Star Light, Star Bright

How backyard observers can help unlock the secrets of supernovae. | By Tim Lyster & George Silvis

The death of a giant star is a wonder to behold.

During its middle age, there is a battle of sorts within every star. Enormously high temperatures in the stellar core create outward pressure, finely balanced by gravity pulling inward. These competing forces stay locked in stalemate for millions of years.

Until they don’t.

For Type II supernovae, when the raw fuel powering thermonuclear reactions runs out, inner temperatures cool, the pressure drops, and the force of gravity begins to overwhelm its opposition. The star rapidly collapses. This implosion happens within seconds, and the resulting shock waves cause the outer part of the atmosphere to be blown into surrounding space.

Supernovae in History

Observed supernovae in the historical record are exceedingly rare. There are recorded eight examples during the past two millennia. Since 1604, there have been two supernovae observed with the unaided eye: SN 1885A in Andromeda and SN 1987A in the southern constellation of Dorado, and none in our own galaxy.

Credit: A Brief Review of Historical Supernovae, International Journal of Astronomy and Astrophysics, Vol.11 No.1, March 2021

Models in stellar physics predict a much more prolific rate, perhaps one or two per century within the Milky Way alone. Astronomers have explained the discrepancy by positing that many supernova events are missed because they occur in the thin, star-filled disk of the galaxy, where dust obscures our view. Quantitative estimates suggest we are missing six out of seven stars undergoing core-collapse and two thirds of all Type Ia events.

Thus, supernovae provide a rare opportunity to record an extreme astronomical event that could lead to a deeper understanding of the physics of stars. But being largely unpredictable, how do we increase the odds of observing its early stages?

A New Type of Telescope

For most of human history, the first indication of a supernova was of a bright star in an area of the sky where there was none before. That changed when the Kamiokande Observatory near Mount Ikeno, Japan, detected a burst of neutrinos—ubiquitous, near-weightless particles—from the direction of the Large Magellanic Cloud. This happened 2-3 hours before visible light from SN 1987A reached Earth. The observations were consistent with theoretical supernova models in which 99% of the energy of the collapse is radiated away in the form of neutrinos. Neutrino astronomy was born.

SNEWS of the World

SNEWS, the Super Nova Early Warning System, is an interesting collaboration between professionals and amateur astronomers, and an example of so-called multi-messenger astronomy.

The program is run by professionals that oversee underground neutrino facilities, like Kamiokande Observatory, all around the world. Despite being unable to see the sky, they are astronomers, as they are listening for that signature neutrino burst expected to precede the optical breakout of the nova by several hours.

This is the multi-messenger part: the SNEWS program coordinates observers who practice astronomy at different wavelengths and signals. The goal is to have telescopes aim toward the target area shortly after the neutrino burst is detected.

The neutrino detectors will provide only a vague location of the star. It will take many people looking to catch the flare given its imprecise coordinates. This is where amateur sky-gazers can contribute real science, as they have the large number of observers that can cover the sky.

The AAVSO plays a pivotal role, ensuring participating observers are aware of this challenge [PDF], prepared for the anticipated alerts, and trained on how to search for and report the event, so optical professionals can turn their large scopes toward the target.

If SNEWS works as it should, scientists will have a front-row seat witnessing the supernova’s optical break-out in detail, and the science garnered has the potential to enlighten our understanding of physics in extreme environments.

Scrutinizing Supernova Candidates

Another part of the amateur's contribution is to capture regular observations of the stars thought to be most likely to be the next local supernova. This requires time and patience to observe stars too bright for the sensitive detectors on large telescopes. Amateurs can collect light curves that could indicate a star nearing core collapse, and provide information for post-event analysis, adding to our knowledge of a supernova’s internal mechanism.

Beyond individual contributions, the AAVSO is deploying the resources of AAVSOnet, the organization’s global network of research-grade telescopes. Leveraging a "subscription" model of operation, AAVSO members will sign up as Bright Star Monitors to observe 192 candidate stars.

The advantage of this approach? Rather than wait for individual AAVSOnet proposals to subsets of the star list, the network will observe them all. Participants can then subscribe to process images and create light curves as they come in.

Perhaps the death throes of giant stars will soon be in reach? Only time—and active participation in this citizen science project—will tell. 

Image of M101:  International Gemini Observatory / NOIRLab / NSF / AURA / J. Miller, Gemini Observatory & NSF’s NOIRLab / M. Rodriguez, Gemini Observatory & NSF’s NOIRLab / M. Zamani, NSF’s NOIRLab / T.A. Rector, University of Alaska Anchorage & NSF’s NOIRLab / D. de Martin, NSF’s NOIRLab.