Seeing double

Well, it's a slow news week astronomically speaking, so let's go the mailbag--or rather the in-box that is.

Interesting question from a reader, "What are double stars?"

Ok, the simple answer is a double star is a pair of stars that appear close to each other in the sky as seen from Earth. A double star can also be called a binary.

The complicated answer is there are several kinds of double stars. The first is a "visual" double which is two stars that are bound together by a mutual gravity.

A visual double can be observed with either binoculars or a telescope or even with just the eyes alone as two distinct stars.

A variety of this is an optical double where two stars are not related and only look like they are close but are actually very far from each other.

But, what often happens is that there are other stars, also gravitationally bound, that cannot be seen. That is the third kind of double -- the spectroscopic double. More about them later.

Let's go to one of the best known double stars -- Mizar and Alcor -- which are the stars at the bend in the handle of the Big Dipper. They can be seen with just the eyes alone (seeing this pair used to be an eyesight test in the Roman army), with binoculars, or a telescope.

The Big Dipper can be found in the north almost overhead. The dipper is upside down as if to dump its contents on the Earth, April showers you know.

Find the handle extending to the right, and the middle star in the handle is the famous pair. Can you see both of them with just your eyes?

It is still up for debate if these two are really an associated double, or just two stars that happen to be along the same line of sight -- a visual double. Alcor has been measured to be about a light year beyond Mizar.

However, there is more. Remember that spectrograph I mentioned. Using the spectrograph, it has been discovered that Mizar does have an associated double. It is too close to be seen, but it is there.

What's more, Alcor has also been found to be a double and there are other stars associated with these spectrographic stars which brings the total number of stars in the Alcor/Mizar system to at least five, some astronomers even say there are six stars there.

Take a look with a telescope and see what you think.

SKY WATCH: New moon, Thursday, May 9. If you have access to a totally unobstructed western horizon, look that way about a half-hour after local sunset on Friday, May 10 for a nice conjunction of Venus and a less-than one-day old moon. It won't be easy, but binoculars will be of great help. The Pleiades star cluster will be just to the right. Venus is making a return to the evening sky after a long absence. On Saturday evening look for the last conjunction of Jupiter and the moon for a while. They will not be as close as some of the past meetings, but it will be close enough; they will be closer on Saturday, May 10, with Jupiter to the moon's lower right. On Monday, May 13, look for the growing crescent moon between the feet of Gemini. Also try to see any Earthshine or the reflection of sunlight off of Earth back onto the surface of the moon. The non-lit portion of the moon will have a ghostly, partially lit look to it.

NEXT WEEK: More about double stars and more astronomical blather.