OUR GALAXY

A Milky Streak in the Sky

The Milky Way – our galaxy! Those of us who are lucky and have skies that are sufficiently uninvaded by streetlights and such see it spreading across the heavens as a band of what appears to be very, very distant and unresolvable stars clustered together with intermingling dark areas.

So, what exactly are we seeing when we witness this band of stellar objects? We are viewing the awesome spectacle of part of the dinner plate of our own galaxy, seeing into our arm and a couple of adjacent arms of the spiraling matter that constitutes our galaxy. And our own earth, the sun, the moon, the planets, all of the closer stars and fuzzy splodges (bar one or two) that we see dotted around the entirety of the sky are also part of this spiraling dinner plate! The dark areas are clouds of dust and gas, presumably ejected from dying stars. This is known because these clouds are observable with radio telescopes.

In the UK and many other countries, we can only ever see part of this band at any given time of year. I describe below how it looks to me from my home in Scotland, during evenings throughout the year, a description which is approximately correct for the whole of the UK:

In winter, we see a straight band running north-west to south-east. The fuzz of this distant band is seen passing behind a number of the prominent constellations in the sky, namely Orion, Gemini, Auriga, Perseus, Cassiopeia, and Cygnus, before descending towards the area of the horizon in the direction of Sagittarius and Ophiuchus. The stars that make up these foreground constellations, which of course are themselves a closer part of this Milky Way of ours, are very nearly all within what is known as the Orion spur, the galactic home of the Solar System.

In summer and autumn, we see the part of the Milky Way that is beyond the horizon in winter, the stretch from Cygnus to Sagittarius and Ophiuchus. It still appears as a straight band, running approximately north to south in summer and east to west in autumn. The southern area, near Sagittarius and Ophiuchus, widens and appears to split into two, divided by a large dark area. This area is known as the Sagittarius Star Cloud and is close to the centre of our galaxy, which is about 30,000 LY away. It is unsurprising that the Milky Way brightens around this region.

In the spring time, the Milky way is low, curving around the horizon from north to west to south, so it doesn't present a very good view. However, this is the only time of year when we see the other end of the galaxy, the end that is towards the outer rim, which stretches from Perseus, through the Orion area and on to Puppis and Vela.

No matter what the time of year is, it looks as if this spectacle that we are viewing is a single, continuous band of stars. However, this is not the case; as we gaze along its extent, it we are in fact viewing across as many as three of the spiral arms of our home galaxy.

So where are we looking from, to see this astounding view? Let's step outside of our galaxy, for a moment, and think about what we might see. Most people are aware that our galaxy is a spiral galaxy and looks like a giant Catherine Wheel, with spiral arms rotating slowly around a central core.

Let's examine these arms in more detail.

Our Neighbouring Arms

It's hard to be exact as to the configuration of these arms, as the various information sources seem to vary, but I've tried to convey an approximate idea in Fig. 1, below, which shows a cross-section of the spiral arms that are thought to constitute our galaxy. Note the central galactic bulge, the core of our galaxy. This core is actually barred, with the Perseus arm connected to one end of the bar and the Scutum-Centaurus arm connected to the other end.

It is thought that our Solar System is located towards the inner facing edge of the Orion spur, which is a small branch protruding from the huge Sagittarius arm. 

Given the vast distances involved, it seems unlikely that we could see any of the other arms at all, but they spiral quite tightly around each other near our Orion spur, as shown in Fig. 2. It is a mere 5,000 light years (LY), or thereabouts, for us to leap across to the facing edge of the adjacent Sagittarius Arm.

Suppose we use a telescope? We can now see various star clusters and nebulae, the ones that amateur astronomers are familiar with, like the great Orion Nebula (M42/43), the Crab Nebula (M1) and the Ring Nebula (M57), and a myriad of other ones. With very few exceptions, these are all scattered within the Orion spur, the Sagittarius arm and the Perseus arm.

A closer look at our local galactic neighbourhood, the dotted area in Fig. 1, is shown in Fig. 2. This diagram shows the distribution within the arm structure of the various clusters and nebulae that are visible with a telescope.

