Measuring distances: the furthest away objects

[First up, apologies for the unscheduled hiatus. Being sick and moving house (mercifully not simultaneously) sort of quashed any blogging plans I may have had. OK, so I don't mean so much quashed as put out of my mind entirely, but whatever.

Also, thanks to changing life routines, I think I'll be changing my update schedule from Wednesday nights to weekends. For the time being it seems more manageable.]

This week I thought I'd talk about something a little bit outside the realms of foreseeable future SF: extragalactic distances. I say outside in the sense that there is currently no plausible way to travel to neighbouring galaxies, let alone galaxies at the edges of the observable universe.

How far is far?

The thing to understand here is that the universe is really big. Like, amazingly, mind-blowingly large. Douglas Adams said it well:

Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.

Light travels 300 000 km in one second (in a vacuum). It takes about eight minutes for light to travel from the sun to Earth. About five and a half hours from light to travel from the sun to Pluto. It takes a hundred thousand years for light to travel from one edge of our galaxy to the far edge (along the disc). By this stage, I'm sure you've heard the term light year. It's the distance that light can travel in one year and is about 9 460 000 000 000 kilometers (about 10¹³ km or ten trillion kilometres). The nearest galaxy is two and a half million light years away. See how all these distances stack up?

Most astronomers don't actually measure or think about distances in light years. They're somewhat useful for conceptualising things, but parsecs (abbreviated to pc) are more common. One parsec is 3.26 light years. That puts the nearest galaxy, Andromeda almost 800 kpc away (kpc is kiloparsec where the kilo indicates a factor of 1000). Mostly galaxy distances are measured in megaparsecs (Mpc) where one megaparsec is a million parsecs.

Even larger distances are measured using a scale called redshift. The universe is expanding. When light leaves a distant galaxy and travels towards us, it takes time. Possibly millions or billions of years, depending on just how distant that galaxy is. In all that time while it's travelling, the universe continues expanding. The expansion stretches out the light so by the time it gets to us, it has a longer wavelength. Longer wavelength means redder (on the visible spectrum, although in reality this light could have started at any wavelength, depending on what emitted it, and could finish up stretched out all the way into the radio region). Hence the term "redshift".

Doing the Measuring

I touched upon spectroscopy when I talked about the Doppler effect in this astronavigation post. Basically, you look for some known lines (often hydrogen lines) and see how far they've shifted towards the red end of the spectrum.

So we should just be able to use this to measure distances, since we know how fast the universe is expanding, right? Not quite. In the post I linked above, I talked about Doppler shift. This is different to redshift but it can look very similar. Far away galaxies can be moving relative to their neighbours (galaxies in a cluster orbiting a central galaxy, for example). These local motions are called peculiar velocites. This means that you could work out spectroscopic redshifts for galaxies in a cluster, which are all around the same distance from us, but get different results because of their peculiar velocities.

Instead we have to use a combination of different methods, two of which I'll talk about now. The same principle underlies both. Basicaly, in the 70s and 80s, it was discovered that there are a few "scaling relations" which galaxies obey. For example the Tully-Fisher relation relates a spiral galaxy's luminosity (how much light it gives out in total) and the rotational velocity of it's stars (how quickly they orbit the centre).

A spiral galaxy. Although, for a Tully-Fisher measurement, you'd
want it to be side-on, not face-on. But face-on looks prettier.
Source: Hubblesite.org

We can measure the rotational velocity of the stars in a spiral galaxy using the Doppler effect (even though the galaxy is moving away, the stars on one side will be moving towards us and on the other side away from us -- the difference between the two sides can be used to work out the rotational velocity). Once we know that, we get a prediction for the luminosity of the galaxy. However, the galaxy is far away and looks dim, much like a light in a high-ceilinged hall that isn't too bright too look at although it would hurt your eyes if you were up close. Exactly how dim a galaxy looks depends exactly on how far away it is. So if we know how bright it should be, we can compare with how bright it appears and work out how far away it is. Huzzah.

There is a similar, albeit slightly more complicated, relationship for elliptical galaxies called the Fundamental Plane.

Both the Tully-Fisher relation and the Fundamental Plane are constantly being improved upon as we build better telescopes that give more precise measurements and also as we understand the underlying physics governing them better. (At the moment, we're not super sure why they exist.) However, we're still able to use them to measure things like Hubble's constant. Who knows, maybe there are super advanced aliens out there who can set up extragalactic wormholes and need to know how far away to place them.

Probably not, though. I did say this post was outside the realm of plausibility.

Habitable Galaxies - Part 3: Galaxy Environments

This is part three in a series of posts about habitable galaxies. Post 1, covering types of galaxies and galaxy mergers, is here, last week's post 2 talking about active galaxies is here and this earlier post on the (most) habitable areas of our own galaxy is also relevant.

[Unrelated to the topic, but I wanted to say that I've been playing around with Blogger settings and made a favicon (the little icon that helps distinguish this tab from others) and an iOS home screen bookmark button. So now, if you're using Chrome or the latest version of Firefox (I suppose it should work for other versions of Firefox, but the second to most recent version failed for me) check out the little purple telescope on the white background. And if you're reading this on an iDevice, you can even see said telescope with a glossy Apple finish. What fun!]

So this week I'm talking about different galaxy environments. That is, the environment where a galaxy might be found, not environments within a galaxy (although I briefly covered that earlier). Let's start by looking at what sort of environments galaxies can be found in.

The universe has environments now?

