In his classification scheme, Hubble denoted normal spiral galaxies by S and barred spiral galaxies by SB. The spiral arms wind outward from the edge of this bar structure. In barred spirals, there is a central bar structure extending out on either side of the nucleus. In the normal spiral galaxies, the spiral arms wind outward from the nucleus. There are two types of spiral galaxies, normal spirals and barred spirals. A few spiral galaxies have more than two spiral arms. There are usually two spiral arms that wind around each other several times in a whorl from the nucleus to the edge of the disk. Our goal now is to compare the model predictions with observations of more distant galaxies seen in images obtained with the Hubble and those of the soon to be launched James Webb Space Telescope“, says Devereux.The disk of a spiral galaxy contains the spiral arms that give class of galaxy its name. “These new findings set a clear direction for future research. Our Milky Way galaxy’s barred spiral shape suggests it has seen a complex evolutionary history, with only a few minor collisions and at least one episode where the inner disk collapsed to form the large central bar. Their model suggests that the number of mergers between these haloes and their galaxies drives the final outcome-elliptical galaxies result from multiple mergers whereas disk galaxies have seen none at all. Galaxies are thought to be embedded in very large haloes of dark matter and Benson and Devereux believe these to be crucial to their evolution. Just 4 percent of the universe consists of the familiar visible or ‘baryonic’ matter that makes up the stars and planets of which galaxies are comprised. Here ‘Lambda’ is the mysterious ‘dark energy’ component believed to make up about 72 percent of the cosmos, with cold dark matter making up another 23 percent. The astronomers’ model is underpinned by and endorses the ‘ Lambda Cold Dark Matter‘ model of the universe. “It really boosts my confidence in the model”, adds Benson. “We were completely astonished that our model predicted both the abundance and diversity of galaxy types so precisely”, says Devereux. To their surprise, their computations reproduced not only the different galaxy shapes but also their relative numbers. The different types clearly result from different evolutionary paths, but until now a detailed explanation has eluded scientists.īenson and Devereux combined data from the infrared Two Micron All Sky Survey (2MASS) with their sophisticated GALFORM computer model to reproduce the evolutionary history of the universe over thirteen billion years. (Credit: Ville Koistinen)įor comparison, the galaxy we live in, the Milky Way, has between two and four hundred thousand million stars and is classified as a barred spiral.Įxplaining the Hubble Sequence is complex. The lower right line shows the barred spirals that range from the tightly wound SBa to loosely wound SBc types. The upper right line of objects stretch from Sa (tightly wound spiral) to Sc (loosely wound spiral). Type S0 is intermediate between elliptical and spiral galaxies. On the left are elliptical galaxies, with their shapes ranging from spherical (E0) to elongated (E7). This figure illustrates the Hubble Sequence. elliptical, where the galaxy’s stars are distributed more evenly in a bulge without arms or disk.barred spirals, where the arms wind out in a disk from a larger bar of material,.spiral, where arms of material wind out in a disk from a small central bulge,.The smallest have a few million and the largest as many as a million million (a trillion) stars.Īmerican astronomer Edwin Hubble first developed a taxonomy for galaxies in the 1930s that has since become known as the ‘ Hubble Sequence‘. Galaxies are the collections of stars, planets, gas, and dust that make up most of the visible component of the cosmos. Their results appear in the journal Monthly Notices of the Royal Astronomical Society. The scientists, Andrew Benson of the California Institute of Technology (Caltech) and Nick Devereux of Embry-Riddle University in Arizona, tracked the evolution of galaxies over thirteen billion years from the early universe to the present day. Benson/University of Durham, NASA/STScI)ĬALTECH (US)-For the first time, two astronomers have explained the diversity of galaxy shapes seen in the universe. The yellow objects are most distant and therefore appear as they were 13 billion years ago, whilst those closer are seen as they looked more recently. The image shows some of the galaxies generated by the computer model.
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