It should be noted that all of these clusters and nebulae except for one are in approximately the  plane of our view of the galactic arms, i.e., they appear in or close to the Milky Way when we look up at the sky.

This one exception is M97, the Owl Nebula, in Ursa Major. This is actually about 2000 LY distant from us, but it appears significantly closer on the diagram below. The reason for this is that its direction from us is significantly inclined with respect to the plane of the galactic arms (it is much higher up than us in the galactic arm - by higher, I mean inclined towards the north galactic pole). We can see this if we look for Ursa Major in the sky, it is significantly distant from the Milky Way.

Time to get back to earth now, and our Milky Way view stretching across the sky; if we view the part of the stellar band that passes the region of sky inhabited by the constellations Sagittarius and Ophiuchus, we are seeing mainly this Sagittarius arm, a gigantic spiral arm that continues running alongside our Orion spur, after our branch-off junction.

Moving our gaze along to the mottled area in the direction of the constellation Cygnus, we're now seeing our own Orion spur, with much obscured by the dark gas and dust clouds of the Cygnus Rift. These clouds prevail in all of the arms of our galaxy. Along even further, by the Cassiopeia region, our gaze has shifted direction, out towards the edge of our galaxy, showing us the Perseus arm. The section in the direction of  the constellations Perseus, Auriga and Taurus is also in this arm. Continuing along the section beyond the constellations Gemini and Orion, we are looking within our own spur, once again.

This is hard to visualise. Looking at Fig. 3 should help. It is similar to Fig. 2, but I have replaced the cluster and nebula locations with constellation directions. We can now correlate the constellation directions with the position in the Milky Way of the constellations and really get a feel of whereabouts within the arms we are looking as we view the band of the Milky Way stretching across the night sky:

To reiterate where we are looking in terms of galactic direction as we view the glowing band of the Milky Way, it is clear from Fig. 3 that:

*    If we look in the Scorpius/Sagittarius/Ophiuchus direction, or the directions of the southern hemisphere constellations, we are looking in towards the interior of our galaxy. 

*    Looking in the direction of Cygnus and its neighbours, we are looking along our Orion Spur.

*    Scanning from the Cassiopeia region and right along to the part of the sky where Orion and Canis Major are, we are looking increasingly outwards towards the outer rim of the galaxy.

Neighbouring Arms - Deep Sky Object Listings

As an `at a glance' guide that gives an idea of the spread of various clusters and nebulae, I've listed their arm locations, together with their constellation association:

Orion spur:

M 42, M 43, & M78 - Orion

M 45 - Taurus

M 35 - Gemini

M 44, & M 67 - Cancer

M 41 - Canis Major

M 50 - Monoceros

M 34, M 76 - Perseus

M 29, & M 39 - Cygnus

M 57 - Lyra

M 27 - Vulpecula

M 97, & M 40 - Ursa Major

M 6 & M 7 - Scorpius

M 23 & M 25 – Sagittarius

M 73 - Aquarius

M 48 - Hydra

M 47, M 46, & M 93 - Puppis

Perseus Arm:

M1 - Taurus

M 36, M 37 & M 38 - Auriga

NGC 869 & NGC 884 - Perseus

M 52 & M 103 - Cassiopeia

Sagittarius Arm:

M8, M17, M18, M20, M21, M24 & M55 - Sagittarius

M11, M26 - Scutum

M16 - Serpens

NGC 3372 - Carina

Looking Towards the Central Region

The professional astronomers can start to penetrate further towards the centre of the galaxy, beyond the Sagittarius arm, where the next arm is found. There are one or two small areas of much thinner dust cloud within the Sagittarius arm which allow onward peeping using infrared telescopes which are capable of penetrating the obscurance of dust and nebulosity. One notable area like this is around the area of M24, a cluster which is actually in the direction of the constellation Sagittarius. This cluster is believed to be located in the Sagittarius arm, but with some of its stars in the Centaurus Arm.

