Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Flattest Star Ever Seen


VLT Interferometer Measurements of Achernar Challenge Stellar Theory

The shape of the bright southern star Achernar; from VLTI observations (model)

To a first approximation, planets and stars are round. Think of the Earth we live on. Think of the Sun, the nearest star, and how it looks in the sky.

But if you think more about it, you realize that this is not completely true. Due to its daily rotation, the solid Earth is slightly flattened ("oblate") - its equatorial radius is some 21 km (0.3%) larger than the polar one. Stars are enormous gaseous spheres and some of them are known to rotate quite fast, much faster than the Earth. This would obviously cause such stars to become flattened. But how flat?

Recent observations with the VLT Interferometer (VLTI) at the ESO Paranal Observatory have allowed a group of astronomers to obtain by far the most detailed view of the general shape of a fast-spinning hot star, Achernar (Alpha Eridani), the brightest in the southern constellation Eridanus (The River).

They find that Achernar is much flatter than expected - its equatorial radius is more than 50% larger than the polar one! In other words, this star is shaped very much like the well-known spinning-top toy, so popular among young children.

The high degree of flattening measured for Achernar - a first in observational astrophysics - now poses an unprecedented challenge for theoretical astrophysics. The effect cannot be reproduced by common
models of stellar interiors unless certain phenomena are incorporated, e.g. meridional circulation on the surface ("north-south streams") and non-uniform rotation at different depths inside the star.

As this example shows, interferometric techniques will ultimately provide very detailed information about the shapes, surface conditions and interior structure of stars.

The full text of this Press Release, with three photos (ESO PR Photos 15a-c/03) and all related links, is available at:

VLTI observations of Achernar

Test observations with the VLT Interferometer (VLTI) at the Paranal Observatory proceed well, and the astronomers have now begun to exploit many of these first measurements for scientific purposes.

One spectacular result, just announced, is based on a series of observations of the bright, southern star Achernar (Alpha Eridani; the name is derived from "Al Ahir al Nahr" = "The End of the River"), carried out between September 11 and November 12, 2002. The two 40-cm siderostat test telescopes that served to obtain "First Light" with the VLT Interferometer in March 2001 were also used for these observations. They were placed at selected positions on the VLT Observing Platform at the top of Paranal to provide a "cross-shaped" configuration with two "baselines" of 66 m and 140 m, respectively, at 90° angle, cf. PR Photo 15a/03.

At regular time intervals, the two small telescopes were pointed towards Achernar and the two light beams were directed to a common focus in the VINCI test instrument in the centrally located VLT Interferometric Laboratory. Due to the Earth’’s rotation during the observations, it was possible to measure the angular size of the star (as seen in the sky) in different directions.

Achernar’s profile

A first attempt to measure the geometrical deformation of a rapidly rotating star was carried out in 1974 with the Narrabri Intensity Interferometer (Australia) on the bright star Altair by British astronomer Hanbury Brown. However, because of technical limitations, those observations were unable to decide between different models for this star. More recently, Gerard T. Van Belle and collaborators observed Altair with the Palomar Testbed Interferometer (PTI), measuring its apparent axial ratio as 1.140 ± 0.029 and placing some constraints upon the relationship between rotation velocity and stellar inclination.

Achernar is a star of the hot B-type, with a mass of 6 times that of the Sun. The surface temperature is about 20,000 °C and it is located at a distance of 145 light-years.

The apparent profile of Achernar (PR Photo 15b/03), based on about 20,000 VLTI interferograms (in the K-band at wavelength 2.2 µm) with a total integration time of over 20 hours, indicates a surprisingly high axial ratio of 1.56 ± 0.05 [3]. This is obviously a result of Achernar’s rapid rotation.

Theoretical implications of the VLTI observations

The angular size of Achernar’’s elliptical profile as indicated in PR Photo 15b/03 is 0.00253 ± 0.00006 arcsec (major axis) and 0.00162 ± 0.00001 arcsec (minor axis) [4], respectively. At the indicated distance, the corresponding stellar radii are equal to 12.0 ± 0.4 and 7.7 ± 0.2 solar radii, or 8.4 and 5.4 million km, respectively. The first value is a measure of the star’’s equatorial radius. The second is an upper value for the polar radius - depending on the inclination of the star’s polar axis to the line-of-sight, it may well be even smaller.

