GALAXIES
A galaxy is a massive, gravitationally bound system that consists
of stars and stellar remnants, an interstellar medium
of gas and dust, and an
important but poorly understood component tentatively dubbed dark matter. Examples of galaxies range
from dwarfs with as few as ten million (107)
stars to giants with a hundred trillion
(1014) stars, each orbiting their galaxy's own center of mass.
Galaxies contain varying amounts of star systems, star clusters and types of interstellar clouds.
In between these objects is a sparse interstellar medium
of gas, dust, and cosmic rays. Dark matter appears to account for around
90% of the mass of most galaxies. Observational data
suggests that super massive
black holes may exist at the center of many, if not all, galaxies.
They are thought to be the primary driver of active galactic
nuclei found at the core of some galaxies. The Milky Way galaxy
appears to harbor at least one such object.
There are an estimated 100-300 billion galaxies in
the observable universe. Only about 3,000
of these galaxies are visible to us.The Milky Way Galaxy has
between 200 and 400 billion stars. The Sun is just one small star among these.
How Galaxies are formed:
Some scientists believe small clusters
of about 1 million stars, known as globular clusters, formed first and later
became galaxies. Others believe galaxies formed first and stars inside them
formed later. Galaxies are grouped into three distinctive types.
1). Spiral galaxies: Earth is located in
a spiral galaxy called the Milky Way. Spiral galaxies are a flat disk with a
bulging center and spiral arms. They're made up of stars, planets, dust and gas
that rotate around the center.
Milky Way is a spiral shaped galaxy.
2). Elliptical Galaxies
Elliptical
galaxies are the universe's largest known and are generally round, but they
stretch longer along one axis than the other. Elliptical galaxies are often
made up of up to 1 trillion older stars without as much dust and other
interstellar matter.
The elliptical galaxy NGC-1132.
3). Irregular Galaxies
Some galaxies aren't spiral or elliptical. These are called irregular galaxies because they look misshapen and lack a distinct form.
I Zwicky 18 irregular galaxy.
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