The next arm which can be viewed through these few gaps is the Scutum–Centaurus arm (also known as the Scutum-Crux arm), about which little seems to be known. It is difficult to discern a picture of what is happening in much of our galaxy, as we ourselves are located within it and cannot see very much of it from our vantage point! Even our current ideas about the structure of our galaxy are tentative in many areas.

It doesn't help that the galactic centre is obscured from us by gas and dust clouds, and, of course, the inner spiral arms. Thanks to the advent of modern technology and the various cloud viewing gaps, astronomers have now managed to glean at least some information about the central area of the galaxy using gamma ray, X-ray and various different radio frequency telescopes, and they have seen that there is a central bulge at the galactic core. This comes as no surprise to them, having observed many other spiral galaxies.

An intense radio source has also been detected at the galactic centre, thought to be indicating the presence of a black hole. Further evidence of a black hole comes from studying the movements of some of the most distant stars that can be individually detected around the galactic central region; the most famous of these is a star that lays about 26,000 LY from us and is simply called Source 2 (S2). The movements of this star, together with those of a number of its neighbours, have been studied in detail since 1995, and its eccentric and tight orbit is consistent with models of how a star would behave near to the edge of a black hole. 

Even closer to the galactic centre than S2 and its companions, we find the densest known star cluster in the Milky Way, known as the Arches Cluster. This cluster contains many, many giant young stars, as does the nearby Quintuplet Cluster, the home of the brightest known star in the Milky Way, called the Pistol Star. At an estimated 80 to 150 times the mass of the sun, this star shines very brightly, earning its name by illuminating a surrounding nebula, known as the Pistol Nebula.

The Galactic Halo

The aforementioned star clusters around the galactic centre are young clusters, full of large, hot, young stars that will burn ferociously to a relatively early death.

In contrast, the most elderly star clusters that we know of in the galaxy are the most distant ones from the centre, and they are very densely packed with stars, sometimes containing more than a million of them. Known as globular clusters, due to their spherical shape, most of them are thought to be nearly as old as the universe itself. Their stars are low in mass and because of this, they burn slowly and live long lives, unlike the fiery giants in the galactic central zone. 

One good example of an aging globular cluster is M56, located about 33,000 LY from us in the direction of the constellation Lyra. Where does it live? Well, based upon the direction of M56, maybe it's just about in the Scutum–Centaurus arm. 

Unlike M56, the majority of this tight cluster's elderly relatives can be found in directions that are significantly inclined from the plane of the spiral arms, like the magnificent M13 in Hercules, the richly compact M2 cluster in Aquarius and M53 in Coma Berenices, the latter being almost perpendicular to the plane of the galactic arms at a distance of about 60,000 LY. 

The reason that most of these wizened old globulars are significantly inclined from the plane of the spiral arms is that they are on the outskirts of our galaxy, more than 150 of them being distributed across both sides of the galactic spiral disk in a spherical halo enveloping the galactic spiral arms. The most distant one known is NGC2419, at a distance of about 200,000 LY in the direction of the constellation Lynx.

The almost spherical halo is also believed to contain gas, thinly scattered stars and much dark matter. The main reason that professional astronomers believe that this halo exists is because it is evident surrounding various other spiral galaxies. Looking up into the halo at high angles of inclination, we have less stars and dust clouds obscuring our vision, so we well beyond our galaxy, viewing the most galaxies in these directions. At 90 degrees inclination from the plane of the spiral arms, the galactic north pole is in the direction of the constellation Coma Berenices, the south pole being in the direction of the constellation Sculptor. We see the most distant galaxies that we can spot in these directions, with many of those of the Coma Cluster being around 300 million LY from us! A good example of one of these galaxies is NGC4889 which lays just off the celestial north pole and is listed as 308 million LY away!

Just beyond the Galactic Halo, a number of dwarf galaxies orbit our Milky Way. These include, in the southern hemisphere, the famous naked eye visible Small and Large Megallanic Clouds and the Sagittarius Dwarf Elliptical Galaxy, and in the northern hemisphere the Ursa Major and Ursa Minor dwarfs.

©  2014 Nigel Joslin

With thanks to Chris Rolston for research contributions.