* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Classification of Galaxies By Daniel Underwood Contents The
Perseus (constellation) wikipedia , lookup
International Ultraviolet Explorer wikipedia , lookup
Fermi paradox wikipedia , lookup
History of supernova observation wikipedia , lookup
Dark energy wikipedia , lookup
History of astronomy wikipedia , lookup
Outer space wikipedia , lookup
Rare Earth hypothesis wikipedia , lookup
Aries (constellation) wikipedia , lookup
Non-standard cosmology wikipedia , lookup
Space Interferometry Mission wikipedia , lookup
Star formation wikipedia , lookup
Dark matter wikipedia , lookup
Hubble's law wikipedia , lookup
Chronology of the universe wikipedia , lookup
Gamma-ray burst wikipedia , lookup
Physical cosmology wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Andromeda Galaxy wikipedia , lookup
Modified Newtonian dynamics wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Observational astronomy wikipedia , lookup
Lambda-CDM model wikipedia , lookup
High-velocity cloud wikipedia , lookup
Observable universe wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Structure formation wikipedia , lookup
The Classification of Galaxies By Daniel Underwood 1 Contents The History of Galaxy Discovery – Page 3 The Different Types of Galaxies – Page 4 Galaxies Connected – Page 7 Conclusion – Page 8 References – Page 9 2 The History of Galaxy Discovery It was only last century in the 1920s that the presence of certain nebulae outside the Milky Way we realised, and it became universally accepted by astronomers that there were other galaxies than our own in the cosmos. However, it wasn’t immediately recognised that these nebulae were actually galaxies like our own, it took time to realise that they weren’t gaseous, but actually massive collections of stars. These masses outside the Milky Way were becoming more and more noticed by astronomers, and they had their own characteristics which helped identify them. But it didn’t end there... Even though galaxies these days are widely recognised, even by the average man on the street, their characteristics are not always consistent. It was pointed out by certain astronomers (namely Wolf, Hubble) that the structure of galaxies could be furthermore sub divided. That is to say galaxies come in all shapes and sizes, but fall into certain categories. It was only natural that rational physicists decided it was necessary to develop a classification scheme in order to identify types of galaxy. Classification of galaxies was first attempted by Wolf in 1908. In 1926 Edwin Hubble published his own scheme for classifying, which is what is still used today, but includes additions and amendments by Holmberg (1958), de Vaucouleurs (1956, 1959) and van den Bergh (1960). 3 The Different Types of Galaxies When it was established that galaxies had distinguishable features which even differentiated them from them from other galaxies, a classification scheme was devised. The classification scheme used today was devised by Edwin Hubble and later extended to include further ideas proposed by de Vaucouleurs. The scheme can help identify evolutionary patterns and other properties of the galaxies. The types of galaxies are: - Elliptical - Spiral/Barred Spiral - Lenticular - Peculiar They are explained under the following subheadings. Elliptical Galaxies These galaxies have no general structure, and are basically large groups of matter compacted together in an elliptical shape, with a drop in brightness from the centre to the edge of the galaxy, suggesting that there is a larger density of stars in the centre, or brighter ones. These galaxies are denoted by the letter ‘E’ followed by an integer n (e.g. E3), where the integer is the product of the galaxy’s ellipticity: , where e is the ratio: 4 , where a and b are the major and minor axes of the galaxy respectively. The value of n ranges from 0 to 7, where 0 denotes a circularshaped elliptical galaxy and 7 a more ‘cigar-shaped’ galaxy. Any galaxy with a value of n greater than 7 are classed as lenticular galaxies, explained later. Spiral/Barred Spiral Galaxies Spiral galaxies can be easily recognised by their structure; arms of matter spiralling out from the bulging centre of the galaxy. There are two types: barred spirals (SB) and non-barred (SA), although if distinction between the two happens to be difficult then “SAB” is used for denotation. A barred spiral is a spiral type (pictured on the right) but with a bar-like feature in the centre off which the spirals emanate. Like with elliptical galaxies, spiral galaxies can be sub-divided by adding a letter to the SB/SA symbol (e.g. SBc). The letters run from “a” to “d”, with a further sub type “m” at the end. As you move through “a” to “d” the following occurs: - The central bulge becomes smaller compared to the galactic disc; - The spiral arms spread further and become less compact; - Increased clumping of arms into stars and star forming regions (clumping means the degree of condensation of the regions). The “m” type classes galaxies where no nucleus is clear and the galaxy is generally loosely spread. An irregular galaxy is denoted by “Im” 5 (or “IBm”) which is very similar to Sam/SBm types. The classification of these galaxies relies mostly on the bulge and the disc-condensation. Lenticular Galaxies Lenticular galaxies appear to be intermediate between elliptical and spiral galaxies, where their structure shows an elliptical style that is more elongated, where 10e is greater than 7, although spiral galaxy features may be present, if very slightly. These galaxies are denoted by “SA0” or “SB0”. Peculiar Galaxies These galaxies have no features that lie under the common classification scheme. They are simply labelled “P”. However, sometimes galaxies can be classified and still have peculiarities, and so “p” is added to the end of the class denotation. 6 Galaxies Connected The galaxy types aren’t completely connected in any way that we currently know of, but they are commonly visualised together by the famous ‘tuning fork’ diagram: SA0 SAa SAb SAc SAd SAm Im E0 E1 E2 E 3 E4 E5 E6 E7 SB0 SBa SBb SBc SBd SBm IBm ‘Early type’ ‘Late type’ The reason the two sides are labelled ‘early’ and ‘late’ respectively is because it was previously believed that the diagram was an interpretation of an evolutionary line, but this view doesn’t hold anymore. However there are a few apparent physical trends along the sequence that are being recognised moreover. 7 Conclusion The development of the galaxy classification scheme may seem trivial, but in the greater scheme of things it has the substance to help astronomers and physicists in general to discover more about the universe. Simply analysing deep field images of space in concordance with this scheme can lead to conclusions about the evolution of the universe with respect to galaxies, by recognising patterns and differences as the universe ages, ultimately helping towards the ultimate goal and understanding the universe’s origins and its fate. 8 References Zeilik & Gregory – Introductory Astronomy and Astrophysics, Chapter 21 University Of London Observatory – 1B30 Practical Astronomy, Classification of Galaxies report sheet Images http://galaxyzoo.org http://sci.esa.int/ http://content.answers.com/ http://www.cosmiclight.com/ 9