Double Star Observations
Most of the stars we see in the sky are not single, isolated stars but
in fact have one or more close companions. Many of these double and
multiple stars can be separated with binoculars or small telescopes.
This exercise is to observe a few such double or multiple stars.
It is divided into two parts: planning the observation and
carrying it out.
Background Reading: Stars & Planets,
p. 278 (Double and Multiple Stars).
There are two kinds of double stars. One kind, called optical doubles,
occurs because a second, unrelated star happens to be in the same line of sight.
But these are fairly rare, because the fainter star can't be too much
farther away than the bright one or it would be too faint to see.
Most double stars are actually a pair of stars that are physically
close together and orbit about their common center of gravity
This is called a binary system.
There can be three or more stars in a common system, too—all tied
together by gravity. If two stars are too close to each other
to tell apart, even with the largest telescopes, it is sometimes
possible to detect that there are two sets of spectra in the light
from the object. This is called a spectroscopic binary.
One attraction of double star observing for the amateur is the challenge
of seeing both components of a difficult double—difficult either
because the stars are very close together or because their brightnesses
are very different. But another reason to look at doubles or multiples
is just that they are very pretty, particularly if the two have quite
different colors.
Objective
Each person will observe three double or multiple stars.
The chosen targets will be selected in a team planning session,
then the observations will be conducted at a later outdoor session.
We will use both binoculars
and the Dobsonian telescopes, as appropriate.
Planning
Planning the observations involves choosing target objects,
then figuring out how to locate them and what tools to use.
Choosing objects
Picking a target involves three steps:
-
Determine which part of the sky is visible. Start by picking
a range of right ascension (RA) that will be visible, since
it's difficult to find something that's below the horizon.
The chart in Dark Sky Observations
may be useful to choose an appropriate range of right ascension.
These observations will be made in April, when the Sun is
at 1h to 2h right ascension. We have to wait until the sun is
about an hour below the horizon after sunset, and we can't see
things too near the horizon. And on the chart, the dotted line
separating "Evening Sky" from "Morning Sky" represents the RA
of objects that are rising at sunset. This gives a possible
range of RA.
-
Then, by searching through the descriptions in the textbook pages
for likely constellations (those that are visible and not too far south),
pick a tentative double. Base your choice mostly on interest
at first. You can use the index under stars, double or stars, multiple stars
to help find candidates. Although open clusters are technically
multiple stars, that's not really what we're looking for here.
Stick to doubles or multiples with a few closely related stars.
The Sky and Telescope article
Pretty Double Stars for Everyone has some suggestions,
and some comments about what kind of doubles are the most
satisfying to observe.
-
Now decide if your candidate is going to be findable and resolvable.
The primary (brighter of the pair) should probably be at least 6th magnitude,
because fainter stars are harder to locate and not as impressive
in the telescope. Look at the description to see if it says
"resolvable in small telescopes" (or binoculars). Avoid spectroscopic
binaries, because as mentioned above you won't ever be able to resolve them.
If the primary is much brighter than the secondary, it may be more
difficult to see the secondary—but then that's a challenge too!
Finding charts
You will probably want to prepare two charts for each target object:
one small-scale (large field, maybe 45 degrees diameter) chart to
locate the area of interest using bright stars and known constellations;
and another large-scale chart, with a field maybe about 7 degrees in
diameter. Our binoculars, and our finder scopes, have about a 7 degree
field.
The telescope field of view is, as we know, determined by the eyepiece
chosen. The eyepieces have an apparent field of about 50 degrees
diameter, but this is reduced by the magnification to a field
of 50/m degrees, where m = 1200/fl, and fl is the eyepiece focal length.
It's useful to have a loop of wire, or a circular overlay of some kind,
with the size of the finder field, and another with the field of one
of the eyepieces.
You can work out your group strategy for making the finder charts.
One option is that each member provide the charts for one of the objects.
The web site
Your
Sky from Honolulu may be very useful for this, since you can adjust
the scale. Change the color scheme to black on white background (this is
under Display Options) before printing your charts; they will be easier
to see and won't use so much printer ink.
Make good charts, at two or three different scales. This will really
help when you get out in the field.
Part of the observing plan is to plan how to get the telescope pointed
at the target object, starting with known stars and moving from one star
to another (it's called star-hopping) until you're pointed at the right
place, even if you can't see it in the finder. Read the article in
Sky and Telescope called
Using a Map at the Telescope for a good description of how
star-hopping can be used to systematically move the telescope to
a desired location using a finding chart.
For each object, make a detailed plan of which stars will be used
to hop from a known location to the position of the desired object.
Some plans may be very simple, such as when you already know just
where the object is and it's so bright you can't miss it. Others
should be much more detailed, or you'll end up wasting lots of
time with the telescope pointed the wrong direction.
Observing strategy
The idea here is to plan the order of observations, so as to have
the best chance of accomplishing all of them. There are several
issues to consider. Are any of your target objects in the west?
These will need to be observed before they get too low in the sky.
Likewise, are any of them so far east that you can't observe them
early in the evening? You may want to do something else first,
and save these for later in the session.
Are any of the objects likely to be easier to locate than the rest?
You need to decide whether to do these first, or start with the
hard ones.
The Observations
The observations will be conducted at another lab session.
Be sure to come prepared with an observing plan and finder charts.
After locating each object, each person
should spend several minutes looking at it.
Try pushing gently on the telescope tube, just to wiggle the
image a small amount without really changing the pointing.
A little motion helps to make faint detail more apparent.
Sketch the stars, noting colors and relative brightness.
Try to figure out the orientation: if you push the front end of
the telescope toward the east (Don't really move it or you'll lose
your object! Just lean on it a little.) stars to the east move into
the field of view; likewise you can find north or south by leaning
gently on the telescope in that direction. If you get it, indicate
the directions on your sketch.
Report
The report should be a full report, describing the goals of the project,
the methods and results. Include finder charts for one of the objects;
then for each target object tell why you chose it,
describe the observation process, and include
your sketch and comments. Please comment on observing conditions.
mickey@ifa.hawaii.edu
Last modified: March 30, 2005
http://www.ifa.hawaii.edu/users/mickey/ASTR110L_S05/doublestars.html