From the very beginning of my interest in astronomy I remember seeing colors in the stars. As I learned the constellations it was fairly easy to see that Betelgeuse and Antares are orange, Arcturus is yellow, Sirius and Vega are sparkling white. Once I acquired a telescope these colors from the glowing gases in the outer layers of our stellar neighbors became even more interesting. There are lots of nice colors to be seen in the Universe, let us take a look at the colors in the stars.
COLOR CONTRAST DOUBLE STARS
Most often, colors are associated with binary stars. Binaries are stars that have formed in pairs, a common occurrence in nature. Because human eyes are sensitive to contrast effects, subtle differences in the tints of these stars stand out when observed in a telescope. The colors in these stars arise from the fact that the outer layers of gas that form a star can be much different in temperature. Much like heating a piece of steel from a dull red to hotter orange and then white hot, stars display different colors depending upon their temperature.
Therefore, cooler stars appear reddish or orange and hotter stars seem white or blue-white. Our Sun falls in the middle somewhere and so it looks quite yellow. Because we evolved under the rays of Old Sol, humans developed an eye that is centered on the wavelength of the maximum brightness of Our Sun. As a sidelight, I have always been amazed that an optical system which evolved to allow our ancestors to accurately grab fruit from trees is so adaptable that it can also show us the Universe. Binary stars form a large part of that Universe.
Seeing as how multiple stars are two (or more) stars in orbit about their common center of gravity, astronomers should be able to compute their orbital path. The problem is that many of the wider binary pairs have an orbital period (the time of one revolution) that lasts for hundreds or thousands of years. To calculate an accurate orbit, at least one half of the period must be completed so that the rest of the curve can be predicted. For this reason, no more than a thousand or so stars have accurate orbital paths available.
One of the best things about double stars is that they can be done from even light polluted cities. My backyard in the metropolitan Phoenix area only allows me to barely see a hint of the Milky Way. On the best of nights from the sidewalk in front of my house a star of 5.5 magnitude can be located at the zenith. That is because there is a streetlight in my front yard!
The light can be hidden behind a corner of the house, but obviously this will never match a good, clear night in the Arizona desert. However, the only reason to observe double stars from a dark sight is to look for dim companions. Many of the observations I will present here are from in town.
This article is not going to discuss how to measure the values needed for calculating orbits of double stars, to be honest I know very little about it. But we need to understand what those values mean so that as an observer we can make certain that the telescope is pointed at the correct pair. The first of these important readings is the separation. This tells the observer how far apart the stars will appear in the telescope. The easiest way to get a feel for separation is to go out with your scope and observe some of the easier pairs on the list that accompanies this article. The easy to split pairs will be the ones with large values in the separation column. Any telescope will split a pair of 20 arc seconds or larger. When you have found some of these stars, notice how far apart they seem in your telescope at a magnification you use often. As you get better at this technique, try some of the tougher pairs (smaller separations in arc seconds). Soon you will have a good estimate of what 30", 20", 10" and 5" looks like in your scope at certain magnifications.
The closer pairs of stars are going to need higher magnification to split them cleanly and show two distinct stellar points of light. Very close pairs may only be "notched", that means that they display a figure 8 pattern with a connecting bridge between the two stars. Trying extremely high power on a tight binary will rapidly teach you about one of the major problems that astronomers face. That problem is called "seeing."
Seeing is the distortion effect of our atmosphere as it mixes different temperatures of air together. A glance at a weather map will tell you that the mixture of gases that we breathe is constantly in motion. Remember looking down a hot road on a Summer day? The turbulent air distorts the field of view along the road and makes things appear to move that you know are not moving, like mountains. This effect is quite obvious in the Arizona desert on the Fourth of July.
So, what can be done about seeing? First, choose a place to set up the telescope that will not be changing temperature through the night. An asphalt or concrete parking lot is a poor choice, a grass field is best. Next, pick your observing nights carefully. After a powerful storm has passed through your area the sky will remain turbulent for at least 24 hours. Let the sky settle down before going after those tough 0.8 arc second pairs. Last, if the sky is turbulent, stick with the wider pairs at low power. You must learn to adapt your observing to whatever nature provides.
The next important piece of information given for a binary star is the position angle. Often shortened to "PA", position angle is the measure of the relative position of two stars. Just connect the stars with an imaginary line from the primary (brightest) to the secondary (dimmest) and the angle that line makes in the field of view is the PA. The angle is always measured clockwise from North, through East, South then West. As an example, a star with the dim companion to the Northeast would have a PA of 45 degrees. The obvious question is: what do I do if the stars appear to be the same magnitude? Do what I do, guess. If the angle appears to be 180 degrees removed from the value in a catalog, then you guessed wrong, if your estimate agrees with the published PA you can say "I knew it all the time."
Now for the tough part about PA determination; figuring out the direction of North and East in your telescope. East is the easy one, let's do it first. If you have a drive, turn it off, if you are a Dobsonian user, let go of the scope. Now, watch stars enter and exit the field of view. Stars will always enter on the East side and leave on the West side of the field.
