Measuring Sirius: An Exercise in Patience

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Sometimes important astronomical advances require the newest and fanciest observatories and technologies — but sometimes they just require decades of work and a lot of patience. Patience is finally paying off for a team of scientists who have been observing the Sirius star system for nearly 20 years.

Sirius observations

Historical (black, blue and green) and Hubble (red) observations of the relative orbit of Sirius B around Sirius A. [Adapted from Bond et al. 2017]

Bright Neighbors

Located a mere 8.5 light-years away, the Sirius system consists of the main-sequence star Sirius A and its white-dwarf companion Sirius B. Sirius A is the brightest star in our sky, and Sirius B is the brightest and nearest white dwarf we’ve observed. The unusual proximity and brightness of these stars make them excellent targets for learning about stellar and white-dwarf astrophysics.

In order to interpret our observations, however, we first need to pin down the basic information about these stars. In particular, we want to measure the precise masses and orbital elements for the system — but because the stars orbit each other only once every ~50 years, these properties take time to measure well!

Toward this end, a team of scientists began an observing campaign in 2001 to regularly image the Sirius system using the Hubble Space Telescope. Now, 16 years later, they have enough data to make precise statements about the system.

Precision Measurements at Last

In a recent publication led by Howard Bond (Pennsylvania State University and Space Telescope Science Institute), the team details nearly two decades of precise photometric and astrometric measurements using Hubble. In addition, they supplemented these data by dredging through 150 years’ worth of historical observations of Sirius and critically analyzing 2,300 of these as well.

white-dwarf theory

Comparisons of white-dwarf theory with the observed parameters of Sirius B, both on the H-R diagram (top) and in a mass-radius plot of cooling white dwarfs (bottom). Sirius B’s measured parameters matches the theoretical models very well. [Bond et al. 2017]

The result? Bond and collaborators were able to make very precise measurements of the masses of Sirius A and Sirius B — 2.063 ± 0.023 and 1.018 ± 0.011 solar masses, respectively — and of their orbital elements. They find that the position of Sirius B on the Hertzsprung-Russell diagram is beautifully consistent with models based on cooling white dwarfs of Sirius B’s measured mass. Similarly, stellar models of Sirius A are nicely consistent with Bond and collaborators’ measurements if the star has a slightly low metallicity of ~85% that of the Sun.

The high-precision measurements also allowed the authors rule out the possibility of a third body in the system — an idea that’s been tossed around for decades — unless the third body is smaller than 15–25 Jupiter masses.

Bond and collaborators enumerate some open puzzles of the Sirius system, such as like conflicting signs that the two stars might have interacted, long ago. Though these puzzles remain unresolved, the painstaking decades of observations of Sirius have already revealed much about the system and improved our understanding of stellar evolution. What’s more, these measurements give us an ideal launching point for future studies of these two objects. In the case of the Sirius system, patience has definitely paid off.

Citation

Howard E. Bond et al 2017 ApJ 840 70. doi:10.3847/1538-4357/aa6af8

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