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| Andromeda Galaxy |
cosmic objects, clouds and dust. By shape it looks like a ‘discuss’ (used is sports) rotating about its centre with immense speed. Now what’s the size of the Milky-way? Through its diameter from one periphery to another light takes about hundred thousand years to travel. (The distance light travel in one year with a speed of 300,000 km per second is called ‘Light Year’). Hence the diameter of Milky-way is about 100,000 light-years. At its centre its thickness is about 2500 light years and at the periphery it’s about 1000 light years. Now you can imagine the shape and size of a galaxy. The universe consists of several such galaxies.
Our own galaxy, Milky-way, and
some other galaxies in the universe are in a very orderly system. Here rotation
of the galaxy dominates other internal motions and it takes the shape of a
disk. This orderly system has not been the characteristic of a galaxy since its
birth rather it is achieved through evolution of billions of years. The more
active star-forming blue galaxies (they are named such for their colour) do not
look like a disk rather their shapes are irregular. New stars are being formed
within them. Their internal motions are also quite haphazard.
So long Astronomers thought that the disk
galaxies in the nearby universe had settled into their form by about 8 billion
years ago with little development since then, but recently a comprehensive
study of hundreds of galaxies observed by the Kek telescope in Hawaii and
NASA’s Hubble Space Telescope has revealed the opposite; that galaxies were
steadily changing over this time period.
The researchers studied all the galaxies
with emission lines bright enough to be used to determine internal motions.
Emission lines are the discrete wavelengths of radiation characteristically
emitted by the gas within the galaxy. These emission lines also carry the
information about the galaxy’s internal motions and distance. They are revealed
when the light emitted by a galaxy is analyzed splitting it into its
components.
In the study, Blue galaxies show less disorganized motions
and ever-faster rotation speeds the closer they are observed to the present.
This trend holds true for galaxies of all masses, but more massive systems
always show higher level of organization. Researchers say the distant blue
galaxies they studied are gradually transforming into rotating disk galaxies
like our own Milky Way.
The team studied a sample of 544 blue galaxies from the Deep Extragalactic Evolutionary Probe 2 (DEEP2) Redshift Survey, a project that employs Hubble and the twin 10-metre telescope at the W. M. Kek Observatory in Hawaii, located between 2 billion and 8 billion light-years away, the galaxies have stellar masses ranging from 0.3 percent to 100 percent of the mass of our home galaxy.
Our home galaxy must have gone through the same
rough-and-tumble evolution as the galaxies in the DEEP2 sample, and gradually
settled into its present state as the sun and solar system were being formed.
In the past 8 billion years, the number of mergers between
galaxies large and small has decreased sharply. So has the overall rate of star
formation and disruptions of supernova explosions associated with star-formation. Scientists speculate these factors may play a role in
creating the evolutionary trend they observe.
The DEEP2 survey is led by Lick Observatory at the University
of California at Santa Cruz in collaboration with the University of California
at Berkeley, the University of Hawaii at Manoa, Johns Hopkins University in
Baltimore, Md., the University of Chicago and the California Institute of
Technology in Pasadena.
News Source- NASA's press release.
Picture Courtesy- NASA
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