This Galaxy Has Been Home to 5 Supernovae in the Last 20 Years

Some of the most dramatic events in the Universe occur when certain stars die — and explode catastrophically in the process. Such explosions, known as supernovae, mainly occur in a couple of ways: either a massive star depletes its fuel at the end of its life, become dynamically unstable and unable to support its bulk, collapses inwards, and then violently explodes; or a white dwarf in an orbiting stellar couple syphons more mass off its companion than it is able to support, igniting runaway nuclear fusion in its core and beginning the supernova process. Both types result in an intensely bright object in the sky that can rival the light of a whole galaxy. In the last 20 years the galaxy NGC 5468, visible in this image, has hosted a number of observed supernovae of both the aforementioned types: SN 1999cp, SN 2002cr, SN2002ed, SN2005P, and SN2018dfg. Despite being just over 130 million light-years away, the orientation of the galaxy with respect to us makes it easier to spot these new ‘stars’ as they appear; we see NGC 5468 face on, meaning we can see the galaxy’s loose, open spiral pattern in beautiful detail in images such as this one from the NASA/ESA Hubble Space Telescope.

When stars die, they don’t die quietly but prefer to go out with a bang! This is known as a supernova, which occurs when a star has expended all of its fuel and undergoes gravitational collapse. In the process, the outer layers of the star will be blown off in a massive explosion visible from billions of light-years away. For decades, NASA has been monitoring galaxies beyond the Milky Way and detected numerous supernova taking place.

For instance, over the past 20 years, the Hubble Space Telescope has been monitoring the galaxy NGC 5468 – an intermediate spiral galaxy located roughly 130 million light-years from Earth in the constellation Virgo. In that time, this galaxy has experienced 5 supernovae and, thanks to its orientation (perpendicular to our own), astronomers have been able to study this galaxy and its supernovae in glorious detail.

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It’s Been Three Years Since We’ve Had a Supernova This Close

Artistic impression of a star going supernova, casting its chemically enriched contents into the universe. Credit: NASA/Swift/Skyworks Digital/Dana Berry

A supernova is one of the most impressive astronomical events anyone can possibly witness. Characterized by a massive explosion that takes place during the final stages of a massive star’s life (after billions of years of evolution), this sort of event is understandably quite rare. In fact, within the Milky Way Galaxy, a supernova event is likely to happen just once a century.

But within the Fireworks Galaxy (aka. the spiral galaxy NGC 6946), which is located 22 million light years from Earth and has half as many stars as our galaxy, supernovae are about ten times more frequent. On May 13th, while examining this galaxy from his home in Utah, amateur astronomer Patrick Wiggins spotted what was later confirmed to be a Type II supernova.

To break this magnificent astronomical event down, most supernova can be placed into two categories. Type I Supernovae occur when a smaller star has consumed all of its nuclear fuel, and then undergoes core collapse with the help of additional matter accreted from a nearby orbiting star. Type II Supernovae are the result of massive stars undergoing core collapse all on their own.

The confirmed supernova, “SN 2017aew”, which can be seen on the top right side of the “Fireworks Galaxy”. Click to see animation. Credit: Patrick Wiggins

In both cases, the result is a sudden and extreme increase in brightness, where the star blows off its outer layers and may become temporarily brighter than all the other stars in its galaxy. It then spends the next few months slowly fading until it becomes a white dwarf. It was while surveying the Fireworks galaxy with his own telescope that Wiggins noticed such a sudden burst in brightness, which had not been there just two nights before.

Wiggins finding was confirmed a day later (May 14th) by two experts in supernovae – Subo Dong and Krzysztof Z. Stanek, two professors from Peking University and Ohio State University, respectively. After conducting observations of their own, they determined that what Wiggins had witnessed was a Type II supernova, which has since been designated as SN 2017eaw.

