If we talk about universe’s first stars ignited some 180 million years ago after the big bang, the explosive moment 13.8 billion years ago that marks the origins of the universe as we know that the first galaxy is shaped by the time universe turned 400 millions years old , or less than 3 percent of its current age. Astronomers now think that nearly all galaxies with possible exceptions are embedded in huge haloes of dark matter. Theoretical models also suggest that in the early universe vast tendrils of dark matter provided normal matter the gravitational scaffold it needed to coalesce into the first galaxies.
In general we know that galaxies are sprawling system of dust, gas, dark matter, and anywhere we can say form billion to trillion stars that are held together by gravity. Nearly all large galaxies are thought to contain also supermassive black holes at their centers. The deeper we look into cosmos, the more galaxies we see. We really don’t know how various galaxies formed? How they took into many shapes? but, we have some ideas about their origin and evolution.
Types Of Galaxies
If we talk about before 20th century, we didn’t know that galaxies other than the milky way existed. Earlier in ancient times astronomers has classified them as “nebulae”, because they looked the fuzzy clouds. But in 1920s, the astronomer Edwin Hubble showed that andromeda “nebula” was a galaxy in its own right. hence, in this immense distance, Andromeda is the closest large galaxy to our Milky Way, and its bright enough in the night sky that it’s visible to the naked eye in the northern hemisphere.
Then, after 1920s, In 1936 , Hubble debuted a way to classify galaxies, grouping them into four.
In these, More thn two-thirds of all observed galaxies are called spiral galaxies.A spiral galaxy has a flat, spinning disk with a central bulge surrounded by spiral arms. That spinning motion, at speeds of hundreds of kilometers a second, may cause a matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel.
These galaxies contain many older stars, but little dust and other interstellar matter. Their stars orbit the galactic center, like those in the disks of spiral galaxies, but they do so in more random directions. Few new stars are known to form in elliptical galaxies.
These galaxies are called lenticular because they resemble lenses. Like spiral galaxies, they have a thin, rotating disk of stars and a central bulge, but they don’t have a spiral arms. Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form in more often in densely populated regions of space.
If the galaxies are not spiral, elliptical or lenticular then, these galaxies are irregular galaxies. Irregular galaxies such as the large and small magellanic clouds that flank our Milky Way – appear like misshappen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by.
As we have to know that, how galaxies formed? How they took shape? So, Most theories about the early universe make two assumptions here
- It was filled with hydrogen and helium.
- Some areas were slightly denser than others.
Frome these assumptions, astronomers believed that the denser areas slowed down the denser slightly, allowing gas to acculumate in small protogalatic clouds. In these clouds, gravity caused the gas and dust to collapse and form stars.The rotating disks attracted more gas and dust with with the gravity to form galactic disks, hence, new stars formed.
we can conclude the two assumptions from this process for the difference between spiral and elliptical galaxies.
- Degree of spin – protogalatic clouds with more angular momentum could spin faster and form spiral disks. slowing spin could have formed elliptical clouds.
- Cooling – high density protogalactic clouds cooled faster, using up all dust and gases in forming up stars and even leaving none for making a galactic disk. hence, Low density protogalactic clouds cools more slowly, leaving gas and dust for disk formation.
Galaxies do not act alone, the distance between galaxies do seem large, but the diameter of galaxies are also large. Even they are very closer to one another, they interact and more importantly, collide. Collisions tend to distort galaxy’s shape. Even scientists estimate that as many as half of all galaxies have been involved in some sort of collision.
Gravitaional interactions between colliding galaxies could cause several things :
- New waves of star formation
- Stellar collapses that forms the black holes or supermassive black holes in active galaxies.