February 25, 2008

Leo Minor

Under the hind paws of Ursa Major and just north of the sickle in Leo the Lion, you can find Leo Minor, a constellation introduced by Johannes Hevelius in 1687. It has no mythology associated with it, and this small area of the sky is basically neglected or treated as a minor feature in a larger pattern in most ancient star mythologies.

According to several sources, the brightest star in this constellation doesn't have an alpha designation due to an oversight. It is 46 Leo Minoris, also known as Praecipua, which translates to "chief" since it is the brightest star in the constellation. Being a more modern constellation, the star name is more on the practical than the mythological side. This is an orange giant with a somewhat cooler outer temperature than the Sun, though its core is much hotter, fusing helium into carbon in an advanced evolutionary stage.

Beta Leonis Minoris is a very close binary system, containing a yellow giant star of about eight solar masses and a companion main sequence star similar to our Sun only a little bit hotter and more massive. A closely watched star for variable star observers is the long-period (Mira-type) variable known as R Leonis Minoris, the brightness of which varies by 7 magnitudes (a factor of about 600) over the course of about a year. These pulsations are not very well understood, though there are many theories.

One way Astronomers try to unravel the puzzle is by examining the spectrum of the star at each step of the pulsation cycle to learn about the temperature, density and other properties of each layer we can see in the extended atmosphere. Since all of the spectral lines from each layer are overlapping each other on the star's spectrum, though, there is still uncertainty about the interpretation. My guess is that when we have the angular resolution to measure the center of such a star and the limb of the star (it would have to be fairly nearby) separately, it will be easier to distinguish between these layers, and that's still some years off.

Another star of note is 20 Leonis Minoris, a sunlike star less than 50 light years from Earth with a dim red dwarf companion star. This star has a fairly high metallicity and also a relatively large proper motion compared to stars at similar distances, so it likely comes from a population mixed into the Milky Way sometime in the past, perhaps part of a tidal tail of stars from a dwarf galaxy devoured in the distant past.

There are no deep sky objects in Leo Minor that are easily accessible to small telescopes, but there is still plenty to look at courtesy of some of the larger telescopes used by professional astronomers. Starting at the front (West) part of the constellation and working back toward the rear as we did with Leo, we first come to is NGC 2859, a ring-shaped spiral galaxy with an image here taken from the Sloan Digital Sky Survey.

This galaxy is about ten times furthest away than the nearest large spiral, Andromeda, and so only visible as a faint blob in a small telescope. The ring (perhaps caused by the presence of the central bar or perhaps the result of a recent interaction with another smaller galaxy) only comes out in larger telescopes. This is a good test for astronomical imaging because there is such a wide range of surface brightnesses in this small object.

Next up, just a degree or so east of Beta LMi is NGC 3294, seen in a very nice image. Another couple of degrees to the East is the distorted galaxy NGC 3432 which has a couple of different tidal features indicating interaction with some nearby galaxy. This is also mentioned in Halton Arp's catalog of peculiar galaxies, a catalog he put together to study galaxy evolution, but he ultimately used this to support his hypothesis about redshifts.

In standard cosmology, redshift is a measure of distance due to the expansion of the Universe. The greater the spectral line shift of a galaxy, the faster it is moving away from us and so the further away it is. According to Arp, though, redshifts were caused by something else, perhaps material ejected from the cores of active galaxies. To support his hypothesis, Arp pointed to several images of galaxies apparently interacting but found at dramatically different redshifts. As our observational capabilities improved, the data generally support the standard explanation for redshifts, but the Arp catalog is still a great guide for amateur observers to interesting interacting (and fairly bright) galaxies.

Next up, a few galaxies in the Southeast corner of the constellation, within a triangle formed by 46 LMi, Algeiba and Zosma (both of the latter in Leo). NGC 3344 is the southern most of the set, a ghostly face-on spiral, the nicest image I've found in this constellation. Also nearby is NGC 3486, another face-on spiral. In the image linked, you can see lots of reddish dots sprinkled throughout the disk. Those are H II regions, which are regions of ionized Hydrogen gas surrounding young bright stars or star clusters similar to the Orion Nebula region. Notice, too, the small bar and ring at the center of this spiral.

Closer to 45 LMi, maybe a half a degree or a degree to the South, is the interacting galaxy pair NGC 3395 and NGC 3396, another pair of interacting galaxies from Arp's catalog. Not as spectacular as some that we saw in Leo itself, but it is still a fairly easy target for an 8" or 10" telescope in good conditions. Going much deeper with the Hubble Space Telescope also reveals a more distant galaxy cluster which is causing gravitational lensing in a more distant quasar. The discovery of such lensing, in which objects at lower redshift act like lenses to bend the light of objects at higher redshifts, was one of the definitive tests showing the standard interpretation of redshifts is likely the proper one.

Posted by Observer at February 25, 2008 07:06 PM

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