00:00TYPES OF GALAXIES A galaxy is a gravitationally bound system
00:08of stars, planets, stellar remnants, interstellar gas, dust and dark matter.
00:18The word is derived from the Greek Galaxia.
00:22Milky Way is the galaxy that contains our solar system.
00:28Galaxies range in size from dwarfs with just a few hundred million stars to giants with
00:36one hundred trillion stars, each orbiting its galaxy's center of mass.
00:44Galaxies are categorized according to their visual morphology as elliptical, spiral or
00:52irregular.
00:54Many are thought to have supermassive black holes at their centers.
00:59The Milky Way's central black hole, known as Sagittarius A star, has a mass four million
01:07times greater than the Sun.
01:09GN-Z11 is the oldest and most distant galaxy observed.
01:17It has a distance of 32 billion light-years from Earth and is seen as it existed just
01:25400 million years after the Big Bang.
01:29There are three main categories of galaxies – elliptical, spiral and irregular.
01:38These types of galaxies are further divided into sub-categories and elliptical galaxy.
01:46An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and
01:53a smooth, nearly featureless image.
01:57They are one of the three main classes of galaxy.
02:02Most elliptical galaxies are composed of older, low-mass stars with a sparse interstellar
02:10medium and minimal star formation activity and they tend to be surrounded by large numbers
02:17of globular clusters.
02:21Elliptical galaxies are in various shapes.
02:24Some of them should be in round shape and some appeared as oval in shape.
02:31Elliptical galaxies are believed to make up approximately 10-15% of galaxies in the Virgo
02:40supercluster and they are not the dominant type of galaxy in the universe overall.
02:48They are preferentially found close to the centers of galaxy clusters.
02:54Elliptical galaxies range in size from tens of millions to over 100 trillion stars.
03:02Spiral Galaxies
03:05Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust and a
03:15central concentration of stars known as the bulge.
03:21These are often surrounded by a much-painted halo of stars, many of which reside in globular clusters.
03:30Spiral galaxies are named by their spiral structures that extend from the center into
03:38the galactic disk.
03:41The spiral arms are sites of ongoing star formation and are brighter than the surrounding
03:48disk because of the young, hot O-B stars that inhabit them.
03:54Number of arms are not constant in every galaxy.
03:58It may vary from galaxy to galaxy.
04:01Spiral galaxies may consist of several distinct components.
04:07A flat, rotating disk of stars and interstellar matter of which spiral arms are prominent
04:14components.
04:15A central stellar bulge of mainly older stars which resembles an elliptical galaxy.
04:23A bow-shaped distribution of stars.
04:27A near-spherical halo of stars including many in globular clusters.
04:33A supermassive black hole at the very center of the central bulge.
04:39A near-spherical dark matter hollow.
04:43Barred Spiral Galaxy A barred spiral galaxy is a spiral galaxy
04:49with a central bar-shaped structure composed of stars.
04:55Bars are found in about two-thirds of all spiral galaxies.
05:01Bars generally affect both the motions of stars and interstellar yards within spiral
05:07galaxies and can affect spiral arms as well.
05:13The Milky Way galaxy where the solar system is located is classified as a barred spiral
05:19galaxy.
05:20Barred galaxies are apparently predominant with surveys showing that up to two-thirds
05:28of all spiral galaxies contain a bar.
05:31The current hypothesis is that the bar structure acts as a type of stellar nursery fueling
05:39star birth at the Earth's centers.
05:42The bar is thought to act as a mechanism that channels gas inwards from the spiral arms
05:49through orbital resonance, in effect fueling the flow to create new stars.
05:55The creation of the bar is generally thought to be the result of a density wave radiating
06:02from the center of the galaxy whose effect reshapes the orbits of the inner stars.
06:09This effect builds over time to stars orbiting further out which creates a self-perpetuating
06:17bar structure.
06:18Irregular Galaxy An irregular galaxy is a galaxy that does
06:27not have a distinct regular shape unlike a spiral or an elliptical galaxy.
06:35Irregular galaxies do not fall into any of the regular classes and they are often chaotic
06:42in appearance with neither a nuclear bulge nor any trace of spiral arms.
06:49Collectively they are thought to make up about a quarter of all galaxies.
06:55Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an
07:02uneven external gravitational force.
07:07Irregular galaxies may contain abundant amounts of gas and dust.
07:12This is not necessarily true for dwarf irregulars.
07:17Irregular galaxies are commonly small, about one-tenth the mass of the Milky Way galaxy.
07:24Due to their small size, they are prone to environmental effects like crashing with large
07:30galaxies and intergalactic clouds.
07:34Milky Way is the galaxy that contains our solar system.
07:39It is a bed spiral galaxy.
07:42Milky Way have four arms.
07:44Our solar system is in the inner edge of Orion arms of Milky Way.
07:51The Milky Way's central black hole is known as Sagittarius A star.
07:57It has a mass 4 million times greater than the Sun.
08:01There are 200 plus million stars in our galaxy.
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