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How Small are Small Stars Really? VLT Interferometer Measures the Size of Proxima Centauri and Other Nearby
Stars
by the European Southern Observatory
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Summary
At a distance of only 4.2 light-years, Proxima Centauri is the
nearest star to the Sun currently known [2].
It is visible as an 11-magnitude object in the southern constellation of
Centaurus and is the faintest member of a triple system, together with
Alpha Centauri, the brightest (double) star in this
constellation.
Proxima Centauri is a very-low-mass star, in fact barely massive
enough to burn hydrogen to helium in its interior. It is about seven times
smaller than the Sun, and the surface temperature is "only" about 3000
degrees, about half of that of our own star. Consequently, it is also much
fainter - the intrinsic brightness is only 1/150th of that of our Sun.
Low-mass stars are very interesting objects, also because the
physical conditions in their interiors have much in common with those of
giant planets, like Jupiter in our solar system. A determination of the
sizes of the smallest stars has been impossible until now because of their
general faintness and lack of adequate instrumentation. However,
astronomers have long been keen to move forward in this direction, since
such measurements would provide indirect, crucial information about the
behaviour of matter under extreme conditions.
When the first observations with the VLT Interferometer (VLTI),
combining the light from two of the 8.2-m VLT Unit Telescopes (ANTU and
MELIPAL), were made one year ago (ESO
PR 23/01), interferometric measurements were also obtained of
Proxima Centauri. They formed part of the VLTI commissioning and
the data were soon released to the ESO community, cf. the special
website.
Now, an international team of astronomers from the Geneva Observatory
(Switzerland), ESO/Chile and the Canada-France-Hawaii Telescope (CFHT) has
successfully analysed these observations by means of newly developed,
advanced software. For the first time ever, they obtained a highly
accurate measurement of the size of such a small star.
Three other small stars were also measured and the results are in
excellent agreement with stellar theory, indicating that our present
understanding of the structure and composition of very small stars is
reasonably correct. More VLTI observations are soon to follow,
eventually also of even smaller objects, like Brown Dwarfs.
PR
Photo 27a/02: Proxima Centauri, the nearest star
known.
PR
Photo 27b/02: The "Hertzsprung-Russell (HR)" diagram of
stars
PR
Photo 27c/02: Diameters and masses of small stars.
PR
Photo 27d/02: Interferometric fringes at VLTI/VINCI of the
small star GJ 887. |
Proxima Centauri - barely a real star
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ESO PR Photo 27a/02
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ESO PR Photo 27b/02
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Caption: PR Photo 27a/02 shows a small sky field
with the nearest known star, Proxima Centauri, close to the centre.
This object is cool and red and it is brighter on the near-infrared image
(in the I-filter at 900 nm) to the left than on the red image (in the
R-filter at 600 nm) to the right. The rapid motion is easy to perceive.
The field measures 10 x 10 arcmin2; North is up and East is
left. Reproduced from the Digital Sky Survey (STScI Digitized Sky Survey,
(C) 1993, 1994, AURA, Inc. all rights reserved - cf. http://archive.eso.org/dss/dss).
PR Photo 27b/02 is a colourful rendering of the basic
Hertzsprung-Russell diagram for stars [3].
Very-low-mass stars (also known as "M dwarfs") are faint and cool and are
located to the lower right of the diagonal line (the "main sequence") on
which most stars lie and which corresponds to where (and when) stars burn
hydrogen into helium. The present location of the Sun is also
indicated. |
From its spectrum, Proxima Centauri (PR Photo 27a/02) is
classified as a "late M-dwarf star". Such stars are among the smallest and
faintest, but also the most numerous in our Milky Way galaxy. PR Photo
27b/02 displays their particular location relative to other stars in a
famous astronomical presentation of basic stellar parameters, known as the
"Hertzsprung-Russell (HR) diagram" [3].
Here, stellar temperatures are plotted against brightness to yield a typical
distribution that also represents the diverse stages of stellar evolution and
ages.
