Adventures in Observational Astronomy : Deep Space Celestial Objects

In amateur astronomy, one of the most interesting objects to see using a pair of binocular on a clear dark night are the Messier objects (M objects). 

Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his Catalogue des Nébuleuses et des Amas d'Étoiles  (Catalogue of Nebulae and Star Clusters). Because Messier was interested only in finding comets, he created a list of those non-comet objects that frustrated his hunt for them. This list, which Messier created in collaboration with his assistant Pierre Méchain, is now known as the Messier catalogue. The Messier catalogue is one of the most famous lists of astronomical objects, and many objects on the list are still referenced by their Messier numbers. The catalogue includes most of the astronomical deep-sky objects that can be easily observed from Earth's Northern Hemisphere; many Messier objects are popular targets for amateur astronomers.

We have already written about deep space objects such as M objects Other M objects are M42 (Orion Nebula), M45 (Pleiades), open clusters like M44 (Beehive Cluster). All these deep space objects can be seen with the  naked eyes. So are binary stars such as binary stars like Sirius (Alpha Canis Majoris) that are also naked eyes objects.

However, some deep space  objects like Albireo (Beta Cygni), open clusters like M67 require binoculars or a telescope for colour.

Open clusters like M67 and the dumbbell nebula  (M 27) require binoculars or a telescope. Although some of these objects can be seen with the naked eye, others require a telescope or a pair of binoculars to gather more light through the lens.

Their apparent brightness is measured by what we call  ‘apparent magnitude’ (m).  Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light caused by interstellar dust along the line of sight to the observer.  Compare this with absolute magnitude which is a measure of the celestial intrinsic (actual)  brightness, or how bright it would appear if it were located at a standard distance of 10 parsecs (approximately 32.6 light-years) from Earth.  

Unless stated otherwise, the word magnitude in astronomy usually refers to a celestial object's apparent magnitude. The magnitude scale likely dates to before the ancient Roman astronomer Claudius Ptolemy, whose star catalogue popularized the system by listing stars from 1st magnitude (brightest) to 6th magnitude (dimmest). The modern scale was mathematically defined to closely match this historical system by Norman Pogson in 1856.

Calculating Apparent Magnitude:

Since the apparent magnitude is a logarithmic measure of the brightness of an object as seen from Earth, this  can be calculated using the formula:

m = -2.5 log ₁₀ (F / F₀)

where:

m =  is the apparent magnitude,

F =  is the observed flux (brightness)  of the object

F₀ is the reference flux (zero magnitude flux)

log ₁₀ is the base of 10 logarithm

This formula is derived from the logarithmic scale used in astronomy, where a difference of 5 magnitudes corresponds to a factor of 100 in brightness.

The scale is reverse logarithmic, meaning, the brighter an object is, the lower its magnitude number. A difference of 1.0 in magnitude corresponds to the brightness ratio of, or1005 = about 2.512. For example, a magnitude 2.0 star is 2.512 times as bright as a magnitude 3.0 star, 6.31 times as magnitude 4.0, and 100 times at magnitude 7.0.

The brightest astronomical objects have negative apparent magnitudes, for example, Venus at −4.2 or Sirius at −1.46. The faintest stars visible with the naked eye on the darkest night have apparent magnitudes of about +6.5, though this varies depending on a person's eyesight and with altitude and atmospheric conditions. The apparent magnitudes of known objects range from the Sun at −26.832 to objects in deep Hubble Space Telescope images of magnitude +31.5.

Here are the approximate apparent magnitudes of various objects:

Sun: -26.83

Full Moon: -12.74

Venus: -4.6 to -3.8 (varies)

Mercury: -2.0 to +5.6 (varies)

Mars: -2.0 to +1.6 (varies)

Jupiter: -2.24

Saturn: +0.5 to +1.2 (varies)

Andromeda Galaxy: +3.4

The Andromeda galaxy for instance lies at a distance of about 2.537 million light-years away from Earth. This is approximately 24.7 million trillion kilometres away (2.537 million light-years × 9.461 trillion kilometres / light-year).

Despite that distance the Andromeda Galaxy (M31) can be seen with the naked eye under dark skies, away from light pollution. It appears as a faint, smudgy patch of light in the sky. The apparent magnitude of the Andromeda Galaxy is approximately +3.4. This makes it relatively bright compared to many other objects in the night sky.

In Malaysia, which is near the equator, you can find the Andromeda Galaxy by facing towards the north-northwest. Here are the details to help you locate the Andromeda galaxy. Look towards the constellation Andromeda, specifically in the area between the constellations Cassiopeia and Pegasus. You can start by locating Cassiopeia, which forms a distinctive "W" shape.

 From Cassiopeia, you can draw an imaginary line towards the northwest and find Andromeda. The best months to view the Andromeda Galaxy are from October to January.

As you can see, the Andromeda Galaxy is considerably fainter than the Sun and the Moon, but brighter than most planets when they are at their best visibility.

Several deep-space objects can be seen with the naked eye or through binoculars. Here are a few categories and some examples once again :

Deep-space objects bright enough to be visible to the naked eye are:

Andromeda Galaxy (M31) – The closest major galaxy to the Milky Way, visible as a faint smudge in the Andromeda constellation.

1.      Orion Nebula (M42) – A bright star-forming region in the Orion constellation, easily visible with the naked eye as a fuzzy patch in Orion’s "sword."

2.      Pleiades (M45) – A beautiful open star cluster in Taurus, often mistaken for a small dipper.

3.      Beehive Cluster (M44, Praesepe) – An open star cluster in Cancer, appearing as a faint hazy patch.

4.      Omega Centauri (NGC 5139) – The largest and brightest globular cluster, visible from the Southern Hemisphere.

5.      Lagoon Nebula (M8) – A bright emission nebula in Sagittarius, visible under dark skies.

Deep-space objects visible through binoculars (7×50, 10×50, or 15×70) are:

1. Hercules Globular Cluster (M13) – A dense cluster of stars in the Hercules constellation.

2. Dumbbell Nebula (M27) – A planetary nebula in Vulpecula, appearing as a small, faint blob.

3. Ring Nebula (M57) – A tiny, doughnut-shaped nebula in Lyra, requiring steady hands to see clearly.

4. Triangulum Galaxy (M33) – A spiral galaxy, dim but visible in dark skies.

5. Wild Duck Cluster (M11) – A dense, compact open cluster in Scutum.

6. Swan Nebula (M17) – A nebula in Sagittarius, resembling the shape of a swan.

Most of these objects can be seen with the naked eye under good conditions, while others may require binoculars or a telescope to get a clearer view or to see details. My recommendation, a pair of 7 x 50 mm binoculars is probably the best. It has wider angle of view, brighter images due to its 50 mm objective light-gathering capacity, lighter in weight to hold up steadily for a longer time, and the images are less shaky due to its lower magnification.