May 06, 2008

Virgo

Just North of the Corvus/Crater/Hydra combination is the second largest constellation in the sky, Virgo. Many cultures share a similar story of this constellation as a maiden or virgin carrying a stalk of wheat (indicating by Alpha Virginis, or Spica, which translates to ear of wheat). Even in Greek mythology, this constellation sometimes represents Persephone, who was lured into the underworld by her uncle, the God Hades, where she ate the seeds of a pomegranite and was eventually forced to spend half of her life there as a princess after some negotiation by her father Zeus (this is related to the seasonal cycle).

Other stories identify her as Callisto, who also figures prominently in the legends of Ursa Major and Ursa Minor. Or perhaps she is Astraea the goddess of justice (which is fitting given the nearby Libra the Scales in the sky), who ruled a benevolent age of the Earth until the ways of mankind became so violent and upsetting that she retreated back to the heavens. The story that seems to have passed the test of time is that of the winged harvest goddess holding the ear of wheat.

The bright star Spica is surely the easiest to spot in Virgo, found most easily by following the arc of the handle of the Big Dipper through Arcturus and on to Spica. Spica is very close to the ecliptic plane, so the moon and the planets frequently pass very close to it, and the moon even occults Spica occasionally. Spica is a closely-paired system of two hot blue stars, the combined light of which is over 2000 times the luminosity of the Sun, making it a bright 1st magnitude star even at 260 light years away (30 times further than Sirius).

While these two stars do not eclipse each other from our perspective, the light curve does vary a bit as the stars orbit. That's because the two stars are so close together that their shapes are tidally distorted, so sometimes we see them elongated and sometimes end-on, making the total light from the system vary. Kaler reports that lunar occultation studies confirm the presence of more than two stars in this system, perhaps as many as three more dim companions, but they are very hard to study individually due to the intense amount of light given off by the central binary.

Spica was important to the Egyptians, and it was identified with their goddess of love Hathor. A temple was built at Thebes in such a way as to align with the rising of Spica on the autumnal equinox, when it was time for the harvest, but over time, the precession of the Earth has caused that alignment to shift noticeably. This is an example from the field of archeoastronomy, and it provided some of the first evidence that precession means the celestial sphere is not fixed.

About 20 degrees Northwest of Spica, a bit less than halfway along a line drawn from Spica to the tail of Leo the Lion, Denebola, is the 2nd brightest star in this constellation (though it is designated Gamma Virginis), named Porrima after the goddess of prophecy. Porrima makes up the vertex of a right-angled cup that extends about 20 degrees North and 20 degrees West, opening up toward Leo (see the star chart here for visual help).

Between this "cup" and Leo, you will find all of the Messier objects in Virgo except for M 104 (the famous Sombrero Galaxy). This is no coincidence as the nearby Virgo cluster of galaxies is located in this direction, so this an extremely rich field for extragalactic astronomy. The stars in the cup are all five in the neighborhood of 3rd or 4th magnitude. Porrima is actually a double star, with the two virtually identical stars separated by a very small angle, currently about 3 arcseconds apart (an arcsecond is the diameter of a stellar image in a telescope if the viewing conditions are excellent). The two stars are both on the main sequence and only a little more massive and hotter than the Sun, about 40 light years away.

Kaler points out that stars like Porrima mark an interesting transition point for stellar behavior. First of all, at about this mass, the nuclear fusion process in the core becomes more efficient (using Carbon, Nitrogen and Oxygen as a facilitator instead of purely the proton-proton chain), so the luminosity increases a bit faster as mass increases. Also, at this mass and higher, stars begin to transition away from having large convection zones in their outer layers like the Sun. Instead, their interiors are dominated largely by their radiative zones (at the extreme, very hot stars have no convection and are ionized all the way through, meaning spectral absorption lines are very weak or non-existant).

Convection helps a star generate a strong magnetic field, which in turn helps slow down the star's rotation. When the field interacts with the gas and dust that usually exists in a surrounding disk after the star forms, this acts as a brake on the star's rotation. For more purely radiative stars, this braking action doesn't exist and so they can become very fast rotators. Remember Regulus in Leo? Or Gamma Cassiopeia? There are plenty of hot blue stars in the sky that are very fast rotators thanks to this.

