Starship Asterisk*

Small starbursting galaxy NGC 1705 looks remarkably similar to the supercluster in NGC 6946.


Note that both NGC 1705 and the supercluster in NGC 6946 display a small dense "super-condensation" of stars, surrounded by a loose "halo" of stars.



But the supercluster in NGC 6946 is just a small part of the galaxy - visible at upper right in the picture above - while in NGC 1705 the brilliant cluster is the galaxy.


NGC 1705 is slightly similar to the "extreme compact blue galaxy" I Zwicky 18:


NGC 1705 also resembles supercluster R136 in the Tarantula Nebula in the Large Magellanic Cloud:



In compact blue dwarf galaxies, a supercluster (or possibly two superclusters) dominate the entire galaxy. In larger galaxies, these superclusters are just a small part of the galaxy as a whole.

Read about the new Hubble image of NGC 1705 here.

Ann

I said in my previous post that in small starbursting galaxies, the cluster is the galaxy. But that is not correct.

In a cluster, all stars were formed at more or less the same time. But a galaxy has undergone several episodes of star formation and contains stars of different ages, except in the very first galaxies, when no previous episodes of star formation could have occurred.


Astronomers used to believe that the extreme starbursting blue compact galaxy I Zwicky 18 might be a truly young galaxy formed just a few million years ago, but it turned out not to be the case:

Wikipedia wrote:

Galaxies resembling I Zwicky 18's youthful appearance are typically found only in the early universe. Early observations with the Hubble Space Telescope suggested an age of 500 million years old for I Zwicky 18.

From a theoretical point of view, astronomers have hypothesised the existence of young dwarf galaxies in a cold dark matter-dominated Universe. Dwarfs as young as 500 million years would be, however, much fainter than I Zwicky 18. I Zwicky 18 is thus unlikely to have formed all its stars recently in a cold dark matter Universe.

Later observations with the Hubble Space Telescope found faint and old stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies.

So I Zwicky 18 is not "a cluster", but a true galaxy containing stars of different ages.

Ann