Determining the Qualities of Stars
 

I've decided to start working some more on my space opera setting, and have determined a number of things I had initially put off, but need help with some of the numbers. Specifically, I need help with the stars that humans are able to settle.

A quirk of the setting's FTL travel is that it is only possible to use it to travel between very specific stars - all G0V's (comparable to the Sun, but a bit bigger and brighter) with an undefined peculiarity to its solar (stellar?) wind. Additionally, within the star's heliosphere (astrosphere?), it's possible to use boost drives that get incredibly high pseudovelocities. Naturally, humans were unaware of this FTL travel until after they sent a generation ship fleet to a nearby G0V star that had been determined to have an orbiting planet that would support life. As no such nearby star exists, I had to make one up, and to give it a decent name, I decided to make up a new constellation. Said constellation is the Harpy, and the star in question is the "eye" of the Harpy, named Harpyia Mita (if I butchered that, let me know and I'll adjust).

So, here's what I need to know.

How bright would a G0V star be? Specifically, what would one that is 5 light years away (as is the case for Harpyia Mita) look like from Earth - would it be the brightest star in the night sky, or would Sirius still hold that title? Where would its habitable zone be located, and would there need to be any peculiarities to look out for (I recall Proxima Centauri was found to have a planet in its habitable zone, but it was determined the planet would be battered too much by the solar wind to actually be habitable)? Finally (for now), how large would such a star's heliosphere likely be?

EDIT: If the answers to all of these questions are hidden within GURPS Space, a response of "Just buy GURPS Space" is acceptable (considering doing so regardless), but page numbers would be appreciated.

Re: Determining the Qualities of Stars
 

GURPS Space page 103 has the Stellar Evolution table with most of the information you want. The short answer is 'depends on the age of the star'. G0V can spend 7.7 billion years in the main sequence according to the book.

To get the apparent magnitude, you'll need GURPS Traveller: First In, page 55. Although, you'll need to reverse the equation and there's no accounting for distance. But I don't know how important that is.

Re: Determining the Qualities of Stars
 

Gurps space won't give you this, but you should really buy it if you're going to be building a space opera setting.



This tool, on the other hand, will give you the brightness of a star at a given distance and brightness. You still need to determine the luminosity (and from there the absolute magnitude) of the star, and the G0 classification isn't actually sufficient to give you that. For that, gurps space IS useful.



But I can tell you right now that the number is going to be between 0 and 1, which will put it in the top 20 stars but not in the top 5 (alpha centari will be brighter because it is a binary)

Re: Determining the Qualities of Stars
 

I had thought Space might have (some of) my answers, so I'll probably be picking that up shortly. The fact things vary depending on age might give me my "only specific G0V's" criteria, but I'll have to wait until I read the relevant bits.

Well, all the inhabited planets are pretty much going to have the Earth flora and fauna humans brought with them (the native life was, at best, very simple plant and fungi, and were rapidly outcompeted), and will physically be very similar to Earth, and I could just handwave the rest of the planets in the star systems, but I think you're right - Space is the way to go.

Outstanding, thank you. I figured I could work out apparent brightness myself once I knew the star's actual brightness, but a calculator to do the work for me would be quite useful.

Even better - I didn't want Harpyian Mati (still would like to know if this is appropriate for something roughly meaning "the harpy's eye") to be too bright, so not being in the top 5 is sufficient for me.


Incidentally, while not directly related to stars, is there a legitimate reason for a generation ship using reaction drives (albeit superscience ones that get probably-physically-impossible delta-v's) to not use a multistage design? An important plot point is the original colonists having a choice of returning to Earth (first shortly after passing the heliopause and a massive solar flare cut off communications with Earth, then 10 years later when Earth's new government reestablished contact and ordered the ships to return), but I'm pretty certain a multistage design wouldn't have had a choice. As it stands, the fleet was overengineered enough they could have transferred people, fuel, and supplies between them, leaving half of the ships derelict, and limped back home, so they did have a choice, and opted to continue forward.

