A faint double near Neptune
8/25/2014 9:35 AM
When looking at planetarium software for objects I discuss in weekly Eyes on the Sky videos, I often see interesting objects I wouldn't necessarily find if I were looking for them in the night sky. That is because I am usually looking at the software with a wider field of view than I can usually see through an eyepiece. In addition, this summer has been absolutely terrible for observing where I am located; some forest fires up in Canada coupled with weather systems that keep a near constant cirrus-to-full-stratus-cloud cover has made it hard to get much good observing accomplished.
So while looking for the appropriate star hop waypoints to find Neptune in the sky, I noticed there were some interesting sights nearby to the last landmark - ummm... skymark? - to reach Neptune, Sigma Aquarii. Now that star itself has a bit of an interesting look to it, what with a triangle of three 10-th magnitude stars and a pair of 8-th ones nearby. Given how loose and small some open clusters are (M29 anyone?), one could almost be forgiven for mistaking this little grouping for an open cluster. Alas, it is not, but it's worth giving it a quick look before starhopping on towards our solar system's outermost planet.
If you do make your way out to Neptune, finding this next true double star is easy; with a one degree or one and a half degrees field of view at your eyepiece, turn the right ascension slow motion control on your mount one degree towards the west. Or, simply use a long focal length eyepiece, put both Sigma and 58 into your eyepiece field of view, and move along a similar line to find the "bow" shape of stars.
Just over one full magnitude dimmer than Neptune is a double star that is easily split: HD 213067 or HIP 110974A (see chart below to help you identify it). This is a G0 spectral class star, similar to our G5 dwarf-sized Sun, but much larger, indicated by the "0" after the G.
This star lies at 374 light years distance, so the light from it left the star in 1640 (if you're reading this in 2014), not too many years after Galileo first peered up at the sky with a telescope. I always find it amazing how far light travels and how many years, decades and indeed, centuries pass before that light even reaches our own eyes. Truly incredible to consider the vastness of just our galaxy, when you stop to think about it!
The companion star, a dimmer 9.6 magnitude object, makes this a dim apparent double to locate. (More on why it's an apparent double below.) But both should be visible in most telescopes from most areas. This star is even farther away, at 461 light years. It's light left that star in the year 1553, so it's light had traveled for 87 years when it passed the star HD 213067 / HIP 110974A, joining that stars light on it's way towards Earth for us to see now. So looking at these two stars, you're seeing light that is quite literally, hundreds of years old, and one of which is 87 years older than the other. Yet you see them simultaneously.
I love thinking about the stars this way. I hope you enjoy thinking of them like that as well. These stars are about 36 arc seconds apart.... BUT!
That's not actually the double star.
The brighter of those two stars, however, IS a double star. The distance between them though? That is VERY close together indeed - so much so, you'll need a 12" telescope to split them cleanly, according the Dawes limit of theoretical resolving power anyway. At a separation of just 0.40" they will be very tight to split... but at the very least, it's worth a go if you have a large light bucket with good optics and a clear sky. Then you can turn your attention to Neptune to see that planet at high magnification too.
There's a lot more here on Eyes on the Sky. For example, every week on the homepage there is a new astronomy video about observing objects in the night sky. They're only 5 minutes long - why not check out the latest one right now? For those new to astronomy, don't miss Eyes on the Sky's Ultimate Beginner's Guide!