Magnus Frater

I'll be on the road for the next two weeks and away from my computer, so I thought I'd sign off (for now) with one more posting, this one about our own Sun's probable sibling, born out of the very same cloud of gas.

From left to right

90 Herculis, HD 162989, and HD 162826

I am speaking, of course, of HD 162826 in Hercules. This star is currently 110 light years from our Solar System, and has an apparent magnitude of 6.7. Its chance alignment with two slightly brighter stars, HD 162989 (magnitude 6.04),  and 90 Herculis (magnitude 5.16) is a huge aid in finding it. The 3-star asterism lies approximately halfway between super-bright Vega and the globular cluster M92 and just three degrees north of Theta Herculis (magnitude 3.85). It is ridiculously easy to find through binoculars. Theta Herculis is a naked eye star, to the left of the northwest corner of Hercules' "Keystone" (which contains the mighty M13 globular). Find that, and place it at the lower edge of your binocular field of view. The 3 stars shown in the above image will be near the top of that same field of view. [see footnote] The star on the far right is the one you're looking for. It's that easy!

Red arrow points to HD 162826

So why do we believe that HD 162826 was born in the same stellar litter as our own star? Three reasons:

1. It is the same age as our Sun (4.6 billion years).

2. Its chemical composition is a close match to the Sun, especially in elements that typically vary wildly between different stars, such as barium and yttrium. There are only 30 stars known in the entire Milky Way galaxy that are as close a match to our Sun as HD 162826.

3. It has been calculated (by computationally "winding back the clock" of the orbits about the galactic center of HD 162826 and the Sun) that 4.6 billion years ago (that is, when the two stars were born), they pretty much occupied the same space.

HD 162826 may indeed be the Sun's litter mate, but it is no twin. It is 15% more massive than our star and shines at an absolute magnitude of 3.92 (as compared to 4.83 for the Sun). If our Earth were circling round HD 162826, it would have be orbiting at a distance of Mars to be the same temperature.

Hmm... HD 162826 is a rather awkward moniker for so close a relative. It was a pain typing it nine times for this posting. Considering its mass, perhaps we ought to just call it "Big Brother" (or, more formally, we could use the Latin Magnus Frater).

Footnote: This assumes the top of your field of view is to the north. At the time of this posting (early July), HD 162826 will be very close to the zenith, so your own orientation may be quite different. So with that caution, let us say that the 3-star asterism will be to the north of Theta Herculis in your binocular field of view, and leave it at that.