Saraswati supercluster

Recently, a group of scientists from IUCAA (Pune), IISER (Pune), NIT Jamshedpur and Newman College (Thodapuzha) announced that they have discovered a very large “supercluster” of galaxies. They have named it Saraswati. Here is a brief explainer on the discovery and its background.

What are galaxy clusters? How big are they?

Galaxies are like the building blocks of the universe, they contain a huge number of stars, something like 100 billion at a count. Galaxy groups can have three to 20 galaxies, the richest systems are called clusters (like the Virgo cluster) which can have several hundred galaxies.

Superclusters are clusters of clusters. They can have as few as two clusters, and superclusters with two to four clusters are common. Saraswati has 42.

Within superclusters, clusters are connected by filaments and sheets of dark matter with galaxies embedded in them.

It is supposed that the galaxies are born in the filaments and then migrate towards the intersection of the filaments where they are assimilated into clusters.

What are superclusters?

These are the largest coherent structures seen in the universe. Firstly there are clusters of galaxies together with associated gas and dark matter. Large groups of such clusters, linked by filaments, separated by voids together form the superclusters. Though initially a supercluster was used to describe groups of two-four clusters, now it is understood that much larger superclusters, comprising clusters that number an order of magnitude higher, exist. The first such large supercluster to be discovered was the Shapley supercluster.

How does the Saraswati supercluster compare with the Milky Way?

The newly discovered Saraswati supercluster is 600 million light years across. The Milky Way is 150,000 light years across.

Where does the supercluster Saraswati lie in the sky?

The supercluster Saraswati lies in the Stripe 82 of the Sloan Digital Sky Survey. It is about 4000 million light years away from us. It is in the constellation of Pisces.

What is “Stripe 82 region of SDSS”?

SDSS stands for the Sloan Digital Sky Survey. This is an ambitious plan to make a digital 3D map of the universe. Started in 2000, it has, over eight years, mapped more than a quarter of the sky. It has mapped nearly 930,000 galaxies. The SDSS has found nearly 50 million galaxies so far.

In its third phase, SDSS-III, which started in 2008 and ended in 2014, gave out sets of data that were released in 2011, 2012 and 2013. It produced a map of the North galactic cap which stretched to 7500 square degrees and of three stripes in the South Galactic Cap which added to 740 square degrees. The central stripe is known as Stripe 82.

What is the significance of this discovery? Is there a puzzle posed by the discovery of Saraswati?

Spotting a supercluster which is 4000 million light years away means that you are looking at light that has come in from four billion years ago. This is because a light year is the distance travelled by light in one year. Since the universe is believed to be 13.8 billion years old, this means we are looking at light from when the universe was about 10 billion years old, just about 70% of its present age.

This poses a puzzle. According to present theories, it is difficult for such a huge galaxy to have formed so early in the universe’s lifetime.

When was the term Galaxy cluster first used, in what context?

In 1926, Harlow Shapley and Adelaide Ames were the first to coin the term “cluster” to describe a collection of galaxies. They used this to describe the Coma-Virgo region. The term “Virgo cluster” was first used by Edwin Hubble and Milton Humason in 1931.

Is there any special way of discovering a supercluster? For instance, a star has a definite boundary, but how do you discern the boundaries of a supercluster?

It is very hard to outline the boundaries of a supercluster. It is done by studying maps such as the Sloan Digital Sky Survey in an exceedingly detailed manner. In the context of Saraswati supercluster, the astronomers could delineate 43 clusters of galaxies connected by a network of filaments.

Is Saraswati the first supercluster that Indians have discovered? Which was the first?

Professor Somak Raychaudhury, who is presently director of Inter University Centre for Astronomy and Astrophysics, Pune, had discovered a supercluster as part of his PhD thesis work at University of Cambridge. It was named “Shapley Supercluster,” after the American astronomer Shapley who first coined the term cluster. Shapley’s work in measuring the extent of galaxies is remarkable and a part of extending the Copernican programme – establishing that the earth is not the centre of the universe.


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