December 20, 2007


Now that I have the luxury of a little more time to spare, I will return to some constellation posts, and today I'll talk about the constellation Taurus the Bull. The Bull that this constellation represents appears in Babylonian mythology as the symbol of fertility and power. In Greek mythology, this is perhaps a form of Zeus who carried off Europa, a princess of Phoenicia. Half-immersed, the bull swims across the sea to the isle of Crete, where he reveals his true self. In China, this V-shaped group of stars is part of the larger White Tiger constellation, and the Bull played a major role in Egyptian and Celtic mythologies.

The bull has been perhaps the most important constellation in the ancient world. This is because the location of the Vernal Equinox (the sun's position in the sky when it crosses the Celestial Equator heading North, marking the first day of Spring) was in Taurus from about 4000 to 1700 BC. Most ancient calendars begin their year on the first day of Spring. From there it moved into Aries and then Pisces, as I discussed before when talking about the Age of Aquarius, which began around the year 7 BC.

The brightest star in Taurus (13th or 14th brightest in the sky) is named Aldebaran, which translates to "the follower" since it follows the Pleiades cluster across the sky. It is often considered the eye of the Bull. Though it is in the same direction as the Hyades cluster, it is actually much closer than that cluster and so not a member. Aldebaran was also considered to be one of the four Royal Stars of Persia, along with Antares, Regulus and Fomalhaut. I talked about Antares (in Scorpio) earlier as being ascendant in the sky during Fall. Aldebaran is ascendant during the Winter.

The star itself is a giant star, about 40 times larger in diameter than our Sun and somewhat cooler (hence its red appearance). It is about 65 light years away. Like many big stars, it is not entirely stable, so its brightness varies about a few percent. Aldebaran is one of only three bright stars (along with Spica and Antares) that are currently close enough to the ecliptic and the moon's orbital plane to be occulted by the Moon. The Moon is scheduled to occult the Pleiades this year on December 21 (more on this below) and shortly thereafter on the 24th will be a grazing occultation of Mars. The Moon won't occult Aldebaran until the year 2015 due to gradual changes in its orbital alignment.

Burnham tells an interesting story about occultations. Edmund Halley learned that the Greeks observed the Moon occulting Aldebaran in the year 509 AD, but Halley realized that the Moon's orbital alignment should've been different at that time. Either that, or Aldebaran had moved considerably over the past 1300 years or so. By doing a little further research on other observed occultations, Halley concluded that, yes, stars are gradually moving across the sky in very small increments that can only easily be measured over centuries. We now refer to this routinely observed phenomenon (with our more accurate instruments) "proper motion". Aldebaran has moved about one quarter of a full-moon diameter in the past 2000 years from our perspective!

The second brightest star in Taurus is El Nath (or Alnath), "The Butting One", is located at the tip of the Northern horn, right on the border with the constellation Auriga (this is technically the third brightest star in Auriga also). The third brightest, or Gamma Tauri, is at the vertex of the V in Taurus, and it is often called Hyadem I, being the brightest member of the Hyades cluster as seen from Earth. It is quite similar to Aldebaran, actually, just a little bit hotter and about twice as far away.

Zeta Tauri marks the tip of the southern horn. It is a bright B-type giant star with a sun-like companion orbiting with a period of about 1/3 of a year (this is an eclipsing binary system). This is a very odd rapidly rotating star, with a turbulent expanding atmosphere and a lot of glowing gas surrounding the system, apparently thrown off of the star. Another interesting but faint star in this constellation is T Tauri, a newly formed star with a very strong stellar wind, affecting the surrounding clouds of gas and dust (the nebula NGC 1555 is probably lit up by the outflow of T Tauri).

The most famous deep-sky object in Taurus (omitting for now the two clusters, Pleaides and Hyades) is M1, the Crab Nebula, the most notable supernova remnant, an expanding, glowing cloud of debris from an explosion first witnessed on Earth in the year 1054 AD. This cloud is located roughly along a line connecting the tips of the horns (Alnath and Zeta Tauri), maybe about 10% of the way along that line from Zeta Tauri, and it is about 6000 light years distant. It was the initial confusion of this glowing cloud with possible comets that prompted Messier to begin his famous catalog.

Since this is pretty faint, you won't see it too well in detail with the naked eye without a 10-inch or larger telescope (you can only make out the oval glow most of the time depending on conditions with smaller scopes). High resolution photographs over the years have shown the expansion of this nebula, and astronomers used that to work backwards to the date of the beginning of the expansion. That's when they made the connection with the supernova observed by the Chinese and (perhaps by) some Native Americans (but no records from the Europeans, strangely, even though it was as bright as the full moon for many days after first becoming visible). Perhaps religious prejudices of the time forced medieval historians to ignore it (suggests Burnham)?

