11 mins read 03 Nov 2020

The Red Giant That Pulsed in the sky

In 2019, one of the brightest stars in the sky suddenly started dimming, raising speculation it was about to go supernova. Now, a new study led by ANU Scientists, takes a look at the astronomically local red supergiant star Betelgeuse and redefines some of its parameters – extending its life and changing its distance and size.

The red supergian star, Betelgeuse. Credit: Cristian Cestaro.

For thousands of years, one star's deep red, ultimately bright twinkling light has captivated human attention - with its presence recorded throughout many human cultures around the world. Its modern name also shares that of a devious, scruffy, obnoxious ghost from a popular 1998 Tim Burton feature film. Whilst both these things are intertwined ever so slightly through popular culture, scientists from the Australian National University have looked a little deeper at one of the most well-known and studied stars in human history and revealed some surprising new detail.

Betelgeuse (pronounced like the film version, “Beetlejuice”) - is the name of the infamous red supergiant star which resides as the shoulder of the well-known summer constellation Orion (winter for northern hemisphere skywatchers).

A new study, led by Dr. Meridith Joyce from The Australian National University (ANU) and published in The Astrophysical Journal, not only gives Betelgeuse a new lease on life but shows it is both smaller and closer to Earth than previously thought. 

Dr. Joyce says the supergiant has long fascinated scientists, but lately, it has been behaving in a rather strange manner.

“It’s normally one of the brightest stars in the sky, but we’ve observed two drops in the brightness of Betelgeuse since late 2019,” Dr Joyce said. 

“This prompted speculation it could be about to explode. But our study offers a different explanation. 

“We know the first dimming event involved a dust cloud. We found the second smaller event was likely due to the pulsations of the star.” 

The great 2019-2020 Dimming

Betelgeuse’s dimming magnitude between 2015 and 2020. Credit: NYTImes.

In late 2019, astronomers around the world started observing that the great red star in Orion’s shoulder began dimming – not just through their sensitive equipment, but also to the naked eye. The sudden dimming came as a surprise to scientists around the world, as it had not been recorded to the extent at which it reduced in brightness (known as magnitude) from previous years in modern times.

Given Betelgeuse is a known supernova progenitor star – that is, it will likely detonate in a relatively short time (astronomically speaking) – this generated lots of excitement amongst the astronomy community, media and general public. 

"It is hypothesized that sudden dimming could immediately precede a supernova. These are rare celestial events, so catching one live would be significant. I think the public was excited by the possibility of catching a supernova in their lifetimes," said Dr. Joyce.

The dimming dropped to such an extent, that astrophysicists turned their ground-based telescopes to the star, and the Hubble Space Telescope was brought in to conduct studies of what could be causing such an extreme event where the brightness of the supergiant would fall by several magnitudes. To put it in perspective, during the 2019-2020 dimming, Betelgeuse went from being one of the top ten brightest stars in the sky to dropping outside the top 20.

"For scientists, this would be exciting because the late evolutionary stages of high mass (giant) stars are rarely observed (because stars of this mass are so rare to begin with), so we usually rely on models to study them rather than direct, empirical measurements. Observations of a real late-stage-to-supernova transition would provide a way to test our models of rare events."

Was Earth about to experience its most brilliant supernova event of recent times? According to the findings of several studies, no. 

The 2019-2020 dimming of Betelgeuse is expected to be due to ejected material obscuring our line of sight view from Earth. Credit: NASA, ESA, and E. Wheatley (STScI).

The Hubble study found that the culprit for the dimming was likely caused by an ejection of material from the star which cooled in space and formed dust across our line of sight, partially obscuring some of the light we received here on Earth.  Another study found that it could have been related to large starspots – regions of dimmer temperatures and energies – which likely came into view from our perspective that may have caused it.

In all scenarios, most scientists agreed that Betelgeuse was not about to go supernova, much to the dismay of the public and media.

Stellar Evolution

The different evolutionary pathways that stars follow depending on their mass. Credit: R. N. Bailey/Wikipedia.

Our understanding of the stars is unlike the ancient knowledge banks, in which stars and the heavens were both eternal and related to spiritual purposes. Instead, we now know that stars evolve and undergo massive physical changes along this cycle.

One of the key factors involved in stellar evolution is the mass of the star when it is born. This important value determines how much gravity the object will have and since gravity is the main driving force of ‘squeezing’ the star inwards, it is the mechanism that also translates to how much energy the star will output.

In short, the mass of the star creates gravity which squeezes matter in the core to high pressures and temperatures creating the environment for nuclear fusion, which then unleashes the energy in the form of electromagnetic radiation (heat, light, x-rays, etc.) that we observe. A good rule of thumb is that more massive stars have stronger gravity, and as such have more pressure to drive more furious nuclear reactions that output higher energies.

Stars like our Sun are considered middle range in mass, with many more stars in the lower-mass range. However, a smaller population of stars are many times more massive than the Sun – which include many of the stars observable to the naked eye under urban skies.

As stars age, they consume most of their hydrogen fuel and begin to burn newer, synthesised elements like helium, carbon and oxygen which begins to change their shapes through expansion and contraction. This results in their colour also changing – as an example, towards the end of its life even our Sun will expand from the yellow star we note today to a huge red supergiant, engulfing the planets Mercury, Venus and potentially Earth.

