Editing Galactic bulge (section)

== Classical bulges ==
[[Image:Messier 81 HST.jpg|thumb|An image of [[Messier 81]], a galaxy with a classical bulge. The spiral structure ends at the onset of the bulge.]]
Bulges that have properties similar to those of [[elliptical galaxies]] are often called "classical bulges" due to their similarity to the historic view of bulges.<ref>[[Allan Sandage|Sandage, Allan]], ''The Hubble Atlas of Galaxies'', Washington: Carnegie Institution, 1961</ref>  These bulges are composed primarily of stars that are older, [[Population II|Population II stars]], and  hence have a reddish hue (see [[stellar evolution]]).<ref name=Review />  These stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form.<ref name=Review />  Due to the lack of dust and gases, bulges tend to have almost no star formation. The distribution of light is described by a [[Sersic profile]].

Classical bulges are thought to be the result of collisions of smaller structures.  Convulsing gravitational forces and torques disrupt the orbital paths of stars, resulting in the randomised bulge orbits.  If either progenitor galaxy was gas-rich, the [[tidal force]]s can also cause inflows to the newly merged galaxy nucleus.  Following a [[Galaxy merger|major merger]], gas clouds are more likely to convert into stars, due to [[Shock wave|shocks]] (see [[star formation]]).
One study has suggested that about 80% of galaxies in the field lack a classical bulge,
indicating that they have never experienced a major merger.<ref name="kormendy2010">
{{cite journal
   | last1 = Kormendy
   | author-link = John Kormendy
   | first1 = J.
   | last2 = Drory | first2 = N.
   | last3 = Bender | first3 = R.
   | last4 = Cornell | first4 = M. E.
   | title = Bulgeless Giant Galaxies Challenge Our Picture of Galaxy Formation by Hierarchical Clustering
   | year = 2010
   | journal = [[The Astrophysical Journal]]
   | volume = 723
   | issue = 1
   | pages = 54–80
   | doi = 10.1088/0004-637X/723/1/54
   | arxiv = 1009.3015
   | url = http://esoads.eso.org/abs/2010ApJ...723...54K
   | bibcode = 2010ApJ...723...54K
| hdl = 2152/35173
 | s2cid = 119303368
 }}
</ref>
The bulgeless galaxy fraction of the Universe has remained roughly constant for at least the last 8 billion years.<ref name="sachdeva2016">
{{cite journal
   | last1 = Sachdeva | first1 = S.
   | last2 = Saha | first2 = K.
   | title = Survival of Pure Disk Galaxies over the Last 8 Billion Years
   | year = 2016
   | journal = The Astrophysical Journal Letters
   | volume = 820
   | issue = 1
   | pages = L4
   | doi = 10.3847/2041-8205/820/1/L4
   | arxiv = 1602.08942
   | bibcode = 2016ApJ...820L...4S
| s2cid = 14644377
 | doi-access = free
 }}
</ref>
In contrast, about two thirds of galaxies in dense [[galaxy cluster]]s (such as the [[Virgo Cluster]]) do possess a classical bulge, demonstrating the disruptive effect of their crowding.<ref name="kormendy2010" />