Porrima probably should've been named Zavijava, which is the name given to Beta Virginis, at the Western tip of the cup, 15 degrees due South from Denebola in Leo. Zavijava translates to "the angle of the dog kennel" and was originally the name given to what is now Porrima, but confusion over the years has changed our conventions on this and so we're stuck with Zavijava at the end of the line rather than at the true vertex of the angle.

Zavijava is very similar to either of the two stars in the double-star system Porrima that we discussed previously, and it is at about the same distance, 36 light years away. About halfway between Porrima and Zavijava on the Western arm of the cup is Eta Virginis, or Zaniah. Zaniah is almost exactly on the Celestial Equator, which splits the constellation Virgo into a Northern and Southern half, and one of the two intersections of the Celestial Equator and the Ecliptic is located halfway between Zaniah and Zavijava: the autumnal equinox. This is the location in the sky of the Sun on the first day of Autumn. Recall that Virgo is usually associated with harvest time because the Sun has historically been near this constellation at this time of year. This is likely also why Persephone (who spent Fall and Winter in the Underworld, then Spring and Summer in freedom) is associated with this constellation.

Zaniah is a very close triple of hot blue stars. They are so close (and the system so distant at 250 light years) that it is very difficult to separate the light from each component to study them all individually, though this was recently accomplished using optical interferometry. The name originates from another name for this angle of stars, the mouth of the barking dog or perhaps the kennel of the barking dogs (al zawiah translates to kennel). Moving North from the vertex at Porrima, about six degrees NNE, we run into Delta Virginis, which some have named Awwa or Auva for "angle" or "turn", so now you can see where Zavijava got its name, from a mish-mash of al Zawiah and Awwa. Awwa is a cool M-class red giant star about 200 light years away.

Finally, at the Northern tip of the cup, about 12 degrees North (and 1 degree West) from Porrima, we find the third brightest star in Virgo, for some reason designated Epsilon Virginis: Vindemiatrix. This name translates to the grape gatherer, as this star rises just before dawn during the grape harvest. The star is a yellow giant, about 100 light years distant, very similar to one of the components of the bright double star Capella in Auriga. Wikipedia notes that this star shares some characteristics (distance, motion or composition, not sure which) with the stars of the Hyades cluster in Taurus, but I have not been able to find a source to back up this claim.

It seems unlikely to me as the Hyades is on nearly the opposite side of the sky, and I'm certain we are not completely enveloped by the Hyades cluster so that we would have members all over the sky. The cluster is about 150 light years away and estimated to be less than 100 light years in diameter with most stars being in an inner 10 light year diameter sphere. Just goes to show that while Wikipedia can be very useful, it is important to read what's there with a skeptical eye and always look for credible backup sources if it is an important topic.

Moving on, I will now talk about some of the eleven Messier objects in the constellation Virgo. I suppose one way to do it systematically is to move from the Southernmost and work North toward the center of the Virgo cluster, so I'll start with Messier 104, also known as the Sombrero Galaxy. This is an outlier but generally accepted as a member of the cluster. The Virgo Cluster of galaxies is found mostly in a square whose opposite corners are Spica and Denebola (the tail of Leo). The central concentration is about halfway between Denebola and Vindemiatrix, a star I talked about previously.

The Virgo Cluster is a large group of galaxies (between 1000-2000) centered on a point about 60 million light years from us, and our own Local Group is a little bubble of 30-40 galaxies about 10-20 million light years in diameter that forms another part of the supercluster we share with Virgo. Most of the cluster galaxies are in the direction of Virgo, but some lie just across the border into Coma Berenices, a constellation I will talk about next time. Getting back to M104, this is a very nice nearly edge-on spiral (tilted about six degrees toward us) on the front side of the Virgo Cluster, about 30 million light years away from us. At 8th magnitude, it shows up nicely in a 8" or 10" telescope, which is ideal for most of the bright Messier objects I will talk about in this constellation.