Once you have found the East-West line through the field, it is obvious that North is going to 90 degrees to that line. But, which right angle do I use to find North? This is a situation where an equatorial mount makes things easier. In the Northern Hemisphere, just move the telescope toward Polaris while looking in the eyepiece. The side of the field of view where stars are entering the field is North. In the Southern Hemisphere, move the scope southward and stars will enter the field on the South side. I realize it is tougher to pick out Sigma Octans(the Southern Pole Star); try moving the telescope in the general direction of the Small Magellanic Cloud. Dobsonian owners can utilize the same technique but they will have to be careful while trying to adjust the scope in both altitude and azimuth at once. It gets to be second nature with some practice.
The reason that a double star observer needs to become skilled
at determining the PA and separation of pairs of stars is simple. There
are lots of stars which travel in pairs and when you point your telescope
at a rich star field and there are five or six candidates which could
be your target, what do you do? If you are good at estimating the PA and
separation of star pairs, then you can figure out which binary is the
one you seek.
Because of the contrast effects, binaries can come in a wide variety of colors. However, many of the cooler stars in the sky will appear red even if they are not part of a multiple system. Viewing red stars is a much forgotten aspect of astronomy. Some professional work has been done acquiring the spectra and photoelectric magnitudes of these objects, but little has been written about how they look in a modest telescope.
The best and reddest of these stars belong to Class N. These stars are the coolest known, about 2500 degrees Kelvin. All have a spectrum that displays the characteristic lines of carbon molecules. These carbon molecules absorb the blue wavelengths of light very efficiently. The combination of blue light absorption and cool temperature makes the Class N star very red indeed.
All Class N Carbon Stars vary in magnitude somewhat, usually about 2 magnitudes. They appear reddest to my eye when they are at the dimmest in their cycle.
Many of these lovely tinted stars are contained within an
open star cluster. Observing these groupings of stars afloat in the Milky
Way is a great way to pass an evening at the telescope.
NGC 2362 in Canis Major is a
cluster of about 45 stars in a compressed group surrounding Tau Canis
Majoris. The central bright star has two companions, both on the east
side. The primary is white and both secondaries are light blue. Then the
cluster fans out from Tau CMA. It is rare to have a nicely tinted triple
star in the center of a pretty rich open cluster. This object has been
a favorite of mine for many years and a fine view can be had in the 13"
f/5.6 at 165X.
NGC 4755 in Crux is the Jewel Box cluster. In 1986 I travelled to Australia to view Comet Halley and to acquaint myself with Southern Skies. Using Jim Barclay's 12.5" f/6 at 120X, this magnificent cluster lives up to its' name. There are about 30 stars in a 10' field and the brightest of the cluster members are gold, blue, orange, creme and light red. A stunning open cluster.
Another beautiful object from Australia is Alpha Centauri. This multiple star system is the closest star to Our Solar System. The two main stars in this system are easily split at 70X in a 6" Newtonian. The sight of a first magnitude and second magnitude star only 15" apart is unique in the sky. The primary is blue-white and the secondary is very light yellow.
To give you some more information on specific objects that show color in my telescope, here are some observations I have made with my 13" f/5.6.
Almach--Gamma AND. RA 02hr 04min
Dec +42 18
Eta (ï) CAS- RA 00hr 49min
Dec +57 54
Iota ORI- RA 05hr 35min Dec
24 Comae RA 12hr 35.1min Dec
Once you have gotten proficient at observing doubles, hopefully you will try making some sketches of what is in the eyepiece. If you give it a try using the traditional techniques for drawing deep sky objects or planets, there is a major problem. To preserve night vision while drawing or taking notes, a dim red flashlight is generally used by dark adapted observers. Drawing tinted stars under the red light is quite impossible, because the colors will not appear correct.
One way to overcome this problem is to use a white light. This leads to two problems. One, you will not be very welcome at star parties with other people trying to photograph or observe. Second, your own dark adaptation is reduced every time the white light goes on.
The solution lies in taking good notes. There are only so
many colors to be seen in double stars. In many years of observering tinted
binaries I see: red, orange, yellow, green (rarely), blue, and grey. There
are light and dark varieties of each. So, to make a drawing, sketch the
PA and separation you see at the eyepiece and label the colors with a
#2 pencil under the red light. Once you are at the kitchen table you can
re-draw the object with colored pencils or ink.
I realize that the last paragraph would tempt someone unfamiliar with binaries to think that there are very few types of doubles and "seen one, seen them all" is the rule. Nothing could be further from the truth. The combination of PA, separation and color make tinted double stars come in a bewildering and fascinating variety.
All observers of color in stars soon realize that the hues they are reporting apply to only their eye. There will be several people who are in line to look right after them who disagree with the color they have seen. These disagreements have always part of the fun in the Saguaro Astronomy Club. On star party nights you will often find several members going back and forth looking through each others' telescopes discussing colors.
On one such evening, I mentioned that I saw 107 Aquarii as white and light green. Gerry Rattley immediately stepped to the eyepiece and after a moment asked "which star are you calling green, the orange one?" Hopefully, the controversy will never stop.