S3C8: Galaxies

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a) Our place in the Universe

The entire Part B of Series 3 on the Solar System conveyed the incredible scale of interplanetary distances and the complexity of star formation as well as their energy output. Incredible in the human context and of planet Earth, without a doubt. Not on a cosmic scale though and if the previous Chapter 7 provided insights on the past and future of our universe, it did not invest many lines in describing its content other than wielding the broad categories of ordinary matter, dark matter and dark energy.

This chapter will therefore bridge the gap between stellar systems and the broader universe, extinguishing in the process any sentiment that Earth and the Sun have a very special place in the cosmos.

We’ll start the journey where we left it at the end of Chapter 6 on the Giant Planets and beyond, when it was mentioned the Oort Cloud theoretically ends, “where galactic gravitational pull balances out with that of our star”, around 50,000 to 200,000 AU from the Sun. From now on astronomical units measuring 150 million kms won’t do, this is way too small… Instead, we will be using one of two units: the light-year or the parsec.

The light-year is exactly what it sounds like, the distance light travels in a vacuum over one Earth-year. The speed of light was discussed in S3 Section 7.f, it is noted by the symbol c and clocks 299,792,458 metres per second. The maths are based on a Julian year lasting exactly 365.25 days and yields 9,460.7 billion kms or 63,241 AU. The parsec is not as intuitive a distance but can quite easily be explained: if you were to draw two straight lines starting from the same point with an angle of 1/3600 of a degree, also called arcsecond (a degree can be divided into 60 arcminutes and each arcminute can be subdivided into 60 arcseconds), moving away at the same speed along those two lines, you would reach one parsec of distance from the departure point when the distance between the two lines is exactly 1 AU. The technical definition of the parsec is thus “the distance at which 1 AU subtends an angle of one arcsecond”, and it measures 206,265 AU or about 3.26 light-years.

Zooming out of our own stellar system, the next one on our radar, the closest, is called Proxima Centauri, a manageable 4.25 light-years away. It is well-known not simply because of its relative proximity (after all, it’s in the name) but also because it forms a triple star system called Alpha Centaury, one in which the other two stars form a much closer pair that Proxima Centauri, also known as Apha Centauri C, orbits.

Widening the angle some more, our galactic neighbourhood is uncreatively named the “Local Bubble”; it has a diameter of about 1,000 light-years and various sub-areas such as the Local Interstellar Cloud the Solar System is currently in and the G-cloud it is moving towards and expect to reach in less than 2,000 years. These clouds are not isolated stellar systems or cluster of them, instead they are regions of interstellar space with variation in the atomic or molecular density of the interstellar medium, which is the matter and radiation contained in the space between star systems. These various clouds are thought to be the end product of supernovae, the matter ejected by the explosion of a massive star, or possibly of a white dwarf that underwent a runway thermonuclear reaction, as we saw in S3 Section 4.d.

Currently, the Local Bubble happens to find itself in the Orion Arm of our galaxy, the Milky Way, about 26,000 light-years away from its centre, which lies in the direction of the Sagittarius constellation – we’ll revert to the concepts of galactic arms and constellations in sections b) and e) respectively. 26,000 light-years is a long distance, which also suggests a lot of stars in there. Indeed, the Milky Way harbours an estimated 100 billion to 400 billion stars, so unique the Sun is not. The Milky Way was named thus because all those billion stars, with high concentration in its arms, form a hazy strip of lights in our nightly sky which at its most luminous in the direction of the galactic centre.

With so many stars available for observation, no wonder we have been able to work out entire classes of stars, their origin and lifecycle (refer to S3 Section 4.c). Some of those conclusions are the product of theories and modelling and others of direct observation. Indeed, since light takes thousands or years to reach us from distant points in the Milky Way then we are able to literally observe the past. A few thousand years won’t cut it though when it comes to understanding the origins of stars and more broadly of the universe. Fortunately, it so happens the universe is vast in comparison to our galaxy and there are plenty more galaxies out there, an estimated 200 billion to 2 trillion in the observable universe, each comprising on average 100 million stars. So 100 quadrillion stars it is, or more. Difficult to fathom but it should provide some perspective on the absurdity of the notion of us versus them, regardless of the group-level at which this mindset plays out.

And so, we can observe galaxies and stars within them, billions of light-years away. One may ask which of the stars we see are part of the Milky Way and which belong somewhere else, to another galaxy. The answer is simple: considering the distances involved, all the stars we see with our naked eyes are part of our galaxy though we can also sight the Andromeda galaxy, the closest major galaxy from us 2.5 million light-years away, as a patch of light but we can’t make out individual stars within.