M-dwarfs like Proxima Centauri are located in the cool and faint
corner, at the lower right of this diagram. They define the bottom of the
diagonal band referred to as the "main sequence" that is occupied by
hydrogen-burning stars.
With a mass of only 15% of that of the Sun, Proxima Centauri is in the
extreme low-mass end of the M-stars. In fact, if it had only half of its present
mass, it would be too light for hydrogen fusion to ignite in its interior. It
would then have been a "Brown-Dwarf" rather than a real star (cf. ESO PR
14/02).
In the border zone between real stars, brown dwarfs and planets
For a normal star like the Sun whose matter behaves like a perfect gas, the
stellar size is proportional to the mass. However, for low-mass stars like
Proxima Centauri, quantum effects become important and the stellar matter
becomes "degenerate", resisting compression much more than does a perfect gas.
For objects with half the mass of Proxima Centauri or lighter, the matter
is fully degenerate and their size does not depend on the mass.
In the case of Proxima Centauri, both the mass and the diameter are
about 1/7 of those of the Sun. Contrarily, while it is 150 times more massive
than Jupiter, it is only about 1.5 times larger than that planet.
Its location in the border zone between stars, brown dwarfs, and
planets, makes Proxima Centauri a highly interesting object. A direct
measurement of its size, until now impossible because of instrumental
limitations, would represent a significant contribution to the study of the
physics in this critical transition region.
The VLT interferometric measurements
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ESO PR Photo 27c/02
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Caption: PR Photo 27c/02 shows the radii and
masses of the four very-low-mass stars now observed with the VLTI , GJ
205, GJ 887, GJ 191 (also known as "Kapteyn's star") and Proxima Centauri
(red filled circles; with error bars). For comparison, planet Jupiter's
mass and radius are also plotted (blue triangle). The two curves represent
theoretical models for stars of two different ages (400 million years -
red dashed curve; 5 billion years - black fully drawn curve). As can be
seen, theory and observations fit very
well. |
This kind of measurement has now become possible, thanks to the VLT
Interferometer, the VINCI instrument and new software for the complicated data
analysis.
Observations were made of Proxima Centauri and three other small
stars, soon after the light beams from ANTU and MELIPAL, two of the 8.2-m VLT
telescopes, were first brought together in the VLT Interferometric Laboratory at
the top of the Paranal mountain, cf. ESO PR
23/01.
The VLTI, working with the two large VLT telescopes separated by 102.4 metres
(when the moveable 1.8-m telescopes become operational, the maximum baseline
will be 202 metres), provides the image sharpness needed to resolve such small
stellar disks, and these telescopes are sufficiently large to observe fainter
objects than what can normally be studied interferometrically.
The measured angular diameter of Proxima Centauri is 1.02 ± 0.08
milliarcsec, or about the size of an astronaut on the surface of the Moon as
seen from the Earth (or a head of a pin on the surface of the Earth, as seen
from the International Space Station).
Didier Queloz of the Geneva Observatory is content: "The measured
sizes agree well with theoretical predictions, based on numerical models of
planets and low-mass stars. The same holds for the sizes of a number of more
massive stars that were measured at the same time. This gives us new confidence
in the models of these extreme objects".
The use of wavelets
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ESO PR Photo 27d/02
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Caption: PR Photo 27d/02: Optical long baseline
interferometers do not produce images but interference patterns. The
amplitude and the position of the fringe pattern is related to the shape
of the observed astrophysical object. This information is "corrupted" by
the turbulence in the Earth's atmosphere and it is a difficult task to
distinguish astrophysical information from the atmospheric perturbation. A
new, powerful technique based on wavelet analysis was developed by the
present group of astronomers and now allows to remove this "atmospheric
noise" from the data. For illustration, the black curve represents a "raw"
fringe pattern obtained on the low-mass star GJ 887 (i.e., as it is
recorded and still corrupted by the atmosphere), while the red curve
represents the same fringe pattern after most of the signal noise has been
removed. |
These new results did not come easily, though. The early VLTI data need very
extensive processing before they can be astrophysically interpreted. One
specific problem is that the two large telescopes here used (without which it
would not have been possible to observe such a faint star) have not yet been
equipped with "adaptive optics" to eliminate the effects of the turbulence in
the Earth's atmosphere [4].
