A Brief Guide to the Images

The Scale of the Universe

One of the most difficult things to comprehend about the Universe is its enormous scale in both space and time. Here is a brief summary:

A light year is about 9,500,000,000,000 km
The Earth is 8.3 light minutes from the Sun (150,000,000 km)
Pluto is about 5½ light hours away
The nearest star is 4.3 light years away
The nearest galaxy (the Large Magellanic Cloud) is 160,000 light years away
Our Galaxy contains more than 200 billion stars
The observable Universe contains more than 200 billion galaxies
The Universe is about 13.8 billion years old
The radius of the observable Universe is 46 billion light years

Nebulae

Nebulae are clouds of gas and dust that occupy areas of the Milky Way and other galaxies. There are four broad classes, all except the planetary nebulae being many light years across and very diffuse.

Emission nebulae are clouds of gas that have been ionised by nearby hot stars. The gases are predominantly hydrogen, sometimes oxygen, plus other much less abundant ionised gases: sulphur, nitrogen etc. The predominant colours are magenta (hydrogen) and teal (oxygen) but these can be modified by dust and gas between nebula and observer. Commonly the magenta tends toward red due to this scattering, and the teal can be severely attenuated or absent.

Reflection nebulae are clouds of gas and dust reflecting the light from stars which has not ionised the gas. Their colour is generally somewhat bluer than the starlight they reflect due to stronger scattering of blue light. It can range from brown through to blue.

Dark nebulae are clouds of gas and dust unlit by stars, which attenuate the light coming from more distant areas of the galaxy, sometimes almost completely blocking it. There are very large expanses of dark nebulae in the plane of the Milky Way, which are readily seen if you look up at the Milky Way on a moonless night well away from city light pollution.

Planetary nebulae are expanding shells of gas ionised by star cores that have collapsed because they have run out of fuel. They are relatively small and usually red and teal in colour. If the collapsing star is big enough, it explodes as a supernova, leaving a rapidly expanding remnant of turbulent ionised gas.

Galaxies

Galaxies consist of stars, planets, and vast clouds of gas and dust, all bound together by gravity. The largest have trillions of stars and can be more than a million light-years across. The smallest can contain a few thousand stars and span just a few hundred light-years. Most large galaxies have supermassive black holes at their centers, some with billions of times the Sun’s mass.

Galaxies come in a variety of shapes, mostly spirals and ellipticals, as well as those with less orderly appearances, usually dubbed irregular.

Most galaxies are between 10 billion and 13.6 billion years old. Some are almost as old as the universe itself, which formed around 13.8 billion years ago. Astronomers think the youngest known galaxy formed approximately 500 million years ago. Data from the James Webb Space Telescope may change some of these estimates.

Galaxies can organize into groups of about 100 or fewer members held together by their mutual gravity. Larger structures, called clusters, may contain thousands of galaxies. Groups and clusters can be arranged in superclusters, which are not gravitationally bound. Superclusters, empty voids, “walls” of galaxies, and other large-scale structures make up the cosmic web of matter in the universe.

Globular Clusters

A globular cluster is a spherical conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centres. They can contain anywhere from tens of thousands to many millions of member stars.

Globular clusters are found in nearly all galaxies. In spiral galaxies like the Milky Way they are mostly found in the outer spheroidal part of the galaxy – the galactic halo. The Milky Way has more than 150 known globular clusters, and there may be many more.

The origin of globular clusters and their role in galactic evolution are unclear. Some are among the oldest objects in their galaxies and even the universe, constraining estimates of the universe’s age. Star clusters were formerly thought to consist of stars that all formed at the same time, but nearly all globular clusters are now known to contain stars that formed at different times, or that have differing compositions.

Sources: ESO, NASA, Wikipaedia

The colours of nebulae, galaxies and globular clusters are generally not visible to the eye, except through quite large telescopes, because the eye does not see the colour of very faint objects. However, the camera does, so the best way to appreciate the wonders of these clouds is through photography.