This dramatic event, counted as Seleucus' first regal year, was continued as the Seleucid Era, the first continuous count of time in world chronology, soon to inspire the similar Arsacid Era of Parthia. There is also the residual uncertainty about Hellenistic dating. Bickerman, for instance, positively asserted that Seleucus reconquered Babylon "in August of " [Chronology of the Ancient World, Cornell University Press,p. Seleucus] was given a thousand men by Ptolemy and set out from Palestine to Babylonia" [Dividing the Spoils, Oxford,p.
In Greek mythologyZeus places his son born by a mortal woman, the infant Heracleson Hera 's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: The English term Milky Way can be traced back to a story by Chaucer c.
Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulaeand were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxybegan resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
For this reason they were popularly called island universes, but this term quickly fell into disuse, as the word universe implied the entirety of existence. Instead, they became known simply as galaxies.
Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxythe Magellanic Cloudsthe Whirlpool Galaxyand the Sombrero Galaxy. All of the well-known galaxies appear in one or more of these catalogues but each time under a different number.
Observation history The realization that we live in a galaxy which is one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae.
Milky Way Main article: In his view, the Milky Way is celestial. Actual proof of the Milky Way consisting of many stars came in when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.
The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk. He produced a diagram of the shape of the galaxy with the Solar System close to the center.
A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: The Magellanic Cloudssatellite galaxies of the Milky Way, appear near the left edge.
Distinction from other nebulae A few galaxies outside the Milky Way are visible on a dark night to the unaided eyeincluding the Andromeda GalaxyLarge Magellanic Cloud and the Small Magellanic Cloud.
In the 10th century, the Persian astronomer Al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud". These views "are remarkably close to the present-day views of the cosmos. Photograph of the "Great Andromeda Nebula" fromlater identified as the Andromeda Galaxy Toward the end of the 18th century, Charles Messier compiled a catalog containing the brightest celestial objects having nebulous appearance.
Subsequently, William Herschel assembled a catalog of 5, nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.
Slipher discovered that the spiral nebulae have high Doppler shiftsindicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us. Searching the photographic record, he found 11 more novae.
Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.
Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variablesthus allowing him to estimate the distance to the nebulae:§ Implementation of Texas Essential Knowledge and Skills for Science, High School.
(a) The provisions of this subchapter shall be implemented by school districts.
|Galaxy morphological classification - Wikipedia||Scientific Considerations Galaxies Types and Classifications The identification of other galaxies, or independent stellar systems, goes back towhen Edwin Hubble found Cepheid variables in the nearby galaxies Messier 33 and Messier|
|Galaxy - Wikipedia||Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images.|
Galaxy - Types of galaxies: Almost all current systems of galaxy classification are outgrowths of the initial scheme proposed by the American astronomer Edwin Hubble in In Hubble’s scheme, which is based on the optical appearance of galaxy images on photographic plates, galaxies are divided into three general classes: ellipticals, spirals, and irregulars.
Irregular Galaxies UGC , an irregular galaxy from the SDSS Most galaxies fit one of the three previously described types, but about . Irregular galaxies (Ir) show little, if any, symmetry in their luminosity structure; their appearance really does appear irregular, and therefore they were defined by Hubble as a separate class of galaxy.
In modern modifications of Hubble's classification system, some astronomers consider them to be a morphological extension of the spiral. adshelp[at]alphabetnyc.com The ADS is operated by the Smithsonian Astrophysical Observatory under NASA Cooperative Agreement NNX16AC86A.
A third classification of irregular galaxies are the dwarf irregulars that are classified as dI or dIrr. Astronomers believe these galaxies may .