Elliptical and Lenticular Galaxies

Elliptical galaxies are very different from spirals in that they have very little in the way of internal structure. Even to the largest telescopes, they only appear to be uniform patches of light that form an elongated shape. These galaxies have very little in the way of interstellar matter such as dust and gas traveling between stars. This gives the suggestion that the elliptical galaxies are extremely old. Elliptical galaxies vary tremendously in size from the tiny companion galaxy M32 to the massive M87. These galaxies are characterized in the Shapely system with the letter "E" followed by a number 0 through 7 with the lower numbers being the closest to spherical in shape. In reality the elliptical galaxies for the most part are shaped like footballs, with different measurements on each of its three axes. The Messier catalogue lists a total of eight elliptical galaxies.

Lenticular galaxies are disk-shaped galaxies that also lack any internal structure, so they were often mistaken for elliptical galaxies. The Shapely system actually gives them an "S" designation followed by a zero. These "S0" galaxies can also be found among the Messier list in small numbers, there are three and possibly four. Galaxies take on a lenticular shape as the result in many billions of years of uninterrupted organization. In both elliptical and lenticular galaxies, the common theme is extreme age.

With the many thousands of galaxies in the western Virgo area, there have to be some choice elliptical galaxies to look at and our first is M49. The best time for viewing this galaxy is during early spring when western Virgo is in opposition to the Sun. At a distance from Earth of some sixty million light years, it shines at magnitude +8.4 and spans about 9 by 7 arc minutes. At that distance from Earth, that means the galaxy shines at absolute magnitude -22.8 and joins M60 and M87 as one of the truly giant galaxies of the Virgo Cluster. It is about 160,000 light years along its major axis, or the longest dimension of the ellipsoid. That number is subject to considerable uncertainty because with an elliptical galaxy it's hard to tell exactly what direction that major axis is pointed in. The galaxy is very elongated and rates an E4 on the Shapely system, which suggests that it may well be shaped exactly like a football. The galaxy has an integrated, or average spectral type of G7. This makes its overall color somewhat more yellow than typical galaxies in the Virgo cluster. This suggests that it may be a bit younger than the average age of galaxies in the cluster.

M87 is the signature galaxy for elliptical galaxy chasers. This monstrous galaxy may also be the gravitationally dominant galaxy of the entire Virgo supercluster. Unlike the more football-shaped M49, M87 is a Shapely-type E1, indicating it is nearly spherical in shape. It is also large. Spanning 7 arc minutes of sky from sixty million light years away makes it some 120,000 light years across. The galaxy shines at magnitude +8.6 from that distance also making it very intrinsically bright, around absolute magnitude -22. Though M87 is slightly smaller in size than M49, it appears to be significantly denser than M49. M87 is also noteworthy for three other reasons that are not readily observable to amateurs. First, it has the largest known system of globular clusters of any galaxy that we've been able to account for. While the Milky Way has some 200 globulars (of which we know of 151) M87 may possess as many as 15,000 of them! A second fascinating feature that required very large telescopes to observe is a jet emanating from the galaxy's core. Early Hubble images (predating the 1993 repair) show the jet clearly emerging from a supermassive central object at the core that may possess as much as three billion times the mass of the Sun. The jet was first discovered in 1918. In 1966 a second jet was discovered extending in the opposite direction, although since it points more away from our line of sight, it is not as obvious. Third, M87 is a powerful source of electromagnetic energy from low-energy radio waves (designated Virgo A) to being the center of an enormous sphere of high-energy x-rays. The center of M87 is extremely active although there is very little gas and dust in the galactic core are, it does qualify as having an active galactic nuclei although without large amounts of gas to be excited, it cannot become a Seyfert-type galaxy. Still not even many Seyferts have a feature like M87's powerful jet. M87 is one of the most amazing places to study in the entire universe as it shines in every wavelength of light in the spectrum. It is conveniently placed in Virgo allowing anyone on Earth to see it. And it may well be home to the largest known suspected black hole in the cosmos. There's quite a bit in that little smudge of gray light.

From the enormous to the tiny, the elliptical galaxy M32 is a satellite of mighty Andromeda. This galaxy also shines at magnitude +8.1, which actually makes it slightly brighter than giant M49 or M87, but this is a function of perspective since M32 is less than 1/20th the distance from Earth of the two giant ellipticals. The galaxy spans 8 by 6 arc minutes and has a mass of about three billion Suns. That's about the same mass as the supermassive central object at the center of M87. The galaxy spans a linear diameter of some 8,000 light years along its major axis. A large amount of mass in M32 is concentrated close to the core; some 5,000 solar masses per cubic parsec and these stars are in rapid motion around a supermassive central object in the core. M32 has an average spectral type of G3, which would indicate that this old galaxy is contaminated with a sprinkling of younger stars of about two to three billion years in age. Otherwise, M32's stellar population is largely intrinsically faint orange and red dwarves and non-nuclear shining degenerate white dwarves. M32's population strongly resembles that of a much larger elliptical, say like M49. It is possible and even likely that M32 was once a much larger galaxy that has since lost of its stellar mass to Andromeda.

Going back into the Virgo supercluster once again, just about 1.3 degrees to the west of M87 is the lenticular galaxy M86. This Shapely type S0 galaxy shines at magnitude +8.9 and spans about 7 by 5 arc minutes from sixty million light years away. This makes it another intrinsically bright giant like M49 and M87 spanning perhaps a diameter slightly larger than that of the Milky Way. Another interesting facet of Virgo supercluster galaxies that we talked about previously is their speeds as they move around the cluster's center. M86 has the largest blue shift of all the Messier galaxies, approaching Earth at some 419km/sec. M86, though it is not an elliptical by shape, is probably populated much like ellipticals, consisting of intrinsically faint small main sequence dwarves and degenerate white dwarves.

While spirals are galaxies that are vibrant with life, forming new stars regularly in its arms, ellipticals and lenticulars are galaxies that have ended their star-forming lives and shine with the light of expired white dwarves and faint low-mass main-sequence stars. Let's next have a look at unusual galaxies that seem to have no organized structure at all.

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