Define the 4 types of galaxy in brief??
Answers
Answer:
Spiral Galaxy - Spiral galaxies are characterized by a distinct flattened spiral disk with a bright center called the nucleus. Our own Milky Way is a spiral galaxy. Spiral galaxies are represented by the letter S and are divided into four subgroups. These are S0, Sa, Sb, and Sc. S0 galaxies have a bright nucleus but have no spiral arms. Sa galaxies have spiral arms that are wound tightly around the nucleus while the arms of Sc galaxies are wound much more loosely.
Barred Spiral Galaxy - A barred spiral galaxy is very similar to a spiral with one important difference. The arms spiral out from a straight bar of stars instead of from the center. About one third of all spiral galaxies are barred spiral in shape. Barred spiral galaxies are represented by the letters SB and are arranges into three subgroups according to the openness of the arms. These subgroups are labeled SBa, SBb, and SBc. SBa galaxies have a short bar of stars extending from the center while SBc galaxies have a long, well-defined bar. Our own Milky Way galaxy is now believed to be a barred spiral galaxy.
Elliptical Galaxy - Elliptical galaxies vary in shape from completely round to extremely elongated ovals. Unlike spiral galaxies, they have no bright nucleus at their center. Elliptical galaxies are represented by the letter E and are divided into seven subgroups according to their shape. These subgroups are labeled E0 to E7. E0 galaxies nearly circular in shape while E7 galaxies are extremely elongated or stretched out.
Irregular Galaxy - A fourth type of galaxy is known as the irregular galaxy. These galaxies have no discernible shape or structure. Irregular galaxies are divided into two classes, Im and IO. Im class galaxies are the most common and show just a hint of structure. Sometimes the faint remnants of spiral arms can be seen. IO class galaxies are completely chaotic in form. The large and small Magellanic Clouds are examples of Im class irregular galaxies.
Explanation:
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glaxies are categorized according to their visual morphology as elliptical, spiral, spiral barred or irregular. They are sometimes further divided into subcategories.
Key Facts & Summary
There are four main categories of galaxies: elliptical, spiral, barred spiral, and irregular. These types of galaxies are further divided into subcategories while at the same time other types of galaxies exist based on their size and other unique features.
The most common type of galaxy found throughout the universe is the spiral galaxy. Around 77% of the galaxies observed by man are spiral galaxies. A good example of this type is the Andromeda galax.y.
Around two-thirds of all spiral galaxies have a bar-like structure – thus they are classified as barred spiral galaxies. Our galaxy, the Milky Way is an example of this type of galaxy.
The rarest type of galaxies is the elliptical double-ringed galaxy. PGC 1000714 is an example. Estimates suggest that around 0.1% of galaxies are this type. It is sometimes named the Hoag-type galaxy.
Elliptical galaxies are usually comprised of very old stars or stars with low mass. They make up around 10-15% of galaxies in the Virgo Supercluster, a supercluster of which we are also part of. They are very dim in comparison with the very bright spiral galaxies.
Irregular galaxies are commonly small, and collectively they make up about a quarter of all the galaxies. They lack a distinct shape or form often having a chaotic appearance.
The largest galaxies are usually giant elliptical galaxies, containing a trillion or even more stars. They span as much as one million light-years across – 10 times as much as the Milky Way.
Curious enough, the smallest galaxies are also the elliptical types.
The oldest and farthest galaxy ever discovered is an irregular type of galaxy called GN-z11. It is 32 billion light-years / 9.8 billion parsecs away from us. It is estimated that the galaxy formed just around 400 million years after the Big Bang.
Most galaxies are between 1.000 to 100.000 parsecs / 3.000 to 300.000 light-years in diameter.
A system used by astronomers to divide galaxies into groups on their visual appearance is the galaxy morphological classification. Many schemes are in use in which galaxies are classified according to their morphologies. The most famous one is the Hubble sequence, devised by Edwin Hubble, and later expanded by Gerard Vaucouleurs and Allan Sandage.
In the present day, galaxy classification and morphology are largely done using computational methods and physical morphology.
Originally, Edwin Hubble grouped galaxies into four main categories: spiral galaxies, barred spiral galaxies, elliptical galaxies, and irregular galaxies. Observations revealed that spiral galaxies were the most common type of galaxies found throughout the universe.
Spiral Galaxies
They have a flat, spinning disk with a central bulge surrounded by spiral arms. The spinning motion reaches speeds of up to hundreds of kilometers/miles per second. This usually can cause matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel. The bulge located in the center is made up of older, dimmer stars, and is thought to usually contain a supermassive black hole.
The disk of stars orbiting the bulge tends to separate into arms that circle the galaxy. These spiral arms contain a wealth of gas and dust, thus many young stars are birthed in these regions. These young stars shine very brightly before their quick demise. Spiral galaxies can be classified according to the tightness of their spiral, the lumpiness of their arms, and the overall size of their central bulge.
The relative amounts of gas and dust contained within these galaxies can portray these differences. Andromeda is a perfect example of a spiral galaxy.
Elliptical Galaxies
Elliptical galaxies have an elongated spherical shape and lack a nucleus or bulge at the center. Although there is no nucleus, the galaxy is still brighter in the center and becomes less bright toward the outer edges of the galaxy.
Their light is dominated by older reddish stars. They appear to also lack spiral arms. The stars, gases and other materials are spread throughout an elliptical galaxy. Elliptical galaxies can be nearly round or long and cigar-shaped.
Though individual stars orbit the center of an elliptical galaxy, the orbits aren’t all in the same direction, as they appear in spirals. Therefore, ellipticals don’t appear to rotate in a systematic way
Theories suggest that a great deal of the mass in an elliptical galaxy is due to the presence of a central black hole. Elliptical galaxies have very little activity and contain mostly old stars of low mass, because there gasses and dust which is needed to form new stars, is quite lacking.