In this deep image, you can see that it is a large, extended, low surface brightness halo with a couple of tidal tails extended from within the bright center. Like our own galaxy, the Sombrero has some satellite galaxies that it is slowly devouring, though this galaxy is only about half the visible diameter of our own Milky Way. The dust lane in the disk of this galaxy can be very clearly seen in this Infrared image taken by the Spitzer Space Telescope, and it doesn't look very disturbed.

Though we are pretty good at determining distances to galaxies these days, the distance to the Sombrero is still somewhat uncertain. I see quotes of distances anywhere from 28-50 million light years, though I tend to trust the lower end projections since they fit so many other aspects of the galaxy. The problem is that galaxies in the Virgo Cluster are a little bit too far away from our normal distance determination methods (using Cepheids or Planetary Nebulae or star clusters or bright stars) to work well. Normally, with an accurate value for Hubble's constant (which we have now thanks to observations by WMAP), we can use Hubble's Law to find the distance.

That's just d = v/H where v is the radial velocity of the galaxy and H is Hubble's Constant. The problem is that galaxies in Virgo have large peculiar velocities since they are in a cluster. They are swarming about the center of mass like a bunch of angry hornets, and this velocity adds a random component to their Hubble flow speeds. Ideally, a galaxy 60 million light years away should be receding from us at a speed of 1400 km/sec, but the peculiar velocities in Virgo can be as high as 2000 km/sec, overwhelming the "signal" we get from cosmological expansion. Since the cluster is close to us, this random component is very large compared to the cosmological expansion velocity, so that makes using Hubble's Law very tough. As our technology improves, so does our ability to pick out standard candles in these distant galaxies and improve our distance accuracy.

Now, a quick aside about dark matter. I mentioned that the peculiar velocities in this cluster are very high, on the order of 2000 km/sec. This fact and our knowledge that the cluster is bound together is another piece of evidence that tells us of the existence of dark matter. Here's how: when we look at galaxy clusters, we assume they are bound. That means all (or almost all) of the galaxies are gravitationally bound to their clusters. This is a good assumption because most galaxies exist in clusters, and we don't see clusters in varying degrees of dissipation in our Universe.

Compare the state of affairs for galaxy clusters with that of open clusters in our galaxy. Open clusters in our galaxy are NOT bound (due to the strength of tidal forces in the disk of the galaxy), and so we see them in varying degrees of concentration. Some very young ones are very tightly grouped together while most older clusters are very loosely bound, sometimes almost to the point where they are unrecognizable as clusters. With galaxy clusters, they are all pretty similar in terms of their concentration and velocity distribution, much like the globular clusters in our galactic halo, so we assume they are bound together.

The escape velocity of a cluster is like a speed limit. If a galaxy is moving faster than the escape velocity, then it will escape, and it is considered to be not bound to the cluster. The escape velocity depends on the mass and size of the cluster. The more massive the cluster, the harder it is to escape the cluster's gravity. So, if we count up the mass of all of the visible matter in Virgo, which means all of the stars, gas and dust, we can estimate the escape velocity of the Virgo Cluster. We get a number of about 1000 km/sec.

This is a problem, because if the escape velocity of Virgo were really that low while galaxy velocities are often as high as 2000 km/sec, then the cluster should be flying apart! The solution comes with dark matter: When we make the assumption that dark matter is 10 times more common than visible matter, like it is in our own galaxy, the escape velocity from Virgo grows to about 4000 km/sec, easily high enough to contain all of the "angry hornets" buzzing around the cluster's center of mass at 2000 km/sec or so.

There is also very hot gas bound to galaxy clusters, seen in X-ray images due to its high energy radiation. If we measure the velocity of such gas (which is related to the temperature), we also get a very high number. If we assume the hot gas is bound to the cluster, then you need a lot more mass than what you can see to contain that gas, and so the need for dark matter appears. Fritz Zwicky was the first to advance this argument for dark matter way back in the 1930's, but his ideas weren't generally accepted until more evidence came to light decades later. The problem is not so much that Zwicky was unfairly ignored, it's just that he had a lot of crazy ideas, many of which turned out to be wrong, and so when he claimed to discover dark matter, he wasn't taken as seriously as, say, Einstein or Hubble would have been.

