Why are the asteroid trails curved? As the Hubble Space Telescope orbits around the Earth, and the Earth moves around the Sun, the asteroid will appear to move with respect to the vastly more distant background stars, due to an effect called parallax. It is somewhat similar to the effect you see from a moving car, in which trees by the side of the road appear to be moving much more rapidly than background objects at much larger distances. If the Hubble exposure were a continuous one, the asteroid trail would appear like a continuous wavy line. However, the exposure with Hubble's camera was actually broken up into more than a dozen separate exposures. After each exposure, the camera's shutter was closed while the image was transferred from the electronic detector into the camera's computer memory; this accounts for the many interruptions in the asteroid's trail.

Animation of the asteroid path over time. The asteroid has been colorized red, a truer representation of its real color based on measured intensities. (Credit: L. Frattare STScI)

More About the Sagittarius Dwarf Galaxy
by Dr. Yazan Momany (University of Padua, Italy)

This dwarf irregular galaxy was observed by the Advanced Camera for Surveys on-board the Hubble Space Telescope in August 2003. This program marks an international collaboration between Italian (Yazan Momany, Enrico V. Held, and Marco Gullieuszik), E.S.O. (Ivo Saviane and Luigi Bedin), U.S.A. (Michael Rich, Luca Rizzi), and Dutch (Konrad Kuijken) institutions.

"One characteristic of SagDIG of particular interest to astronomers is the fact that it is among the most metal-poor dwarf galaxies known. Being metal-poor simply means that the material (gas and dust) from which stars are yet to be made has not been as enriched in elements heavier than hydrogen and helium by the ejecta of earlier generation of stars. For comparison, the matter that formed our Sun almost five billion years ago has a higher proportion of heavy elements than that now present in SagDIG.

The interest in low-luminosity, dwarf galaxies derives from the fact that these galaxies are a dominant population of present-day universe and galaxy clusters. Dwarf galaxies are thought to be the first objects to have formed, that later contributed (through merger events) to the assembling of larger systems. Dwarf galaxies may hold the key for a deeper understanding of the formation and evolution of bigger, Milky Way-like galaxies.

Typically, all dwarf irregular galaxies (dIrrs) studied so far began forming stars some 10 billion years ago. Following this first star-forming episode, dIrrs show a low global star formation rate throughout the galaxy, and enhanced star formation localized in ~100 parsec complexes, with typical lifetimes of ~100 million years. Indeed, dwarf irregular galaxies owe their "irregularity" to the way these star-forming complexes are distributed: all over the face of the galaxy and rarely showing ordered patterns as in giant spiral galaxies like our Milky Way.

In order to understand the mechanisms by which the interstellar gas gets enriched, it is important to understand when the galaxies started forming its first stars, and ascertain the presence of old (~10 billion year old) stars in SagDIG. If truly old stars are present, astronomers can conclude that a very metal-poor chemical composition, typical of newly born galaxies, can be observed also in galaxies that formed a long time ago.

"One of the main features of this galaxy is the relation between the stars (age and distribution) and the surrounding gas (from which they are made). To this end, we came up with a movie showing the footprint of the ACS camera on a HI [ionized hydrogen] gas map. On this map, we plot the distribution of stars in different age intervals. It is very instructive. Moreover, it could not have been done without the HST/ACS deep photometry." - Yazan Momany (University of Padua)

Analyzing the HST data from ACS, our team has been able to unveil the presence of a very old stellar component in SagDIG. This result, added to a confirmation of extremely low-metal content of SagDIG, points in favor of scenario where a low-metallicity does not necessarily imply a young age.

The second important result is that concerning the spatial distribution of stars (in different age intervals) in comparison with the distribution of the neutral hydrogen in SagDIG, studied a few years ago by Dr. Lisa Young (NM, U.S.A) and collaborators (see the gif movie in Netscape). We see clearly that the youngest stars are located near the major peaks of emission on the HI shell, whereas relatively older stars define a more extended halo or disk. In particular, the spatial distribution of stars with ages between 30 and 60 Million years indicate that the most recent star formation episode occurred in a small region located between the three densest gas clumps, somehow offset from the central hole seen in the distribution of the gas. As stars become older, their distribution shows a lower central concentration. Interestingly, the movie shows that stars of less than ~1 Billion years old seem to surround a zone devoid of stars (a hole), that is offset from the HI central hole. Overall, star formation seems to proceed in a stochastic manner. Yet, a hint of "tails" and spiral structure in the distribution of stars younger than 500 million years might be the signature of star formation on the rim of the gas shell. "