In addition to being an amateur astronomer, Patrick Wiggins is also the public outreach educator for the University of Utah’s Department of Physics & Astronomy and the NASA Solar System Ambassador to Utah. This supernova, which was the third Wiggins has observed in his lifetime, is also the closest to Earth in three years, being about 22 million light years from Earth.

The last time a supernova was observed exploding this close to Earth was on January 22nd, 2014. At the time, students at the University of London Observatory spotted an exploding star (SN 2014J) in the nearby Cigar Galaxy (aka. M82), which is located around 12 million light years away. This was the closest supernova to be observed in recent decades.

Animation showing a comparison between M82 on Jan. 22nd, 2014 Nov. 22nd, 2013. Credit: E. Guido/N. Howes/M. Nicolini

As such, the observation of a supernova at a comparatively close distance to Earth just three years later is a pretty impressive feat. And it is an additional feather in the cap of an amateur astronomer whose resume is already quite impressive! Besides the three supernova he was observed, Wiggins has received many accolades over the years for his contributions to astronomy.

These include the Distinguished Public Service Medal, which is the highest civilian honor NASA can bestow. In addition, he discovered an asteroid in 2008 which the IAU – at Wiggin’s request – officially named “Univofutah”, in honor of the University of Utah. He is also a member of the Phun with Physics team, which provides free scientific lessons at the Natural History Museum of Utah.

Further Reading: University of Utah UNews

A Lurking Companion Star Explains Enigmatic Supernova

The above sequence depicts a rare supernova explosion. The topic panel

Massive stars end their lives dramatically. Once the nuclear fuel deep within their cores is spent, there’s no longer any outward pressure to push against gravity, and the star collapses. But while the inner layers fall in to form a black hole or a neutron star, the outer layers fall faster, hitting the inner layers, and rebounding in a huge supernova explosion.

That’s the textbook definition. But some of these supernovae defy explanation. In 2011 one such explosion, dubbed SN 2011dh, pierced the Whirlpool galaxy, roughly 24 million-light years away. At the time astronomers were baffled. But now, thanks to NASA’s Hubble Space Telescope, they’ve discovered a companion star to this rare supernova and fit the final puzzle pieces together.

SN 2011dh is a Type IIb supernova, unusual in that it contains very little hydrogen and unexplainable via a textbook definition. Even so, astronomers can shed light on the progenitor star simply by digging through archived images from HST. Thanks to HST’s wealth of data and the fact that it observes the Whirlpool galaxy often, two independent research teams both detected a source — a yellow supergiant star — at the right location.

But astronomers don’t think yellow supergiant stars are capable of becoming supernovae … at least not in isolation.

At this point, controversy arose within the astronomical community. Several experts proposed that the observation was a false cosmic alignment and that the actual progenitor was an unseen massive star. Other experts proposed that the progenitor could have been the yellow supergiant, but that it must have belonged in a binary star system.

When a massive star in a binary system overflows its Roche lobe — the region outside that star where gravity dominates — it can pour material onto its smaller companion, therefore losing its hydrogen envelope and shrinking in mass.

At the time the mass-donor explodes, the companion star should be a massive blue star, having gained material during the mass transfer. Its high temperature should also cause it to emit mostly in the ultraviolet range, therefore rendering it invisible in any visible images.

So Gastón Folatelli from the Kavli Institute for the Physics and Mathematics of the Universe (IPMU) and colleagues decided to take a second look at the mysterious supernova in ultraviolet light. And their observations matched their expectations. The original supernova had faded, and a different point source had taken its place.

“One of the most exciting moments in my career as an astronomer was when I displayed the newly arrived HST images and saw the object right there, where we had anticipated it to be all along,” said Folatelli in a news release.

The research illustrates the intricate interplay between theory and observation. Astronomers often rely on theories long before they gain the technology necessary to provide the correct observations or spend years trying to explain odd observations with complex theoretical modeling. More often, however, the two coexist as theory and observation banter back and forth.

The findings have been published in the Astrophysical Journal Letters and are available online.