@garynewport doesn’t show all the ways in which element and individual element variables e.g., hydrogen are used in the code.
Chances are high the code is setup to load some data into the rank-2 array for element and the purpose of equivalence is as an alias to the values.
If so, an even braver option, one that will be “kosher” with respect to current Fortran standard, might just be to employ the TARGET and POINTER attributes. And apply the PROTECTED to the aliases i.e., the objects with the POINTER attribute in order to some guard against the pitfalls of working with POINTERs in the code.
That is, what is currently
module helium
use stellar_values, only: me, mh, mj
real, save :: element(mj, me), velement(mj, me)
real, save :: hydrogen(mj), vhydrogen(mj)
..
Equivalence (hydrogen, element(1,1))
..
Equivalence (vhydrogen, velement(1,1))
can instead be as follows:
module helium
use stellar_values, only: me, mh, mj
real, target, save :: element(mj, me), velement(mj, me)
real, pointer, save, protected :: hydrogen(:) => element(:,1)
real, pointer, save, protected :: vhydrogen(:) => velement(:,1)
..
At the point of use in the rest of OP’s code with objects such as hydrogen and vhydrogen, etc., no change be needed.
An advantage with working with TARGET ↔ POINTER paired attributes as a use case for aliases is type safety besides the fact it is in line with the use case that led to the introduction of the facility in Fortran 90 revision circa 1991.