23 November 2014
In a broad sense, galaxies may be grouped into three varieties: spirals, ellipticals and irregulars. Of those, ellipticals and spirals are by far the commonest. A lot of the galaxies we see round us are spiral galaxies, however all through the universe ellipticals are extra frequent, making up about 60% of all galaxies. The categorization of galaxies was first made by Edwin Hubble in 1926, and have become referred to as Hubble’s tuning fork. It was so-named as a result of the number of galaxies may very well be laid out alongside a scale of ellipticals which then branched into two sorts of spirals. When Hubble proposed this scheme, some prompt that it indicated the character of galactic evolution, the place spherical, elliptical galaxies regularly become flat, spiral galaxies. Hubble himself was cautious to not assume an excessive amount of concerning the classification scheme. Nonetheless, it does increase an fascinating query about a majority of these galaxies. Why do some change into spirals and others not?
After we take a look at elliptical and spiral galaxies, we discover some fairly clear variations (past their obvious shapes). Spiral galaxies are typically brighter, with numerous younger blue stars. Spirals even have numerous gasoline and mud obtainable to make new stars. It’s this propensity of mud in our personal Milky Approach that creates issues just like the zone of avoidance so bothersome to astronomers. Ellipticals, then again, comprise principally outdated, crimson stars. In addition they are typically largely devoid of the gasoline and mud crucial for star manufacturing.
This would appear to suggest an evolution of galaxies transferring from spiral to elliptical. On this mannequin a younger galaxy develops a spiral construction by density waves within the galactic cloud, which inspires the manufacturing of stars. The biggest and brightest stars dwell brief lives ending in supernovae, which feeds the manufacturing of recent stars for some time. However ultimately the obtainable gasoline and mud is consumed by star manufacturing, and solely the small crimson stars (which can proceed to shine for a trillion years or extra) stay.
Whereas this appears an affordable mannequin, it seems to be a bit too simplistic. After we take a look at the metallicity of stars (that’s, the quantity of components apart from hydrogen and helium they comprise) we discover that stars in elliptical galaxies can have metallicities 3 times increased than that of spiral galaxies. Metallicity is a tough indicator of the age of a star, because the first technology of stars could be nearly totally hydrogen and helium, whereas subsequent generations fashioned from the remnants of earlier stars have more and more increased metallicity. If the distinction between elliptical and spiral galaxies had been merely that ellipticals are older, they should have fashioned earlier within the universe, and thus their crimson stars ought to have decrease metallicity, not increased.
One other clue come from the very fact of the place we discover several types of galaxies. Elliptical galaxies are typically seen in massive superclusters of galaxies, and in dense galaxy clusters. Spirals are typically present in much less clustered areas, and infrequently exist in dense clusters. Within the early universe, the primary clusters of galaxies would are inclined to type bigger and tighter clumps as a consequence of gravity and darkish matter. Extra dense clusters would are inclined to have extra galactic collisions early on.
So the present mannequin is that ellipticals did start as small spiral galaxies, in early galactic clusters, however frequent collisions stirred them up, inflicting them to enter a time of fast star manufacturing referred to as a dusty starburst interval. This brought about them to eat their gasoline and mud early on. Subsequent collisions disrupted any remaining spiral construction, and with out gasoline and mud the density wave impact seen in fashionable spirals couldn’t happen. The dusty starburst interval would clarify why ellipticals have excessive metallicity, and this mannequin additionally explains why we see them within the massive superclusters that fashioned within the early interval of the universe.
The spiral galaxies we see right this moment may very well be youthful galaxies, however they is also older galaxies that weren’t supercharged by early collisions. Their star manufacturing may thus happen at a extra sedate tempo.
As we observe galaxies at rising redshifts (and thus farther again into the historical past of the universe) we see issues corresponding to small spirals in early clusters, so the mannequin appears to work effectively. So evidently Hubble’s quest to know galaxy evolution was heading in the right direction, it was simply the alternative of what his tuning fork implied.