Re: Determining the Qualities of Stars
 

"Harpyian Mati" seems... odd. Couldn't tell you why. Maybe the Y. Is is pseudo-Latin? (If it is supposed to be, I would expect "oculus" to be in there somewhere. "Mati" makes me think "mother".) If you want Latin I might go with Oculus Strix or even better Oculus Strigea- a Strix instead of a Harpy, just because you'll lose the odd Y. Maybe... Someone help me out- what is the genitive singular of Strige? (Why do I think of that scene in The Life of Brian?) And since it would presumably be the brightest star in the constellation, you could better call it "Alpha Strigeae" (α Strigeae). That looks better, like Xi Ursae Majoris.

It has been a very long time since I studied Latin in high school...

Anyway, you needn't limit yourself to Latin and Greek. A lot of stars have Arabic-descended names. Aldebaran, Betelgeuse, Mizar, etc. Going with Strix, google translate tells me that the Arabic for "owl's eye" is eayan albawma. Corrupt that however you wish. Yanalbawma? Yanabama? Analbama? Alabama? :) The Arabic phrase for "harpy" is long, though, meaning basically "old woman". It would be unwieldy.

EDIT--- The Latin word for Strix was Striga, not Strige. So "Alpha Strigae"? Seriously, someone who is better with Latin, help me out here!

Hurting. My. Brain.

Re: Determining the Qualities of Stars
 

Harpyia is the romanization of the Greek word ἅρπυια - the harpy. I can't remember why I added the n on the end of it (probably to make it possessive or similar), although I do know the colonists refer to themselves collectively as Harpyians, as the colonization fleet was called the Harpyian Fleet (being as it was going to a star in the Harpy constellation). They also refer to all the human worlds aside from Earth collectively as "Harpyia," despite only Gateway, the world most distant from the others (and closest to Earth), being part of the Harpy constellation. Meanwhile, Mati is the romanization of μάτι, the Greek word for eye. It should really be written Máti, but I'm generally too lazy to add in accent marks.

All of this is complements of kludging something together using Wikipedia and Google Translate, of course, so there's a pretty good chance I got something horribly wrong.

EDIT: Actually, while it won't influence the name of the constellation and star, the Arabic word for harpy might be of some use. If the infidels refuse to return home in spite of explicit orders to do so from the Mahdi, and want to call themselves "harpies," I could certainly see the Earth Caliphate taking to calling them that, particularly if it roughly translates to "old woman" in an insulting way. Google Translate gives me khafaash, which seems to translate into something more like vampire. I can see the "old woman" translation there, but it just tells me it's إمرأة عجوزة and doesn't deign to tell me how that's pronounced.

Re: Determining the Qualities of Stars
 

The translation / transliteration I got via Google was "Arpias Mati" (being equally lazy with diacritics). Actually, it was "to máti tis arpías", which is closer to "the eye of the harpy" thanks to the extra words.

(I don't know Greek, much less ancient Greek, but apparently you can put adjectives before or after the noun, which has a subtle effect on the emphasis, whether or not you're distinguishing this eye from other eyes, and so on. Putting it after means you need the articles. Leaving out all the articles is -- I gather -- incorrect, as it's then a sentence with an implicit "to be" -- "The harpy is an eye". But no doubt someone in medieval or modern times was equally lazy :) I went for putting "harpy" first, since that's usually what we do with constellation names. IMO, it looks better without the 'H', which is closer to the Greek spelling anyway -- το μάτι της αρπίας.)

Re: Determining the Qualities of Stars
 

The proposed star has a slightly greater luminosity than Alpha Centauri (G2V) but is slightly further away (5 ly vs 4.3); on average it will be slightly dimmer. This would place it as the fourth brightest star in the sky, between Alpha Centauri and Arcturus.