The Milky Way retains in its gravitational wake dozens of smaller galaxies called dwarf galaxies and so does Andromeda, forming with the Triangulum Galaxy the core of a galaxy cluster we call the Local Group with a diameter of a mind-numbing 10 million light-years. The Local Group, together with a 100 or so of their closest galactic cluster friends, form a broader structure called the Virgo Supercluster or Local Supercluster. This time, we are looking at a diameter of more than 100 million light-years and a total mass corresponding to 1500 quadrillion Suns. Dizzying. And think there are 10 million or so of those superclusters in the observable universe.

b) The formation of galaxies

The most incredible things about galaxies perhaps, is that they exist at all, almost 14 billion years after the original outpouring of matter and energy. Based on our understanding of the attraction and repulsion dynamics between different interactions as well as the hypothetical dark energy, it is not hard to see how in other universes (real or imaginary) with slightly different ratios from those, starting with a newly released homogeneous content would not have given rise to any meaningful structure or would only have done so for a much shorter timespan before gravitational forces completely took over and everything was compressed and merged again, a gravitational collapse of universal scale.

Models developed to explain the formation of galaxies have tried to evaluate whether they started with massive rotating gas clouds with clumps forming within and evolving into what would eventually become stellar systems or if, on the contrary, they came about bottom-up, with the molecular clouds giving rise to the stellar systems and those over time congregating into groups, clusters and eventually the structures we call galaxies. The latter seems to be the favoured scenario and lines up well with the repeating pattern of agglomeration under gravitational forces. What is not well explained at this point, however, is how the disk shape of a large portion of galaxies can remain stable instead of undergoing continuous contraction; the answer may lie with the repartition of dark matter.

This mention of shape naturally brings up the topic of galaxy classification and indirectly that of galactic life cycle. The two dominant shapes sported by those galaxies anchoring local groups are spiral, like the Milky Way, and elliptical. A spiral galaxy is characterized by four key features: most of its stars lie on a horizontal plane (as opposed to forming a sphere), this disk rotates, they have spiralling arms originating from the centre, and they have a much higher concentration of older stars towards the centre, which forms a bulge. Further differentiations can be made according to the degree of luminosity and the existence or not of a bar-like structure of stars linking the two arms and traversing the centre. If you wish to read more about this, I included a link to the Wikipedia entry on the morphological classification of galaxies at the end of the chapter.

It is important to understand what those galactic arms are and are not. The arms are essentially the visual outcome of a high concentration of igniting stars, a phenomenon called starburst, which is triggered by the compression of molecular clouds in the interstellar medium. It is hypothesized some type of density wave ripples around the galaxy and results in a higher-than-normal rate of star formation. Thus, the stars in a spiral arm are unlikely to stay in this spiral for the next few billion years, unless the rotation speed of the mechanical wave exactly matches the orbit period of the stars within. Even then, starburst is a transient period in any point of space and eventually the amount of available interstellar gas is no longer sufficient to birth new stars.

Since I mentioned stars orbiting the galaxy, in case you are wondering, Earth revolves around the Sun, and the Sun revolves around the centre of the Milky Way at a leisurely 230km/s on average. One such orbital period lasts 225 million Earth-years and is called a galactic year.

Unlike spiral galaxies, the elliptical kind doesn’t appear as a disk and can be quite spherical, or something in between. It is also differentiated by the much lower rate of star formation and the presence of older stars, in particular as one nears the centre, thus their spectrum is tilted towards the red. In addition, the largest ones are always home to a supermassive black hole and they tend to be found in galaxy groups or clusters. All this indicates the elliptical galaxies are generally older than the spiral ones and are actually some kind of end stage, essentially spiral galaxies having lost their structure because of gravitational interference – as we will see in the next section.

The third and most frequent type of galaxy is called dwarf galaxies, they are much smaller than the spiral and elliptical kind with a star-count generally in the order of millions of stars rather than billions, though the number can vary anywhere from a few thousands up to one billion. In fact, this is only a handy name to designate a structure that is smaller in scale and is gravitationally bound to a larger structure. For example, there are over 60 smaller galaxies within a radius of 1.4 million light-years of the Milky Way, most of which are orbiting our galaxy.

