Star sizes are . . . variable.

[senko]

Ok first off the last paragraph in this post is just why I got to thinking about this and isn't relevant to the point but its there if your curious.

I was looking at the sizes of known star's in the universe to see how large and small they got and I found the results surprising. The largest known star in the universe detectable by our instruments is 1,708 times the size of our sun and that's radius. To put it another way side to side you can fit 3416 suns inside this other star and the same amount top to bottom thousands of our suns fitting inside it. Sure its a pulsating star that gets bigger and smaller but still if my calculations are right the sun to earth is only 215 solar radius so we'd fit neatly inside it easily. Now on the other side of the scale the smallest known star is only the size of Saturn. I find  it interesting that one star would probably swallow our inner solar system whole while another isn't as large as the largest planet in our solar system and both are red stars on the colour spectrum.

Research continues and reveals that the star mentioned above is only 30 times as massive as our sun while the most massive known is 300 times as massive and interestingl only 30 times the suns radius nice symmetry there. If it were swapped with our sun, it would outshine our closest star as much as the sun currently outshines the moon, would reduce earth's year to 3 weeks and lets not talk about the radiation. The scary thing is its already lost a huge amount of mass since the big bang.

Meanwhile the least massive has only 1.5% the mass of our sun. It only has 15 times as much mass as Jupiter and astronomers are debating if it should even be classed as a star at all. Poor things may join Pluto in being reclassified.

Then we get things like the Pistol Star which radiates as much energy in 20 seconds as our sun does in a year.

I was consideirng my theory (not based on canon sources just house rule equivilent) that you could fit a smaller eye of harmony in a TARDIS than the one on Gallifrey. Think small generator vs nuclear power plant so while both eyes of harmony generate unlimited energy reserves they have different accessible reserves. That is they'll run forever but can only provide so much power per second so the larger one on Gallifrey can power their entire civilization and thousands of TARDIS's during the war but one inserted on a TARDIS to allow operations in other univeres and beyond the range of Gallifrey only has enough power at any one time to power said TARDIS it will last forever but wont run a second one. I also need to consider star colour spectrum but that's for the future. Anyway that's just backstory but I think my theory works Rassilon created the original Eye of Harmony from a supermassive star long ago and during the time war they started making much smaller, less powerful eyes of harmony like the one we see in the Doctor's tardis generating far less power per second than the true or original eye of harmony but contained inside a TARDIS supplying it with that power in any circumstance such as visiting another universe.

[Catsmate]

Red giant stars are extremely tenuous, being basically a 'hot vacuum' or, more accurately, a hot cloud of gas surrounding a small fusion core.

Now, IIRR, the Eye of Harmony is linked to Rassilon and Omega's detonation of Qqaba, converting it into either a singularity or a black hole (or both). A black hole is massive but extremely small; a stellar mass black hole would be 5-6km in diameter, but a Terrestrial mass one would be about 3cm. Perhaps the power source of a TARDIS is such a 'micro' black hole, formed by gravitational compression of matter, and fed matter?

[senko]

Good point on red stars and why I went from "largest" to "Most Massive" as RMC 136a1 would output far more energy (power) than VY Vanis Majoris because its smaller but hundreds of times more massive. Converting that into a singularity/blackhole would thus theoretically generate far more power than say something like our yellow dwarf star sun.

My current theory revised after yesterdays research is that Rassilon and Omega detonatd some super hot, super massive star to create something on the order of a supermassive black hole. I'd say TON 618 66 billion times the mass of our sun but I think that's still theoretical on whether its one black hole or many in a close radius. Anyway point is Gallifrey has a supermassive black hole and thus requires more "space" to contain it safely.

Which ties in with your idea its just a question of scale Terristrial vs Stelllar or Stellar vs Supermassive. Which brings us to this image . . .

Which in New Who is apparently his Eye of Harmony Power Source now I have no idea of that star is super small and close or large and still disturbingly close. What we do see is that its yellow so its probably not that massive or powerful in stellar terms. Enough to run an old Type 40 TARDIS (mostly) but if you compare that to stars we know exist its easy to see even a normal blue supergiant in the same situation taking up far more spacce and generating far more power. So really the idea seems sound you just need to adjust the star size/colour for whether your talking about powering an Obsolete/Ancient/Modern/Advanced TARDIS or Gallifrey itself. If a star like this can handle a type 40 I can see one like RMC 136a1 doing the same for Gallifrey's needs.