Fields of Interest

Note : Terms and definitions are quoted from Wikipedia,The Free Encyclopedia.

Stellar Population

Open Clusters

Open Clusters are young, metal rich and weekly bound stellar systems. An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud. They have formed and resided in the galactic disk. So their physical properties give us important implications about disk formation and evolution. Open clusters have been doing an important role of a basic yard stick for measuring the astronomical distance scale. Due to their close distances, their distance can be determined directly by using parallax or "moving cluster method". Accurate knowledge of open cluster distances is vital for calibrating the period-luminosity relationship shown by variable stars such as cepheid and RR Lyrae stars, which allows them to be used as standard candles. These luminous stars can be detected at great distances, and are then used to extend the distance scale to nearby galaxies in the Local Group.

Globular Clusters

Globular Clusters are the oldest, metal poor and tight and strongly bound stellar systems. Most of them are located in the galactic halo - far outside from the galactic center. By tracing their age and metallicity , we might have important information describing the conditions of the galaxy at the time of its birth. Globular Clusters are fairly numerous; there are about 150 currently known globuar clusters in the Milky Way (with perhaps 10~20 more undiscovered), and larger galaxies such as M31 tend to have more (as many as 500). Some giant elliptical galaxies, such as M87, may have as many as 10,000 globular clusters. In many galaxies (especially massive elliptical galaxies) there appear to be two populations of globular clusters, which appear to be of similar ages (nearly as old as the universe itself) but of different metal abundances. These subpopulations are generally known as "metal-poor" and "metal-rich", although the metalicities of the metal-rich clusters are generally less than that of the Sun. Many scenarios have been suggested to explain these subpopulations, including violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In our Milky Way, the metal-poor clusters are associated with the Halo and the metal-rich clusters with the Bulge.

RR Lyrae Variables

RR Lyrae are pulsating Horizental branch stars, with a mass of around half the Sun's. RR Lyrae stars are old, relatively low mass stars. Therefore, they are much more common than Cepheids, but less luminous. The average absolute magnitude of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun. Their period is shorter, typically less than one day, and sometimes down to 8 or 9 hours. There are three main types of RR Lyrae stars : RRab, RRc and RRd. The RRc stars are of shorter period and the RRd stars are double mode pulsators. The relationship between an RR Lyrae's variability period and absolute magnitude makes them good standard candles for relatively near objects, especially within the Milky Way. In addition to this, their minimum light colors reveal the line of sight extinction, and their pulsation properties (e.g. amplitudes and periods) are can be used to obtain their metalicities.

Blue Stragglers

Blue Stragglers are stars in open or globular clusters that are hotter and bluer than other cluster stars having the same lumnosity. Thus, they are separatefrom other stars on the cluster's HR digram. Their formation mechanism is not yet clearly knwon, but the leading hypothesis is that they are current or former binary stars that are in the process of merging or have already done so. The merger of two normal stars would create a single star with larger mass, making it hotter and more luminous than stars of a similar age. The resulting star would have more hydrogen in its core making it behave like a much younger star. On way to test this hypothesis is to study the pulsation of variable blue straggler stars. The astroseismological properties of merged stars may be measurably different from those of normal pulsating variables of similar mass and luminosity. However,the measurement of pulsations is very difficult, given the scarcity of variable blue stragglers, the small photometric amplitudes of their pulsations, and the crowded fields the stars are often found in.

Galaxy Formation & Evolution

How do galaxies form and evolve?

This is a huge question that we try to answer using various ways. One of the way to answer this question is that we investigate each galaxy and try to find out it's star formation history, chemical evolution, merging and morphological evolution as a function of mass redshift and environment.

The Astronomical Distance Scale