The sky is full of wonderful deep-sky objects to observe and photograph, but the variety of these can also be overwhelming. Let’s have a look at the main types of DSO!
First of all, in astronomy and astrophotography alike, a distinction is made between Solar System Objects and Deep Sky Objects. The main reason is that these are very different kind of targets, and they generally require a slightly different type of equipment to be observed and photographed.
Deep-sky objects represent anything that is alien to our solar system, such as distant stars, galaxies and nebulae. On the other hand, Solar System Objects include the Sun, the Moon and all the planets, but also comets and asteroids orbiting the Sun.
1. Galaxies (and Galaxy Clusters)
Galaxies are extremely big systems, and are actually the only type of deep-sky objects located outside the Milky Way. They host billions of stars, as well as nebulae and even black holes…
In the observable universe, scientists estimate the number of galaxies to be between 100 and 200 billions.Some of these are regrouped in clusters. But that is very hard to estimate, and the light that our telescopes receive, is sometimes billion years old… So who knows how many of them there are today, and what happened to them while the light they emit was reaching us?
Galaxies come in various sizes and shapes, and they actually work like a giant solar system. The billions of objects contained in a galaxy are orbiting its core, obeying the rules of gravity. Today, scientists believe that the core of some galaxies (including our galaxy, the Milky Way) contains a supermassive black hole, something so dense and heavy, that the whole galaxy would be orbiting it.
Evidence suggests that galaxies were formed by the action gravity, bigger galaxies attracting and eventually merging with smaller galaxies. These galactic collisions can be observed and photographed, like the famous Whirlpool Galaxy. The observation of these galaxies (and a phenomenon called red shift) also helped demonstrate that the universe is expanding.
From the Earth, the closest and biggest galaxy that can be photographed is the Andromeda Galaxy. And it will be easier to do so in the future: in a few billion years, the Andromeda will collide with our own galaxy, the Milky Way, and they’ll both create a new structure. I can’t imaging what a show that would be from the Earth!
In our skies, the Andromeda Galaxy is 4 to 5 times wider than the Moon… And, sadly, much dimmer. But still bright enough to take amazing pictures with standard equipment!
2. Star Clusters
Star clusters are groups of stars, held together by the gravitational force they exert on each other. There are two types of clusters:
- Globular clusters, which can be made of up to about a million stars.
- Open clusters, much smaller groups (up to about a hundred stars) but also much younger. The Pleiades are an open cluster, and can easily be spotted in the winter skies as a bright and compact group of stars.
Nebulae are probably the most beautiful deep-sky objects you can ever photograph. It’s difficult not to be amazed by the wonderful nebulae photographed by the Hubble Space Telescope! They are very colorful objects, and come in various shapes and sizes. Interestingly, some of these nebulae are the birthplace of our stars.
The term nebula comes from the Latin for cloud. Until recently, they were mysterious objects, and the term nebula actually included some galaxies and star clusters as well (i.e. anything diffused, cloudy).
Today, however, we know a bit more about these interstellar clouds. Scientists identified different types and sub-types of nebulae:
- Emission Nebulae
- “Standard” Emission Nebula (e.g. Ha region)
- Reflection Nebula
- Planetary Nebulae
- Supernova Remnants
- Absorption Nebulae (or Dark Nebula)
To photograph a nebula, a simple camera is enough. However, you can get much better results with a modified camera (especially for emission nebulae, mostly red), as well as with dedicated filters (e.g. Ha and OIII).
3. Emission Nebula
An emission nebula is a giant cloud of ionized gas and particles, glowing due to its proximity with one or several young and very hot stars.
The energy emitted by these nearby stars ionizes the atoms of gas, which in turn emit photons and produce a colored glow. The color of such nebula mainly depends on its composition:
- Hydrogen alpha (Ha) produces red, and is usually the most present element (about 90%)
- Hydrogen beta (Hb) produces blue
- Oxygen III (OIII) produces green
- Sulfur II (SII) produces red as well
4. Reflection Nebula
Reflection nebulae are different. They also reflect the light of nearby stars, but these stars aren’t hot enough to ionize the gas surrounding them (usually because the nearby stars are in formation).
Instead, we witness a phenomenon called light scattering: the light emitted by the nearby stars, is reflected by the dust and particles in the nebula. Therefore, acting like a mirror, this reflected light is very similar to that emitted by the nearby stars.
However, the amount of light being scattered depends on the wavelength: short ones (blue) are more scattered than long ones (red). This is the reason
why reflection nebulae are most of the time blue, but also why our sky looks blue!
The Pleiades star cluster is a famous reflection nebula. You can clearly see on this picture that the dust clouds near the main stars are blueish, while the rest of the clouds, further away, are much darker and grayer.
A reflection nebula is often seen together with an emission nebula, in which case they form what scientists call a diffuse nebula. The Trifid Nebula is an amazing object, as it contains an emission nebula (red) and a reflection nebula (blue), but also some absorption nebulae.
5. Supernova Remnant
A supernova remnant is another type of emission nebula. Within a star, there’s a constant battle between nuclear reactions (pushing) and the star’s own gravity (pulling). During most of the star’s life, it’s a rather balanced fight and the star is in a stable state.
Eventually, though, the star will run out of combustible. When this happens, there can be different outcomes, depending on the characteristics of the star (type, size, mass…). But sometimes, when the nuclear reactions cease, the gravity takes over and the star start collapsing on itself, before releasing in a huge explosion (a supernova) the gas contained in the outer layers (mostly hydrogen).
The Crab Nebula is an example of a very young supernova remnant. It is believed that the explosion took place in the 11th century, because it was observed and recorded by astronomers in China and Iraq.
6. Planetary Nebula
These nebulae have actually nothing to do with planets! They were named planetary nebulae, because the first observers believed the core was actually a planet.
Planetary nebulae have a lot in common with supernova remnants: they are both created when a star dies and collapses; they are both made of ionized gas, ejected by the star; and they are both hot enough to make this ionized gaz shells glow.
The key difference, however, is that for a planetary nebula, the star doesn’t explode in a giant supernova. They indeed look less chaotic. A supernova remnant is also much hotter and emit X-rays.
7. Dark Nebula
Dark nebulae are most peculiar objects, because they’re… dark. Unlike other observable deep-sky objects, they don’t emit nor reflect any light. Thus the name absorption nebula.
A dark nebula is a very dense cloud of interstellar dust, so dense in fact, that light cannot penetrate it. Any star, galaxy or nebula located behind that cloud of dust, won’t be visible.
A very famous example is the Horsehead Nebula. Located in the constellation of Orion, this dark cloud has the shape of a horse’s head, which contrasts with the red background of the emission nebula behind.
Other absorption nebulae can be seen in our own galaxy, the Milky Way, where they form noticeable dark patches.
- Helix Nebula: NASA, ESA, and C.R. O’Dell (Vanderbilt University)
- Cygnus Nebula: NASA, ESA and J. Hester (ASU)
- Veil Nebula: Ken Crowford
- Messier 15: NASA/ESA
- Messier 78:
ESO/APEX (MPIfR/ESO/OSO)/T. Stanke et al./Igor Chekalin/Digitized Sky Survey 2