badhex

Thanks, another great recommendation. I'll have a dig around for that one too.

Thanks Mr H, I'll look into this, sounds like a good book. I'll dig around and see if there's a used copy or a PDF somewhere in the first instance. After a quick bit of research I believe it is now being reprinted by a US astro outfit, though currently still out of stock.

LOVE this.

Hello all, I'm just about to start the polishing stage on a 10" mirror, somewhere between F5 and F6, though FL needs to be measured properly) and I'm thinking about what direction to take with the remaining build. I'll preface this by saying that I am pretty handy with DIY and very organised when it comes to project work, but don't have a workshop or similar to go with a full from-scratch build, only fairly standard power tools e.g. small circular, jigsaw, orbital sander, drill, impact driver, workmate etc. - though I am looking for any excuse to buy an angle grinder! I'm thinking it would probably be easiest to buy some off-the-shelf bits and bobs which would presumably need to be modified slightly to fit the FL, and put them together. In the first instance I have an old 2" focuser which would probably replace with something nicer eventually, and I can probably get a secondary from the telescope makers club, but I have no mirror cell, tube, truss, base, spider, or anything else. I want to get decent parts, but not looking to splurge on super expensive premium brands, certainly not at the moment, due to funds. So, what are my options? Are truss kits available? I've seen tubes available from TS, not sure of UK suppliers? Are there any good resources people can recommend for starting a build, and example build plans? Anything else I need to think of? TIA!

Uh-oh 😅

I have perused this list several times! Definitely going to have to go through the all at some point 😊

Indeed! And let's not forget my fave solar system body, Enceladus, which has every possibility of harbouring life. Who needs planet status anyway if you have microbes! 😂

I believe that the currently least massive known star is 93MJ (Jupiter Masses) which is where that figure comes from. From 13MJ up to 80-somethingMJ it would be a Brown Dwarf which is not a true star, but very badly defined. Not sure what happens between the 80 something and 90 something though! I imagine they are giving themselves some room for error - perhaps it has been discovered that the most massive known brown dwarf is that 80 something, but I don't know.

Yes, I mean we weren't even that hard and fast on what constituted a planet until 2006, and as the excellent discussion on here proves, not everyone agrees that it was the correct decision. I guess we have to start somewhere, hopefully it will evolve as we learn more and become less of a loose definition. Like with brown dwarfs, there is a lot of grey area to tighten up. One more thing that didn't note earlier: apparently there had been a motion to make dwarf planets a subcategory of Planet which members of the IAU voted against. I'm not sure why I was not accepted but that really does seem like an odd choice.

Can anyone tell I'm really looking forward to studying Astrobiology next year? 😂

I think it needs to be at least 90 times more massive (which is not necessarily that much more massive in comparison with the most massive stars) in order to begin nuclear fusion of hydrogen and become a true star, but it only needs to be 13 times more massive to be able to fuse deuterium and become a brown dwarf - which is a weird grey, or brown, area between gas giants and true stars.

True enough. Though, if one were playing devil's advocate and say, both planets were equal in mass, then they would both be holding each other in their Lagrange points, ie planet A would be in planet B's L4 and planet B would be in planet A's L5. In that potentially unlikely scenario, we would not be able to say which planet was dominant 😅

No worries - yes, and now I come to think - the whole Planet X theory was disproven years ago, which is essentially what we're talking about. I don't know why I didn't think of that.

Yes, understood, but typically we talk about maintaining objects with relatively small masses in lagrange points - the question from Stu was specifically whether two planets of a similar size could share an orbit. I did not think L3 would be stable enough, but it turns out that L4 and L5 can in theory support a planet sized object sharing an orbit with another planet sized object.

I did a bit of digging on whether a planet could inhabit another planet's (well, planet/star) lagrange points and it turns out yes, with caveats. L1, L2 and L3 are not stable enough but L4 and L5 are both theoretically possible, and now we may even have proof: https://earthsky.org/space/2-planets-share-an-orbit-photo-pds-70/#:~:text=July 23%2C 2023-,Can 2 planets share an orbit%3F,orbit appears to be true!

