The last days of a massive star before it went supernova

Supernovae do not enter gently into good night.
It rages, oh my! It rages even before it explodes in one the most epic events in the cosmos.

We don't know exactly how it happens. It is known that some stars that explode have material around them (called circumstellar media), but it can be hard to determine how much and how often the star ejects this material. We can now see the supernova SN2020fqv, which is quite close, and the rapid response of a team astronomers, that it had a powerful paroxysm right before it blew.

SN 2020fqv refers to a Type II (or core collapse) supernova. This is a huge star that ran out nuclear fuel. The core then collapsed, releasing a lot of energy and the blast wave that ripped the outer layers of the star. They were flung away at a fraction of the speed light.

These supernovae are detected in thousands every year. But this one is unique. It was very special. It happened in NGC4568, a galaxy located in the Virgo cluster, a collection made up of thousands galaxies bound to their own gravity. It is only 60 million light-years from us. This is very close on an cosmic scale.

The TESS observatory in orbit. Credit: NASA's Goddard Space Flight Center/CI Lab Image: NASA's Goddard Space Flight Center/CI Lab

Another reason was that this spot was being observed at the time by TESS (Transiting Exoplanet Survey Satellite). TESS monitors the light of millions of stars to find planets. If there is a dip in brightness, it will reveal a planet passing right in front of it. It scans the same area of sky multiple times and does not distinguish between bright stars or galaxies.

TESS measures brightness of an object in 30 minutes. In this case, it detected the sudden brightening. The Zwicky Transient Facility was the ground-based telescope that looked at the sky for brightening objects, and it actually detected the supernova on April 1, 2020. The TESS observations show the early stages of the explosion, but it does not take many images.

A team of scientists quickly seized on the confirmation of the explosion. I was truly stunned by this part: Less than an hour after supernova was identified as a core collapsing, astronomers contacted the Space Telescope Science Institute (responsible to Hubble Space Telescope operations) to inform them that they were going trigger a clause in an accepted proposal to capture as fast as possible observations of a nearby supernova core collapse supernova.

Two and a quarter hours later, the revised observing plan proposal and the ultra-rapid Target of Opportunity were submitted. This is what I find most amazing: Hubble began observing the explosion 32 hours after it was discovered, just 79 hours (3.3 days) later. Although Hubble normally takes about two weeks to point anywhere, there have been times when it took less time. It's amazing that observations can be collected in a matter of days and a third.

Composite image of merging galaxies NGC4567 and NGC4568 (with Supernova 2020fqv highlights) is a composite image that combines Hubble and new Hubble observations. It was taken from ground using the Lick Observatory. Photo credit: NASA, ESA Ryan Foley (UC Santa Cruz); processing done by Joseph DePasquale(STScI).

This rapidity is critical. The outer layers of the star explode at an extreme speed. This wave of many octillion tons stellar matter will catch up to the star in a matter of days if there is any material around it. This expanding debris will light up the circumstellar medium, which can be used to determine how dense it is, its distance and how it changes in density.

It worked in this instance. Astronomers were able, using Hubble and TESS data (including observations of these galaxies that occurred long before the supernova erupted) to reconstruct the probable layout of matter around it. This gave them information about the final days of this star...

It is likely that it was a star with a mass between 13.5 and 15 times that of the Sun. It's quite large. These stars fuse hydrogen into hydrogen for millions of years to create energy. Then, helium is fused into carbon and oxygen, and then into neon and magnesium. Each stage takes less time and runs hotter than the last. The core will be silicon by the time it fuses the whole core into iron. The core will no longer be able to fuse iron into any heavier material without absorbing the energy that is necessary to keep the core upright. The core bursts and everything is gone.

The outer layers of the Earth's crust may not change much during this period, but the action below will. The star can also blow away a lot of material, which is destined to become the circumstellar media, at an average rate of about one-third of the Earth's annual mass. Astronomers have calculated that SN 2020fqv ejected a remarkable 0.23 solar mass of material into space one year before it blew itself up. This is a tremendous rate, 200 times more than the Earth's daily mass. It's likely that the star's unstable fusion of oxygen caused huge amounts of energy to be emitted into outer layers of the star's core. This increased wind speed greatly.

The material was not able to get far before the star exploded. It was again only detectable due to the amazing response speed of our team.

Supernova debris smashed into the junk and turned it on. It was filled with so much stuff, that the brightness remained almost constant for approximately 114 days. This supernova is known as a Type II P.

It was how bright? Astronomers estimate that it was approximately a billion times brighter then the Sun, given its distance and the brightness of the image. One billion. Fast 4 months, every second of every day.


We don't know much about what a huge star like this does just before it explodes or how that affects the supernova. They've demonstrated that very rapid response times can be profitable, so I expect to see more observations similar to this. We'll be able to see what these stars do as they age, raving at night.