Some nuclear isomers (like tantalum-180m) have never even been observed to decay....
How do we know it is in an excited state then? That it actually absorbed some form of energy?
Can these includes be induced to decay and emit a gamma ray, even if they never spontaneously do so?
Most of the heavy elements currently present in the Universe were created in Supernova events.
After the big bang, when the Universe had expanded enough for the temperature to have decreased enough to allow elements to form, the elements present were mostly Hydrogen and Helium, with very little heavy elements. It is in stars that fusion creates elements heavier than Hydrogen and Helium, and it is in supernova events that elements heavier than Iron are created.
In supernova events (isotopes of) elements are created in proportion to the probability of each of those creation events.
After a supernova event the remnents of the supernova can join a new coalescence process, with formation of a star out of interstellar gas that is enriched with heavy elements. Some of the proto-stellar material may form planets that orbit that star.
Formation of Tantalum-180m is of low probability, but not zero probability. Given the sheer amount of matter involved in a star: if Tantalum-180m can form, it will form.
For any given nucleus, it is possible to calculate its net binding energy for various configurations (positions) of the protons and neutrons in it, including which way all their spins are organized. The most likely packing of the nucleons is the one which maximizes the binding energy, resulting in the most stable nucleus.
You can then perturb the nucleus in your binding energy model by pulling it out of round or re-arranging the spins and recalculating the binding energy for any weird configuration you want. the difference in energy between the most stable or "ground" state of the nucleus and what it has in the deformed state is the energy which must be put into the nucleus to get it deformed, and will be the amount released when this excited state decays back into the ground state.
Now, the most likely lifetime of the excited state can itself be calculated knowing the size of that energy difference and a bunch of other quantum considerations (including, for example, the tunneling probability for nucleons to leak through a potential barrier that traps them in a metastable state) and from that you get a half-life for the nuclear isomer.
Then you discover that the theoretically-calculated half-life for certain isomers is really, really long while for others it is extremely short.
Since the energy release in a nuclear isomer decay process is of the order ~gamma rays, the expected decay product will be a gamma-ray photon- which led some physicists to conclude that if you had a large number of metastable isomers and beamed them with gamma rays, you could trigger their simultaneous decay and thereby make a bomb far more powerful than any chemical bomb yet less powerful than a fission bomb- and that bomb would be extremely compact and easy to carry around.
But no conclusive evidence exists that you can in fact trigger that all-at-once decay with a gamma ray beam- so the isomer bomb remains a science-fiction weapon only.
