Australasian Science Magazine

By David Reneke

Astronomers take a close look at planetary formation around a binary star and examine one of the biggest stars in our galaxy.

Astronomers have always struggled to understand how planets form in binary star systems. Early models suggested that the gravitational tug-of-war between two stellar bodies would send young planets into eccentric orbits, possibly ejecting them completely from their home system or crashing them into each other. Observational evidence, however, reveals that planets do indeed form and maintain surprisingly stable orbits around double stars.

To better understand how such systems evolve, astronomers using the Atacama Large Millimetre/Submillimeter Array (ALMA) took a new, detailed look at the planet-forming disc around HD 142527, a binary star about 450 light-years from Earth in a cluster of young stars known as the Scorpius–Centaurus Association.

The HD 142527 system consists of a main star a little more than twice the mass of our Sun and a smaller companion only about one-third the mass of our Sun. They are separated by almost 1 billion km – about the distance from the Sun to Saturn.

“This binary system has long been known to harbour a planet-forming corona of dust and gas,” said Andrea Isella of Rice University in Houston. “The new ALMA images reveal previously unseen details about the physical processes that regulate the formation of planets around this and perhaps many other binary systems.”

Planets form from the expansive discs of dust and gas that surround young stars. Small dust grains and pockets of gas eventually come together under gravity, forming larger and larger agglomerations and eventually asteroids and planets.

However, the fine points of this process are not well understood. By studying a wide range of protoplanetary discs with ALMA, astronomers hope to better understand the conditions that set the stage for planet formation across the universe.

ALMA’s new high-resolution images of HD 142527 show a broad elliptical ring around the double star. The disc begins incredibly far from the central star, about 50 times the Sun–Earth distance, and consists mostly of a crescent-shaped dust cloud, possibly the result of gravitational forces unique to binary stars and may also be the key to the formation of planets.

One of the Biggest Stars in Our Galaxy

Westerlund 1 is a beautiful cluster that contains one of the largest stars in the Milky Way. The red supergiant Westerlund 1-26 (W26) is so huge that if it was in the centre of our solar system it would cover all four rocky planets and extend beyond the orbit of Jupiter.

Westerlund 1 is located 15,000 light-years away in the Ara constellation. Most of the stars in the cluster have similar ages and compositions as it’s believed that the cluster formed in one large burst of stellar activity.

This beautiful collection of stars is among the most massive and brightest star clusters in our solar system, and it’s only a few million years old. For comparison, our Sun is almost 4.6 billion years old.

Westerlund 1 contains six yellow hypergiants, four red supergiants and 24 particularly rare objects called Wolf-Rayet stars, which can be up to a million times brighter than the Sun. Perhaps the most special thing about Westerlund 1 is that it contains a luminous blue variable, an extremely rare type of star whose brightness changes unpredictably over time.

The Hubble Telescope’s Wide Field Camera 3 captured the latest images with both visible and infrared data. However, brighter doesn’t necessarily mean bigger, which makes it tricky to figure out which of the bright dots is W26. According to Prof Alan Duffy of Swinburne University, it’s most likely the bright orange star indicated in the image.

The red supergiant W26 has a radius of about 1530 solar radii (star size is measured against our own Sun), and is thought to be so big, it easily sits within the top five of the biggest stars we’ve ever found. UY Scuti, a red supergiant in the Scutum constellation, is 1708 Solar radii wide.

The lifespan of those massive stars in Westerlund 1 is very short. The bigger the star, the more fuel it must burn to remain stable. In 2013, astronomers announced that W26 is in fact already nearing the end of its life.

The Royal Astronomical Society states: “Stars with masses tens of times larger than that of the Sun live very short and dramatic lives compared to their less massive siblings. Some of the most massive stars have lifetimes of less than a few million years before they exhaust their nuclear fuel and explode as supernovae.”