Two Recent Supernovae
It’s hard to exaggerate the explosive power of supernovae. When triggered, a single supernova outshines the brightness of an entire galaxy. The explosion itself takes only a couple of minutes, but we can observe the afterglow for weeks.
Supernovae are exploding stars. Two recent supernovae have been in the news — SN2019hgp and SN2020tlf — each for their own reason.
SN2020tlf would have been a garden variety, otherwise uninteresting Type II, “core collapse” supernova if not for the fact that astronomers caught it exploding in essentially real-time. What fascinated the authors who reported on this star’s demise was what happened in the months just before it went supernova.
SN2019hgp, on the other hand, appears to be an exploding Wolf-Rayet star. It would be the first observation of a Wolf-Rayet supernovae ever made, which is important as some recent proposals suggested these exotic stars might collapse silently into black holes.
We will visit the details of both events in turn. But first, we should probably explain why stars explode in the first place.
Some Nuclear Physics
Stars are a physical manifestation nuclear power. They are not the familiar, fission reactors that we’ve had on Earth for the past century. Fission reactors break apart the nuclei of extremely heavy atoms, releasing energy and other nuclear material in the process. Starlight is powered by nuclear fusion, which brings the nuclei of smaller atoms together.
If two smaller nuclei fuse to form a bigger nucleus, the individual particles collectively tighten naturally. That tightening involves the release of energy. That’s the energy that powers a star.
It might seem peculiar that both breaking and joining nuclei can release energy. While many kinds of nuclei are stable, the most tightly bound nuclei are medium sized elements like Iron or Nickel. These have 26 and 28 protons, respectively, and each carry an additional 30 or so neutrons.
On balance, any process — fusion or fission — that gets you to something like Iron or Nickel can generate energy.
The Stellar Fusion Reactor
Stars start out as a large cloud of gas — perhaps the gas left over from past supernovae. Since most of the gas in the universe is made up of hydrogen, those clouds are mostly comprised of hydrogen gas.
Gravity pulls the gas clouds together, condensing into planet-like objects similar to Jupiter. If enough gas is present, the pressure and heat at the center of that newly formed “planet” or “protostar” can become so intense that the hydrogen atoms get pulled uncomfortably close together. When this happens, their nuclei fuse into helium.
These fusion reactions generate a lot of energy, which is what causes the stars to shine. The associated heat also creates an outward pressure which balances inward pressure of gravity. Most of a star’s life exists “burning” hydrogen into helium.
Life After Hydrogen
When a star burns up the hydrogen, things start to get weird. How weird depends on the mass of the star.
In normal stars, the heavier helium molecules gravitate to the center. The star collapses, drawing hydrogen from the outer layers of the star inwards.
As the new hydrogen atoms aggregate near the boundary of the helium core, they begins to fuse, which pushes out much of the outer stellar material further. With this new scenario, the gas in the outer layers of the star begin to convect strongly, plumping the star into a red giant.
Depending on the star’s overall size, that helium core will ignite. For smaller stars — like our sun — it goes all at once, for bigger stars it slowly burns just like the hydrogen did.
For really big stars this process repeats. The carbon fuses into neon. The star contracts again and the pressure rises, which causes the neon to start burning. This process repeats until a core of iron is formed, which being as tightly bound as it get, is the end of the road.
Since iron will not fuse, the outward pressure given by the nuclear reactions stops and gravity causes the core of the star to collapse in on itself. In what must be some of the most extreme conditions in the universe, the collapse stops abruptly as the star essentially becomes one titanic nucleus.
The sudden stop of the gravitational collapses creates the shockwave that rips through the outer layers of the star. That destructive shockwave and the resulting explosion is what we call a supernova.
How to Spot a Supernova
For a galaxy like ours with a billion or so stars, about one or two goes supernova in a human lifetime. With billions of galaxies in the universe, these events are not terribly uncommon.
Despite this, and despite being the brightest objects in the universe, unless one happens sufficiently close to us, the unaided eye still cannot see them.
But research telescopes can.
Many astronomy research programs involve scanning the sky looking for supernovae. The Pan-STARRS telescopes are a pair of telescopes each with over gigapixel camera sensor — and. that scan a large portion of the sky every night.