The fourth type of galaxies is termed “irregular” and could have been called “miscellaneous”; they lack an obvious structure and are most likely the remnants of the deformation induced by an intergalactic interaction. There are further sub-divisions in the classification reflecting their relative size and absence or presence of (surviving) inner structure.

c) Intergalactic interactions

The distances separating galaxies, often spanning in the order of millions parsecs, may look too vast for gravity to really matter, especially since its effect decreases in proportion with the square of the distance, yet the sheer magnitude of the masses involved, such as 1.15 quadrillion solar masses for the Milky Way (quadrillion is 10¹⁸), and the time scales on which all these interactions are playing out means galaxies not only cluster together, they interact with each other and, eventually collide or absorb one another.

Going by what we know of gravity, all masses interact, however far, it is only a matter of magnitude and competing attractions. Our own Solar System is subjected to tidal interactions with the rest of the Milky Way, and likewise the smaller dwarf galaxies in orbit or who happen to be passing in the same celestial neighbourhood will experience a tidal interaction, stretching them in the direction of the centre of mass of our galaxy. This is what is happening to the Magellanic Clouds for instance; the Large Magellanic Cloud is located about 163,000 light-years away and the Small Magellanic Cloud slightly further at 206,000 light-years but both can be seen without optical instrument from the southern hemisphere. They are formally listed as dwarf galaxies though they do have spiral structures which have clearly been disrupted by the Milky Way and, correspondingly, their mass has distorted parts of the Milky Way. Considering the velocity at which these travel in relation with our galaxy, it is thought they are not gravitationally bound with us or only are with orbiting periods in the order of several billion years.

The result of a tidal interaction can be a wholesale transfer of matter from the smaller galaxy towards the larger one, and since interaction is two-way, this also translates into a deformation at the outer edge of the more massive structure. This phenomenon is sometimes dubbed galactic cannibalism and is thought to have occurred several times in the Milky Way’s past. Actually, all that mass has to come from somewhere so this is just the term for a one-sided merger whereas what is called galactic merger is a collision where both sides lose some of their previous structure to form a single larger one, perhaps an elliptical galaxy.

Whether a collision ends up with a merger or a “pass through” interaction is a function of relative masses and velocities and, in some cases, an initial pass through will be succeeded by several other passes and eventually a proper merger. The notion of “passing through” is not a euphemism, galaxies may be home to hundreds of millions or even several billion stars but it is mostly still empty space so in the vast majority of cases stars will not collide and the overall galactical structures are even likely to be maintained, at a large scale. However, the consequences of such a tidal surge at the level of stellar systems can be more than disorienting, they can be de-orbiting.

The same alteration to stellar orbits will be observed when the result of the collision of two galaxies is a merger, which supports the argument that elliptical galaxies are born from such cosmic events. One need not look far, at least by galactic standards, to see this play out. Indeed, Andromeda also known as M31, the closest major galaxy from the Milky Way and twice as massive with its trillion stars, is 2.5 million light-years away and both are getting closer at the speed of 225km/s, indicating a collision will take place in 4 to 4.5 billion years, probably forming an elliptical galaxy in the process. Solar System: fate unknown, but ejection is on the cards.

In addition, the local increase in gas density creates triggers for a concentrated period of starburst, with potentially hundreds of stars being born every year compared to an average of two in the Milky Way. Consequently, since there is only so much gas and molecules to go around, an intense period is followed in the next billions years by a markedly decreased rate of starburst, a symptom exhibited by elliptical galaxies. Furthermore, this extraordinary accretion of mass and energy at the conjoined centre of merged galaxies can give rise to a core-like region named active galactic nucleus (GCN).

d) At the centre

We have seen in S3 Section 5.a on the formation of the Solar System that density increases dramatically towards the centre of a rotating mass of matter, as more mass begets more gravitational pull. With protoplanetary disks, this results in the formation and ignition of stars but where more matter is available there comes a point when gravitational collapse cannot be prevented and these regions of space will experience the formation of black holes, a process discussed in S1 Section 10.b. At the centre of large galaxies, the result is astronomical objects called supermassive black holes characterized by masses in the order of several hundred thousands or millions of times the mass of the Sun.