This strongly affects the interferometric measurements by adding "noise".
This particular difficulty had to be solved before the present observational
data could be used, cf. PR Photo 27d/02. "We overcame the problem by
means of a novel technique based on so-called 'wavelet analysis' and specially
developed for the present purpose", explains Damien Ségransan of the
Geneva Observatory, "it is a mathematical device we took over from fluid
mechanics - another demonstration of how we astronomers keep our eyes open for
useful developments in other fields!".
Prospects
This new scientific result, one of the first obtained with the VLT
Interferometer, is a fine achievement of this new facility, especially when
considering that it was obtained already during tests with the VINCI instrument
that was designed to verify the functionality of the VLTI.
With the coming installation of more performing and specialized instruments
like the Mid-Infrared
interferometric instrument for the VLTI (MIDI) and near-infrared/red VLTI focal instrument
(AMBER), the possibility to use also the four 1.8-m Auxiliary
Telescopes, as well as the introduction of Adaptive Optics systems for the VLT
Unit Telescopes, the astronomers now hope to move quickly forward towards the
direct detection of short-period extra-solar planets with the VLTI.
More information
The information presented in this Press Release is based on a Letter to the
Editor ("First radius measurements of very low mass stars with the VLTI" by
Damien Ségransan (Geneva Observatory), Pierre Kervella (ESO
Chile), Thierry Forveille (Canada-France-Hawaii-Telescope [CFHT]) and
Didier Queloz (Geneva Observatory) - see also http://obswww.unige.ch/~segransa/proxima.html
- that will soon appear in the European research journal "Astronomy &
Astrophysics".
More information about the VLTI and photos of many of the components of the
facility are available at the VLTI website, as well as in
ESO PR
06/01 ("First Light" in March 2001 and explanation of the
interferometric measurements), ESO PR
23/01 (observations with two 8.2-m telescopes in October 2001) and ESO PR
16/02 (observations with four 8.2-m telescopes in September 2002).
Notes:
[1]: This press release is issued in coordination between
ESO and the Geneva
University in Switzerland.
[2]: Proxima Centauri, the nearest known star to the
solar system, is a member of a triple stellar system that includes the bright
double star Alpha Centauri. Proxima is the nearest of the three. It was
discovered in 1894 by a Scottish astronomer, Robert Thorburn Ayton Innes
(1861 - 1915), during a stay at the Cape Observatory (South Africa), due to its
very fast motion on the sky, about 3.9 arcsec/year. It is also designated GJ
551, the 551st entry in the "Catalogue of Nearby Stars", published in 1969
by two German astronomers, Wilhelm Gliese (1915-93) and Helmut
Jahreis. The visual magnitude is 11, or 100 times fainter than what
can be perceived with the unaided eye on a dark sky; the parallax measured by
the ESA Hipparcos
astrometric satellite is 772.33 ± 2.42 milliarcsec, corresponding to a
distance of 4.22 light-years.
[3]: The "Hertzsprung-Russell" diagram is named
after the Danish astronomer Einar Hertzsprung (1873-1967) and the
American astronomer Henry Norris Russell (1877 - 1957). At the beginning
of the 20th century they independently noticed that red stars come in very
different sizes, pioneering subsequent studies of stellar parameters (e.g.,
temperature, size and mass). In its basic version, this diagram plots stellar
temperature (or colour) against brightness (or magnitude) and is therefore also
referred to as the "colour-magnitude diagram". The position of a particular star
in the diagram also provides information about its evolutionary stage (and
age).
[4]: The first Adaptive Optics instruments for the VLT
Interferometer will be installed in 2003.
Contact
Didier Queloz
Observatoire de Genève
Sauverny,
Switzerland
Tel: +41-22-755-2611
email: didier.queloz@obs.unige.ch
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