Enhanced Method for Measuring Masses of Millions of Solitary Stars

Astronomers have now introduced a new and upgraded method suitable for measuring the masses of millions of solitary stars, particularly those with planetary systems.

Obtaining accurate measurements of how much stars weigh not only plays a vital role in comprehending how stars are born, evolve and die, but it is also important in evaluating the true nature of the thousands of exoplanets presently known to orbit most of the other stars.

The method is custom-made for the European Space Agency’s Gaia Mission, which is presently performing a process of mapping the Milky Way galaxy in three dimensions, and NASA’s forthcoming Transiting Exoplanet Survey Satellite (TESS), which is planned for launch in 2018 and will review the 200,000 brightest stars in the firmament looking out for alien earths.

We have developed a novel method for ‘weighing’ solitary stars. First, we use the total light from the star and its parallax to infer its diameter. Next, we analyze the way in which the light from the star flickers, which provides us with a measure of its surface gravity. Then we combine the two to get the star’s total mass.

Keivan Stassun, Stevenson Professor of Physics and Astronomy

Stassun and his colleagues — Enrico Corsaro from INAF-Osservatorio Astrofisico di Catania in Italy, Joshua Pepper from Leigh University and Scott Gaudi from Ohio State University — explain the method and prove its accuracy by using 675 stars of known mass in an article titled “Empirical, accurate masses and radii of single stars with TESS and GAIA” that has been accepted for publication in the Astronomical Journal.

Usually, the most accurate method for determining the mass of distant stars refers to measuring the orbits of double star systems, known as binaries. Newton’s laws of motion permit astronomers to determine the masses of both stars by measuring their orbits with significant accuracy. Fewer than half of the star systems in the galaxy are indeed binaries, and binaries make up just about one-fifth of red dwarf stars that have now become expensive hunting grounds for exoplanets, thus astronomers have come up with an extensive range of other methods for assessing the masses of solitary stars.

The photometric method that categorizes stars by brightness and color is the most general, but it is not extremely accurate. Asteroseismology, which measures light fluctuations brought about by sound pulses that pass through a star’s interior, is greatly accurate but works only on several thousand of the closest, brightest stars.

Our method can measure the mass of a large number of stars with an accuracy of 10 to 25 percent. In most cases, this is far more accurate than is possible with other available methods, and importantly it can be applied to solitary stars so we aren’t limited to binaries.

Keivan Stassun

The technique is considered to be an extension of an approach that Stassun produced four years ago along with graduate student Fabienne Bastien, who is presently an assistant professor at Pennsylvania State University. With the help of special data visualization software created by a neuro-diverse team of Vanderbilt astronomers, Bastein was able to discover a refined flicker pattern in starlight comprising of valuable information dealing with a star’s surface gravity.

In 2016, Stassun and his collaborators came up with an empirical method to determine the diameter of stars employing published star catalog data. It deals with incorporating information on a star’s temperature and luminosity with Gaia Mission parallax data. (The parallax effect is considered to be the obvious displacement of an object caused by a variation in the observer’s point of view.)

By putting together these two techniques, we have shown that we can estimate the mass of stars catalogued by NASA’s Kepler mission with an accuracy of about 25 percent and we estimate that it will provide an accuracy of about 10 percent for the types of stars that the TESS mission will be targeting.

Keivan Stassun

Determining the mass of a star that comprises of a planetary system is a vital factor in defining the size and mass of the planets circling it. An error of 100% in the estimate of the mass of a star, which is typical employing the photometric method, can lead to an error of as much as 67% in calculating the mass of its planets. This is approximately equal to the difference between a Mercury and an Earth. Hence, it is extremely essential to correctly assess the nature of all the alien worlds that astronomers have started to detect in recent years.

National Science Foundation PAARE grant AST-1358862 and the European Union’s Horizon 2020 research and innovation program funded the research.