3.1 What is a double or binary star ?
It's a pair of stars in orbit around a common centre of gravity under
their mutual gravitational attraction. The individual stars in the system
are called components: component A is the primary or the brightest and component B is the secondary
or the faintest (in general). There are several types of binaries according to
the techique of detection used. Here we mention four of them.
The two components can be resolved visually through a telescope. Such systems
have mostly been investigated for astrometric purposes: highly accurate measurements of
the position of the star on the celestial sphere are obtained in order to gain
knowledge of the orbit characteristics as well as visual estimates of
the difference of magnitude (measure of the brightness) between
the two components. In more recent times, observers used photometric techniques
to get high precision measurements of the global magnitude and colours of these systems.
Example: HIC A: 71683 B: 71681
Name: alpha Centauri A and B Ref: Belikov Catalogue, 1995
Distance
[pc] | Period
[years] | Angular Separation
["] | Spectral Type A | Spectral Type B | Mass A
[m(sol)] | Mass B
[m(sol)] |
| 1.34±0.01 | 79.9 | 19 | G2V | K1V | 1.09±0.01 | 0.9±0.01 |
HIPPARCOS data » HIP 71683(A)/71681(B)
These are apparently single stars (not (yet) resolved). In this case the binarity is
revealed by perturbations of the position with respect to the "background"
or "reference stars". These systems are studied by photographic means:
measurements show the motion of the photocentre (this is the weighted centre
of the light intensity of the two components). From such a study we can obtain
e.g. the luminosity, the semi-major axis of the
astrometric orbit, the position of the center of mass of the system.
This figure shows the motion from 1987 to 1996 in right ascension and declination
of HIC 87895 as observed by the HIPPARCOS satellite.
Example: HIC 87895(AB) (In Hercules) Ref: A.J. 110,366 (1995)
Distance
[pc] | Period
[years] | Angular Separation
["] | Spectral Type A | Spectral Type B | Mass A
[m(sol)] | Mass B
[m(sol)] |
| 26.6±0.9 | 2.41 | 0.1 | G0-2V | K2-5V | 1.16±0.12 | 0.77±0.05 |
HIPPARCOS data » HIP 87895(AB)
Systems where the components are too close to each other to be resolved as separate stars,
are studied by spectroscopy (study of the stellar atmospheres,
radial velocity measurements via the Doppler effect).The study of their spectra shows periodic shifts
due to the orbital motion around a mean velocity which is the motion
in the radial direction of the system as a whole. We thus can obtain the
spectroscopic orbit. It gives us the orbital period, information on the orbit itself such as the eccentricity and very useful,
the mass ratio of the system.
For example, the left figure illustrates the radial velocity curves for alpha Aurigae A and B.
Example: Star: HIC 24608(AB)
Name: alpha Aurigae A and B Ref: Pourbaix (private communication)
Distance
[pc] | Period
[10-5 days] | Angular Separation
["] | Spectral Type A | Spectral Type B | Mass A
[m(sol)] | Mass B
[m(sol)] |
| 13.2±0.1 | 104.06±7.40 | 2.0 | G6III | F9III | 2.63±0.09 | 2.59±0.09 |
HIPPARCOS data » HIP 24608(AB)
Note: Capella is also resolved by speckle technique
When the orbit is aligned with respect to Earth, one component may occult the
other one. These "eclipses" are seen as periodic perturbations (drops) on the light
curve (luminosity as a function of time, e.g. HIC 31173 see figure). These double stars are called eclipsing
binaries. From the study of their light curves, we have access to the
period, the inclination, the luminosity ratio and the radii of the stars.
The Hipparcos Catalogue, Vol 12 (ESA SP1200), 1997.
Only one of the two eclipses has been observed.
Example: HIC 31173(AB) Name:WW Aurigae A and B Ref: Belikov Catalogue, 1995
Distance
[pc] | Period
[days] | Angular Separation
["] | Spectral Type A | Spectral Type B | Mass A
[m(sol)] | Mass B
[m(sol)] |
| 80 | 2.525 | 0.0007 | A4Vm | A5Vm | 1.97±0.02 | 1.79±0.01 |
HIPPARCOS data » HIP 31173(AB)
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