I agree, it does feel a bit 'loose' for a scientific definition. I think part of the reason for this is that nature tends to gives us spectrums rather than hard stops. To the questions (btw I am absolutely not an expert!): - The neighbourhood means approximately the same non-resonant orbit. I suspect the size of the neighbourhood depends on the mass of the body. With sufficient gravity in play, anything nearby in its own orbit will either be ejected from the orbit or accreted, or maybe turned into a satellite of the body (or sucked into a lagrange point, which counts as cleared). Not sure if you could have two Jupiters opposite each other, but if they were, they'd be in each other's L3 lagrange points, so technically cleared, I guess? - I am not sure about this similar size thing. In general, the objects being cleared are dramatically smaller than the body itself even when combined, and this is where the hard definition lies: the planetary discriminant µ, which is the ratio of the mass of the body vs the combined mass of all the neighbourhood objects is significantly large (>100:1) for all planets except Pluto and Eris and the other dwarf planets. I found the graphs I was on about which makes it very obvious when you plot the planetary discriminant vs various other properties like mass or diameter, or just on their own even:

Did you buy direct from Svbony on Aliexpress?

Oh yes, no doubt - it really is sad. I was quite adamant at the time that I would continue to call and think of it as a planet. That said though, since then I have sort of changed my thoughts on the matter in that Pluto is really bloody awesome regardless, so it doesn't really make it 'less than' in my eyes. Just after the change, one of my favourite geeky songwriters, Jonathan Coulton, released this tune written about Pluto from the perspective of Charon, which sort of sums it up:

I could have just looked up clearing the neighbourhood on wikipedia instead of vaguely stabbing in the dark of my memory. I should have also added for clarity that as well as clearing the neighbourhood, it would need to also satisfy the previous two criteria of being in hydrostatic equilibrium and orbiting the sun, of course, but obviously we were only really debating the Trojans etc. https://en.wikipedia.org/wiki/Clearing_the_neighbourhood

Haha, I was trying to say the same thing, and realised upon re-reading I had said they *are not* included in the definition when in fact I mean they *are*. As you said and per my original post: I've definitely read somewhere that the IAU essentially didn't specifically define planets that well because basically everybody knew what a planet was until Eris came along, which, given how similar it is to pluto, prompted the question of whether Pluto was indeed a planet after all; if Pluto was, then Eris must be - if Eris was not, then Pluto could not be either. Once more had been done to come up with a definition along the lines of this total-mass-of-satellites-in-the-same-orbit-vs-planet-ratio-thing (which I'm probably remembering incorrectly as evidenced by my highly technical description), and not including satellites trapped in L4 and L5 thanks to the body's gravity, it's pretty clear that there is a definable thing that has either happened or not happened, i.e. has the body cleared its neighbourhood or not, by the above criteria.? If yes, it's a planet, if not, it's not.

I admit I was very sad at Pluto's demotion. I saw a cool (if you like that kind of thing) graph somewhere before that plots planetary mass against combined mass of all the other stuff in its orbit, and basically it does show a significant line in the sand between what we now deem to be planets vs dwarf planets. Regarding lagrange points I am pretty sure they are included in the definition - mainly because anything caught there is due to, not in spite of, the planet's gravity. I could be wrong about this but that's how I understood it.

Presumably an energy saving measure due to cost of living 😂

I feel like I've probably also seen something that qualifies as greenish too, but I'm going to guess that I've never seen a star that I'd describe as green on its own in the eyepiece - i.e. only ever as a consequence of colour contrast as you mentioned. I haven't seen the talk but would like to track it down, I think there's a YouTube channel with past talks on somewhere. From my astro studies last year, my admittedly basic understanding is that a black body spectrum sits on the planckian locus in the colour space, which does not pass through green. Any black body in space emitting green light also emits lots of red and blue light, and we would perceive this as white. It is similar, although not the same as why an iron bar does not pass through green when being heated (though an iron bar is not truly a black body, it's a close real world example).

That would be pretty cool, and given the size of the universe it probably exists, somewhere. I'd imagine that you'd have to have no other large bodies in the system in order to keep the planet from being pulled out of the lagrange point and/or ejected or smashed into, since it's only a relatively stable area to inhabit.

I'd have said that a dwarf planet does not have enough mass and therefore gravity to clear its orbit, whereas the Greeks and Trojans are hanging about in Jupiter's lagrange points *because* of its massive gravity, rather than it being a failure it clear its orbit of objects.