Frequently Asked Questions
- What’s a split?
- How do you evaluate seeing?
- What is the difference between binaries, optical doubles, spectroscopic
- What is the difference between a separation of “4 arc seconds” and one of “10 AU?”
What’s a split?
But frequently you will at least see some hint that the star you are looking at is a double because it’s just not the nice round shape of other stars of its brightness. That’s what we call “squashed” – when a star look like a beach ball that someone sat on!
When a squashed stars shows some sign of a developing gap, it becomes a figure eight. It’s not a split – it’s a split wannabee.
Then there’s that most frustrating of categories – kissing! You want to see black sky between those stars. You are straining to see it. But it just isn’t there. There are obviously two stars – but they’re, well – kissing.
And then we get a split! It may be just a hairline, but we are sure we see black sky between the stars.
Anything beyond that is what we call a solid split. It would be obvious to the most casual observer.
That’s how we see them. That’s how we’ll try to call them to avoid confusion. And do keep in mind – a star can go through all these stages in the course of a few minutes! There are many variables contributing to a split, not the least of which is the physical condition of the observer at the moment – but obviously the telescope, the eyepiece, and the most obvious variable – seeing! Seeing can change rapidly so that a star may offer a figure eight one moment and a hairline split the next.
How do you evaluate seeing?
This image, from the Canadian weather service which produces the ClearSkyClock data, almost says it all:
We say “almost” because there is one caution. Don’t trust the naked eye view. Sometimes it’s accurate. When stars are twinkling madly to the naked eye, seeing will be bad. But when you don’t notice this twinkling, it still can be bad. So save your evaluation of seeing for the telescope. And more importantly, save it for the individual star – or at least for the area of the sky – you are observing.
Seeing can vary from moment to moment and can be different in different areas of your sky because it can be dependent on the terrain you are looking over. Looking over the roof of a heated building, for example, or over a parking lot still cooling down from the heat of day, can cause bad seeing in that portion of the sky. But looking in the telescope makes evaluation easy and we use a five-point system because we don’t see an advantage in trying to cut it too fine.
So what are you looking for? For us the ideal star image is perfectly round with very crisp edges and a faint diffraction ring or two that circles it completely and is evenly lit. That’s a “5” – or as the Romans say it, “V.”
A “4” is when the diffraction ring is a bit too bright, and the star’s edges a tad rough, and the whole affair appears to vibrate a bit.
Average seeing – a “3” – is when the diffraction rings are broken in several places and quivering quite a bit. I call this the airplane propeller look because it reminds me of a picture of a spinning propeller.
When seeing is bad – “2” – then it’s hard to separate the light of the star’s disc from the light in the diffraction rings. It’s all jumbled together and nothing will hold still. The general impact is of a bloated star.
In really poor seeing you lose any sense of the star disc as a distinct, crisp entity. You see a dancing blur of light, more intense towards its center. This a “1” – and time to go hunting nebulae and galaxies – something where transparency is more important than seeing and since bad seeing is frequently accompanied by excellent transparency, well, go with the flow!GS
What is the difference between binaries, optical doubles, spectroscopic doubles, etc. ?
OK, this can get confusing because a number of similar terms are used to mean the same thing but there are clear differences. So you may not encounter the exact words used here, but you should see something similar to them and it is all really quite simple and logical.
First, “double star” is used loosely to cover all that follows.
“Binary” is the term applied to multiple stars that are close enough to orbit around a common center of gravity. Most double fall into this category either because their orbits have been studied (about two thousand), or simply because the odds favor that two stars appearing visually close really are close to one another.
There are “optical doubles” however. Optical double simply means that from our viewpoint two stars appear to be very close to one another, though they may be many light years apart and have no grvaitational relationship. It’s just that they appear close in our line of sight. It is rare for two stars to appear really close and yet not be related and a lot of the optical doubles are really quite well separated. Perhaps the bets known of these is Albireo – Beta Cygni – though some astronomers think this may be a binary.
Many times doubles are so close that our telescopes simply can’t separate them. Such doubles form a spectroscopic binary. They can only be detected by looking at the spectrum of the stars.
Multiple stars is the term used when there are more than two stars involved. Frequently there is a visual binary that has caught the interest of astronomers and when they examine it closely they may discover more stars in the system that can be detected either visually, or spectroscopically. Again, a good example of this is the well known “double,” Albireo. It is believed to be an optical double that can be split visually with any small telescope. However, the spectroscope reveals that the primary star – the brighter of the two – is in fact a spectroscopic binary. We just don’t see that with our eyes and small scopes. Castor (Alpha Gemini) is a beautiful close binary that we can be split fairly easily and, in fact, we can see a third member of the system that is quite a bit fainter than the other two. So that makes it visually a multiple star – however, the spectroscope reveals that each of these three is a spectroscopic binary – so the Castor system really consist of six stars.
And again – we speak of all these in a rather loose fashion as “double stars.”GS
What is the difference between a separation of “4 arc seconds” and one of “10 AU?”
These are two much different measures. The first is a measurement of an angle as seen from Earth. The second is a measure of linear distance using the distance between the Earth and Sun as a unit. That distance is 93 million mile and equals one AU – that is, one Astronomic Unit. To put things in perspective,there are about 63,000 AU in a light year.
To those of us wishing to split double stars, arc seconds are very important. That is a measure of the angle between two stars as it appears to us from Earth. Such angular measurements are very helpful to observers. The belt of Orion, for example, measures about three degrees; the full Moon (or Sun) is about half a degree, or 30 arc minutes. Jupiter, when closest to Earth, has a diameter of about 49 arc seconds. Mars, when closest to Earth has a diameter of nearly 25 arc seconds, but when farrthest away can appear to be less than five arc seconds -barely detectable as a disc in a small telescope.
But none of these measures tell you the actual linear distance involved. With the Moon the actual diameter is about 2,100 miles. But the size it appears to be depends on its distance from us. This becomes clear when you think about the Sun. The Sun appears in our sky to be about the same size as the Moon – yet the Sun really is about 400 times as large as the Moon. So why does it appear the same size? Because it is 400 times farther away than the Moon! So when it is said a double star is split by 10 AU, it means that the distance betwen the two stars is 10 Astornomical Units – that is, 10 times the distance between the Earth and Sun, or about 930 million miles. The “AU” is usually the most convenient unit to use when talking about the actual linear distance between most double star components, though some times light years are used.
Bottom line. Knowing the actual distance between stars in AU is interesting to us as double stars observers – it helps us visualize the system we are observing. But, it doesn’t tell us a thing about what we will see, or the difficulty of splitting the stars. For that we need to know the separation in arc seconds. As a very rough rule of thumb, separations less than two arc seconds are going to be difficult for any backyard telescope and call for good seeing conditions. Separations greater than 10 arc seconds are going to be easy – of course, all bets are off if the double in question involves one star that is very bright and the other very faint. So please take this as just a rough guide. The separation in arc second is a good starting point when trying to understand what to expect when you point your telescope at a double.