standard or Dense in terms of star population.

So the big picture thinking at this scale doesn't have to go into a lot of detail. Features that are smaller than say 50pc or smaller than a Sector don't have to be accounted for.

 

[estar]

There is no better site for this stuff (and all things space sci-fi wise) than Winchell Chung's Atomic Rockets

http://www.projectrho.com/public_html/rocket/spacemaps.php

http://www.projectrho.com/public_html/starmaps/mapindex.php

 

[G. Kashkanun Anderson]

I would also suggest biasing rift-stars in favor of K and M-type dwarfs and/or white dwarfs. You should have very few blue, white, or yellow "stragglers", as they would have lived out the course of their lives long ago.

Not that many G-type stars have left the main sequence; some of the dimmer ones, in fact, couldn't have yet, as the universe simply isn't old enough. And F-type stars can live for about 3-7 billion years too, depending on their subtype.

Population-wise, you could have about one star in ten be a main sequence yellow, with about a 3/1 ratio of G-types to F-types. Ones in-between the spiral arms might incline towards a lower metallicity than those in the spiral arms, but they still should be well-represented in the galactic population.

 

[Reban]

There is no better site for this stuff (and all things space sci-fi wise) than Winchell Chung's Atomic Rockets

http://www.projectrho.com/public_html/rocket/spacemaps.php

http://www.projectrho.com/public_html/starmaps/mapindex.php

Excellent Estar. I haven't kept up with the ProjectRho updates.

The Galactic Map 2.0 seems exactly the guide I was looking for.

There are even several maps from 'The Guide to the Galaxy by Nigel Henbest and Heather Couper' which is referenced by the GT Best of JTAS article I mentioned earlier.


We are here, famous star over there, big nebulae that way, the big evil empire hides behind that dust cloud. The borders of our mighty empire stretch 10,000ly in all directions

By this i mean not every know star has to be represented on a map of the OP's ~200pc scale. It just has to give a general idea of the geography within reach. Detail comes when a referee sits down to generate a sector.

 

[whulorigan]

Ones in-between the spiral arms might incline towards a lower metallicity than those in the spiral arms, but they still should be well-represented in the galactic population.

That brings up an interesting follow-up question: Of the low-metallicity and/or Population II stars, what is the relative probability of them having meaningful planetary systems? Many of the current astronomical surveys are revealing that even low metallicity stars have planetary systems, but how much less likely is it that such a star would have Terrestrial or Jovian worlds (or even some analog to a Kuiper Belt) as compared to higher metallicity or Population I stars, based on current observation?

This might be meaningful to Reban's initial post if inter-arm systems have fewer refueling points or meaningful worlds as stop-over points in the inter-arm regions.

 

[Reban]

The Orion Spur

So I've come up with some figures for the size of the Orion Spur based on information from the Spitzer Science Center

- 3500 Light Years across
- 2000 Light Years deep
- ~10,000 Light Years long

Converting that to parsecs (3.26LY to 1Pc)

- 1074 Parsecs wide
- 614 Parsecs deep
- 3068 Parsecs long

For Traveller mapping purposes the depth figure isn't needed (Jumpspace is flat right? :CoW

Using the 200pc x 320pc megasector grid that is eminently map-able. The original SecurityLeak map seems to get the width about right.

 

[JimMarn]

5000 ly with major stars (zoomable)

Orion Arm - 2000ly from Sol (including clusters and nebulae)

Perseus, Orion and Sagittarius arms (10,000 ly)
It shows the Orion spur is about 2000ly across at our point.

I was going to suggest them, and thanks for posting the link to ProjectRho. I've book marked it.

 

[G. Kashkanun Anderson]

That brings up an interesting follow-up question: Of the low-metallicity and/or Population II stars, what is the relative probability of them having meaningful planetary systems? Many of the current astronomical surveys are revealing that even low metallicity stars have planetary systems, but how much less likely is it that such a star would have Terrestrial or Jovian worlds (or even some analog to a Kuiper Belt) as compared to higher metallicity or Population I stars, based on current observation?

Kapteyn's Star (an 11 billion year old Population II star) has at least two super-terrestrial worlds orbiting it, so we know that not just rocky worlds, but large ones, were capable of being formed within about 2-3 billion years of the Big Bang. At the time of its discovery, Kapteyn b was regarded as the oldest potentially habitable world yet discovered (a distinction I believe it still holds). And this is just the example we currently know; I'm sure we'll find even older examples in due time.

Gas giants are an even easier proposition, since even the smallest ones are about 98-99% hydrogen-helium by mass, and we know that the early universe had plenty of that! Their rocky cores might be smaller (to the point even of nonexistence -- although that would technically make them brown dwarfs by my definition), but there still should be plenty of them showing up concurrently with the first Population II stars, making them almost as old as the universe itself.

This might be meaningful to Reban's initial post if inter-arm systems have fewer refueling points or meaningful worlds as stop-over points in the inter-arm regions.

I do not know that the difference in metallicity is going to be that different between the spiral arms as it is within them. It's arguable, in fact, that life-bearing worlds might be slightly more common between the arms, as there is less possibility for apocalypse-grade interstellar events (i.e. supernovae, gamma ray bursts, etc.) happening within your immediate neighborhood. If Population II stars are surrounded by ancient, coreless gas giants (micro brown dwarfs?), however, they might be considered something of a prize, as the 'fuel' they provide could be as close to pre-purified as you're going to get in nature (assuming a hydrogen, not deuterium, based fusion process).