Galaxies like our Milky Way, are gravitationally bound systems composed of stars, interstellar gas, dust, stellar relics, and dark matter. Dark matter is thought to be composed of exotic non-atomic particles that do not interact with light or any other form of electromagnetic radiation, making it invisible. However, most astronomers think that it really exists in the Universe because it does interact gravitationally with objects that can be observed. Dark matter is a much more abundant form of matter than the “ordinary” atomic matter that composes the Universe that we are most familiar with.
The word galaxy itself is taken from the Greek galaxias, translated literally as “milky”. Galaxies can range in size from dwarfs that host only a few hundred million stars to galactic behemoths that contain an astounding one hundred trillion stellar inhabitants, each orbiting around its galaxy’s center of mass.
Relatively small, spherical, and tightly bound collections of stars termed globular clusters are among the most ancient objects in our Milky Way. The ages of individual stars in our Galaxy can be estimated by measuring the abundance of long-lived radioactive elements such as thorium-232 and uranium-238. Astronomers can then compare the results to estimates of their original abundance, by way of a technique termed nucleocosmochronology.
Several individual stars have been discovered in our Galaxy’s halo with ages measured very close to the 13.80-billion-year-old Universe. In 2007, a star inhabiting the Galactic halo, dubbed HE 1523-0901, was estimated to be approximately 13.2 billion years old. As the most ancient known object inhabiting our Milky Way at that time, this measurement placed a lower limit on our Galaxy’s age.
The age of stars dwelling in the Galactic thin disk was also estimated by astronomers using nucleocosmochronology. Measurements of stars inhabiting the thin disk indicate they were born approximately 8.8 billion years ago–give or take about 1.7 billion years. These measurements indicate that there was an interval of almost 5 billion years between the formation of the Galactic halo and the thin disk. More recent studies of the chemical signatures of thousands of stars indicate that starbirth might have plummeted by an order of magnitude at the time of disk formation, 8 to 10 billion years ago, when interstellar gas was much too hot to give birth to new baby stars at the same rate as before. Although it seems counterintuitive, things have to get very cold in order for a fiery new stellar baby to be born.
Satellite galaxies surrounding our Milky Way are not dispersed randomly. Indeed, they seem to be the result of an ancient break-up of some larger system that produced a ring structure about 500,000 light-years in diameter and 50,000 light-years wide. Close and catastrophic encounters between galaxies tear off enormous tails of gas which, over time, can coalesce to create dwarf galaxies.
In November 2018, astronomers reported the discovery of that tiny tattle-tale star that is one of the oldest inhabiting the Universe. This little star may also be one of the very first stars to be born in the Cosmos, and it is classified as an ultra-metal-poor (UMP) star composed almost entirely of matter formed in the Big Bang. Astronomers refer to such stars which are depleted of heavy metals as low metallicity stars. “But this one has such low metallicity, its known as an ultra metal poor star–this star may be one in ten million,” Dr. Schlaufman commented in the November 5, 2018 Gemini Observatory Press Release. 720p mkv movie download with hd quality 480p & 720p .