zakaria blogger: Shrinking Betelgeuse

Sunday, June 14, 2009

Shrinking Betelgeuse

This is news I can't let uncommented, after all my recent posts about interferometry: In a recent Astrophysical Journal Letter, a team of astronomers lead by Charles H. Townes, Nobel laureate for his contribution to the development of the laser, report "A Systematic Change with Time in the Size of Betelgeuse".

Betelgeuse, or α Orionis, is the bright red star in the shoulder of the constellation Orion. In 1921, Albert Michelson and Francis Pease succeeded in measuring its diameter as about 50 milliarcseconds (mas), using an interferometer mounted onto the main telescope of Mount Wilson observatory. This was the first time the diameter of a star could be measured. 50 milliarcseconds is a tiny angle, corresponding roughly to the apparent size of an object 100 metres big on the Moon. But given the distance to Betelgeuse, this means the star, when put in the place of the Sun, would fill the solar system up to the orbit of Jupiter.

In the meantime, Betelgeuse could be resolved directly using the Hubble space telescope, and its distance measured to be 200 parsecs (650 light years), though there remains an uncertainty of about 25 percent. And it has been the object of many interferometric studies.

But given the diffuseness of the outer limits of this Red Supergiant star, which on average has a density less than 1/10000 the density of air, measurements of its diameter depend on the wavelength of observation, as the thin outer atmosphere has different transparency for different wavelengths. Here is a comparison of the shape of Betelgeuse, as reconstructed from interferometric measurements taken within two weeks at three different wavelengths in the infrared:

Source: Surface imaging of Betelgeuse with the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WHT).

The two images on the right were reconstructed using data from the Cambridge Optical Aperture Synthesis Telescope (COAST), and the image on the left with data from the William Herschel Telescope (WHT). Each image has a side length corresponding to 100 mas, and one sees big differences in the diameter of the star, and of its surface features, depending of the wavelength at which it is observed.

This fact makes it difficult to compare directly angular diameters of Betelgeuse obtained with different instruments at different wavelengths. Now, the letter of Charles Townes' team reports on data collected with one instrument at one wavelength over the last 15 years. Using the Infrared Spatial Interferometer (ISI) on Mount Wilson, not far from where Michelson and Pease did their pioneering work, they observe at a wavelength of around 11000 nanometres, and write:

The diameters measured by the ISI are not particularly inconsistent with previous measurements, are rather accurate, and show clearly that the star has systematically decreased in size by about 15% over the past 15 years.

(Click for larger view – data from Townes et al., APJ Lett 697 (2009) L127-L128)

The graph shows their data (note that the zero-line is suppressed, so the change appears bigger than it actually is), together with a best fit by a parabola in grey, and the Michelson & Pease result in orange. Michelson and Pease estimated that the actual diametre of Betelgeuse could be larger by 10%-15% because of systematic errors in their measurement method. I have added the line in magenta, which shows, for comparison, the radius of the orbit of Jupiter, using the latest Hipparcos/VLA parallax distance to Betelgeuse to convert angles in actual distances.

Betelgeuse has been known to be a star with variability, but it seems that this change in size is odd and unexpected, even more so as its luminosity has been roughly constant. It's fascinating to me that such a change can be measured. As the APJ Letter concludes:

ISI measurements over the last 15 years clearly show a systematic change in the diameter of α Ori. This change may or may not be periodic; if it is, the period is likely rather long, perhaps a few decades. [...] It should be valuable to continue accurate measurements of α Ori's size and other characteristics in order to understand the dynamics involved in this striking change, and to have systematic long-term measurements of similar stars.