Basic Types of Variable Stars
"Cepheid" Variables (Delta Cephei)You've probably heard alot about these type of
variables. They're often mentioned along with the distance estimates of galaxies. The reason
is, they vary in a very predictable way. Their variation is
directly proportional to their mass in both intensity and period. You measure how bright the
star appears to get and how long it takes it to complete one cycle and you can calculate its
absolute magnitude and then its distance, to within a very small error factor. They are very
massive stars, typically more than 3 solar masses. They swell and shrink in size as they vary.
When they contract, they burn more intensely and "light up" the outer layers, making them
appear brighter. Delta Cephei completes a cycle in 5.36 days, varying by just under 1
magnitude. Others Cepheid stars vary much faster. RR Lyrae, for example, completes a cycle
in 0.6 days while others of this sub class can vary in under an hour.
Long Period Variables (Mira)Mira type stars are called long period variables
because Mira takes a long time to complete one cycle of variation, but there are many more
complex examples. These stars that are usually slightly more massive than our own sun,
but much larger and therefore less dense. They actually swell and shrink as they burn fuel,
like the Cepheids, but when they swell, low densitycarbon is brought to the surface, thus
dimming their appearence. The neat thing about Miras is their wide rangingcharacteristics.
There are so many subclasses it's hard to keep up! Some stop brightening for a while before
reaching maximum, (R Leonis), others pause on the way down. Some have wide ranges and change
quickly, (Chi Cygni), and vice versa. Mira itself changes by as much as 6 magnitudes,
while other sub classes vary much less. The Miras include a sublass called, "semiregular"
that changes character from one cycle to the next.
Eruptive Stars (U Geminorum)
Also called "Cataclysmic Variables" or "CV's", these are stars that have a companion. In
general, one star is larger and less dense, what's called an M class giant, and one star
that's denser and hotter. A dense white dwarf, a neutron star or even a black hole.
The denser star pulls matter from the larger star where it collects most often in a disk
around the star. When it reaches a certain amount and/or temperature, it
triggers a thermonuclear explosion. As you may know, these are very bright. :-) So, the
stars overall magnitude rises dramatically, in a short amount of time. Now, the variability
that we see most often comes from the changes in the disk, rather than the stars involved.
The characteristics of the variation can tell us a lot aout what's happening there. For
instance, if the system brightens quickly and fades slowly, perhaps preceded by a burst of
x-rays, we can tell that disk suddenly heated from the outside, (toward the M giant), to the
inside. If several brightenings are seen, called "humps", before it stabilizes, we can tell
that the brightening started on the inside of the disk and moved outward.
There are almost as many mechanisms for CV's as there are CV stars. Sometimes, they combine with eclipsers so we can learn even more about them!
Eclipsing Binaries (Algol)
These are also 2 star systems, but usually neither star varies in brightness by itself. It
needs the other star to make it look like it changes. The 2 stars are orbiting each other,
and it just so happens their orbital plane is in our line of sight. Therefore, every so
often, one of them passes in front of the other from our perspective and blocks the light
from the other star. The system as a whole drops in magnitude by the amount of the "eclipsed"
star. If one star is intrinsically brighter than the other to begin with, we can then tell
which one is being blocked by measuring how much the system dims. This is kinda neat!
In studying variable stars, it's important to keep good records to keep track of the brightness changes. Plotting the estimates you and others make creates a "light curve" chart of the star's brightness over time. What follows are a few examples of the general types we've discussed, complete with captions.
This is a graph of the observed magnitude of Mira over the last year or so. They call this the "light curve" of the star. It shows the magnitude along the left and right vertical axes, date along the bottom by Julian Day, the Harvard Designation of the star at top left, (both of which I'll explain later) and the name of the star at top right.
Light curve for Delta Cephei-THE Cepheid variable. You see the pattern emerge? Along with its main up and downs, it has a small hump at the minimum, before it goes all the way up. This pretty much defines how these type stars behave. The height of the max/min, along with the intermediate hump is what's used to calculate the mass and absolute magnitude. Cool, huh?
U Geminorum is a cataclysmic variable. See how the star stays at a low level, then suddenly brightens by 5 orders of magnitude? What can you say, but KABOOM! :-)
This is the curve for SS Cygni. An extremely complex star, it varies not only in magnitude, but its period changes, too. Note the distance between the peaks, and length of low points.
Some links to international VSO organizations.
|AAVSO: The American Association of Variable Star Observers, of which I am a member.||British VSO: How they see stars from there, I'll never know.||Czech VSO or here.: Strong group of dedicated people.||VSNET: Variable Star mailing list archive.||Japanese VSO: Why not? They do most everything else well...Besides, the site shows a lot of enthusiasm.|