The Space And Beyond !!

 GALAXY

    A galaxy is a gravitationally bound system of stars, stellar, emanates, interstellar gas, dust and dark matter. The word is derived from the Greek , literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies range in size from dwarfs with just a few hundred million (10⁸) stars to giants with one hundred trillion (10¹⁴) stars, each orbiting its galaxy's center of mass.

Formation Of A Galaxy 

    Current cosmological models of the early universe are based on the Big Bang theory. About 300,000 years after this event, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "dark ages". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos. These primordial structures eventually became the galaxies we see today.

    Early galaxy formation 

    Evidence for the appearance of galaxies very early in the Universe's history was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and earliest-to-form galaxy seen at that time. While some scientists have claimed other objects (such as Abell 1835 IR1916) have higher redshifts (and therefore are seen in an earlier stage of the universe's evolution), IOK-1's age and composition have been more reliably established. In December 2012, astronomers reported that UDFj-39546284 is the most distant object known and has a redshift value of 11.9. The object, estimated to have existed around 380 million years after the Big Bang (which was about 13.8 billion years ago), is about 13.42 billion light travel distance years away. The existence of galaxies so soon after the Big Bang suggests that protogalaxies must have grown in the so-called "dark ages". As of May 5, 2015, the galaxy EGS-zs8-1 is the most distant and earliest galaxy measured, forming 670 million years after the Big Bang. The light from EGS-zs8-1 has taken 13 billion years to reach Earth, and is now 30 billion light-years away, because of the expansion of the universe during 13 billion years.

Evolution

    Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added. During this early epoch, galaxies undergo a major burst of star formation.

During the following two billion years, the accumulated matter settles into a galactic disc. A galaxy will continue to absorb infalling material from high-velocity clouds and dwarf galaxies throughout its life. This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets.

The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology. Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen in NGC 4676 or the Antennae Galaxies.

The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two might collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing.

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked about ten billion years ago.

Why Is Our Galaxy Called The Milky Way ?

    According to the American Museum of Natural History (AMNH), our galactic home is called the Milky Way after its apparent milky white appearance as it stretches across the night sky. In Greek mythology, this milky band appeared because the goddess Hera sprayed milk across the sky. 

Around the world, the Milky Way is known by different names for example in China it is called "Silver River" and in the Kalahari Desert in South Africa it's called the "Backbone of Night".  

The Milky Way : Size , Structure And Mass  

•  Galaxy type: Barred spiral
•  Age: 13.6 billion years (and counting)
• Size: 100,000 light-years across
• Number of stars: about 200 billion
• Rotation time: 250 million years

Where Is The Sun In The Milky Way Galaxy ?

    The sun orbits about 26,000 light-years from the black hole Sagittarius A*, roughly in the middle of the galactic disk. Traveling at the speed of 515,000 mph (828,000 kph), the sun takes 230 million years to complete a full orbit around the galactic center. 

    The sun sits near the edge of the Local Arm of the Milky Way, one of the two smaller spiral arms of the galaxy. In 2019, using data from the Gaia mission, astronomers found that the sun is essentially surfing a wave of interstellar gas that's 9,000 light-years long, 400 light-years wide and undulates 500 light-years above and below the galactic disk according to ESA. 

    Planets of the solar system do not orbit in the plane of the galaxy but are tipped by about 63 degrees. 

"It's almost like we're sailing through the galaxy sideways," Merav Opher, an astrophysicist at George Mason University in Virginia, told Space.com.

Types Of Galaxies : 

• Interacting 

    Interactions between galaxies are relatively frequent, and they can play an important role in galactic evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust. Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but the gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures.

• Starburst   

   Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions. These stars produce supernova explosions, creating expanding remnants that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star-building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the activity end.
Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity.

                                

• Active Galaxy 

    Some observable galaxies are classified as "active" if they contain an active galactic nucleus (AGN). A significant portion of the galaxy's total energy output is emitted by the active nucleus instead of its stars, dust and interstellar medium. There are multiple classification and naming schemes for AGNs, but those in the lower ranges of luminosity are called Seyfert galaxies, while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects or quasars. AGNs emit radiation throughout the electromagnetic spectrum from radio wavelengths to X-rays, though some of it may be absorbed by dust or gas associated with the AGN itself or with the host galaxy.

                               

The Future Of The Milky Way Research 

     Since the beginning of its operations, the Gaia mission has provided three updates to its massive stellar catalog. Astronomers from all over the world continue analyzing the data in search of new patterns and revelations. Gaia data currently generates more research papers than even the famous Hubble Space Telescope. In a recent "space telescope tournament", a jokey poll run by astronomers on Twitter, Gaia beat the veteran Hubble by five votes as the current darling of the astronomical community. 

Gaia will continue charting the galaxy until at least 2022, and the catalog it has compiled will keep astronomers busy for decades to come. 

Before Gaia, the largest dataset about positions and distances of stars in the Milky Way came from a mission called Hipparcos, after an ancient Greek astronomer who began charting the night sky 150 years before Christ. Hipparcos only saw about 100,000 of the brightest stars in the sun's neighborhood, compared to Gaia's one billion. The data was also less precise. 

Even though Gaia sees less than 1% of stars in the galaxy, astronomers can expand their findings and model the behavior of the entire Milky Way. 

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