February 24, 2008

Leo

In mythology, near the Greek city of Nemea, there lived a lion who frequently came out of his cave to hunt down and kill the locals. As one of his 12 labors, Hercules was tasked with killing this lion, but it was impervious to ordinary weapons. So Hercules chased the lion into the cave and throttled it with his bare hands. Leo the lion has had a prominent place in the nighttime sky ever since those times. Leo had great importance to the Egyptians. The time of year when the Sun first entered Leo coincided with the rising of the Nile river most of the time, and the famous Sphinx may have the body of a lion for this reason.

Alpha Leonis is Regulus, which translates as "little king" and was named by Copernicus, in honor of the king of beasts. In Arabic, the name is Cor Leonis, which translates as "heart of the lion", and this is sometimes the name used in star charts. Regulus held great importance in many ancient civilizations, partly because it is a rather solitary bright star in its region of the sky (though it doesn't make the top 20 brightest stars in the whole sky) and also because it is found very close to the ecliptic plane in the sky. This means it is frequently occulted by the Moon (see image) and often approached very closely by the other planets.

Regulus was one of the four royal stars of Persia, the ancient guardians of the four corners of heaven. Each of the four stars (Regulus in Leo, Aldebaran in Taurus, Antares in Scorpius and Fomalhaut in Pisces Austrinis) is located along the ecliptic roughly 90 degrees apart in the sky, and so each is dominant in the sky during a particular season. Regulus is dominant during the Spring.

Leo is pretty easy to find thanks to the sickle asterism, with Regulus at its base. This looks like a backwards question mark in the sky and is formed by several bright stars in Leo, representing the mane of the lion. Regulus is a main sequence star, about 3.5 times the sun's mass and probably less than a billion years old. It is only about 80 light years away, which makes it fairly bright to us. It is also one of the few stars that is close enough and bright enough to enable us to measure its angular size and shape directly. The results published in 2006 were rather surprising!

As this picture indicates, Regulus is one of the fastest spinning main sequence stars known, completing its rotation about 16 hours (compare to our Sun, which takes a month). If it were rotating only 10 or 20 percent faster, it would literally fly apart. As it is, Regulus is highly oblate, so much so that the poles (closer to the core) are much hotter and brighter than the equatorial regions.

Regulus is actually a triple star system, with two small close-knit companions very far away, dwarf main sequence stars that are only fractions of the brightness of our Sun. A small telescope can spot the brighter of the two companions about 3 arcminutes away (one-tenth of a lunar diameter), but the fainter one is tough to see for small telescopes in part due to the brightness of nearby Regulus.

Moving on from Regulus (by the way, there is a nice image showing the Moon occulting Regulus in the year 1999) to the next brightest star in the constellation Leo, Denebola (Beta Leonis), from the Arabic for Lion's Tail. This star is very similar to the much brighter Sirius, just further away (about 40 light years).

Gamma Leonis is Algieba, from the Arabic for Lion's Mane, a fine double star consisting of two giant stars similar to the Sun in external temperature and color about 90 light years away. The velocity of this pair relative to the Sun is extremely high, nearly four times the average for stars in our vicinity, which leads us to believe it originated a couple of billion years ago from some different population of stars, perhaps from a merger between the Milky Way and some small dwarf galaxy.

The rest of the sickle going up from Regulus are Eta Leonis, Algieba, Adhafera, Ras Elased Borealis and Ras Elased Australis (sometimes called Algenubi). Eta is one of the pivotal stars in the outline of the constellation, but no traces of a proper name have been found in historical records. It is perhaps the most distant star in Leo from us, being a luminous supergiant about 2000 light years away.

Back to the other end of the lion, we find Zosma and Chertan, a couple of bright blue main sequence stars nearing the stage in their lives when they will run out of fuel and turn into red giants. Another useful star in this region is Rho Leonis, a hot blue star with a very smooth spectrum. Stars like this are so hot that most of the simple atoms in their atmospheres are ionized, which means a simple continuous spectrum with few absorption lines. That makes it a nice backdrop for studying the intervening gas and dust in the interstellar medium, because don't get ISM absorption lines mixed with with lines from the background star.

Another interesting faint star in Leo about halfway between Regulus and Subra (the forepaw of the lion) is R Leonis, one of the brightest and easiest to observe variable stars in the sky. This is a long period variable, changing its brightness by a factor of about 600 over the course of 312 days. R Leonis is a very luminous red giant with a loosely held atmosphere that pulsates gently over time in response to variations in the inner part of the envelope. CW Leonis is nearby, another giant star with a very interesting spectrum that reveals the presence of a surrounding shell rich with heavy elements and molecules such as water. We believe it is in the early stages of becoming a planetary nebula.

