What’s a double star?

[Username556]

Not the best picture of it but it’s that bright thing at the bottom of the moon. I aligned it with sky safari and it said it was a double star 

[markbc01]

Hi! 
I think a binary star is a pair of stars that orbit each other.

The star app may have indicated that you were looking at  Nunki (Sadira) [AKA  sigma Sagittarii,Magnitude 2.05 Sagittarius].  

Nunki (https://www.star-facts.com/nunki/) is part of Sagittarius -where the moon is sitting right now.

Nunki is a double star, or maybe better said a binary star. (https://www.space.com/22509-binary-stars.html)

It is a bright star and in your picture/view it is to the right of the Moon, and maybe halfway between Moon and Venus on the lower right.

The really bright thing lower down on the right -halfway to the horizon is not a double star. It appears to be Venus and is seriously bright.  

Last night Venus followed the Moon down and tonight the Moon follows Venus below the horizon.!

One of the nicest pair of double stars to see ("split") are Mizar and Alcor on the handle of the Big Dipper -literally the middle star on the handle. 
If you look quickly, they may appear as a single bright star. When you look more closely you will see there are two stars in close proximity -very pretty.

Discerning both stars in a careful viewing is called "splitting" double stars.  It's fun -and sometimes easy (Mizar/Alcor), sometimes not possible ?Nunki?

[wibblefish]

Double stars are either two stars in close proximity (binary) or just two that appear to be close together on the same plane (optical double). 

When viewed at lower magnification they appear to be a single star but if you can increase your magnification enough you can “split” them so you can see them as two stars. 

Some doubles are very pretty (different colours) and it can be a fun challenge to find and split them. They are also good to hunt when the moon is up as they are unaffected by its brightness unlike most deep sky objects.

https://www.skyatnightmagazine.com/advice/double-binary-stars-guide/

There is an excellent book by Sissy Haas if you want a really good guide to them (its mostly just tables of locations and notes on what powers to use / what they look like) but its hard to get hold of:

https://www.goodreads.com/book/show/271431.Double_Stars_for_Small_Telescopes

 

 

 

Edited November 9, 2021 by wibblefish

[Louis D]

From Wikipedia:

In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes.

This occurs because the pair either forms a binary star (i.e. a binary system of stars in mutual orbit, gravitationally bound to each other) or is an optical double, a chance line-of-sight alignment of two stars at different distances from the observer.

[Starslayer]

WOw - nobody mentioned Albireo - beginner's delight. 👌. Easy too.

 

 

Edited November 9, 2021 by Starslayer

[wibblefish]

Finally found the diagram I was looking for, describing what the terms are for double star resolution  

[Ian McCallum]

38 minutes ago, Starslayer said:

WOw - nobody mentioned Albireo - beginner's delight. 👌. Easy too.

 

 

I came across Almach by accident, last week.

[Pixies]

It is also estimated that 50%+ of star systems are binary (or even higher-multiples). In our solar system, some consider Jupiter a failed-star. It's made up of mainly star-like material: hydrogen and helium, it just never became dense enough to fire up!