(This press release from September
14, 1995, is reproduced courtesy of the Space Telescope Science Institute.)
This is a Hubble Space Telescope picture of one of the least massive
and coolest stars even seen (upper right). It is a diminutive
companion to the K dwarf star called GL 105A (also known as HD 16160)
seen at lower left. The binary pair is located 27 light-years away in
the constellation Cetus.
Based on the Hubble observation, astronomers calculate that the
companion, called GL 105C, is 25,000 times fainter than GL 105A in
visible light. If the dim companion were at the distance of our Sun, it
would be only four times brighter than the full moon.
The Hubble observations confirm the detection of GL 105C last year by
David Golimowski and his collaborators at Palomar Observatory in
California. Although GL 105C was identified before, the Hubble view
allows a more precise measurement of the separation between the binary
components. Future Hubble observations of the binary orbit will allow
the masses of both stars to be determined accurately.
The Palomar group estimates that the companions mass is 8-9 percent of
the Suns mass, which places it near the theoretical lower limit for
stable hydrogen burning. Objects below this limit, called brown
dwarfs, still shine -- not by thermonuclear energy, but by the energy
released through gravitational contraction.
Two pictures, taken with Hubbles Wide Field Planetary Camera 2 (in PC
mode) through different filters (in visible and near-infrared light)
show that GL 105C is redder, hence cooler than GL 105A. The surface
temperature of GL 105C is not precisely known, but may be as low as
2,600 degrees Kelvin (4,200 degrees Fahrenheit).
This image was taken in near-infrared light, on January 5, 1995. GL
105C is located 3.4 arc seconds to the west-northwest of the larger GL
105A. (One arc second equals 1/3600 of a degree.) The bright spikes
are caused by diffraction of light within the telescopes optical
system, and the brighter white bar is an artifact of the CCD camera,
which bleeds along a CCD column when a relatively bright object is in
the field of view.
These observations are part of a Guaranteed Time Observing Program for
which William Fastie (the Johns Hopkins University, Baltimore, MD) and
Dan Schroeder (Beloit College, Beloit, WI) were co-principal
Credit: D. Golimowski (Johns Hopkins University), and NASA