In this list we present some of the “best” observing targets in the Milky Way, but bear in mind that for many observers the Milky Way is all but invisible. In fact, some estimates put the number of people in Europe who have never seen the Milky Way at about 60%, therefore even if an observer looks at an object through an optical instrument he may not necessarily be able to tell whether the object being observed falls inside or outside of the Milky Way Galaxy.
Consider the above image of the Milky Way produced by the GigaGalaxy Zoom project during the International Year of Astronomy in 2009, which shows a 360-degree panoramic view of our home galaxy almost as an observer would see it from the outside. From this perspective, all the major components of the galaxy come into view; the bright central bulge at center, most of the dark nebulae that mottle the edge of the main disc, vast star fields, as well as some of the galaxy’s satellites. However, from a practical standpoint, we can only see a small portion of the Milky Way when we look outwards from Earth, and the river of stars we sometimes see overhead represents but a minute fraction of the galaxy as a whole.
The small portion of the Milky Way that we can see from Earth, however, contains many millions of possible observing targets, which makes choosing the “Best Milky Way Targets for Stargazers” a difficult task. The few objects subsequently included on this list were all chosen because they are known to be the biggest, brightest, or scientifically most interesting examples of their class that occur either within the galaxy’s disc, or in the halo that surrounds the galaxy proper.
NGC 869/NGC 884 (The Double Cluster)
There are many examples of objects that appear to be close to one another, but in most cases, this is an illusion, since many objects lie along the same line of sight from Earth. Since human eyes cannot perceive depth beyond a few miles, we often see objects as being close together when they are in fact separated by enormous distances along their common line of sight.
However, the two star clusters shown above are physically close to each other, being separated by only a few hundred light years between the constellations of Perseus and Cassiopeia. Located about 7,600 (NGC 884) and 6,800 (NGC 884) light-years from Earth respectively, both clusters are included in the Perseus OB1 association, and under dark skies can sometimes be observed without optical aid as a large detached part of the Milky Way.
Using binoculars, both clusters then cover over half a degree of sky, while an 8-inch telescope reveals hundreds of stunning stars, including blue and white supergiants, as well as a few fainter red supergiants around cluster NGC 884.
NGC5139 (Omega Centauri)
Although Omega Centauri is best seen from the southern hemisphere, its status as the Milky Way’s biggest and brightest globular cluster ensures its place on this list. Located about 15,800 light years away in the constellation Centaurus, this cluster contains about 10 million stars that collectively weigh about 4 million times as much as the Sun. Omega Centauri is also the second biggest and brightest globular cluster in the entire Local Group of Galaxies after Mayall II, the Andromeda Galaxy’s biggest globular cluster.
Omega Centauri is one of the few globular clusters that can be seen with the naked eye, and from a dark site appears as a hazy area almost as large as the full Moon. Binoculars will then reveal a large fuzzy disk, although a 10-inch (250mm) telescope is needed to really do it justice as a celestial wonder with thousands of stars seen extending from a dense core.
With a diameter of about 150 light years, the average spatial separation between individual stars in the cluster is only about one tenth of a light year, but the most interesting aspect of Omega Centauri is the fact that it contains several, distinctly different stellar populations. These populations range from relatively young Population III stars to several million ancient, roughly 12 billion-year-old Population II stars, among which are some of the oldest stars known to exist. In fact, Omega Centauri is so different from other globular clusters that it is almost certainly not a true globular cluster, but the remains of the core of a dwarf galaxy that was destroyed by the Milky Way in the distant past.
While initial investigations suggested that an intermediate mass black hole existed in the cluster’s core, recent studies have refuted these findings, although they cannot entirely rule out the possibility that a black hole may exist in the cluster.
Messier 17 (Omega Nebula)
Also known as the Swan Nebula, Checkmark Nebula, and the Horseshoe Nebula, M17 is located between 5,000 and 6,000 light years away in the constellation Sagittarius, where some of the richest star fields in the galaxy are located.
