Andrew Wood

This series of articles will give basic information about those objects beyond our solar system. These stars, star clusters, nebulae and galaxies are given the collective name of Deep Sky Objects.

Within our solar system, everything we see; Moon, planets, comets and asteroids, we see only because of light generated by the Sun reflected back to Earth. We observe objects in the solar system as they were a matter seconds, for the Moon, or minutes or hours ago for planets and other objects. This reflects how far away they are and how long their reflected light takes to reach us at the speed of light.

Once beyond the solar system, the nearest stars are light years away. So, we move from hours, beyond days, weeks and months, to years of light travel once we are out of our solar system. Most of the stars we see with the naked eye are hundreds or thousands of light years away; and some objects for which we need a telescope are millions of light years away. Hence the term Deep Sky.

Naming Deep Sky Objects

Some well-known objects are named e.g. Great Nebula, Butterfly Cluster.

All objects are named in one or more catalogues. The most common catalogues are:

Messier: This catalogue contains 110 objects which are given the designation M.

New General Catalogue: contains more than 7,000 objects with the designation NGC.

Index Catalogue: contains more than 5,000 objects with the designation IC.

Bayer designation: the brightest star in a constellation is identified by the Greek letter alpha (α), the next brightest beta (β) and so on to omega (ω). Beyond that, other designations take over. (Some stars also have common names, such as Antares, the brightest star in Scorpius; which is also designated α-Scorpii).

There are other catalogues, though those above are the major ones used by amateur astronomers.

Magnitude

The Magnitude of a deep sky object is simply a number that reflects how bright it is. The larger the number, the fainter it is. In a dark sky, around magnitude 6 is the limit of what can be seen with the naked eye. Most deep sky objects are beyond naked eye brightness and require a telescope.

Deep Sky 101.1 – Double Stars

A high proportion of stars do not exist in isolation, existing in a Double Star system (another term often used is Binary Star). Stars form from rotating disks of gas and dust under gravity. Double stars form when the disk fragments, with a second star forming within the disk, surrounded by its own disk. The two stars form an orbiting pair.

The result is that a planet in a solar system with a double star at its centre will have two “suns” around which it orbits.

The images above are artistic interpretations. When viewing double stars from Earth through a telescope, each pair has its own unique set of properties, as illustrated below.

(CPM in the diagram above lower right stands for Common Proper Motion. These systems appear double though measurements show they are probably a chance line-of-sight alignment).

Seeing two stars close together through a telescope can also accentuate any colour difference – as seen in the figure above – between the stars.

Whether or not your telescope will “split” a double star depends on:

• The angular separation of the two stars in arcseconds [1 degree = 60 arcminutes. 1 arcminute = 60 arcseconds. A an arcsecond is 1/360^th of a degree]. We normally say a double star system is separated by ‘x’ seconds (“).
• The brightness difference between the stars. Sometimes in a widely separated pair the luminance of the brighter star (usually referred to as the A component) makes it difficult to see its fainter companion (the B component).
• Atmospheric conditions – higher magnification can help in good “seeing”, when the atmosphere is steady.
• The aperture – diameter of the mirror or lens that collects the light. Larger aperture telescopes have better theoretical resolution – ability to split two points of light. Smaller high-quality refractors, however, often have better resolution than larger reflectors, which are better at seeing fainter deep sky objects. Ignore any formulae for theoretical limit and test out your own telescope.

Most star atlases list tables of double stars that you can observe and test out your telescope and observing prowess. Many double stars can be observed in less-than-optimal conditions, under suburban skies with light pollution or with a bright moon present.

A few well-known examples visible from the southern sky are:

1. Alpha Centauri – the brighter of the two ‘pointers’ that aim at the Southern Cross has components of magnitude 0.0 and 1.2. One of the brightest naked-eye stars, the A and B stars are currently separated by about 5” and easily separated by most telescopes. This gap will widen until 2030 then start to narrow until 2035, then widen again to its greatest separation around 2060.
2. Acrux – the brightest star in the Southern Cross is an example a 2+1 system as shown in the earlier figure. The closer components are of magnitudes 1.3 and 1.7 separated by 4”. There is a third component (so it’s actually a Triple Star) at a much wider separation to the close pair.
3. Antares – or alpha Scorpii – is a challenge. Separated by 2.5”, the A component at magnitude 1.0 completely overwhelms the B component at magnitude 5.4, making it very difficult to see. If you do happen to have the telescope and conditions that do split the pair, the fainter component is a distinct green colour, next to the red of the brighter supergiant.
4. Sirius – or alpha Canis Majoris is another challenge. The brightest star in our night sky, at magnitude -1.5, has a white dwarf companion of magnitude 8.5. This large brightness difference makes the fainter star very difficult to see, despite a wide separation. Currently about 9”, this separation would normally be an easy split in any telescope.
5. Rigel – or beta Orionis, is another pair with a large magnitude difference (0.1 and 6.8) with a separation of 10”. In this case, though, it is generally easy to split.

Deep Sky 101.2 –Star Clusters

As illustrated by Double Stars, most stars do not exist in isolation. In fact, there are a multitude of clusters of stars containing tens, hundreds, and thousands of stars. Star Clusters fall into two groups: Open Clusters and Globular Clusters.

101.2.1 – Open Clusters

Open star clusters are groups of stars that are loosely bound together by gravitational forces and were formed from the same giant molecular cloud. Typically containing a few hundred to a few thousand stars, these clusters are found within the galactic plane of galaxies, including our Milky Way. They are relatively young, often just a few million years old, and can provide valuable insights into the processes of star formation and the evolution of galaxies. Unlike their more tightly bound counterparts, globular clusters, open star clusters tend to disperse over time due to interactions with other stars and molecular clouds. [via Copilot]

Observing Open Clusters

• Some are bright enough and large enough to see with the naked eye e.g. Pleiades (Seven Sisters)
• Some can easily be seen with binoculars, making it easy to then aim a telescope
• Generally bright enough for small telescopes – great visual objects for those starting out in Astronomy
• Great backyard objects. In fact, when out under dark skies, Open Clusters can be difficult to pick out from the background stars in the rich star-fields of the Milky Way, and most are bright enough to be seen in fairly light-polluted areas.
• Also great for starting out in Astrophotography. They generally do not require long exposures.

The Pleiades cluster can be seen to the right of centre in the upper portion of the above image. It is easily seen with the naked eye.

The “Jewel Box”, (NGC 4755), above, can be seen with the naked eye, or easily picked up using binoculars, as a small knot of stars near the second brightest star in the Southern Cross, Mimosa (beta-Crucis). It makes a wonderful telescopic object.

101.2.2 – Globular Clusters

Globular Star Clusters are spherical collections of stars that orbit the core of a galaxy. These clusters are densely packed, containing thousands to millions of stars, which are bound together by gravity. They are typically composed of older, metal-poor stars and are found in the halo of galaxies, including our own Milky Way. Globular clusters are important for astronomers as they provide insights into the early stages of galactic formation and evolution, and their study can help unravel the history and dynamics of galaxies. [via Copilot]