Binary stars

In a previous article, I mentioned some strange words. One is binary systems. In the universe,
less than 50% of stars are only single stars like our sun, all the other stellar systems are multiple
Systems.
We sometimes see some stars closer to each, but in a difference distance between them. They
are called as optical binaries. They are also like our sun. we see them as closer stars for some
period of time, because there is no relationship between them.
But in most of stellar systems built as binary systems. Binary system is a physical system in
which two stars are orbiting each other. Multiple systems has hierarchical structure: a star and
a binary orbiting around each other in a triple systems, two binaries orbiting around each
other’s in quadruple systems. Thus most multiple systems can be described as binaries with
several levels.
The stars in a binary system are move in an elliptical orbit around the center of the mass of the
system.

Image: The components of a binary system move around their common centre of mass. A1, A2 denotethe positions of the stars at a given time A, and similarly for B and C

(Image ref: fundamentals in Astronomy – Hannu Karttunen et al. (Eds.), Binary Stars and Stellar Masses. In: HannuKarttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 221–227 (2007) DOI: 11685739_9 © SpringerVerlag Berlin Heidelberg 2007)

These are classified on the basis of their discovery and on the basis of their mutual separation.
First, let’s consider the classification on the basis of the method of their discovery.

Classifications are as follows,

1. Visual binaries
2. Astrometric binaries
3. Spectroscopic binaries
4. Photometric binaries or eclipsing variables

Visual binaries

These can be seen as two separated components. The separation between the stars is larger
than about 0.1 arc seconds. The relative position of the stars changes over the years as they
move in their orbits.

Image: When a visual binary is followed for a long time, the components can be seen to move withrespect to each other. Picture of Krüger 60. (Yerkes Observatory)

(Image ref: fundamentals in Astronomy – Hannu Karttunen et al. (Eds.), Binary Stars and Stellar Masses. In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 221–227 (2007) DOI: 11685739_9 © SpringerVerlag Berlin Heidelberg 2007)

Astrometric binaries

In this type, only one star can be seen. But its variable proper motion shows their a second
invisible star must be present.

Image: The apparent paths of Sirius and its companion in the sky

(Image ref: fundamentals in Astronomy – Hannu Karttunen et al. (Eds.), Binary Stars and Stellar Masses. In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 221–227 (2007) DOI: 11685739_9 © SpringerVerlag Berlin Heidelberg 2007)

The spectroscopic binaries

These types of binaries are discovered on the basis of their spectra. Either two sets of spectral
lines are seen or else the doppler shift of the observed lines varies periodically, indicating an
invisible companion.

Image: Spectrum of the spectroscopic binary κ Arietis. In the upper spectrum the spectral lines are single,in the lower one doubled. (Lick Observatory)

(Image ref: fundamentals in Astronomy – Hannu Karttunen et al. (Eds.), Binary Stars and Stellar Masses. In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 221–227 (2007) DOI: 11685739_9 © SpringerVerlag Berlin Heidelberg 2007)

Photometric binaries or eclipsing binaries

The components of the pair regularly pass in front of each other, causing a change in the total
apparent magnitude.

Image: Typical lightcurves and schematic views of Algol, β Lyrae and W Ursae Majoris type binary
systems. The size of the Sun is shown for comparison

(Image ref: fundamentals in Astronomy – Hannu Karttunen et al. (Eds.), Binary Stars and Stellar Masses. In: HannuKarttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 221–227 (2007) DOI: 11685739_9 © SpringerVerlag Berlin Heidelberg 2007)

We classified binary stars on the basis of their mutual separation. This classification is as
follows,

1. Distant binaries
2. Close binaries
3. Contact binaries

Distant binaries

We named it distant binaries because their separation between the stars is tens or thousand
Astronomical units. In distant binaries their orbital are from tens to thousand years.

Close binaries

In these, separation is about one AU down to the radius of the stars and the orbital period and
ranges from a few hours to a few years.

contact binaries

These type stars are close to touch each other’s

Author: Induwara Bandara