Star*Points for June, 2013
Siblings of our Sun

It's the month of June, and that means schools will be letting out soon.  For the northern hemisphere this year, summer begins on Thursday, the 21th according to the U.S. Naval Observatory.

Each year there are two Solstices and two Equinoxes marking the start of the four seasons.  For us north of earth's equator, these are the summer and winter Solstices  in June and December, and the Vernal  (or Spring) and Autumnal (or Fall) Equinoxes in March and September.

Some may consider it intuitive to assume that the different seasons are caused by the varying distance between the earth and the sun.  It's hotter in the summer, so we must be closer to the sun at that time, correct? Not really.  The earth is actually farthest from the sun in July two weeks after the Summer Solstice, and July is not the coolest month of the year by far.

While it's true that the earth-sun distance varies during the year, the difference in distance is not enough to account for the temperature swing between the summer and winter seasons, and vice versa.   The average sun-earth distance is about 93 million miles. And although earth's orbit around the sun isn't quite circular, the difference between the farthest and nearest separation is only three percent. 

As we all learned in school, the seasons are due to the tilt of the earth's rotational axis, the imaginary line passing through the earth's north and south poles, intersecting the celestial sphere near North Star in the Little Dipper, which is in the constellation Ursa Minor - the lesser bear.

During the colder months, the sun seems to pass in front of the more southerly constellations such as Scorpius and Sagittarius. It is low in the daytime sky and its rays coming in at a shallow angle and the northern hemisphere experiences winter.

In warmer months, the sun rides through northern constellations such as Taurus and Gemini.  It transits high in the sky during the day and its rays are more direct, causing the northern hemisphere to heat up.

The commonly accepted main stream astronomical theories put the age of the sun at approximately 4.6 billion years.  Astronomers look out and see stars being born in nebulous regions of gas and dust throughout our Milky Way galaxy.  Pockets of this material coalesce and form stars. As the stars heat up the remaining nebula is driven off leaving a cluster of new born stars.

As a cluster rotates around the galaxy, gravitational and drag forces may eventually cause its members to disperse.   Indeed, as we look around us, our sun is all alone and not part of any known star cluster.  

So what happened to the siblings of the sun, is there a way to identify them?  After all, even if the sun's natal nursery is dispersed, the original members could still be traveling in galactic orbits whose characteristics are similar to the sun. At least that's how the theory goes.

Estimates in the number of siblings within the volume of space reaching out to a distance of 330 light years (100 parsecs) from the sun ranges from three stars to more than 50.  Astronomers are looking for certain stars whose motion, composition, and estimated age is similar to the sun's.  

The goal of the search is to ascertain which stars that are similar to the sun are its true siblings.  It's a daunting search with many hurdles.  A 2012 summary identified only three candidates stars.  The brightest of these is a magnitude 5.5 double star 268 light years away in the summer constellation of Lyra, known as HIP 92831.