Two lamps in a shop window: one glows with a cool, bluish light, the other a bit warmer and yellower. That's roughly how the two types of supernovae, Ib and Ic, look when astronomers view them through a telescope. Both flare up at the end of a massive star's life, after it has blown off its hydrogen envelope, much like a dandelion losing its fluff. But one retains a bit of helium, while the other has almost none. As it turns out, this difference can be detected by color.
A team of scientists sifted through data from the Zwicky Transient Facility — an automated telescope that scans the sky every night for anything that moves or changes brightness. They obtained light curves for hundreds of Type Ib and Ic supernovae. Comparing their colors at peak brightness, the researchers noticed that Type Ib supernovae are, on average, bluer than Type Ic. The difference is small but statistically robust — the probability of a chance coincidence is less than five percent. Dust in the host galaxies also slightly tints the light, but its contribution isn't enough to explain the whole effect.
Interestingly, among these supernovae there's a special subgroup: those with narrow lines in their spectra (Ibn and Icn). They turned out to be even bluer than their "broad-lined" counterparts. This might involve interaction with material the star ejected shortly before the explosion — something like a shock wave slamming into an invisible cocoon. Such events are sometimes linked to fast blue optical transients — mysterious flashes that last only a few days.
The practical takeaway is simple: the color of a supernova, measured from images in different filters, can serve as a quick indicator of how stripped the star was before death. For large surveys where there's no time to take spectra, this is a valuable tool. It will help better predict which stars end their lives in an explosion and which become neutron stars or black holes. So color becomes not just decoration but a key to a star's biography.
as written by the authors arXiv:2605.01200
Type Ib and Ic supernovae (SNe Ib/Ic) are the bright finale of massive stars that have lost their hydrogen envelopes, making them powerful probes of mass stripping in massive star evolution. The advent of modern large photometric and spectroscopic surveys presents a unique opportunity to investigate systematic differences between these two kinds of SNe. In this study, we analyze a large, homogeneous sample of SNe Ib/Ic light curves from the Zwicky Transient Facility. We find a systematic difference in their apparent optical colors at peak: SNe Ib are, on average, bluer than SNe Ic, with two-sample tests confirming that the distributions differ (p<0.05). The difference in their host galaxy reddening, as currently constrained, is too small to fully explain it. The color difference therefore most likely has an intrinsic origin, reflecting progenitors with different degrees of stripping -- helium-rich for SNe Ib and helium-poor for SNe Ic. In addition, we find that SNe Ib/Ic with narrow lines (SNe Ibn/Icn) are bluer than those without, which might originate from circumstellar matter interaction, with potential connection to fast blue optical transients. We demonstrate that SN colors offer a promising probe of mass stripping in massive stars, potentially providing a useful tool for analyzing large photometric data and improving predictions for the final outcomes of stripped massive stars.
Type Ic supernovae are sometimes mistaken for Type Ib if data is lacking, but color can reveal the difference even without a spectrum.
In Stanisław Lem's novel "Solaris," the ocean is studied by the color of its glow — much as astronomers read the history of stars by the hues of their explosions.