About a hundred years ago we thought the Milky Way was the universe. Today we see much further and realise that even the observable part of the universe contains more than 100 billion separate galaxies. They vary in size from dwarf galaxies a few hundred light years across and containing a few million stars, to giants spanning several hundred thousand light years across and containing several trillion stars.
But galaxies contain more than just stars. Vast clouds of gas and dust grains called nebulae (from the Latin word, nebula, meaning cloud) together with an interstellar medium of hydrogen in various states make up about 10% of the visible mass of a typical galaxy. The dust alone is thought to account for 1% of this mass. It is held together, if somewhat haphazardly, by the force of gravity.
Even the mass we see is only part of the story. If astronomers are to be believed far more is hidden from view. Theories abound. But the current perceived wisdom is that “Dark Energy” is responsible for 68% of all mass energy in the universe; and “Dark Matter” makes up 27% (of which 1%-2% is represented by neutrinos). This leaves us with a mere 5% of regular matter, and what we are able to see.
We live in a uniformly expanding universe, a principal established by Edward Hubble. Almost all galaxies are moving away from us, and the more distant galaxies are moving away faster still.
There are an estimated two trillion galaxies in the known universe. It is such a fantastic figure it doesn’t mean much, no more than being told a given domestic bleach kills 99% of all known germs. It’s the unknown that worries me.
Nebula v Galaxy
So what is the difference between a galaxy and a nebula? A hundred years ago astronomers thought they were the same. But probably the only thing they have in common is that they are both deep sky celestial objects. And they are associated.
A nebula is a dense interstellar region of dust and other ionized gasses—particularly helium and hydrogen. Other elements may also be included in smaller but varying amounts, and there will be materials from the remnants of stars left after the end of their lifetime.
On the other hand, galaxies are stellar regions. Nebulae provide the materials to make the stars, and galaxies collect them up into clusters bound by gravity. Galaxies also contain black holes, cosmic dust, and dark matter, and they also contain the interstellar medium, which in turn gives rise to nebulae. In a sense, a galaxy is a star factory forever recycling material.
A nebula is tens to hundreds of light years across. A galaxy is much larger — usually thousands to hundreds of thousands of light years across.
At the centre of most galaxies there is a black hole, extremely small and dense, referred to as a “singularity”. Some galaxies may even have a pair of black holes in a binary system. Black holes are only detected by the behaviour of materials around them, spinning at high velocities, throwing out jets as materials are sucked into the centre.
There are two types of black hole, the supermassive black holes at the centre of galaxies, and “Stellar black holes”, which are the collapsed remains of supernovae. This is where a supergiant star has reached end of life in a massive explosion.
Examples: SS 433 – A stellar black hole; NGC 4438 – A galactic black hole
Naming of galaxies
A galaxy can have many names. As with stars they may have a common name, a Latin derivative and the “M” and/or “NGC” prefix, depending on their catalogue listing. Charles Messier (1730 – 1817) compiled a catalogue of what he determined where “non-comets” and gave them the M prefix. He catalogued 110 in all, but only a few of them turned out to be galaxies. For example, M58 is a normal barred spiral galaxy in Virgo. Probably the most well-known galaxy (after the Milky Way) is the Andromeda Galaxy, under his catalogue number M31. This galaxy is also in the New General Catalogue of Nebulae and Star Clusters (NGC) as NGC 224. As with Andromeda many galaxies are named after their location within a constellation.
Still more galaxies are listed in different catalogues.
The “3C Catalogue of Radio Sources” was first published in 1959 and revised a few years later. It was drawn up at the Mullard Radio Astronomy Observatory, Cambridge, which conducted the first reliable radio survey of the northern sky. Objects in the catalogue are given the prefix 3C. The brightest quasar as seen from Earth, 1.5 to 3 billion light years away, is called 3C 273. The brightest radio source outside the Solar System is the supernova remnant Cassiopeia A (3C 461); and the brightest extragalactic radio source is Cygnus A (3C 405). There is a quasar in the Triangulum Constellation, designated 3C 38 in the catalogue.
There is another, the Markarian Galaxies Catalogue. It is named after Benjamin Markarian who in 1963 drew attention to a class of galaxies that have nuclei with excessive amounts of ultraviolet emissions compared with other galaxies. His catalogue contains a machine-readable version of “Galaxies with Ultraviolet Continuum”. The catalogue contains no galaxies brighter than magnitude 13.0 or fainter than magnitude 17.5.
The catalogue is based on an objective-prism survey using a 40-inch Schmidt telescope at the Byurakan Astrophysical Observatory in Armenia conducted from 1965 to 1978. Taking one entry, as an example, is Markarian 421, sometimes designated MKN 421. This is a blazar located in the Constellation Ursa Major. It is a giant elliptical galaxy with a strong radio source. It is estimated to be from 395 to 435 million light years away.
The Gliese Catalogue of Nearby Stars is a modern catalogue of stars located within 80 light years of Earth. It is named after German astronomer Wilhelm Gliese who in 1957 published his first list of 915 known stars within 65 light years of Earth and detailing their known properties. It was subsequently extended to include stars at a distance of up to 80 light years. An example is GN-z11, a galaxy in the Constellation Ursa Major at a distance of about 13.4 billion light years. Evidently it is the oldest and most distant galaxy known in the observable universe.
It seems the further away an object is in the universe the weirder it is, and the more obscure its nomenclature. What one is to make of “Q0906+6930”, I don’t know. What I am told is that it is the most distant known blazar organised around a supermassive black hole—indeed believed to be the most massive black hole on record. Q0906+6930 is estimated at a distance of 12.3 billion light years away, and further described as a “radio-loud flat-spectrum QSO”. QSO means quasi-stellar object.
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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.
Last updated:Sunday, 21 March 2021