I also shouldn't fail to mention Wolf 359, the third closest star to the Earth at a mere 7.7 light years. The Alpha Centauri system and Barnard's Star are the only closer stars to our Sun. Being a very dim red dwarf star only about 10% of the mass of our Sun, this is way too faint for naked eye observations.

Now for some of the galaxies in Leo. I'll start near the head of the lion and work my way back to the two famous triples, so that means we begin with NGC 2903, a spiral galaxy about 20 million light years away from us very similar to our own Milky Way. Notice in the image the dramatic color difference between the central regions and the disk, caused by the different populations of stars. Typically, older, low mass stars occupy the central regions of galaxies where there is relatively little gas and dust while in the disk, the light is dominated by the rare luminous blue stars that are continuously forming from the abundant gas and dust present.

Near the back of the sickle are NGC 3190 and NGC 3227. NGC 3190 is the brightest of the least well-known galaxy triple in Leo, seen here as an elliptical on the left and two spirals, clearly warped by tidal interactions with one another, and we'll see even more spectacular examples of this later. This group is also known as Hickson 44 from a catalog of small galaxy clusters. This next image is a wider view and shows another galaxy (NGC 3227) all in the vicinity of the bright star Algieba.

In the torso of the Lion is NGC 3370, much tougher for small telescopes being five times further away than NGC 2903, it is quite stunning seen through the Hubble Space Telescope's camera. This is a key galaxy in the Hubble distance determination project because it hosted a particular type of supernova that is well-understood, and it is also close enough that we can pick out individual Cepheid variable stars, so we can use this to establish the distance to a supernova and then bootstrap our way out to more distant galaxies in which supernovae are seen.

Right next to Regulus (about a degree away) is one of the closest galaxies to our own, a little dwarf galaxy known as Leo I, sometimes called the Regulus Dwarf. This is part of our local group of galaxies and is actually a small satellite orbiting our own Milky Way at a distance just under 1 million light years (Andromeda is about 2.1 million light years away).

Now to perhaps the best known galaxy trio in Leo, about halfway between Alpha Leonis (Regulus) and Beta Leonis (Denebola), you will find three Messier objects and a few galaxies all bunched together. This is known as the M96 group and can be seen in wide angle in this image. M95 (seen more closely here) is in the lower right corner while M96 (closeup here) is at the bottom center of the image, both very nice spirals. Then in the upper left you see the M105 group, dominated by the large elliptical galaxy M105 and including a couple of fainter spirals, all of which are far enough away that the expansion of the Universe is carrying them away from us at a rate of about 700 miles per second.

The other triple is a little further on toward Denebola, about three-quarters of the way from Regulus, and it is known as the Leo Triplet or M 66 group, a set of galaxies about 30 million light years away. It contains the spiral galaxies M65 and M66, both very beautiful in deep images, elegantly warped by their mutual interaction. The third galaxy of the set is NGC 3628, which has a spectacular tidal tail seen here. This one is seen almost perfectly edge on and so has a nice dust lane right across the long axis of the image. Not only is the tidal tail of 3628 extremely long, but notice how the disk is puffed up like the hair on an angry cat from the dynamical heating of the interaction with the other two galaxies.

Another pair of interacting galaxies in this region is just a few degrees to the East, NGC 3607 and 3608, also known as Arp 87. What a spectacular collision these two have undergone, leaving behind a huge bridge of hot gas, dust and stars connecting the two enormous spiral galaxies. The edge-on spiral in the background of the linked photo is just that, a non-connected background object. These interactions between galaxies are actually quite common in the Universe because galaxies are so extended compared to the space between them (unlike the huge difference between stellar sizes and distances which make collisions of stars within a galaxy almost unheard of).

In fact, our basic idea of galaxy formation is that galaxies begin their lives as spirals typically, but after many interactions, they can lose their structure and much of their gas and dust, and this eventually turns them into starburst galaxies and subsequently into big piles of chaotically orbiting stars that we call elliptical galaxies or giant ellipticals. What we're seeing here are the beginnings of the kinds of larger clusters we see much further away.

There are lots of other fainter galaxies in Leo. I've only scratched the surface here and talked about the brightest ones and the neatest groups. There is enough here to keep an amateur with a decent-sized telescope busy for many nights.

Posted by Observer at February 24, 2008 12:20 AM
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