Measuring the temperature of stars with an accuracy of 0.1°

Astronomers study stars using spectroscopy, which allows them to analyze the light they emit across all colors. A team led by Étienne Artigau, a researcher at the Trottier Institute for Research on Exoplanets (iREx), has developed a method to extract temperature fluctuations from a star's spectrum with an accuracy of a tenth of a degree Celsius, over different timescales.


"By monitoring the temperature of stars, we can learn a lot about them: their rotation period, stellar activity, and magnetic field. This intimate knowledge of stars is also crucial for finding and studying their planets," explains the researcher.

In an article soon to be published in the Astronomical Journal, the technique's efficiency and wide versatility are demonstrated with observations of four very different stars made using the Canada-France-Hawaii Telescope and the 3.6-meter La Silla telescope.

Knowing stars to understand their planets

The team initially focused on the spectra of stars to improve the detection of exoplanets using the radial velocity method. This method involves measuring the slight wobble of a star caused by the gravitational pull of a planet orbiting the star.

The more precisely we can measure small variations in a star's velocity, the more effectively we can detect low-mass planets. Étienne Artigau and his team developed a radial velocity technique that leverages the entire spectrum of the star, rather than focusing on just a few portions, as was previously the norm. This allows for the detection of planets as small as Earth around smaller stars.

Building on the success of this technique, the researcher had the idea to apply a similar strategy not to determine changes in a star's velocity but rather its temperature.

This measurement turns out to be just as crucial for studying exoplanets, which are most often observed indirectly by closely monitoring their host stars. In recent years, astronomers have faced the challenge of distinguishing stellar features from planetary ones in their observations. This has proven to be a problem both for discovering exoplanets using the radial velocity method and for learning more about their atmospheres using the transit spectroscopy method.

"It's very difficult to confirm the presence of an exoplanet or study its atmosphere without precisely understanding the properties of the host star and how they change over time. This new technique gives us an invaluable tool to ensure that the knowledge we're gaining about exoplanets is solid and helps us go further in characterizing them," says Charles Cadieux, a PhD student at iREx who contributed to the study.

Unmatched precision

The surface temperature of stars is a fundamental property that astronomers are eager to measure, as it allows them to deduce the star's luminosity and chemical composition. In the best case, the exact temperature of a star can be known with a precision of about 20°C (36°F).


With this new technique, the focus is not on the exact temperatures but on their variations over time. And these can now be measured with remarkable precision.

"We don't know if the star is at 5000°C (9032°F) or 5020°C (9068°F), but we can tell if its temperature has increased or decreased by one degree or even less! No one had ever achieved that before. Being able to determine such a temperature change is already quite a challenge for the human body, so just imagine doing it for a ball of gas at thousands of degrees located tens of light-years away!" exclaims Étienne Artigau.

An effective and versatile new technique

To prove that their technique works, the astronomers used data from the SPIRou spectrograph (Canada-France-Hawaii Telescope) and the HARPS spectrograph (3.6-meter telescope of the European Southern Observatory).

In the data obtained by these two telescopes for four small stars in the solar neighborhood, the team clearly observed temperature changes, which they attributed either to the stars' rotation or to processes occurring on their surfaces or nearby.


The new technique allows for the measurement of large temperature variations. For the star AU Microscopii, known for its high stellar activity, the team recorded variations of nearly 40°C (72°F).

By using this technique, they could capture both very rapid changes, such as those caused by the rotation of stars like AU Microscopii or Epsilon Eridani, occurring on the scale of just a few days, as well as much slower changes — a feat very difficult to achieve with ground-based telescopes.

"We're able to measure changes of just a few degrees or less occurring over very long periods, such as those associated with the rotation of Barnard's Star, a very quiet star that rotates once every five months,” says Étienne Artigau. "To measure this subtle and slow variation, we had to rely on Hubble back in the day!"