9 mins read 22 Oct 2019

Stellar Reflection: How Binary Stars Reflect Light

Hot binary stars don't just produce their own light, they also reflect a small portion of their stellar companions

Binary stars. Credit: Palomar Observatory/STScI/WikiSky

Staring at the night sky from our own backyards, we tend to feel connected to the distant past. Sure, the images entering our eyes are of light that has left its celestial origin in a previous era (hundreds, thousands or millions of years ago) - but our human connection to our ancestral astronomy knowledge provides a link of looking at the same heavens that ancient civilisations, high-priestesses, and classical philosophers once looked into. In an age of rapid technological revolutions springing on us daily - the sky still connects us with our history, knowledge, and advancement. 

There are a handful of stars that date back to the artefact tablets of the ancient Assyrians or the recordings of the ancient Egyptians - and in particular, the role of these stars in helping the development of the calendar, our understanding of the cosmos and blooming of the sciences.

Throughout history, one of these stars has continually captured the attention of skywatchers and observers - not only due to its brightness but also because of its proximity to the ecliptic. 

Astronomers, from many cultures, have given it many names (in Arabic it is known as Alerph; in Hindu, it is known as Chitrā and in Chinese, it's called Jiǎo Xiù) as they were continually dazzled by its brilliance in the sky. Today, we know this star as Spica (α Virginis) and when we look towards the constellation Virgo, it is the bright blue star. 

With the invention of the telescope came a scientific revolution - allowing a greater detail of data to be gathered about the eternal objects in the sky and their motions, events, and interactions with each other - often inconsiderate of all our worldly affairs on our home planet. 

Spica, it would turn out, was not one - but two stars (a binary system) orbiting each other. So close, that even the most powerful telescopes could not tell them apart, requiring spectrometry to reveal the two companions. 

Whilst binary stars are very common in the universe, exciting new research released this year by the University of NSW (UNSW) and Western Sydney University (WSU) shows that binary stars not only produce their own light but also reflect the light from their companions into space.

Spica's story, it would seem, is far from over. 

From Hipparchus to UNSW

During his time, Hipparchus (190 BCE - 120 BCE) studied Spica (in addition to the bright star in Leo, Regulus) making relative comparisons of the position of both stars with the detailed recordings of Alexandrian Timochares, who's records dated back to around 300 BCE. 

Through these comparisons, Hipparchus was able to study the precession of the equinoxes - a study upon which was continued into much further detail by Copernicus many years later. 

Over 2,000 years later - new observational data from Spica, captured from the other side of our planet from the heart of a metropolis with a roaring population of five million people - is beginning to challenge our views of how stars work. That is, we've always been told from a young age that stars shine and produce their own light, but this new UNSW and WSU research has shown that they also reflect light

At first, this might seem like a challenging concept to picture (how could a bright shining object be also reflecting light?) but by looking at the polarisation of light coming from Spica, the scientists were able to determine that a small portion was reflective light from each companion. 

Spica Revealed

Here's what we know about the Spica binary system:

  • It's relatively close - approx. 250 light-years away
  • The binary pair was discovered in 1890 using spectrometry as double stellar lines were revealed as having different velocities and corresponding Doppler shifts
  • The binary pair are extremely close - separated by approx. 28 solar radii and orbiting each other every 4.01 days. This close proximity means that they each distort each other's atmospheres. 
  • The primary star (Spica A - Class: B1 III-IV) has a mass of approx. 11.5 times that of our Sun with a radius seven times greater. This results in a surface temperature of approx. 25,000K and brightness of 20,000 Suns
  • The secondary (Spica B - Class: B2 V), is more compact, also containing an approx. mass of seven times that of our Sun, but limited down to a smaller radius of 3.7 times that of our Sun. Whilst this keeps the temperature fairly high at 20,000K its brightness is reduced to approx. 2,000 Suns.

A schematic of the binary stars in Spica, showing four stages of an orbital period. Credit: A. Tkachenko et al.

Sydney sees reflections from 250 light-years away

Do stars like the Sun and Spica, also reflect some of the light that falls on them? This is a question that scientists from UNSW Sydney and Western Sydney University wanted to find out, which quite surprisingly, has been little studied by astronomers. 

“Single stars don't have a light source nearby (such as the binary companion) and so there is no way we could measure the small amount of reflected light,” said Professor Jeremy Bailey, from UNSW’s School of Physics, who led the paper. 

