How old is the Southern Cross?

The Southern Cross holds a special place in the hearts of those Down Under. Its four bright stars and one smaller star criss-cross in the shape of a kite, marking out the south celestial pole to act as a compass for bright-eyed explorers and navigators.

The symbol of Crux – the Latin name for the constellation – is displayed across the flags of five different nations and appears in astronomical lore of First Australians and Māori peoples, where it appears as a possum in a tree, a great eagle claw, or the anchor of the great sky canoe.

But how old is the most famous constellation in the Southern Hemisphere?

An international team of astronomers has, for the first time, unlocked the interior structure of Beta Crucis – the huge bright blue star that makes up the western point of the cross – with asteroseismology, the study of star oscillations.

The team found that Beta Crucis – also known as Mimosa – was 14.5 times bigger than the Sun and could be as young as 11 million years – which might not sound very young, but is just a baby compared to our 4.6-billion-year-old Sun!

Learning about the Southern Cross with asteroseismology

According to the study, published in Nature Astronomy, it is also the heaviest star yet identified with an age determined from asteroseismology.

Asteroseismology relies on seismic waves that bounce around the inside of a star, producing measurable changes in polarised light – a technique called polarimetry. As these light waves change in orientation, they can be measured to get a glimpse of a star’s interior, which is very difficult with other techniques.

“I wanted to investigate an old idea,” says lead author Daniel Cotton, from the Australian National University and Monterey Institute for Research in Astronomy in the USA.

“It was predicted in 1979 that polarimetry had the potential to measure the interiors of massive stars, but it hasn’t been possible until now.”

These changes can be imperceptible without the right equipment.

“The size of the effect is quite small. We needed the world’s best precision of the polarimeter we designed and built at UNSW for the project to succeed,” says study co-author Professor Jeremy Bailey from the University of New South Wales.

Astronomy requires collaboration

The study combined three different measurements of Beta Crucis’ light: space-based measurements of light intensity from NASA’s WIRE and TESS satellites, 13 years of ground-based high-resolution spectroscopy from the European Southern Observatory, and ground-based polarimetry gathered from Siding Spring Observatory and Western Sydney University’s Penrith Observatory.

“It was a lucky circumstance that we could use the world’s most precise astronomical polarimeter to make so many observations of Mimosa at the Anglo-Australian Telescope while TESS was also observing the star,” says second author Professor Derek Buzasi from Florida Gulf Coast University.

“Analysing the three types of long-term data together allowed us to identify Mimosa’s dominant mode geometries. This opened the road to weighing and age-dating the star using seismic methods.”

With this super precise polarimeter, the researchers hope they can learn more about how stars are formed and the age of our stellar neighbours.

“This polarimetric study of Mimosa opens a new avenue for asteroseismology of bright massive stars,” says Professor Conny Aerts of the Catholic University of Leuven, Belgium, who was also involved in the study.

“While these stars are the most productive chemical factories of our galaxy, they are so far the least analysed asteroseismically, given the degree of difficulty of such studies. The heroic efforts by the Australian polarimetrists are to be admired.”