The Gurgling before SN2020tlf
The Young Supernovae Experiment uses the Pan-STARRS data to look for supernovae as they happen. The supernova named SN2020tlf was one of these observations.
SN2020tlf was a fairly ordinary, Type II core collapse supernova. As we discussed above, the end of fusion is believed to result in a lack of outward pressure that drives the collapse which produces the shockwave.
But what happens just before that shockwave? What happens as the star beings to run out of nuclear fuel?
Until recently, the behavior of massive stars prior to these explosions has been “almost entirely unconstrained”.
Going back through the Pan-STARRS data, the YSE team was able to examine what that star was doing just months before it exploded. Apparently, the star was actively ejecting material a few months prior to the explosion.
While such pre-explosion gurgling has been observed in some more exotic supernovae before, it had never been in a “garden variety” Type II explosion. In the YSE’s language:
“Based on the above data reduction, we find evidence for a statistically significant (>3σ) pre-explosion flux excess at the SN location (m ≈ 20.7–21.9 mag) in riz-bands [red into near infrared] from MJD 58971.42–59097.24 (δt = −127.3 to −1.49 days before first light).”
Final Moments. I. Precursor Emission, Envelope Inflation, and Enhanced Mass Loss Preceding the Luminous Type II Supernova 2020tlf. W. V. Jacobson-Galán et al 2022 ApJ924 15
While models certainly exist for how the stars can behave, until 2020tlf, observations were lacking to constrain them. Exotic supernovae measurements can leave room for all kinds of exceptions, statistical uncertainty and exotic explanations. Being an “ordinary” example, 2020tlf behavior now directly constrains the behavior of “ordinary” Type II supernovae.
Astronomers are now entering a new phase of understanding the explosion of these large stars.
Wolf-Rayet Stars and SN2019hgp
Wolf-Rayet stars are a rare but diverse population of stars whose light contains very little spectra from Hydrogen. They are among the hottest known stars, whose light is intense but is mostly in the ultraviolet. These physical characteristics are consistent with a star that has burned through all its hydrogen and has started to fuse helium — or heavier elements.
The lack of surface hydrogen has been modeled in two ways. Either the stars spin too quickly and their outer layers get mixed well, causing the heavier elements to rise unexpectly to the outer later. Or, they simply have lost all of their lighter material into space, owing in part to their immense heat.
While rare, they can be seen from Earth. There are some visible in the southern hemipshere:
- Gamma Velorum of the Constellation Vela
- Theta Muscae of the Constellation Musca
- R136a1 in the Large Magellanic Cloud (magnitude 7+ so you’ll need binoculars)
Previously, no Wolf-Rayet stars had been known to explode as a supernova. Given the lack of observations — and that these stars appear to be constantly losing much of their outer layers — it had been hypothesized that Wolf-Rayet stars might not explode. There might not be enough matter left around their core when it collapsed, to carry a shockwave.
The observation of SN 2019hgp carried definitive signatures of a Wolf-Rayet star. In particular, it was shown that the light generated in the supernovae came from the stellar envelop of gas including carbon and oxygen. Moreover, spectral signatures of neon was observed for the first time in an explosion.
Whether or not all Wolf-Rayet stars are capable of denoting a supernova is not clear, but what is clear now is that we have a first example.
More observations are on the way. A recent supernova, SN2021csp may also turn out to be from a Wolf-Rayet star.
Big Explosions and Big Data
Not all stars will explode, but those that do can outshine their entire galaxy. The shockwave that drives the explosion can accidentally fuse the nearby elements into some of the biggest known nuclei like Uranium and Thorium.
The remnant of the a star’s collapsed core after a supernova is either a neutron star — one titanic nucleus — or even a black hole. Although there is an occasional possibility that nothing will be left behind.
Like Collider Physics, Astronomy is a leader in driving the demand for massive data storage, processing and analysis pipelines. With more of the observations and analyses being automated, and with data being archived for later use, our capacity to constrain behavior of astronomical transistents, like supernovae, will only improve.
Much observational work remains to pin down exactly how these Supernovae occur in context, and new data always beget new models. With modern tools like the Python ecosystem replacing dated languages like IDL and Fortran, data-driven sciences like particle physics and astronomy may will attract more students who can become more productive more quickly.
More discoveries await, and more, perhaps, in the near term.