Moving North about 15 degrees toward the center of the cluster, we move into the Messier Objects contained in Virgo's cup, starting with M61, a 10th magnitude galaxy very similar in properties to the Milky Way and located about 60 million light years away. This makes it one of the largest spirals in the Virgo Cluster. Moving North about four degrees brings us to the next Messier object, M49, one of the giant elliptical galaxies in the cluster with a major axis of 160,000 light years in length or possibly longer depending upon the angle of orientation with respect to us. It is at least five times more massive than our own galaxy, which is itself one of the largest galaxies in our own little cluster.

Going north another four degrees or so, we stumble across a string of galaxies known as Markarian's Chain, at the heart of the Virgo Cluster. In the linked image, that's roughly the same angular size as a standard photograph of the Pleiades. Most prominent on the Western end are two giant elliptical galaxies, M84 and M86. Proceeding East from there, we see a pair of connected galaxies that looks at first glance like a pair of eyes, known as Markarian's Eyes. They are also known as NGC 4438 and NGC 4435, and you can see in the linked image the dramatic effects of tidal forces on these two closely passing galaxies. A full moon would just barely cover the two ellipticals and the two eyes, to give you some idea of how separated these objects are in a line on the sky.

A degree or so Southeast of the Eyes, we find Virgo A, also known as M87, probably the largest galaxy in the Virgo Cluster, probably at least 20 times more massive than the Milky Way galaxy. In deep exposures, the surface brightness of the extended halo of this galaxy fills a region in the sky roughly equivalent to the Full Moon, and this is a galaxy that is 60 million light years away! At the heart of Virgo A is likely a supermassive black hole, from which is emanating an enormously powerful jet, a stream of plasma extended around 5000 light years from the core of the galaxy.

Another couple of degrees East from M87 is a cluster of five messier objects loosely arranged in a 180 degree arc of a circle that it about 3 degrees in diameter. The Southeastern part of the arc is anchored by the giant elliptical galaxy M60 and a close companion spiral galaxy known as NGC 4649 (seen better in this image). A little less than a degree West of M60 is M59, another large elliptical galaxy. To give you some idea of how big these big ellipticals are in Virgo, M59 has about 2000 globular clusters in orbit around it, over ten times more than our own Milky Way galaxy has. Each of these clusters contains anywhere from 50,000 to 10 million stars.

The last three Messier objects in the "cup" of Virgo formed by Vindemiatrix, Porrima and Zavijava about 25 degrees Northwest of Spica are M58, M89 and M90, three galaxies within about a degree of one another and about 1-2 degrees West of Virgo A. M58 is one of the few barred spiral galaxies in Messier's list, and it is also a rare spiral to be found so close to the directional center of Virgo. It is hard to say how far this is from the true center of Virgo. With a velocity about half that of the highest peculiar velocities, it is likely safely outside of the true core and so not subjected currently to the kind of tidal forces that can strip its gas and disrupt its structure.

M89 is another giant elliptical, almost perfectly round in shape. It isn't clear whether it is a true sphere or just an ellipsoid being viewed end-on from our perspective. Deep images of M89 show a little bit more asymmetry, including a jet-like feature similar to that seen in M87 (Virgo A). Most of the scruff surrounding the main body of this galaxy is probably tidally disrupted remnants of galaxies that have been devoured in the recent past.

M90 is another bright spiral with a two-phase structure. The inner disk is lumpy and clearly split into regions thick with dust and clumps of vigorous star formation. Then about a third of the way out from the center to the visible edge of the disk, it transforms into a very smooth appearance similar to that of a lenticular galaxy, which it may be evolving into. M90 also has a high enough peculiar velocity that it should be able to escape the Virgo Cluster. The peculiar velocity is so high that it is currently larger than the expansion velocity of the cluster as a whole and so the net radial motion is toward us, one of a very small fraction of galaxies in the sky that seems to "violate" Hubble's Law. It is only a technical violation, really, as I talked about previously.