A feature of those extremely massive and dense bodies is a luminous ring-shaped concentration of matter around the body. The reason for this luminosity, and here I use the word to signify the emission of electromagnetic radiation across the entire spectrum, not simply visible light, has to do with physics we should be familiar with. As matter is gravitationally pulled towards the dense body, instead of following a circular orbit, friction with other particles causes heat and the loss of energy in the form of the emission of radiation. The quid pro quo is that having lost this quantum of energy, the angular momentum of the particles of matter is reduced and so they fall inward with increased speed, thereby creating more friction and more intense radiation emission, and so forth until, in the case of the black hole, past the event horizon, radiation is no longer able to overcome gravity. This phenomenon is behind the expression of “active galactic nucleus”.

I enclose below in Figure 5 a computer model of what of black hole accretion disk would look like. Note the lensing effect which allows the observer to see the far side of the disk, including its underside. If you have watched the movie Interstellar, this should look familiar, except that now you know the physics behind it.

Figure 5: Accretion disk of a black hole

Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman (CC BY-SA 4.0)

Without going deep into the physics, including because some of the underlying mechanisms are still imperfectly understood, the rotation of the dense body and accretion disk creates a strong magnetic field and concentrates some of the emission of radiation out of the magnetic poles. In the case of the category of neutron stars called pulsars, the rate of rotation can be in the order of tens of thousands per minute and this creates extremely regular beams of light that can be seen provided they are oriented towards Earth.

These jets are features that also frequently occur in active galactic nucleus, especially when their luminosity (the intensity of the electromagnetic radiation) entitles them to the name of quasi-stellar object, or “quasar” for short. The closest of this kind is called Markarian 231, it has a strong UV radiation and lies in the constellation of Ursa Major (also known as the Great Bear), 581 million light-years away. The luminous intensity is so high however that distances are no issue and the furthest quasar detected is named UHZ1, it is 21,000 light-years in diameter and its light has travelled 13.2 billion years to reach us so that its comoving distance today is 31.7 billion light-years away. Not clear? This will be explained in S3 Section 9.g on the observable universe.

Wrapping up the topic of active galactic nucleus, the electromagnetic radiations can be very important in the radio wave range, those with frequencies below 300 GHz. This is due to the process of synchrotron radiation taking place when electrons are being driven to relativistic speeds, approaching the speed of light, in a magnetic field. Galaxies with large regions of radio emission are called radio galaxies.

e) Trivia – Constellations

To conclude this chapter, we’ll revert to our homely Milky Way and discuss the stars within, in particular the way they appear in our sky and have been named long before humans understood about galaxies or even the solar system. When seen from Earth, stars are so far away they appear fixed, though eventually over generations and centuries they do move relative to each other. This means groups of them will sometimes form patterns our human imagination find compelling, or useful as markers in the sky that can be leveraged to orientate ourselves and navigate across long distances without landmarks or even landmasses to set eyes on.

These patterns are called constellations (the etymology essentially means collection of stars) and they are named after the object, animal or divine entity they are supposed to look like. The best-known of them can be seen from the Northern Hemisphere for the simple reason that it is the writings of the early natural philosophers around the Mediterranean basin and all the way to the Indian sub-continent which formed the basis of Western astrology.

Thus, the 12 constellations we are familiar with all occupy a 30° arc of the entire zodiac, this part of the sky around the ecliptic, the path followed by our star in Earth’s sky. Their names are Aries, which the Sun starts transiting after the March equinox, followed by Taurus, Gemini, Cancer (summer solstice in the Northern Hemisphere), Leo, Virgo, Libra (September equinox), Scorpio, Sagittarius, Capricorn (winter in the Northern Hemisphere), Aquarius, and Pisces.

To be “in” one of the zodiac constellations means the Sun is located in the same direction as this constellation, and this alignment of course changes by about 1° each day as the Earth revolves around the Sun. Accordingly, when we are in Virgo we can’t see Virgo because it is right behind or on the edges of the Sun and is too bright for us to see those stars.

The International Astronomical Association recognizes a total of 88 constellations and, besides the aforementioned 12, some of the best-known are Centauri which harbours Alpha Centauri, Cassiopeiae, Orion, Ursa Major with the Big Dipper stellar pattern (the technical term is asterism) designed by the subsection of its main seven stars, and Ursa Minor with Polaris at the end of the imaginary tail. Polaris is also known as the North Star because it currently lies within 1° of the celestial North. Remember, all of these constellations are within the Milky Way, so when we say that Galaxy X is located in Constellation Y, this is shorthand for Galaxy X is located in the same line of sight as the stars forming Constellation Y.

a) Our place in the Universe

b) The formation of galaxies

c) Intergalactic interactions

d) At the centre

e) Trivia – Constellations

f) Further reading (S3C8)