What comes beyond this ‘puffing’ stage is once again determined by the mass of the progenitor star – stars like our Sun will result in a compact remnant core called a white dwarf, whilst more massive stars go on to become neutron stars or black holes.

According to Dr. Joyce, we can learn more about the complex evolutionary pathways of stars like Betelgeuse by understanding their internal mechanisms. 

"Betelgeuse isn't well enough constrained to provide much insight on the models themselves, though our study does demonstrate the power and utility of the seismology technique to help characterize complex stars like Betelgeuse," she said.

Betelgeuse the Super Star

Prior to this study it was thought that Betelgeuse’s radius was so big, it would engulf all the inner planets and extend out to the orbit of Jupiter. Credit: ESO.

This new study has now looked at Betelgeuse and reported on several parameters, for the bright red star that’s so very familiar to our perspective and views of the night sky.

Betelgeuse is classed as a red supergiant star whose radius is an enormous 764 times that of our Sun. If placed at the centre of our own Solar system, Betelgeuse’s radius would extend out to just over twice the orbit of Mars (roughly 3.5 astronomical units), just outside the Asteroid Belt, according to these latest findings.

Dr. Joyce’s team also found that Betelgeuse has a mass range about 16.5 – 19 times the mass of our Sun and is likely located only 530 light-years from us – which is 25% closer than previously reported. Its age is expected to be less than 10 million years old, which is considered relatively young (remember the dinosaurs died out 65 million years ago).

"Our results are not inconsistent with other recent measurements, but our best estimates do place Betelgeuse closer to Earth and find it to be smaller."

Modern technology has also allowed us to determine that Betelgeuse is a pulsating star, as it heads towards the end of its life, and experiences three pulsation cycles: a main period that cycles every 400 days, a shorter period that occurs on a 185-day cycle and a longer period that occurs every 2,100 days. Accurately measuring these multiple pulsation cycles allows astronomers to understand what is occurring inside the star – even though we cannot see it.

"More subtly, we also provide a prediction for its evolutionary phase, which can be deduced by figuring out what fuel it's burning in its core. Our models suggest this fuel is Helium, which tells us that the star is not as old (nor as close to "going supernova") as previously estimated," said Dr. Joyce. 

"Previous estimates placed its supernova timescale at 10,000 years, whereas ours suggest 100,000 years (an order of magnitude). However, since these calculations are largely theoretical, it was still possible that the 2019-2020 dimming event was foreshadowing a supernova imminently-- we cannot be 100% sure without direct measurement, and our models are revised as more and more direct measurements of all types of stars are taken."

Aboriginal Astronomers Record Variability

The recent dimming event of Betelgeuse showcased that humans can indeed observe the variability of massive stars with the naked eye – something that has been studied across Aboriginal cultures who have had a deep connection with astronomy for many millennia, in many regions around Australia.

Both studies of Betelgeuse in particular, or studies which have also included other red supergiants – such as Aldebaran and Antares – also detail the variability of these massive red supergiants in the past. In addition to the regular dimming cycles observed by Aboriginal astronomers, there’s a good chance that stars like Betelgeuse might have previously thrown up large plums of dust which obscured the culturally significant object from the night sky.

The result of these dimming events have been recorded through Aboriginal community songlines and lore for tens of thousands of years.

Humans are also known to predict the variability of stars with short term pulsations as well – a prime example of this is the documented surviving evidence that ancient Egyptians noted the variability of the star Algol (almost to the exact value of 2.85 days) and recorded this variability as a relation to the God Horus in the Cairo Calendar, many thousands of years ago.

Pre-Supernova Opportunity

Artist illustration of a supernova event. Credit: spaceexploration92.com.

Due to its astronomically close proximity to Earth, Betelgeuse offers astrophysicists the opportunity to study the end stages of stellar evolution of massive stars, and how they behave as they age.

“It’s still a really big deal when a supernova goes off. And this is our closest candidate. It gives us a rare opportunity to study what happens to stars like this before they explode,” Dr Joyce said. 

With research, like the paper produced by Dr. Joyce and her team – astrophysicists are starting to learn more about the internal structures and mechanisms of massive stars, the impacts they have on their surrounding environments, and how we observe them from Earth.

Betelgeuse is not expected to detonate as a supernova for another 100,000 years or so – but it is during this time, the here and now when our technology is at its most advanced, that we can discover new and exciting things about how stars evolve and what happens to them when they die.

Like most massive stars towards the end of their lives, Betelgeuse has been synthesising heavy elements like carbon, oxygen, nitrogen – the ingredients that make up other stars like the Sun, planets like the Earth and as a speciality, life, like humans and other Earth beings.

By understanding how these massive supergiants end their lives, we gain an insight into our own beginnings. When we look at this cycle occurring everywhere around the Universe – the cycle of death that creates life – it opens up the possibility to consider through self-reflection our own presence.

If we are here as a result of a former red supergiant star dying in a supernova, which gave the opportunity to create the conditions that spawned life, could another red supergiant, much like Betelgeuse somewhere else in the Universe have also replicated these conditions?

Betelgeuse, the star with the funny name, has not only played an important part of our human history through our observations that have drawn us to its deep red glimmer – it could help us resolve some of our deepest philosophical questions about how and why life emerged on this small blue planet, in a vast and endless cosmos. 

The paper is available on arXiv.org