Important Geography Study Notes for SSC CGL Exam 2016 and other similar Competitive Exams.
Definition 1: The definition of the `Universe’ in simplest words is everything we can touch, feel, sense, measure or detect.
Definition 2: Alternatively, the Universe can also be defined as all of time, space and its contents.
- The Universe includes all living things, stars, planets, galaxies, natural satellites, dust, light and even time. Before the Universe was born, time, space and matter did not exist.
- The Universe includes billions of galaxies, with each galaxy containing billions of stars. The space between stars and galaxies is mostly empty. However, this empty space between stars and galaxies contains scattered dust particles and hydrogen atoms.
- This empty space is also filled with heat, light, magnetic fields and energy particles like cosmic rays.
- The Universe is so huge that a modern fighter jet would take more than a million years to reach the nearest star from the Sun.
- In fact, it would take 100,000 years while traveling at the speed of light (300,000 km per second) to cross the Milky Way galaxy, in which our planet Earth and the entire solar system lie. And the Milky Way galaxy is just a small part of the Universe.
- The exact size of the Universe is not known as its edge cannot be seen, that is if there is one. However, the visible Universe is at least 91 billion light years (light year is the distance light travels in one year) across.
- The scientists believe that the Universe was created by a Big Bang nearly 14 billion years ago. The scientists are also of the opinion that the Universe has been expanding at a high speed ever since it was born and the distance between galaxies has been increasing
Definition 1: `Galaxy’ is a system of millions or billions of stars that are held together by gravitational force along with gas and dust.
Definition 2: Alternatively, galaxy can also be defined as a large, self-contained mass of stars and planets.
Definition 3: A gravitationally bound system of stars, stellar remnants, gas, dust and dark matter is referred to as a galaxy.
- In a galaxy, celestial bodies rotate around a central object. The Earth’s galaxy is known as the Milky Way. The Universe contains billions of galaxies.
- Galaxies range in many sizes from dwarfs which contain only a few thousand stars to giants which may contain over 100 trillion stars. All stars in a galaxy orbit around their galaxy’s center of mass.
- Galaxies are broadly classified into three types based on their visual appearance, namely elliptical, spiral and irregular. Many galaxies have black holes as their active centers.
- Sagittarius A*is the Milky Way’s central black hole and has a mass four million times greater than the Sun.
- GN-Z11 is the oldest and the furthest most observed galaxy, lying 32 million light years away from the Earth.
- In the part of the Universe that is observable, approximately 170 to 200 billion galaxies exist in it. Most galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances of millions of parsecs.
- The space between galaxies is filled with gas, with an average density of less than one atom per cubic meter.
- Most of the galaxies exist in the Universe in the form of gravitationally organized galaxy groups, clusters or super clusters.
Definition: A `Star’ is an astronomical body in which a luminous sphere of plasma is held together by its own gravity.
- Stars form the most fundamental blocks of a galaxy. Stars are primarily responsible for the production and distribution of heavy elements like carbon, nitrogen and oxygen.
- The nearest star from the Earth is Sun, which also forms the center of the solar system. Many stars can be seen at night by the naked eye from Earth. Stars appear as fix luminous points in the night sky due to their distance from the Earth.
- Prominent stars are grouped in constellations and the brightest stars have also been given proper names like the Pole Star.
- However, most stars are invisible to the naked eye, especially the ones outside our galaxy, the Milky Way due to their distance from our planet. In fact, many stars cannot be seen even with the help of a telescope.
- A star shines due to thermonuclear fusion of hydrogen into helium in its core, for the most part of its life. Interestingly each star has a lifespan and near the end of its life a star may also contain degenerate matter.
- With advancement in technology astronomers today can determine the mass, age and chemical composition of a star by observing its motion, luminosity and spectrum.
- The total mass of a star determines its evolution and lifespan. A star’s movement and rotation is influenced by its mass, diameter, temperature and changes that it goes through over time.
- A star is born with the gravitational collapse of a gaseous nebula of materials mostly composed of hydrogen, helium and traces of heavier elements.
- A star’s internal pressure prevents it from collapsing further under its own gravity. When the core of a star gets exhausted of hydrogen, it expands to become a red giant. During this stage a star throws its extra mass enriched with heavier elements which leads to the birth of new stars.
- A star after shedding extra mass becomes a white dwarf, a neutron star or if sufficiently massive, a black hole, towards the end of its life.
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