With a diameter of about 15 light years, the Omega Nebula is only a small part of a larger nebulous region that spans about 40 light years. Nonetheless, the Omega Nebula is one of the biggest known HII, or star forming regions in the Milky Way galaxy, and it contains about 800 solar masses worth of gas and dust. In terms of its overall structure, the Omega Nebula closely resembles the Orion Nebula, but rather than being oriented face on as the Orion nebula, we see it almost edge on, instead.
Messier 17 shines with the reflected light of the open star cluster NGC 6618 that is embedded within the nebula, with the total number of stars in the nebula estimated to be about 800 or so, with another 1,000 stars in the process of forming scattered throughout the nebula’s outer regions. Interestingly, with an age of only about 1 million years, NGC 6618 is one of the youngest open clusters known to exist in the galaxy. Its high surface brightness makes M17 easily viewable with the naked eye from dark locations, with a 7×50 pair of binoculars revealing a hazy omega-shaped bar. A 4-inch (100mm) telescope will then show subtle differences in its brightness, while an 8-inch (200mm) telescope reveals further contours and a hook shaped protrusion at one end.
NGC 4755 (Jewel Box)
Although the Jewel Box is best viewed from the southern hemisphere since it is a part of the constellation Crux (Southern Cross), it is included here because with an age of about 14 million years, it is one of the youngest open clusters known that is not directly associated with an area of nebulosity. The Jewel Box has a combined apparent visual magnitude of 4.2, and it is located about one degree to the southeast of the 1st magnitude star Beta Crucis.
This pretty cluster consists of about 100 stars, mostly of the red or blue variety, and while a pair of 7×50 binoculars will resolve its brightest six members, a 3.1 inch (80mm) telescope is needed to reveal the subtle colors of a dozen or more of its stars.
This pretty cluster was first observed by Nicolas Louis de Lacaille with a 12mm (½-inch) telescope while observing from the Cape of Good Hope in 1751-1752. However, it was John Hershel who gave it the name “The Jewel Box” when he described it thus: “..this cluster, though neither a large nor a rich one, is yet an extremely brilliant and beautiful object when viewed through an instrument of sufficient aperture to show distinctly the very different colour[s] of its constituent stars, which give it the effect of a superb piece of fancy jewellery.”
NGC 6397 in the constellation Ara is one of the closest globular clusters to Earth, being only about 7,200 light years away. However, it is better known for the fact that it was used to estimate the age of the Milky Way galaxy in 2004, with a team of astronomers using the UV-Visual spectrograph of the Very Large Telescope at Cerro Paranal to estimate the time that has elapsed between the appearance of the first Population I stars in the entire Milky Way, and the formation of the first stars in the cluster itself by measuring the beryllium content of two stars in the cluster.
By adding this value (and several other factors) to the estimated age of the stars in the cluster in a hugely complex calculation, the team of investigators arrived at an estimated age for the Milky Way of 13.6 billion years, which makes our home galaxy almost as old as the Universe itself. Moreover, a 2006 study by the Hubble Space Telescope on the brightness of the faintest stars in the cluster revealed that the lowest possible mass a star could have to initiate nuclear fusion in its core comes to approximately 0.083% of the Sun’s mass.
NCG 6357, also known as the Lobster Nebula, is not the biggest, brightest, or most massive nebula in the Milky Way, but it does contain the open cluster Pismis 24, which in turn, contains three of the most massive stars known to exist in the galaxy.
The image above shows the blue giant star Pismis 24-1 in the middle of the cavity with its hugely energetic solar wind having cleared around itself the surrounding nebula. While initial studies suggested that Pismis 24-1 was probably the most massive star in the Milky Way with a mass of about 300 times that of the Sun, follow-up studies have shown that the star was in fact a triple star system. Even so, each of the individual stars in the system turned out to weigh more than 100 times as much as the Sun does, which still places them among the most massive stars known to exist in our galaxy.