As expected and according to modeling, stars are likely poor reflectors of light (as they produce their own). Our Sun, for example, reflects as little as 0.1 percent of the light that falls onto it. 

“However, for hotter stars, such as the components of Spica, with temperatures of 20,000 to 25,000 degrees Kelvin, the amount of reflection increases to a few percent. The total amount of reflected light coming from the Spica system is, however, still very small,” said Bailey. 

Starlight received on Earth is usually unpolarised - indicating that it vibrates through more than one plane. However, Bailey and his team found that when starlight is reflected of a binary companion, the vibrations of the light waves travel in a single plain - they become polarised.

"It is a similar process to the way light is polarised when it reflects off a water or glass surface, allowing us to reduce the glare using polarised sunglasses," he said. 

Polarisation Modeling of Spica. (A) Gravity distribution over stellar surface. (B) Intensity of reflected light. (C) Total intensity and polarisation including emitted and reflected light. Credit: Bailey et al.

A new observational tool

The importance of this discovery is that it provides a new observational technique for astronomers to utilise when studying binary stars by allowing them to measure properties of stars that they can’t easily measure for single stars.

“It provides a way of detecting binary systems that could not be detected by other methods – particularly binaries with face-on orbits – and a way of measuring masses for a wider range of binaries than is currently possible,” said Bailey.

Credit for the observations of reflected stellar light can be given to Prof. Bailey and his UNSW colleagues, who have developed very sensitive astronomical polarimeters (instruments to measure the polarity of light). 

“For this work, we used three different telescopes including UNSW’s own observatory, which is located on campus. The small 35cm telescope here was used to make the majority of observations included in this study,” he said.

The research team is now testing their techniques on other binary systems and believes the polarisation technique could open up new opportunities for the study of binary stars.

“We expect it to work even better for hotter stars, and it could be used to find binary systems that are not detectable by other methods and to study binary star orbits and properties,” he said.

When the lion roared

In 2017, Daniel Cotton (who also worked with Prof. Bailey in the above paper) also released another paper regarding the polarisation of the star Regulus, in the constellation Leo. 

In this paper, Cotton et al. found the first detection of rotational-induced polarisation being produced due to the distortion of a rapidly rotating star. 

Astonishingly, the findings in this paper reveal that Regulus is spinning so rapidly, that if its spin rate increased by 3.5 percent it will tear itself apart. 

The polarisation of Regulus. Upper panel shows show the distribution of effective gravity over the surface of the star, and lower panels show the intensity distribution over-plotted with polarisation vectors. Credit: Cotton et al.

Both the Spica and Regulus discovery highlight how polarisation can be used as new techniques in studying rapidly rotating stars or binary systems across our galaxy, allowing us to gain a better understanding of stellar evolution and the environments in which these stars are first created. 

"Of the four techniques astronomers use, polarimetry is the least developed. So, there is a lot we can learn from looking at bright stars. I love that about this pioneering polarimetry research," said Cotton.

I'm looking at the same stars the ancients first learned to recognise and locate in pre-histroy, the same stars Hipparchos was looking at when pioneering photometry two millennia ago, and the same stars people like Maury and Cannon were looking at a century ago pioneering spectroscopy.

Just like those later astronomers I go to the observatory at night and point the telescope at objects I can see with the naked eye and name. It's exciting to be learning new things about the stars that have been our companions defining the night sky for all of humanity's existence." 

Connecting to the Future

Important stars like Spica and Regulus have always played an important role in humanity's search for answers - and in the space of 130 years, we've been able to identify Spica as an extremely close binary system and Regulus as a rapidly rotating star. 

It's a wonderful connection for Australian scientists to have to be able to contribute to the ongoing knowledge of these two stars, just as Hipparchus and Copernicus once did. 

New methods of observing the polarisation of starlight are only in their infancy today with the advancement of instruments now providing opportunities to further explore this field. As cameras, spectrometers and telescopes increase in efficiency, productivity and accuracy - imagine the findings astronomers 300 years into the future will have, based on this early work in stellar light polarisation. 

The sky will once again bridge the gap of time, connecting science findings from different times through our shared history, knowledge and advancement. 

The paper, titled 'Polarized reflected light from the Spica binary system' is currently available on arXiv