Editing Hubble sequence (section)

==Physical significance==

Elliptical and lenticular galaxies are commonly referred to together as "early-type" galaxies, while spirals and irregular galaxies are referred to as "late types". This nomenclature is the source of the common,<ref>{{cite journal |last=Baldry |first=I.K. |date=2008 |title=Hubble's Galaxy Nomenclature |journal=[[Astronomy & Geophysics]] |volume=49 |issue=5 |pages=5.25–5.26 |doi=10.1111/j.1468-4004.2008.49525.x |bibcode=2008A&G....49e..25B |arxiv = 0809.0125 }}</ref> but erroneous, belief that the Hubble sequence was intended to reflect a supposed evolutionary sequence, from [[elliptical galaxy|elliptical galaxies]] through [[lenticular galaxy|lenticulars]] to either [[barred spiral galaxy|barred]] or [[spiral galaxy|regular spirals]]. In fact, Hubble was clear from the beginning that no such interpretation was implied:
<blockquote>
The nomenclature, it is emphasized, refers to position in the sequence, and temporal connotations are made at one's peril. The entire classification is purely empirical and without prejudice to theories of evolution...<ref name="hubble27">{{cite journal |last=Hubble |first=E.P. |author-link=Edwin Hubble |date=1927 |title=The Classification of Spiral Nebulae |journal=The Observatory |volume=50 |pages=276 |bibcode=1927Obs....50..276H}}</ref>
</blockquote>
The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young [[star]]s and regions of active [[star formation]], while elliptical galaxies are composed of predominantly old stellar populations. In fact, current evidence suggests the opposite: the early Universe appears to be dominated by spiral and irregular galaxies. In the currently favored picture of [[galaxy formation]], present-day ellipticals formed as a result of mergers between these earlier building blocks; while some lenticular galaxies may have formed this way, others may have accreted their disks around pre-existing spheroids.<ref>{{cite journal |bibcode=2015ApJ...804...32G |title=Hiding in Plain Sight: An Abundance of Compact Massive Spheroids in the Local Universe |last1=Graham |first1=Alister W. |last2=Dullo |first2=Bililign T. |last3=Savorgnan |first3=Giulia A. D. |journal=The Astrophysical Journal |date=2015 |volume=804 |issue=1 |page=32 |doi=10.1088/0004-637X/804/1/32 |arxiv=1502.07024 }}
</ref> Some lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation,<ref name=ChristensenMartin2010>{{cite book |author1=Christensen, Lars Lindberg |author2=de&nbsp;Martin, Davide |author3=Shida, Raquel Yumi |date=7 April 2010 |title=Cosmic Collisions: The Hubble atlas of merging galaxies |publisher=Springer Science & Business Media |isbn=978-0-387-93855-4 |page=24 |url=https://books.google.com/books?id=QhRAokTI2RYC&pg=PA24}}</ref> although the galaxy [[LEDA 2108986]] opens the debate on this.