“Variable star” is obviously an anagram clue, isn’t it? While Chambers has a fair number of stars among its entries, not many of them are notable as variables. So Mira and Algol it was. Fortunately it was possible to include them in such a way that solvers would have to identify the theme because there were alternatives for unchecked letters.
Now, A, R, S, T. Obviously there’s going to be words containing the four in that order, while ATRS isn’t very promising. So let’s jot down all the permutations and see which offer potential words. Nine, though STAR won’t quite do, leaving us with eight, and hence four crossing places, using them one each.
That’s probably not enough to ensure solvers get the theme. What can we add? Well, the variable star to out-vary all other variables is the Cepheid. That’s governed by Leavitt’s Law, which gives me a chance to honour a largely unsung female pioneer. So into the length calculation: CEPHEID VARIABLE is a bit long, but DELTA CEPHEI is the same length as LEAVITT’S LAW. I can track the necessary letters for the former through the grid entries (just a little tweaking of alternative entries needed…) so we could have wordplay with one letter replaced by the one in its equivalent spot in the other.
Which means the E of DELTA has to be replaced by the E of LEAVITT. Humph!
So let’s reverse. Now the second E of CEPHEI has to be replaced by the E of TTIVAEL. Humph!
So let’s stagger them…and it did prove possible to find an arrangement where L, E, A and I avoided each other.
And finally the title. This was one where it proved hard to think up something that didn’t give too much away too soon and I retreated (it felt like retreating) to simply the definitions of the two names – Mira the miraculous star (with a huge and unpredictable way of varying) and Algol (the demon, but etymologically ‘ghoul’ – Batman fans might like to recall Ra’s al Ghul).