Using the spectrometer on ESA’s Gamma-Ray Observatory INTEGRAL, a precision measurement of the gamma-ray line from radioactive decay of  ⁶⁰Fe (T_1/2~1.5 Mio years) demonstrates that this radioactivity reflects the entire population of massive, young stars in the Galaxy. Since INTEGRAL (and other instruments) also see radioactive ²⁶Al from the same current population of massive stars, they can use the ratio of these isotopes to test the complex models of massive-star nucleosynthesis. This test reflects our best knowledge of nuclear reaction rates to produce and destroy these isotopes, about the temperature, density, and convection zones insude those stars, and about the frequency of such stars with a particular mass and their spatial distribution in the Galaxy. This sounds like a complex test with many parameters. But note that when we measure elemental abundances in the atmosphere of stars like in our Sun (the traditional method to learn about element synthesis in the universe), we measure an elemental abundance that the interstellar gas had at the time of the formation of that particular star - in the case of our Sun 4.6 billion years ago; and this interstellar-gas composition then is understood to be the result of all previous nucleosynthesis by stars and supernovae in the Galaxy in previous times, as mixed from their production sites towards the location where this particular star is formed; quite indirect, isn't it? Radioactivity from a specific isotope, in comparison, reflects current decay, hence production within the radioactive-decay lifetime. In our case, we see the production of ⁶⁰Fe (and ²⁶Al) by the current population of massive stars in the Galaxy over the last few million years (the Galaxy is over 12 billion years old). We know the current state of our Galaxy quite well, and also can count the current stars. Therefore, nucleosynthesis models are quite well constrained, and our measurement is a good test case.
Radioactive ²⁶Al is ejected into interstellar space together with other new elements, when massive stars reach the terminal phases of their evolution, through an intense wind phase called 'Wolf-Rayet' phase, and then when these stars finally explode as supernovae. Using the gamma-ray spectrometer SPI on INTEGRAL, an international team of researchers led by MPE scientists obtained unpredecented precision measurement of the ²⁶Al and also the ⁶⁰Fe decay gamma-ray line .
Massive stars are distributed in our Galaxy as the ²⁶Al gamma-rays show us. (In other wavelength bands, we cannot see throigh the interstellar clouds of the Galactic plane, hence cannot see massive stars far away from our location in the Galaxy; gamma-rays are more penetrating). The integrated census from these more than 10000 stars is a good average of how massive stars 'on average' function in our current Galaxy.
⁶⁰Fe production is through capture of neutrons on stable Fe nuclei. This competes with beta decay, whereby a neutron converts into a proton and makes a Cobalt nucleus out of the Fe. The abundances of 60Fe in stars is a subtle balance between the ladder of successive neutron captures from ⁵⁶Fe to 57-58-59-60-61-62Fe and those beta decays. In the hot inner shell boundaries of He and C nuclear burning zones inside massive stars, abundant neutrons are present, and the temperature is high enough, for this to happen. Also, these regions are convective; this helps to move ⁶⁰Fe, once produced, away from these hot inner regions, so it does not get destroyed by further neutron captures. All this happens deep inside the star. Its results, in the form of ⁶⁰Fe abundance, is only revealed when the star explodes as a supernova, several 100000 years later. But this abundance then tells us how the star was structured at that earlier time. A "window into massive stars", through ⁶⁰Fe gamma-rays.
Since ²⁶Al is produced in those same stars, although at quite dfferent epochs, a relative measurement is possible. Details of stellar mass and space distribution therefore cancel out in such an abundance ratio, and the test for massive-star nucleosynthesis is even more powerful.

Figure: illustration of ⁶⁰Fe astrophysics: radioactive decay of the ⁶⁰Fe isotope produces a cascade of gamma-ray photons of specific energy upon its transition into its stable daughter nucleus, ⁶⁰Ni, in its ground state. The INTEGRAL satellite measures those gamma-rays since its launch in October 2002. Precision spectroscopy revealed the signature of the decay. Here, both lines are sumperimposed at their laboratory energies, so that the (weak) signal in each individual line adds up. This is necessary, because ⁶⁰Fe emission is so weak, with typically only a few photons a day being seen by INTEGRAL. This intensity is too weak for the sensitivity of INTEGRAL to make an image of 60Fe gamma-rays; more advanced telescopes would be needed for that.  pdf  and jpg versions.)