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Founded January 1970
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THE DAILY ASTRONOMER
Monday, September 16, 2019
The Sun's Lost Siblings

[image:
220px-The_Sun_by_the_Atmospheric_Imaging_Assembly_of_NASA's_Solar_Dynamics_Observatory_-_20100819.jpg]

Our Sun did not form alone.   Four and a half billion years ago, when Sol
developed from a gaseous nebula, it formed with many other stars that would
ultimately become members of an open or galactic cluster.  This cluster
traveled along a circuit within the Milky Way Galaxy, following the arc
that its primordial cloud described.  Eventually, within less than a
billion years of its formation, the cluster dispersed as its component
stars moved slowly, but inexorably, away from its center and along
individual trajectories through the spiral arms. As the Sun and its lost
siblings have turned around the galaxy more than 20 times since their
formation, the cluster components are now scattered far and wide through
the expansive Milky Way Galaxy.  The hope of many astronomers is to
identify these lost siblings.

Considering how scattered our stellar sisters have become, would it be at
all possible to find them?   Well, astronomers have found one already:  HD
162826, a star within the constellation Hercules.   Although it is
relatively close at a distance of 111 light years, HD 162826 is slightly
fainter than the dimmest stars visible to the unaided eye.

[image: 1280px-HD162826-starmap.png]
*The Sun's first identified sibling:  HD 162826     *Located within the
constellation Hercles, HD 162826 was the first star positively identified
as being one of the Sun's siblings, defined as the stars that formed with
the Sun about 4.5 billion years ago.   It is possible that the Sun might
have more than a thousand "siblings" at different locations within the
Milky Way Galaxy.  At a distance of 111 light years, HD 162826 is likely
the closest sibling.


Although astronomers have found one, the Sun's siblings might well number
in the hundreds or even more than a thousand.  How can those lost sisters
ever be located?   We know that in 2014 an astronomical research team from
the University of Texas at Austin determined that HD 162826 was a sibling
based both on its chemical composition and by tracing back its orbital
trajectory as it moved around the galaxy. A gaseous cloud from which the
Sun and its siblings arose had a consistent chemical composition so that
each star that formed within it would be chemically similar.    Astronomers
can ascertain this chemical similarity through spectral analysis of these
stars.   In particular, they look for relative abundance of rare elements
such as Barium.   It is likely that propelled remnants from a supernova
(explosion of a super massive star) both enriched the Sun's birth cloud and
induced the gradual collapse precipitating the star formation.   These
remnants would contain a set amount of the heavier elements which the
"metal-poor" cloud would have lacked prior the supernova particles'
arrival.     As a consequence, stars born out of a specific nebula would be
stamped with a particular spectral signature that would, in theory,
distinguish them from non-sibling stars.

The Galactic Archeology with Hermes (GALAH) survey aims to observe more
than one million stars so as to determine their individual "DNA profile."
 The aim, in part,  is to properly identify some of Sol's siblings:  to
know where the Sun's sisters have strayed.  These observations will also
lend astronomers some insights into the motions within the galaxy, itself:
 as leaves in a wind give us information about the direction and velocity
of air streams.      We don't know how many of the Sun's lost siblings
we'll find through these observations.    We do know, however, that they
lurk out there somewhere..



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