When I look up at the night sky, I’m fascinated by the one very prominent thing that we all can see but we need to look to be certain. I’m talking about the glowing band of stars that is our home, The Milky Way, arching dramatically overhead. The Greek scientists like Galileo observed ‘the milk’ of the Milky Way as a band of infinite individual stars. It took another thousand years for the scientists to prove that Milky Way is one of the few billions of the galaxies in the universe.
The scientists also discovered that various galaxies had different sizes; some were giants and some were dwarfs. Why this difference in sizes? Recent discoveries showed that Milky Way is not simply one big galaxy but has engulfed many dwarf galaxies over time thus integrating their stars into itself. At least 20–25 dwarf galaxies, so ranging from one-hundredth to one millionth of itself, are orbiting around the Milky Way. There are many satellite galaxies that existed earlier which have been drawn into our galaxy and absorbed long ago. This ingestion of dwarf galaxies began billions of years ago when our galaxy was younger and smaller than now and is happening even to this date and slowly the current satellite galaxies may eventually be swallowed up.
Long after their death, these swallowed galaxies leave traces in form of faint stream of stars also called stellar streams that stretch across the sky. Over last two decades a relatively new field called galactic archaeology has helped the scientists observe and study these streams. This has helped the scientists study spiral galaxies such as our milky way in a more comprehensive manner than before.
This field has helped us to know how our galaxy got bigger part by part by adding mass in huge parts. Although the mystery still continues, we would pretty much be close to learning the biography of the Milky Way.
I wouldn’t like to go too deep into the specifics of this study but I’ll explain this in brief. The theory of galaxy formation suggests that the main force for Galactic expansion is not the main matter or the baryonic matter that even we are composed of. It’s the vast dark matter that agglomerates and expands and forces the bigger galaxies to move towards the smaller ones and in time prey on them and devour them. Each galaxy’s dark matter halo is much more massive and effective than its baryonic matter although we cannot really understand how dark matter works; but the secret of expansion lies with the interaction of baryonic matter due to gravitational activities in the halos.
The Milky Way includes a vast sphere of halo surrounding the bulge and the spiral of many diffused stars. These stars possibly also can be a part of the stellar streams formed during the gulping of the other dwarf galaxies. As a dwarf galaxy gets closer to the Milky Way it feels the gravitational pull of the core of the big galaxy. This pull enhances a slightly more gravitational force over the baryonic matter that is closer to the big galaxy than the one at the farther side. As a result the galaxy gets stretched along the line by the galactic tidal forces that work pretty much the same way as the moon affects the sea; but the galactic tidal forces have the strength to actually pull matter and in this way stars get pulled from the dwarf galaxy and hence form the stellar streams. This is the diffusion effect. Although the scientists will take at least a century to determine the activity reports such as how often does our galaxy prey on dwarf galaxies, how long does it take for the incorporation of its stars, etc. studying the star streams that are much fainter and almost defunct would help in the understanding of the underlying facts.
Initially when scientists first observed these stellar stream they got interested and started hunting for it like the archeologists usually do when they have to hunt for fossils. Actually I would like to call them galactic archeologists rather than space scientists because that is specifically their job. A decade ago these ‘archeologists’ had mounds of data to analyze, a database of more than 80 million stars within the milky way along with their detailed self-characteristics like its nature, color, etc. which was the perfect dig-site to begin their analysis of the history of the Milky Way.
The first evidence of an extended stellar stream came in 2003 from our galaxies own ‘dearest’ (:P) neighbor galaxy Sagittarius which emanated giant tails in infra-red light. As a matter of fact, the scientists caught the Milky Way in the act of attacking it’s so called ‘dearest’ neighbor. From the Sagittarius’ tails it was discoverable that the stars were in the process of being eaten for about three to four billion years! Since then scientists moved on to find more than a dozen of such streams hanging out in the sky (although they were pretty faint). Many more streams should be out there too…