Facts About Elliptical Galaxies

Observations about elliptical galaxies were incidentally recorded by a French astronomer named Charles Messier (1730 - 1817), while he was looking for comets.

Charles Messier made a huge contribution to the field of astronomy by cataloging various astronomical objects. Though Messier was only interested in locating comets, he compiled a list of astronomical objects he observed during his search for comets.

In the process, he recorded observations about astronomical objects such as star clusters, nebulae, galaxies, etc. His work Catalogue des Nébuleuses et des Amas d'Étoiles, which translates to Catalogue of Nebulae and Star Clusters, was published in 1771.

While the final list that was published included 103 objects, there's evidence of seven more objects being observed by Messier or Méchain. So, this catalog includes a total of 110 objects. Charles Messier described galaxies as 'fuzzy objects'.

So, what are galaxies? Basically, galaxies refer to a system of aggregates of stars, planets, gas, and dust that are held together by gravity. It is believed that there are more than 100 billion galaxies in the observable universe.

It was an American astronomer named Edwin Hubble who broadly classified galaxies into elliptical and spiral. He used a diagram to explain the classification. This tuning fork diagram was later conceived as being an oversimplification of the complex process of the evolution of galaxies.

Galaxies could also be barred spiral (a type of spiral galaxy), lenticular (disc-shaped with a large bulge in the center but without spiral arms), and irregular (having an irregular shape).

The size of galaxies varies, ranging from dwarf galaxies that comprise tens of millions of stars to giant galaxies that might consist up to a trillion stars. As far as the shape is concerned, the galaxies could be spiral or elliptical. Spiral galaxies are large and have active star formation.

The pinwheel shape of spiral galaxies is created, as dust and gas circles the center at the speeds of hundreds of miles per second. The dust and gas are responsible for the formation of stars, which is why, these galaxies have new stars.

In case of 'barred spiral' galaxies, the dust and gas form a bar-like structure at the center. Spiral galaxies have a dense nucleus and multiple arms.

✦ Elliptical galaxies are round/oval in shape.

✦ These are smaller than spiral galaxies, and have less dust.

✦ The characteristic feature of elliptical galaxies is that new star formation is a rarity.

✦ Elliptical galaxies are commonly found in huge clusters of galaxies.

✦ Due to the distant red elliptical galaxies, it is believed that the stars of these galaxies were formed quite early in the history of the universe.

✦ Stars in elliptical galaxies are present evenly around the center of the galaxy in all directions. The brightness decreases outward from the center. The color appears to be reddish.

✦ In Hubble's tuning fork diagrammatic representation, elliptical galaxies are classified as E (E0 to E7), with the number indicating the shape. While E0 is almost spherical, as the number increases from 1 to 7, the length increases in relation to its width, thereby making the galaxy more elliptical.

Well, it is believed that the reddish color comes from older, cooler stars. This implies that most ellipticals might have been formed long ago. Moreover, the uniformity of the color throughout the galaxy suggests that a majority of the stars might have been formed around the same time. Most elliptical galaxies can be found in galaxy clusters.

A majority of galaxies in such clusters are elliptical. For example, the Coma Cluster is a famous cluster that contains thousands of galaxies. Most of these galaxies are elliptical or lenticular. This cluster is dominated by two enormous elliptical galaxies called NGC 4874 and NGC 4889.

They have a diameter of more than 250,000 light years, and are quite large in comparison to galaxies in the Virgo supercluster.

✦ The largest galaxies in the universe are giant elliptical galaxies. They can contain a trillion stars or more, and span as much as two million light-years - about 20 times the width of the Milky Way.

✦ Some of them appear to contain massive black holes that are as much as three billion times as heavy as the Sun.

✦ Hubble classified galaxies on the basis of their shape. Elliptical galaxies are classified from E0 to E7. These numbers signify their shape or ellipticity, with E0 being circular and E7 being significantly flattened. Here are the elliptical galaxies that were recorded by Messier:

• Dwarf elliptical galaxy NGC 221 Messier number M32
• Elliptical Galaxy NGC 4472 M49
• Elliptical Galaxy NGC 4621 M59
• Elliptical Galaxy NGC 4649 M60
• Elliptical Galaxy NGC 4486 M87
• Elliptical Galaxy NGC 4552 M89
• Elliptical Galaxy NGC 3379 M105
• Dwarf Elliptical galaxy NGC 205 M110

NCG stands for New General Catalog, whereas M stands for Messier designations for objects that were observed and recorded.

✦ Elliptical galaxies include some of the brightest galaxies that are about 40 times as bright as the Milky Way, and dwarf galaxies that are 100 times fainter than the Milky Way.

✦ Some of the elliptical galaxies are found as companions to huge galaxies. For instance, the dwarf elliptical galaxy NGC 221 (Messier number M32) is a companion to the Andromeda nebula, M31).

In the 1940s, Walter Bade classified stars observed in the galaxies into two groups: Population I and Population II. This classification was made on the basis of the abundance of metal. While population I stars are often present in the discs or the arms of spiral galaxies, Population II stars are found in the nucleus of the galaxy or the globular clusters. These are metal-poor (low metallicity).

Population I stars are newer, more luminous, and hotter than Population II stars. These stars are present in elliptical galaxies, and they form very early in the galaxy's history of star formation. Stars might also be classified on the basis of the thickness of the disc, halo, or the bulge of the galaxy.

✦ Dwarf elliptical galaxies are the most common type of galaxies that have been found to date.

✦ Galaxies can exist on their own, or can be found in groups, clusters, or superclusters. Rapid star formation could occur when galaxies merge.

✦ Some of the largest galaxies in the universe are known to be giant elliptical galaxies that are two million light years long and contain a trillion stars.

✦ Many galaxies have black holes at their center.

✦ It is believed that the Sagittarius dwarf elliptical galaxy (a satellite galaxy) is in the process of being absorbed by the Milky Way.

✦ A giant elliptical galaxy called M 87 has a diameter of 120,000 light years.

✦ There is considerable debate in the literature over Centaurus A's Hubble type, which means that some consider it to be a giant elliptical galaxy, while others think that it might be a lenticular galaxy.

✦ M 87 has a collection of about 15,000 globular clusters, in comparison to 200 globular clusters of the Milky Way.

Though Edwin Hubble had thought that elliptical galaxies would later turn into spiral ones, this assumption was proven to be incorrect. Unlike spiral galaxies, elliptical ones are believed to have old stars, with very less likelihood of new star formation.

Though it was believed that star formation was lacking on account of the galaxies being poor in cool gas that is required for the formation of new stars, Herschel Space Telescope shows that elliptical galaxies are present in parts of the Universe with plenty of cool gas.

It is believed that hot gas does cool down, but it doesn't turn into stars, as jets from the central super-massive black hole heat up the gas, or the gas might be expelled from the galaxy.

Recently, young globular clusters were found within old galaxies, which raises questions on the assumption about the formation of these galaxies during a single period at an earlier time in history, as well as the star formation in giant elliptical galaxies. Thus, there might be possibilities of major star formation in recent times.

The vastness of the universe is unfathomable, and many of its aspects remain shrouded in mystery. Though technological advancements have made it possible to study celestial objects in the observable universe, mysteries surrounding its origin and the developments that take place will continue to mesmerize us.