February 05, 2008


Today, I'll talk about a tiny little constellation bordering Andromeda, Pisces, Perseus and Aries in the night sky. This is the constellation Triangulum, a small patch of sky that is home to one of the most spectacular nearby spiral galaxies. This is one of Ptolemy's original 48 constellations and for some, it represents the triangular shaped island of Sicily. First, let's look at the bright stars in this constellation.

The brightest star is actually Beta Trianguli, with no proper name. It is a blue A-class star that is on its way to becoming a red giant. It is in the final stages of hydrogen fusion in its core. It has a sun-like companion orbiting it every 32 days or so, which will lead to an interesting situation once Beta grows into a true red giant. It will engulf its companion and turn the system into two stars with a common envelope. The two stars will spiral closer together and the cores will heat up, eventually ejecting this envelope, but the process takes a long time and is not well understood.

At the tip of the triangle is the 2nd brightest star, Alpha Trianguli which is also known as Mothallah, Muthallath or Metallah, from the Arabic for head of the triangle. The Latin translation sometimes used is Caput Trianguli. This is another binary, very close with a companion period of less than 2 days (!). We don't know much about the companion because it is so faint, but the main star is slightly more massive than the Sun and like Beta at the end of its main sequence evolution.

Gamma Trianguli is an A star like Beta, but it is not part of a binary system despite a couple of visually nearby stars that are actually at much different distances. It is also a very fast rotating star, and we know this because of the broadening of its spectral lines. This star is spinning so fast that it is likely very oblate. Delta is one of Gamma's visual companions, consisting of two sunlike stars orbiting one another at a distance of less than 10 million miles (the Earth's orbital distance is 93 million). For a close binary like this, it is possible to have a system of planets in stable orbits if they are far enough away from the center, and so technically there could be habitable planets here. The system is nearby (35 light years away) and a target of some planetary searches.

Finally, there is TZ Trianguli (or 6 Trianguli), a double-double star much like Mizar and Alcor in Ursa Major. The main star is a yellow giant, and the visible companion is a slightly bluer dwarf star, orbiting with a very long period of over 2000 years. The two binary systems have much smaller periods of 15 and 2 days. Like many closely orbiting stars (and the Earth-Moon system), these individual pairs of stars are tidally locked to their closest companion, leading to some strong magnetic field effects. Burnham indicates that this pair makes for a nice yellow/blue contrast in a small telescope. You can find it if you follow a line South from Almaak (one of the feet of Andromeda) through Beta Tri and then about 2/3 again as far.

Now for some of the more interesting deep sky objects in Triangulum. If you draw a line from the Southern tip of the skinny triangle (Caput Trianguli) through Mirach and onwards to M31, just south of this line, about 4 degrees due west of Caput Trianguli, you will find the Pinwheel Galaxy, also known as M 33 (sometimes called the Triangulum Galaxy to avoid confusion with other galaxies known as "pinwheels"). Under perfect viewing conditions, this is faintly visible to the naked eye, somewhat fainter than the Andromeda Galaxy.

M 33 is about half the diameter of the Milky Way or Andromeda, and it is about 3 million light years away (whereas Andromeda is only 2 million light years away). This distance was established early on by observing Cepheid variable stars in the disk. Like Andromeda, M 33 is a member of our local group of galaxies, and it is likely a satellite of Andromeda. A highlight of M 33 is the brilliant star forming region known as NGC 604, which is reminiscent of the Great Nebula in Orion in our galaxy. A very nice high resolution photo of this galaxy can be found here.

In the Eastern portion of Triangulum, just a degree or so east of the visual group of Gamma, Delta and 7 Trianguli, NGC 925 is one of the HST Key Project galaxies in the hunt for Cepheids, visible as a smudge in 10-inch telescopes, you really need to look with a research-class 1-meter or bigger telescope to make out the full extent of this distant face-on spiral which is over 10 times further than the Andromeda Galaxy.

Going back over to the Western edge, a couple of degrees south of Caput Trianguli is NGC 672, about the same distance away as NGC 925 and an interesting spiral because it is in the process of merging with nearby irregular galaxy IC 1727. Though these galaxies appear separated by about 100,000 light years on the night sky, radio observations of neutral hydrogen show both of these galaxies are tidally extended and mixed with one another.

Moving east a couple of degrees from Caput Trianguli, we find NGC 784, an edge-on barred spiral galaxy. Not much to look at, this relatively small galaxy is even further than NGC 925, but it does play a crucial role in our research of spirals. That's because it is one of the few nearly perfectly edge-on galaxies that is close enough for our biggest telescopes to pick out individual stars. With the edge-on view, Astronomers have been able to figure out what kinds of objects are in the center of the disk, the edge of the disk (the "thick disk") and the inner halo.

Posted by Observer at February 5, 2008 09:18 PM

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