Some other galaxies in this same little area that are not to be missed: NGC 4216 is a pretty nearly-edge-on spiral about 4 degrees West of Markarian's Chain. The pair NGC 4762 and NGC 4754 is also a very nice target a few degrees east of M60, at the eastern edge of the group of Messier objects I've been describing. Four degrees South from M58 is a large, bright face-on spiral NGC 4535. This spiral has been a good target for finding Cepheids and has helped the Hubble Space Telescope establish a proper distance to the Virgo Cluster.
Just half a degree South from NGC 4535 is another beautiful spiral, NGC 4526, reminiscent of the Sombrero Galaxy with its prominent dust lane and bright central region. This one is famous for hosting an elusive Type Ia supernova in 1994, a type of explosion that is an extremely important brightness standard used in distance determination.

Before I move out of the Virgo Cluster region of the constellation Virgo, I should point out a very nice sky chart of the "cup" of Virgo and the cluster galaxies within it. Now for a few more highlight galaxies in the Virgo Cluster. Next up is NGC 4527, a nearly edge-on spiral reminiscent of Andromeda but about 20 times further away on the near side of the cluster. Right next to it on the sky is NGC 4536, but distance estimates seem to put this one on the far side of the cluster, about 70 million light years away (a distance obtained thanks to a bright type Ia supernova, 1981b, which also made this a target for the Hubble Space Telescope years later). These two galaxies are so close together in the sky that they would fit within a full moon angular diameter, and they are found about 4 degrees Northwest of Porrima (the vertex of the cup), just across on the north side of the Celestial Equator.

Nearby, just a couple of degrees East of this pair, we find the giant elliptical galaxy NGC 4636, recently a target of the Chandra X-Ray observatory. The goal was to measure the concentration of dark matter in the galaxy by measuring the detailed properties of the hot gas in the galactic halo. Turns out the dark matter is very concentrated, not a diffuse extended halo as some theories might predict, and this rules out a few models (such as that dark matter can diffuse outward or puff up like a halo of stars in a globular cluster or a galaxy). NGC 4636 isn't much to see visually, just an ordinary elliptical, almost perfectly spherically symmetric (but the X-rays tell a different story).

To finish off the bright galaxies in the Virgo cluster (in this constellation, anyway), we need to look several degrees to the East of the main body, about 22 degrees due Northeast from Spica, and there we find the bright spiral galaxy NGC 5566, a barred spiral with a ring of stars around the center. It is imaged here with a couple of fainter companion galaxies. There are still a few galaxies left to cover, though, and they are in a little group just South of Spica and north of Gamma Hydrae. Their membership in the Virgo cluster is on the iffy side, but some are at the right distance.

NGC 5247 is the most prominent of this group, a face-on grand design spiral about 50 million light years away, and just a couple of degrees to the West is a tilted 11th magnitude spiral about 90 million light years away (probably not a member of the cluster) known as NGC 5054. South of these two is a nearby spiral NGC 5068 only about 20 million light years away but rather faint for this distance, indicating it is a small galaxy. Finally, a little further south is another small member of the group, NGC 5084, seen in this gallery of Virgo galaxies.

Over near the Eastern end of Virgo, following the Celestial Equator east from Porrima about 45 degrees and just a few degrees north is the faint star 109 Virginis, marking one of Virgo's feet. Right next to this star is the edge-on spiral galaxy NGC 5746. This is another galaxy that recently drew the attention of the Chandra X-Ray observatory. Chandra discovered a hot halo of gas surrounding this galaxy, which is a little unusual because most galaxies with hot halos have that gas due to vigorous star formation (and stellar explosions), but NGC 5746 shows a rather quiet history. Instead, the hot gas is actually gas from the intergalactic medium falling into this galaxy and heating up as it does so.

The final object I want to mention in Virgo is a frequently overlooked globular cluster, NGC 5634, just under 5 degrees due East from the bright star Syrma in the Southeast corner of Virgo near its border with Libra. Astronomers speculate that this globular cluster once belonged to the Sagittarius dwarf galaxy, a small irregular galaxy currently being tidally ripped apart thanks to repeated passages through the disk of the Milky Way galaxy. This cluster is located in the extended tidal stream of the dwarf galaxy and also has a similar population of stars (and similar composition) to the clusters associated with this dwarf.

Posted by Observer at May 6, 2008 04:33 PM
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