This is a different APOD, indeed. It doesn't feature the Moon, Mars, Saturn, Pluto or comet 67P/Churyumov–Gerasimenko. It doesn't show us Perseids, auroras, sunsets or awe-inspiring Earth weather. It doesn't show us an artist's illustration of an exoplanet. It doesn't show us a planetary nebula, a globular cluster or a galaxy.
It doesn't even show us a supernova.
It shows us what isn't there in a supernova remnant.
Wow! I love this APOD!
Supernovas type Ia are so incredibly important for today's cosmology, since they are "standard candles" that can be used as yardsticks (make that mega-light-year-sticks) to measure mindboggling distances in space. Supernovas type Ia can be used this way because their real luminosity can be inferred from the shape of their light curve. We have reason to think that these supernovas can be trusted, because their luminosity has been calibrated against other, fainter standard candles that we understand better, such as Cepheid variables
. Yet we really don't understand the progenitors of Supernovas type Ia. Much of today's cosmology is built on the light curves of these supernovas, but astronomers don't really know what makes them go pop. Or at least not a single progenitor of such a supernova has been identified.
What today's APOD is telling us that SN 2014J, a type Ia supernova, wasn't caused by a white dwarf accreting matter from a swollen companion
until the white dwarf underwent a runaway nuclear fusion process, leading to the explosive destruction of the white dwarf. This has been the most popular hypothesis to explain supernovas type Ia, but it didn't happen in the case of SN 2014J.
And that is what today's APOD is showing us. It is telling us what didn't happen to trigger SN 2014J, and it is telling us that the Chandra telescope and the people operating it has been able to prove this non-occurrence. It is also telling us that the painstaking work of astronomer-sleuths is really telling us something
, but that there is so much work to be done, and still so much to know.
I love it!