General features

Galaxies within the interstellar medium have a wide range of temperature regions. In this section:

  • Interstellar medium
  • Galaxy rotation
  • Stellar populations
  • Size of galaxies
Interstellar medium

Galaxies within the interstellar medium have a wide range of temperature regions. In the cooler regions, temperatures can be as low as -260°C. Here clouds of hydrogen and other elements coalesce to form stars. There are also neutrally charged clouds of hydrogen: so-called HI regions where temperatures range from  -170°C to +730°C. Ionised clouds of hydrogen heated by stars are HII regions with temperatures around 10 000°C.

Invisible cosmic rays are also part of the interstellar medium. They are highly energetic and consist mainly of ions and electrons. They tend to travel along magnetic field lines and are very dangerous to life. Overall the material in the interstellar medium is very uneven. Supernova blasts, the action of gravitation, swirling galaxies travelling at enormous speeds, and the pull of black holes, are amongst a myriad of other forces that act in a similar way to the atmosphere on Earth. Think of high-pressure weather fronts against low pressure systems, ebbing and flowing, reflected in the rise and fall of millibars.

Temperature regions of the universe
Cooler regions-260°C
HI regionsFrom -170°C to 730°C
HII regionsAround +10 000°C
Galaxy rotation

Something that is a bit of a mystery is the speed at which the materials in a galaxy are rotating. The insane speed at the centre is not in dispute, reflecting the typical environmental conditions of a black hole. But what about the rest of the materials in the galaxy? It is logical to consider stars further out from the centre would travel at a slower rate. After all, isn’t that what happens with our solar system? Mercury, the innermost planet orbits at a speed of 48 kilometres a second. Earth travels at about 30, while Saturn is below 10 kilometres a second and the outermost, runs at barely over 5.

But that is not the case for a galaxy. Everything is travelling at the same speed. It does not make sense. What also does not make sense is that the speed they do travel at is beyond escape velocity and they should by rights leave the galaxy altogether.

The Andromeda Galaxy
In her study of the Andromeda Galaxy Vera Rubin discovered that the stars in the outer spirals of the galaxy were orbiting at the same speed as the stars near the centre. Image source: Adam Block /Steward Observatory/University of Arizona

In 1968 astronomer Vera Rubin (1928 – 2016) at Kitt Peak Observatory in southern Arizona, was studying the Andromeda Galaxy, and she came up with the answer. She found that its “rotation curve” was flat, meaning the stars in the outer spirals of the galaxy were orbiting at the same speed as the stars near the centre. Indeed, this rate of knots at the outer spirals was so fast that they should have left the galaxy. The mass of the visible matter did not provide enough gravity to counteract the outward centrifugal force. There needed to be more gravity.

And the answer was Dark Matter, lots of it. We now know, of course, that dark matter and dark energy together account for 95% of the mass of the universe, and therefore 95% of the gravitational forces.

Stars in a given galaxy all travel at the same speed (innermost and outermost), at approximately 220 kilometres per second, she noted. A dark matter force had to account for this.

Stellar populations

Stellar populations are broadly classified into two groups (though there is a third theoretical group):

  • Population I – the youngest stars with a greater proportion of heavy elements.
  • Population II – older, metal-poor stars.

Population II stars are the oldest, with typical age ranging from 11 billion to 13 billion years. They are the spherical component of a galaxy and found in the bulge and halo. They are metal poor, with 0.1% or less of metals. In other words, they are mostly made up of hydrogen and helium.

Population I stars are the youngest. Many are around a billion years old though there are some members found by astronomers that are as old as 10 billion years. They are “metal rich” in the sense that between 2%-3% of their materials are metals. They are found only in the disk and spirals of galaxies and follow nearly circular orbits around the galactic centre, .

In the Milky Way, for example, most of the disc hosts Population I stars, which have themselves been created from materials of existing stars. On the other hand, Population II stars can be found in the galactic halo. These are gathered in globular clusters where most of the star-making soup has been depleted, so no new star-forming can take place.

By definition, each population group is characterised by decreasing metal content and increasing age of the stars. There is a hypothetical population group with virtually no metals at all. They are extremely massive, luminous, hot stars and are very old—the Population III group.

Size of galaxies

The average galaxy contains 100 billion stars, and they were all born in nebulae.

Giant galaxies

Giants span several hundred thousand light years across and contain several trillion stars.

At 5.5 million light years in diameter, IC 1101 is the biggest galaxy in the universe, and may contain 100 trillion stars. It is classed as a supergiant elliptical galaxy and lies at the centre of the Abell 2029 galaxy cluster, a billion light years from Earth.

Average galaxies

However, most galaxies are far more modest—a mere 3 000 light years to 300 000 light years across. In this respect the Milky Way is well placed at up to 200 000 light years across. It is certainly not the smallest in the room, but it may be one of the quietest! Indeed, as galaxies go the Milky Way is exceptionally quiet, rarely devouring anything. Astronomers suggest that magnetic fields in its centre may be steering gas and dust away into orbit around the black hole rather than allowing it to pour over the event horizon. The galaxy is also thought to contain many inert stars.

Dwarf galaxies

At the bottom of the league stand dwarf galaxies. Many of these are about 200 light-years across and contain only a few tens of millions of stars, with a mass little more than that of a star cluster.

There are between six to eight dwarf galaxies that are orbiting the Milk Way between 200 000 light years and 650 000 light years away. Each dwarf hosts between one million to 50 million stars.

Like globular clusters dwarf galaxies have no gas and no star formation. Globular clusters are a closely packed group of old stars, whereas dwarf galaxies have very little density. Measurements indicate that dwarf galaxies contain a lot of dark matter.

There are ten times as many dwarf galaxies as there are large galaxies.

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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