Re: Determining the Qualities of Stars
 

Well, unfortunately it doesn't look like Space provides the distance to the heliopause, but I've decided all the hyperspace-accessible stars - called aetheric stars (the hyperspatial medium, and in fact about the only thing there, is called aether) - are going to be roughly 4 billion years old, giving them luminosities around 1.67 relative to the Sun. Playing around with the previously linked tool (as I don't trust myself to calculate the absolute magnitude, and the online calculator I found tells me the Sun has an absolute magnitude of 4.74, when it's defined as 4.83), it looks like a luminosity of 1.67 calls for an absolute magnitude of roughly 4.27 and a relative magnitude of 0.1977, putting it between Rigel and Procyon, and snagging the title of 8th brightest star in the night sky. Below is the general trend for all aetheric stars - I'll work up Harpyian Mati in the near future.

Aetheric Star: Spectral type G0V, mass 1.1 solar masses, age 4 billion years, effective temperature 6,000 K, luminosity 1.67 solar luminosities, radius 0.0056 AU. They have an inner limit radius of 0.11 AU, an outer limit radius of 44 AU. Their snow line is located at 5.1 AU (opted to round up). Their habitable zone is between 1 and 1.14 AU. Aetheric stars all have a similar orbital setup, with orbiting radii right around 0.25, 0.41, 0.67, 1.11, 1.84, 3, 5, 8.25, 13.6, 22.45, and 37 AU, with a shockingly-consistent 1.65 ratio between worlds. The primary gas giant is located at a surprisingly-stable 5 AU (surprising because this is inside of the snow line), an asteroid belt is located at 3 AU, and there seems to always be a planet located at 1.11 AU, although its type can vary based on size and composition (blackbody temperature is around 300 K, making a Standard or Large planet Ocean or Garden, but very high albedo could make it an ice world, while high greenhouse effects could make it a greenhouse world). The remaining orbits can be taken up by most any type of planet - or none at all. Because Earth's heliopause is almost exactly 3x its outer limit, I'll use the same relationship here - the heliopause of an aetheric star is roughly 132 AU from the star.

Re: Determining the Qualities of Stars
 

I dont have the equation for absolute magnitutde from luminosity on hand, but Jon Zeigler (author of GURPS Space) has been publishing an update to the star and planet generation system that incorporates modern planetology here.

Re: Determining the Qualities of Stars
 

Why not just use G-type main sequence stars? It is easier if you can connect Sol.

Re: Determining the Qualities of Stars
 

Neat. I'll keep an eye on it if I need deeper details, but so far I'm happy with what I'm getting out of Space. Well, mostly - I'd really like a way to determine the heliopause (but after trying to look it up, I get the feeling we don't actually have any idea of where the heliopause is for any star other than Sol), and determination of planet subtype with a basis on more than just its orbit would be nice (the mentioned possibility of high albedo meaning an ice planet or high greenhouse meaning a greenhouse planet in the habitable zone), although I think I have a kludge for that, which is roughly actually rolling to determine the ratio of the habitable zone planet, and use the resulting temperature instead of the true blackbody temperature. The further it is from 1.11 AU, the higher its albedo (and beyond 1.14 AU, an otherwise-Ocean/Garden world is Ice, and quite bright), while the closer it is, the more of a greenhouse effect is has (and less than 1 AU, an otherwise-Ocean/Garden world has a runaway greenhouse effect and is a Greenhouse planet.

Sol being cut off is a feature, not a bug. I also feel it's necessary for making FTL travel easy, yet never discovered on Earth - it's literally impossible within our solar system, and indeed in many solar systems. "Aetheric stars" aren't actually the only ones where boost drives and entering/exiting hyperspace is possible, however, but they are much easier than the others, and how you enter hyperspace sort of "attunes" your ship to make it easier to detect and utilize very similar systems, hence humans having only colonized the aetherics. The setting's "aliens" do make use of some of the other eligible star systems but essentially nobody actually knows that (nor do they know the "aliens" are actually engineered lifeforms based on Earth life).

