A galaxy is a system composed of stellar remnants, stars, interstellar gas, dust, and substance and all of these are gravitationally bounded. The term galaxy comes from the Greek word galaxies which literally means “milky”, a reference to the Milky Way. Galaxies vary in sizes even from dwarfs having a few of hundred million (108) stars to giants with 100 trillion (1014) stars, each orbiting its galaxy’s centre of mass. Scientists believe that many galaxies possess supermassive black holes at their centres.
Meaning of Galaxies
The diameter of most of the galaxies is between 1,000 to 100,000 parsecs (approximately 3,000 to 300,000 light-years). They are separated by distances on the order of millions of parsecs (or megaparsecs). For example, our Milky Way has a diameter of a minimum of 30,000 parsecs (100,000 ly) is being separated from its neighbouring Andromeda galaxy by about 780,000 parsecs (2.5 million ly.)
The space between galaxies is crammed with a tenuous gas (the intergalactic medium) having a mean density of but one atom per kiloliter. Gravity plays a very important role in organizing the majority of galaxies into groups, clusters, and superclusters. The Milky Way is a component of the Local Group, which is dominated by it and therefore the Andromeda galaxy and is a component of the Virgo Supercluster.
The name of the galaxy in which we survive is known as Milky Way or Akash Ganga. As the name suggests it’s a hazy band of sunshine seen within the night sky formed from stars that can’t be individually distinguished by the eye. Galileo Galilei first resolved the band of sunshine into individual stars alongside his telescope in 1610. The Milky Way is a barred spiral nebula. The estimated visible diameter is 1.9 million light-years (ly). The velocity of Milky Way is estimated roughly around 600 km per second with reference to extragalactic frames of reference.
The oldest stars within the Milky Way are nearly as old because the Universe itself and thus probably formed shortly after the Middle Ages of the large Bang. The Milky Way contains between 100 and 400 billion stars and a minimum of that a lot of planets. The Milky Way is expected to have ten billion white dwarfs, a billion neutron stars, and 100 million stellar black holes.
There are lots of galaxies which are catalogued, but among them, only a few have well-established names. Some of them are the Andromeda galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, like the Messier catalogue, the NGC (New General Catalogue), the IC (Index Catalogue), the CGCG (Catalogue of Galaxies and of Clusters of Galaxies), the UGC (Uppsala General Catalogue of Galaxies) and MCG (Morphological Catalogue of Galaxies).
Types of Galaxies
Galaxies are available three main types: ellipticals, spirals, and irregulars.
The Hubble arrangement rates elliptical galaxies on the idea of their ellipticity, starting from E0, being nearly spherical, up to E7, which is very elongated. These galaxies have an ellipsoidal profile, giving them an elliptical appearance no matter the viewing angle. The largest galaxies are giant ellipticals. Many elliptical galaxies make thanks to the interaction of galaxies, leading to a collision and merger. They sometimes grow to enormous sizes. The giant elliptical galaxies often exist near the core of huge galaxy clusters.
Spiral galaxies resemble spiralling pinwheels. Though the celebs and other visible material contained in such a galaxy lie totally on a plane, the bulk of the mass in spiral galaxies exists during a roughly spherical halo of substance which extends beyond the visible component. Spiral galaxies contain a rotating disk of stars and interstellar space, alongside a central bulge of generally older stars.
According to Hubble classification scheme, spiral galaxies are mentioned as type S. It is being followed by a small case letter (a, b, or c) to indicate the degree of tightness of the spiral arms and the size of the central bulge. For example, a Sa type galaxy is somewhat tightly wound, poorly defined arms and possesses a relatively large core region. At the opposite extreme, an Sc galaxy has open, well-defined arms and a little core region.
Irregular galaxies are galaxies that can not be readily classified into an elliptical or spiral morphology. Like an Irr-I galaxy has some structural pattern but it doesn’t follow the Hubble classification scheme. In the case of Irr-II galaxies, it does not have any structural pattern that resembles a Hubble classification. Nearby examples of irregular galaxies is the Magellanic Clouds.
Dwarf galaxies are small in size when compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a few billion stars. Most galaxies are dwarf galaxies. Ultra-compact dwarf galaxies are in recent discovery that is only 100 parsecs across. These galaxies are relatively small as compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a few billion stars.
Other Types Of Galaxies
Some of the other types of galaxies are Interacting, Starburst, Active galaxy. We can further divide the active galaxies into Blazars, Liners, Seyfert galaxy, Quasar.
Galaxies have its own magnetic fields which are strong enough to drive mass inflow into the centres of galaxies, can modify the formation of spiral arms and can affect the rotation of gas within the outer regions of galaxies. Magnetic fields provide the transport of momentum required for the collapse of gas clouds and hence the formation of latest stars.
Formation and Evolution
The large Bang theory supports the current cosmological models of the first universe. About 300,000 years after this event, atoms of hydrogen and helium began to make, in an occasion called recombination. Almost all the hydrogen present was neutral (non-ionized) and readily absorbed light, but still, no stars were formed. As a result, this era has been called the “dark ages”. It was from density fluctuations during this primordial matter that larger structures began to seem. As a result, masses of baryonic matter began to condense within cold substance halos. These prehistoric structures eventually become the galaxies.
Within a billion years of a galaxy’s formation, key structures begin to seem. Globular clusters, the central supermassive region, and a galactic bulge of metal-poor Population II star form. The creation of a supermassive region appears to play a key role in actively regulating the expansion of galaxies. The evolution of galaxies is often significantly suffering from interactions and collisions. Mergers of galaxies were common during the first epoch, and therefore the majority of galaxies were peculiar in morphology.
Deep scientific surveys show that galaxies often exist in groups and clusters. On the most important scale, the universe is continually expanding, leading to a mean increase within the separation between individual galaxies. Clusters of galaxies contain hundreds to thousands of galaxies bound together by gravity. A giant elliptical galaxy always dominates clusters of galaxies and is the brightest cluster galaxy. This giant galaxy over time tidally destroys its satellite galaxies and adds their mass to its own.
FAQs about Galaxies
Q.1. Is there anything in between galaxies?
Answer- Intergalactic space is more than just an empty space. The vast voids between galaxies can stretch many light-years across and should appear empty. In reality, these voids actually contain more matter than the galaxies themselves.
Q.2. How do galaxies die?
Answer- Gravity condenses gas into galaxies and stars, stars blow up and kick the gas out, gravity cycles the gas back in and makes new stars. Gas is the primary state of matter in forming new stars; the old stars live out their lives and die, and eventually, the galaxy dies too.
Q.3. What is our closest galaxy?
Answer- The closest known galaxy to us is that the Canis Major Dwarf Galaxy, at 236,000,000,000,00.