galaxies in the universe, types of galaxies, spiral galaxies, elliptical galaxies, irregular galaxies, lenticular galaxies, Hubble Space Telescope galaxies, galaxy formation and evolution, local group of galaxies, astronomical observations of galaxies, universe structure, galaxy classification, number of galaxies in the universe, Milky Way galaxy, Andromeda galaxy, cosmic landscape, galaxy clusters
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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.