For the purposes of the setting, only the aetheric systems really matter. Earth is a distant theocracy on a 5-year communication delay, while the Malakim are a threat that can somehow enter - and apparently exit - hyperspace without being at an aetheric heliopause, although not reliably. Nobody can visit Earth (and while Earth occasionally sends new colonists - primarily to further along the Caliphate's spread into Harpyia so it can seize power and force humanity to return to Earth - said colonists are still a few generations removed from anyone who's actually seen Earth), and hitching a ride on one of the Malakim's biological ships to see where they are based is impossible (it will detect you, and if you kill all of its crew and prevent it from making more, it's more likely to just stay in hyperspace long enough for the aether to kill it than try to return home).

Re: Determining the Qualities of Stars
 

<shrug> The distance to the heliopause should be similar for stars with solar wind activity similar to Sol in areas of space similar to the "Local Bubble" where interstellar space is thin but energetic.

A G0 probably has at least a little more acivity than a G2 though I know of no one who has collected data on this. It' hard for me to say evven in theory what having your star in a colder but thicker interstellar medium might do. My intuition gives a 60/40 chances that it might push the heliopause farther out.

Combine that witht he bigger star and the heliopause might be a long way out though you have to go about 100 ly before you're out of the bubble.

I'm not even sure what the "official" distance for our heliopause is. There ahve been announcemnts that the Voyager probes have crossed the heliopause 3 or 4 times in my lifetime.

It's definitely a long way out. If it was 80 AU that'd be about a month at 1G and turnover velocity would be enormous. WMD issues are a definite thing and a drive that can do it makes STL interstellar flight pretty reasonable.

Re: Determining the Qualities of Stars
 

For the original generation ships, I haven't designed them yet, but the current intent is for them to be roughly 50% fuel, and they had a delta-v a bit north of 2% light speed, but rather low acceleration - something like 0.05G IIRC - thanks to their small drives (the amount of time spent accelerating is so small compared to the whole trip, using sub-system engines doesn't make a lot of difference). They accelerated to 1% light speed, then traveled for 500 years to reach Harpyia Mati. Modern ships of a similar design can manage delta-v's of around 5% light speed, and as the drives can function in a pseudovelocity "boost drive" mode once they enter an aetheric star's heliosphere that also sync's them to the system they are in (functioning as a rather efficient braking system), modern ships coming from Earth can make the journey in "only" 100 years.

One thing I intend to use the heliopause distance to determine is actually the speed of the pseudovelocity boost drives - I want a typical freighter to take roughly 1 month to go from 1.11 AU to the heliopause, making interstellar journeys typically take 2 months, plus the time in hyperspace. With my initial estimate of 88 AU for the heliosphere, that's 86.89 AU per month, or right around 3000 mps. That's roughly Earth's solar orbit to Mars' solar orbit in 4 hours, 20 minutes (or Gateway to the next furthest orbit in 6 hours, 20 minutes).

Re: Determining the Qualities of Stars
 

Are you using real stellar data for position etc.? If so, are you aware of the Extended Hipparcos compilation of stellar data? I find it pretty useful.

I count 295 G0V stars within 200 light-years.

Re: Determining the Qualities of Stars
 

άρπυια is a first-declension feminine noun ending in -α. I think the genitive is άρπυιασ. "Harpyias".

Re: Determining the Qualities of Stars
 

I wasn't planning on it - Earth's night sky looks more-or-less the same as in reality, with the glaring exception of the Harpy constellation, but I intended to pretty much just throw the aetheric systems around semi-randomly. Assuming I'm using the catalog properly, I'm pretty certain I'll still need to do that - the catalogue only contains 4 G0V stars between 3 and 5 billion years old, and maybe only 3 of them (HIP 726, 12099, and and 33460) are around the right age to be aetheric (the 4th is 4.7 billion years old, the other three are 4.1, 4.1, and 4.0, respectively). They're also all quite a bit brighter than Harpyian Mati. I haven't yet decided how distant systems will be from each other, although I initially intended to have them be relatively clustered together, with Harpyian Mati some distance from the main cluster. I may still do so.

I thought that might be the case, actually. I'll probably change it to Harpyias Mati, then.