M33: Alive with Variable Stars

M33, also known as the Triangulum Galaxy or the Triangulum Pinwheel Galaxy, is the third largest galaxy in the local group (after Andromeda and the Milky Way). It lies roughly 2.6 million light years away from us (See for example Argon et al., 2004.) toward the constellation Triangulum. This Sc type spiral galaxy spans more than a degree on the sky (more than twice the size of the full moon), and can even be seen with the naked eye on a dark night.

Because M33 is relatively nearby, and relatively large, it has been a favorite target for variable star observers. Duncan in 1922 reported the first three variable stars found in the galaxy, and was shortly followed by Hubble in 1926 who discovered 42 additional variables, including 35 Cepheids that he used to obtain a first estimate of the distance to M33, establishing it as a distinct galaxy external to the Milky Way.

Recent projects such as DIRECT have increased the number of known variable stars in M33 to more than 1000. An ongoing survey of this galaxy led by David Bersier using the MegaPrime/MegaCam wide-field imager on the 3.6m CFHT telescope in Hawaii has uncovered more than 36000 variable stars. Using data from this CFHT survey we have constructed a movie of a portion of M33. The movie consists of 6 images taken over the course of 2 years (from 2003-2004). Seen on this time-scale, a galaxy, typically thought of as a static entity, becomes alive with variable stars.

Click on the image below for movies of variable stars in M33 at two different zoom scales (both movies are 2.8Mb):

Link to M33 movies

For reference the green box shows the field of view of the larger movie plotted on top of a beautiful image of M33 due to T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and M.Hanna (NOAO/AURA/NSF).

The following image is the view from 1 of the 36 MegaCam chips. The green box shows the field of view of the smaller of the two movies.

MegaCam Chip14

So what are these variable stars? The red variables are predominately cool, evolved, giant stars that are unstable to pulsations. These stars, similar to Mira, a well-known naked eye variable in the constellation Cetus, vary by as much as a few tenths of a magnitude to several magnitudes over the course of 100 to 1000 days. It is thought that most, if not all, cool giant stars are variables at the level of a few tenths of a magnitude.

Many of the yellower variables that appear to flash quickly are likely to be Cepheids. Some of these Cepheids, famous for their period-luminosity relationship, were first used by Hubble to prove that M33 is a distinct galaxy. They typically vary on time-scales of tens of days to a few months and so the movie, with its low sampling rate, shows these stars varying more slowly than they actually do.

Some of these variables may be eclipsing binary stars, that is, systems of two or more stars that pass in front of each other as viewed from Earth. The DIRECT program has been conducting a survey for eclipsing binaries in M33 with the goal of obtaining a geometric distance to a galaxy farther away than the Magellanic Clouds that can be used to provide a better calibration for the Cepheid distance scale.

There may even be a few cataclysmic variables (particularly among the bluer stars) that result from explosive mass exchange between two binary stars. There are also some "spurious variables" that are in fact cosmic ray hits. Although we have attempted to clean the images of these defects, a few remain. These can be identified as small squiggly-lines that appear in a single frame, and are bright red, green, or blue.

There is still much to learn and understand about variable stars. In particular, the processes giving rise to the long period variations observed in the cool, giant stars, are not very well understand. Understanding these phenomena require continuous observations, something that is difficult to do with large telescopes that are under great demand. In this respect amateur astronomers can play a very important role in contributing to the study of variable stars. While the variables in M33 are beyond the reach of most amateur telescopes, there are many brighter stars that can be observed by amateurs, with the results reported to organizations such as the American Association of Variable Star Observers or the British Astronomical Association Variable Star Section.

Some Technical Matters

The movie was produced from 6 images taken in each of the Sloan g,r,i filters. For each filter we matched the images using the image subtraction software due to Alard. Note that the images are not really "true-color", since we mapped i to R, r to G, and g to B. In particular, the HII regions appear to glow green in this movie while they would appear reddish to the human eye. What is displayed as red is mainly in the near-infrared.

Joel D. Hartman
Last modified: Fri Mar 18 12:21:01 EDT 2006