What type of binary star is held together by gravity and appears double?

[nautica]

Having trouble with this question. Any help would be appreciated. I have narrowed the answers down

1) A star which appears double when observed directly, and is held together by mutual gravitation is called?

astrometric binary
spectrophic binary
visual binary

 

[Janus]

The clue the the answer is in the words "observed directly".

 

[Chronos]

it is a visual binary, until proven otherwise.

 

[Chronos]

janus, that really was not fair. it was on honest question.

 

[Chronos]

janus, you are very knowledgeable about these things. don't beat people over the head with it. most of those who come here are trying to learn. help them understand. be tolerant of them.

 

[Chronos]

if you want to hammer me with the math, go ahead. you will win.

 

[Nereid]

A visual binary is one which looks double; whether it's just a chance alignment or a gravitationally bound pair of stars requires further observations.

A spectroscopic binary is a 'star' whose spectrum shows a periodic doubling of the spectral lines (or similar).

What do you get when you enter 'astrometric spectroscopic visual binary' into Google?