To radio astronomers, the Crab Nebula is the 4th brightest "star" in the sky due to emission from excited electrons deep within the nebula. At the center is the famous Crab Pulsar, a neutron star rotating about 30 times per second. Each time one of its poles points toward us, we are swept with a beam of energy much like observers of a lighthouse. Another older supernova remnant is also nearby, known as S147, it is a very wide, faint remnant only visible on large angle deep images.

Perhaps the most beautiful open cluster in the sky is the Pleiades. As Tennyson wrote, "Many a night I saw the Pleiads, rising thro' the mellow shade, glitter like a swarm of butterflies tangled in a silver braid." This is also known as the Seven Sisters in many cultures, and it is Messier 45. One of the reasons it is so easily visible in addition to its proximity (about 440 light years) is that it is very young, so there are several hot blue stars with short lifetimes that are still burning very brightly.

Most of the 1000 or more stars in the cluster are not visible to the naked eye or even with a small telescope since they are so intrinsically faint. In deep images, it would be impossible to tell just by looking which stars in the frame are part of the cluster's membership and which are just foreground or background stars. Fortunately, we have other bits of information. The easiest way is to measure the proper motion of all the stars in the frame.

The stars in the Pleiades cluster have some random motions about the center of the cluster, but the cluster as a whole is moving in a common direction, roughly in the direction on the sky toward Orion's bright star Betelgeuse. It covers half a degree (the width of the full moon) about every 30,000 years. We can also estimate distances to these stars and only include as members stars whose distances are very close to the mean cluster distance, but that's a little tougher to determine in some cases than proper motion.

The blue nebulosity of the cluster is actually not part of the cluster. Any reflective dust that would have remained from the giant molecular cloud that initially collapsed and fragmented to form this cluster would have been blown away by the strong stellar winds of these young stars. Instead, we think the cluster is currently moving through a thick area of dust within the interstellar medium of the galaxy.

In Greek mythology, the Pleiades are the daughters of Atlas and Pleione. Their names (shown in this image) are Sterope, Merope, Electra, Maia, Taygeta, Celaeno and Alcyone. They are sometimes seen as a flock of doves or a mother hen with several chicks. The cluster is very important in Aztec and Mayan mythology. During the time of year when it culminates (crosses the meridian) at midnight, legend had it that marked a time on the calendar when the world might end. The Pyramid of the Sun and the surrounding city of Teotihuacan in Mexico is all aligned in such a way that the East-West roads and temple faces are lined up with the rising and setting of the Pleiades at this time of year.

Among some American Indian tribes (the Kiowa and the Cheyenne), the Pleiades were seven sisters who fled from several giant bears they stumbled across in the wilderness. They sought refuge on a tall rock, and the great spirit lifted the rock up high and put the sisters into the heavens to protect them. The bear claw marks on the tall rock became Devil's Tower in Northeastern Wyoming, the natural wonder made famous in the movie "Close Encounters of the Third Kind."

There are various legends of a "Lost Pleiad", as though one of the stars was visible at some point in the past and then faded from view, but this could be said of at least four of the seven stars for various reasons (either because they are variable or due to the nebulosity surrounding them or due to close visual companions that may vary). There are no definitive answers here in any of my sources (I typically rely on Burnham and Allen).

Like Aldebaran, the Pleiades is occasionally favorably positioned with respect to the Moon's orbit that it can be occulted. On Fri Dec 21 (tomorrow!), the nearly full moon is going to plow right through the middle of the Pleiades cluster. Unfortunately, this will occur in the middle of the afternoon for observers in the US. It should be easily visible at this time for most of Europe, however. Maybe we'll see some pictures, though it is tough to artifically dim the moon properly so that the background cluster can be easily seen. We'll get a near occultation in the US next month, on Fri Jan 18 between about 1am and 3am low in the Western sky before the Waxing Gibbous moon sets.

The other interesting cluster in Taurus, the Hyades, forms a beautiful background behind the relatively nearby bright star Aldebaran. The brighter stars in the Hyades help form the characteristic V shape of the face of Taurus. The Hyades is the nearest open cluster in the galaxy to us, only 150 light years away, and that's one reason it is so hard to see on images. It is just so spread out. Compare it to the Pleiades, which is about three times further away. Both are somewhat similar, but the Pleiades is much easier to pick out because it is at just the right distance so we can still see it, but it is compact enough in an angular sense to be a cluster.

