Lenticular Galaxy is a galaxy that is somewhere between Elliptical and Spiral galaxies. They exhibit characteristics of the other two galaxies. The galaxies will have a central bulge and disks but will not have arms that you would find in spiral galaxies such as our own, the Milky Way. The name comes from the fact that the galaxy looks like a lens, a bulge in the middle but no extending arms outwards. They will have a disk like a spiral but the bulge is more prominant.
As you can see from the illustration below, courtesy of Nova Celestial, the position where Lenticular Galaxies appear in the galaxy classification system. The classification scheme was devised by Edwin Hubble, the same person who the Hubble Space Telescope was named after. Edwin Hubble's greatest achievement was to show that the Universe was expanding. laying the foundatons for the Big Bang theory.
When the galaxies are view head on, they can be mis-identified as being elliptical which are tighter galaxies compared to the other forms. Swinburne. This can come when the central bulge is not very bright. SDSS
There are two types of Lenticular Galaxies, the are the barred Lenticulars such as NGC 936, affectionally known as Darth Vader's Galaxy after the character Darth Vader in the Star Wars films. The galaxy looks like a T.I.E. Fighter.
The second type are non-Barred Lenticulars which don't have a bar, simple... An example of a non-barred lenticular is Lindsay-Shapley Ring which has a bright bulge than the ring even if the ring does look a bit perculiar. If you compare that with the Andromeda Galaxy, you will see that centre is not brighter than the rings.
Lenticular galaxies tend to be old galaxies that have used up most of their gas and dust and are just living the twilight years. If you don't have gas and dust, you are not able to create new stars. The twilight years will be very long.
Its not to say that star creation has stopped totally in the galaxy but the amount is very small compared to other galaxies. They way they determine whether its an old or young galaxy is by looking at the colours of the stars, if the stars are mainly red, then the stars assumed to be old stars whereas blue stars are young stars.
Blue Stragglers and vampire stars are old stars that leach gas and plasma from other close by star and in doing so become hotter and look younger than they really are.
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