The indicated ratio between the equatorial and polar radii of Achernar constitutes an unprecedented challenge for theoretical astrophysics, in particular concerning mass loss from the surface enhanced by the rapid rotation (the centrifugal effect) and also the distribution of internal angular momentum (the rotation velocity at different depths).

The astronomers conclude that Achernar must either rotate faster (and hence, closer to the "critical" (break-up) velocity of about 300 km/sec) than what the spectral observations show (about 225 km/sec from the widening of the spectral lines) or it must violate the rigid-body rotation.

The observed flattening cannot be reproduced by the "Roche-model" that implies solid-body rotation and mass concentration at the center of the star. The failure of that model is even more evident if the so-called "gravity darkening" effect is taken into account - this is a non-uniform temperature distribution on the surface which is certainly present on Achernar under such a strong geometrical deformation.


This new measurement provides a fine example of what is possible with the VLT Interferometer already at this stage of implementation. It bodes well for the future research projects at this facility.

With the interferometric technique, new research fields are now opening which will ultimately provide much more detailed information about the shapes, surface conditions and interior structure of stars. And in a not too distant future, it will become possible to produce interferometric images of the disks of Achernar and other stars.

Richard West | alfa
Further information:

More articles from Physics and Astronomy:

nachricht Hubble sees Neptune's mysterious shrinking storm
16.02.2018 | NASA/Goddard Space Flight Center

nachricht Supermassive black hole model predicts characteristic light signals at cusp of collision
15.02.2018 | Rochester Institute of Technology

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Demonstration of a single molecule piezoelectric effect

Breakthrough provides a new concept of the design of molecular motors, sensors and electricity generators at nanoscale

Researchers from the Institute of Organic Chemistry and Biochemistry of the CAS (IOCB Prague), Institute of Physics of the CAS (IP CAS) and Palacký University...

Im Focus: Hybrid optics bring color imaging using ultrathin metalenses into focus

For photographers and scientists, lenses are lifesavers. They reflect and refract light, making possible the imaging systems that drive discovery through the microscope and preserve history through cameras.

But today's glass-based lenses are bulky and resist miniaturization. Next-generation technologies, such as ultrathin cameras or tiny microscopes, require...

Im Focus: Stem cell divisions in the adult brain seen for the first time

Scientists from the University of Zurich have succeeded for the first time in tracking individual stem cells and their neuronal progeny over months within the intact adult brain. This study sheds light on how new neurons are produced throughout life.

The generation of new nerve cells was once thought to taper off at the end of embryonic development. However, recent research has shown that the adult brain...

Im Focus: Interference as a new method for cooling quantum devices

Theoretical physicists propose to use negative interference to control heat flow in quantum devices. Study published in Physical Review Letters

Quantum computer parts are sensitive and need to be cooled to very low temperatures. Their tiny size makes them particularly susceptible to a temperature...

Im Focus: Autonomous 3D scanner supports individual manufacturing processes

Let’s say the armrest is broken in your vintage car. As things stand, you would need a lot of luck and persistence to find the right spare part. But in the world of Industrie 4.0 and production with batch sizes of one, you can simply scan the armrest and print it out. This is made possible by the first ever 3D scanner capable of working autonomously and in real time. The autonomous scanning system will be on display at the Hannover Messe Preview on February 6 and at the Hannover Messe proper from April 23 to 27, 2018 (Hall 6, Booth A30).

Part of the charm of vintage cars is that they stopped making them long ago, so it is special when you do see one out on the roads. If something breaks or...

All Focus news of the innovation-report >>>



Industry & Economy
Event News

2nd International Conference on High Temperature Shape Memory Alloys (HTSMAs)

15.02.2018 | Event News

Aachen DC Grid Summit 2018

13.02.2018 | Event News

How Global Climate Policy Can Learn from the Energy Transition

12.02.2018 | Event News

Latest News

Fingerprints of quantum entanglement

16.02.2018 | Information Technology

'Living bandages': NUST MISIS scientists develop biocompatible anti-burn nanofibers

16.02.2018 | Health and Medicine

Hubble sees Neptune's mysterious shrinking storm

16.02.2018 | Physics and Astronomy

Science & Research
Overview of more VideoLinks >>>