From Horizon: “How astronomers are piecing together the mysterious origins of superluminous supernovae”


From Horizon The EU Research and Innovation Magazine

26 February 2020
Jonathan O’Callaghan

Superluminous supernovae, though rare, tend to be found in star-forming regions of our universe. Image credit – ESO/L. Calçad/ Wikimedia, licenced under CC BY 3.0

When a massive star reaches the end of its life, it can explode as a supernova. But there’s a unique type of supernova that’s much brighter that we’re just starting to understand – and which may prove useful in measuring the universe.

Known as superluminous supernovae, these events are typically 10 to 100 times brighter than a regular supernova but much more rare. We’ve spotted about 100 so far, but many aspects of these events remain elusive.

Why are they so much brighter than regular supernovae, for example, and what stars cause them? Astronomers are hoping to answer these and more questions in the coming years, with various studies underway to understand these events like never before.


Dr Ragnhild Lunnan from Stockholm University, Sweden, is one of the co-investigators on the SUPERS project, which is attempting to work out what stars lead to the formation of superluminous supernovae. With dozens found already, the team are building the largest collection of these events in an effort to learn more about them.

‘By following the evolution of these supernovae into a very late phase, you can decode their (structure),’ she said. ‘This tells you things about the star that exploded, and possibly how it exploded.’

To find these explosions, Dr Lunnan and her team are making use of a camera called the Zwicky Transient Facility (ZTF), part of the Palomar Observatory in California, US, to survey the sky.

Zwicky Transient Facility (ZTF) instrument installed on the 1.2m diameter Samuel Oschin Telescope at Palomar Observatory in California. Courtesy Caltech Optical Observatories

Caltech Palomar Samuel Oschin 48 inch Telescope, located in San Diego County, California, United States, altitude 1,712 m (5,617 ft)

Only one supernova is expected per galaxy per century, with only one in 1,000 or even one in 10,000 of those being superluminous. But by looking at many galaxies simultaneously with the ZTF, it’s possible to spot these events.

Superluminous supernovae are found more often in star-forming galaxies than older galaxies, which means they are likely explosions of young stars, notes Dr Lunnan.

‘Additionally, you very often find them in galaxies that are kind of chemically primitive, called low-metallicity, and we think this is also a clue,’ she said. ‘We think they’re associated with very massive and metal-poor stars. But beyond that, we really don’t know.’

In 2018, Dr Lunnan and her team discovered a superluminous supernova with a giant shell of material around it [Nature Astronomy], which it must have ejected in the final years of its short life. ‘That discovery (of the shell) is another clue that the stars must be very massive,’ said Dr Lunnan.

Going supernova

The exact process that causes a superluminous supernova is another question. Typically, stars can go supernova either by independently collapsing, or sharing material with a small dense star known as a white dwarf before an explosion takes place, known as a Type 1a supernova. But what happens in a superluminous event?

Dr Avishay Gal-Yam from the Weizmann Institute of Science in Israel, project coordinator on the Fireworks project, has been trying to answer this question. The project has been using observations of the night sky from cameras like the ZTF that have a rapid cadence, meaning they show an event shortly after it occurred, to study cosmic explosions.

Previously we would only see supernovae about two weeks after they happened, but ZTF’s constant observations of the sky allows us to see them within about one or two days. And that’s particularly useful for superluminous supernovae. A regular supernova can brighten and fade over a period of weeks, but a superluminous supernovae can last several times longer, while also reaching its peak brightness slower.

‘They are relatively slowly evolving,’ he said. ‘The time for the explosion to reach its peak could be a couple of months, sometimes even longer. So studies of these objects are not focused on rapid observations, but rather a continuous follow-up campaign which takes months and sometimes years.’

So far Dr Gal-Yam and his team have published several studies [Annual Review of Astronomy and Astrophysics], examining some of the theories for how these events happen. One idea is that a regular supernova leaves behind a rapidly spinning and highly magnetised neutron star, called a magnetar, which acts as a giant magnet and pumps energy into the supernova explosion.

But Dr Gal-Yam’s more favoured theory is the same advocated by Dr Lunnan – that collapsing massive stars are the cause. ‘What can generate so much energy that can power such a luminous emissions, both in terms of the amount of energy and the very long amount of time the emission continues to happen?’ he said. ‘The most intriguing (theory) is an explosion from a very massive star 100 times more massive than the sun.’


While many questions about superluminous supernovae remain unanswered, they are already proving useful as distance markers in the universe. Called ‘standard candles’, bright events like supernovae can tell us how far away a particular galaxy is as we know how bright they should be.

Standard Candles to measure age and distance of the universe from supernovae. NASA

‘The idea here is a standard candle, an object of known luminosity,’ said Dr Mark Sullivan, project coordinator on the SPCND project that looked at how explosive events like this might be useful for cosmological studies. ‘If you can find it in the sky and measure how bright it appears to be to us on Earth, you can tell how far away it is.’

The brightness of superluminous supernovae makes them particularly useful. Using the Dark Energy Survey (DES), a survey of the night sky using the Cerro Tololo Inter-American Observatory in Chile, Dr Sullivan and his team found more than 20 superluminous supernovae in galaxies up to eight billion light-years from Earth, giving us a new cosmic distance ladder. ‘We got a new data set of these objects in the distant universe,’ said Sullivan.

Dark Energy Survey

Dark Energy Camera [DECam], built at FNAL

NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet

Timeline of the Inflationary Universe WMAP

The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

DES is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

Over six years (2013-2019), the DES collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.

With a growing sample size of these events, astronomers will now be hoping to answer once and for all what causes them. Upcoming telescopes like the Vera C. Rubin Observatory in Chile could prove vital, performing new sweeping surveys of the night sky, and finding more of these objects than ever before.

Fritz Zwicky discovered Dark Matter in the 1930s, when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

‘We really are in this era where we’re finding so many objects – even things that are rare,’ said Dr Lunnan. ‘It’s a lot of fun.’

See the full article here .

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