Since this cluster is close, all of the stars moving in a common direction don't quite move parallel to one another. Instead, they move along nearly parallel, converging lines toward a common point in space. Since we can measure the radial velocity and the tangential velocity, with a little clever trigonometry, we can deduce the distance of the cluster from the Earth. This technique is called moving cluster parallax.

One reason this cluster is so important: it is close enough for us to see the very faint, low mass stars. We like to think of clusters like the Hyades as somewhat representative of the way stars form as a whole. It helps our understanding of the galaxy to know how many sun-like stars form for every high mass Sirius-type star. How many faint dwarf stars form for every sun-like star that forms? This distribution of stellar masses that forms is known as the mass function. Only in nearby clusters can we truly count all of the stars of various masses (and luminosities), so what we see in the Hyades, we deduce is also true for much more distant clusters in which we can only see the most massive, luminous members. It is for reasons like this that the Hyades cluster is studied so closely.

Unlike the Pleiades, the Hyades has no hot, bright B-stars (all of the bright members of the Pleiades are B stars). That's because this cluster has been around about 5-6 times longer than the Pleiades. Presumably, the Hyades formed with its fair share of blue stars and once resembled the Pleiades, but by now, all of those stars have used up their Hydrogen fuel and evolved off the main sequence, probably blowing up as supernovae. All that's left in this cluster are the less massive stars that haven't burned up all of their fuel yet. Even with that said, this is one of the younger clusters in the galaxy.

The Hyades is also important as a part of the distance ladder. Because we can measure the distance to this cluster directly via parallax, we can directly figure the instrinsic luminosity of its main sequence (the group of stars that is in the main hydrogen burning part of its lifetime, occupying a strip on the H-R diagram). By measuring the apparent luminosity of the main sequence of other clusters, we can use the differences to figure distances. For example, the main sequence of the Pleiades has an apparent luminosity about 7.5 times fainter than the main sequence of the Hyades, so the Pleiades is about 2.75 times further away. Since we know the exact distance to the Hyades, we can know the exact distance to the Pleiades (instead of just a multiple of the Hyades distance). This technique is known as main sequence fitting.

The Hyades in mythology represent the daughters of the Greek god Atlas and Aethra, half-sisters of the Pleiades. This cluster represents rain and storms, perhaps because in the mythology, the sisters were placed in the sky while still mourning the death of their brother. Also, when this cluster is low on the horizon at sunset or sunrise, that tends to correspond with the rainy season in many parts of Europe. The individual names of the stars have been confused by many different sources, perhaps because the cluster is not as visually well-defined as the Pleiades. Among Native American cultures, the Hyades are considered to be the husbands of the Pleiades, always chasing their wives across the sky.

Posted by Observer at December 20, 2007 03:21 PM

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

Ah, one of mt favorite constellations. It is my astrological sign to if you care about that sort of thing, which I confess I don't. :)

Posted by: Liz on December 21, 2007 12:27 PM

I'd love to see software or a web page that shows the current layout of the solar system from an "above" POV. Say, put Earth on Jan 1st at 12 o'clock, and on July 1st it'd be at 6 o'clock.

Where is Mars in relation to the Earth today, or next summer? etc.

It'd be a kick-ass screensaver.

Posted by: Humbaba on December 21, 2007 12:46 PM

There's a lot of planetarium simulation software out there that does this sort of thing. Some of it may even be freeware. A lot of it is bundled with college textbooks. All of them give you the ability to observe from pretty much anywhere in (or in some cases outside) the solar system. Then you can set the time step for whatever you want, have the planets' paths visible, etc.

Posted by: Observer on December 21, 2007 01:50 PM

If you come across a web-based one or a Mac freeware one, let me know.

Posted by: Humbaba on December 21, 2007 05:06 PM

I'm pretty sure the freeware ("trial") version of Voyager III does that, and it runs on a Mac.

Posted by: Feff on December 26, 2007 10:32 AM

Thanks! I downloaded the trial version of Voyager 4.

If Earth is at noon today, then:

Mercury is at 5pm
Venus is at 9pm
Mars is at noon
Jupiter is at 6pm
Saturn is at 10pm
Uranus is at 3:30pm
Neptune is about 4pm
Pluto is about 6pm

and I now have a better feel for the distribution of asteroids.


Posted by: Humbaba on December 28, 2007 09:10 AM

BTW, epsilon Tau, one of the four red giants in the Hyades, has a planet, published last spring in the ApJ, IIRC. So far it's the only planet known in that cluster. We know Just About All There Is To Know about the Hyades giants. So, although most of the known exoplanets are orbiting solar-mass stars (or sub-solar mass), there's a clear case that planets can exist around 2+ solar mass stars, too.

Posted by: Feff on December 28, 2007 05:27 PM