What we mean when we talk about galaxy environments is more or less talking about how many other galaxies are nearby. It's possible to get isolated galaxies or galaxies clustered together in groups of varying sizes. Our galaxy, the Milky Way, is part of a group creatively labelled the Local Group, which has about forty-five members. Of these, the Milky Way is the second largest (probably), with Andromeda the largest. Other members include the Triangulum Galaxy, the Large and Small Magellanic Clouds and a non-literal pile of dwarf galaxies.

As far as classifications go, smaller collections of galaxies are termed groups, while larger collections—containing upwards of fifty more densely-packed members—are called clusters. This may seem like a bit of an arbitrary distinctions (what makes the Local Group a group and not a cluster if it has almost fifty galaxies in it?) but it's important to remember that while we know the Local Group is full of dwarf galaxies, other groups and clusters are too far away for us to be able to see their smaller members. So when we say a cluster has fifty members, we mean that many medium to large galaxies.

Galaxy cluster Abel 2218, from APOD.  The cluster is also a lens, but that's another story.
Image credit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASA

Spot the difference

So what does it matter where a galaxy is, anyway? Well, when it comes to life, most of what determines habitability is internal rather than external to the galaxy. The only external influence I can think of which could inhibit life (and jump in in the comments if you disagree!) would be a nearby AGN blasting at the galaxy. And even then, I don't think it would prohibit life everywhere in the targeted galaxy, just in the parts being most irradiated.

However, there are some properties of galaxies which are dependent on their environment. In a denser environment, where there are more galaxies, there had to have been initially more matter for those galaxies to form from. Because there was (by chance) more matter in that area, it was more strongly gravitationally attracted to itself and hence formed earlier compared with a lone galaxy in a sparser environment. Our current leading theory of galaxy formation and evolution is called hierarchical assembly and one of its tenets is that larger (more massive) objects form first. In a sparse environment, matter is by definition more spread out and hence, as well as being more weakly gravitationally attracted to itself, has further to travel before it can clump and collapse into a galaxy.

The corollary to this is that big galaxies in clusters are older and more evolved (because have gone through more mergers, partly thanks to there being more proximate galaxies), while isolated galaxies are younger and have undergone fewer interactions with other galaxies.

Living around

As far as life developing in other galaxies is concerned, it seems pretty trivial now to make the conclusion that life would have had the opportunity to arise earlier in cluster galaxies and later in isolated galaxies. Group galaxies such as our own would fall somewhere in the middle.

On the other hand, cluster galaxies would have undergone more mergers, have a greater chance of having been in the path of an AGN and are more likely to be elliptical. Those first two points are merely hazards to the development of life, but the latter also gives rise to different experience for that life compared with our Milky Way existence. What off Earth am I talking about? The night sky.

Our night sky is covered in stars with the disc of the Milky Way running through them. An elliptical galaxy, not having a disc component, would not have such a band of dust, gas and denser stars. In fact, they wouldn't have much dust or gas at all, which means no nebulae and significantly fewer hints as to where stars even come from. Depending on the exact placement of the planet, they would likely have a more or less uniform distribution of stars in the sky, maybe with a brighter patch in the direction of the galactic core. Life that evolved in an elliptical galaxy might not ever get to observe young stars in their vicinity, let alone star formation. How might their understanding of astronomy and, in particular, stellar astrophysics be shaped by this? I think it's an interesting question to explore.

On the other end of the scale, we have isolated galaxies which could also harbour life (if they're big enough to develop sufficient metallicity in sufficient time). But if it's truly isolated, it might be that it's not possible to observe external galaxies with the naked eye. (We can, but they sort of look like stars until you put a telescope to them.)  Maybe such a civilisation would skip past the part of astronomy that labelled Andromeda and the Magellanic Clouds as nebulae but who knows how they might interpret distant blobs and spirals in the sky once they had the telescopes to see them? Also remember that isolated galaxies are going to be smaller and form later. By the time life even evolved there, would there be many other spiral galaxies left? How much more powerful would dark energy be at that point? How quickly would all the other galaxies be retreating from them?

How empty would the sky be?

Habitable Galaxies - Part 2: Active galaxies

This is part two in a series of posts about habitable galaxies. Post 1, covering types of galaxies and galaxy mergers, is here and this earlier post on the (most) habitable areas of our own galaxy is also relevant.

A major attribute of galaxies is whether or not they are active. There are a few different things active can mean—actively star-forming, for example—but what I want to focus on today is whether they have active nuclei.

What's in a nucleus?

At the centre of our galaxy and most other large galaxies, there is supermassive black hole. I have briefly mentioned black holes in the past and I will eventually get around to writing a dedicated post on them. Honest. What you need to know to understand their role in galactic nuclei is as follows:

• They are very small and very dense.
• The supermassive part means that they range from around a hundred thousand times to the mass of the sun to billions of solar masses. The Milky Way's central black hole was calculated in 2008 (by this group) to be about 4.1 million times the mass of the sun. In kilograms that's about 8 x 10³⁶ or an 8 followed by thirty-six zeroes.
• As their name suggests, supermassive black holes are very massive. What massive really means (in any physics context, not just with regards to black holes) is that they exert a strong gravitational force.

Before you ask, we don't really know where these come from—there are theories, but no single one is yet the most accepted—but we do know that they must form early on in a galaxy's life (possibly even before the stars form, depending on which theory you subscribe to) and their evolution is closely tied with the host galaxy's.

Other things that can be found in the centres of galaxies include stars, dust and gas. Although following the orbits of stars in the centre of our own galaxy is what convinced us there was a supermassive black hole there (nothing else could be so massive and so small), most of what those stars do is simply orbit. (Yes, it is possible for one to fall into the black hole and yes, that would be very interesting and would generate a lot of energy but from what we've observed, the Milky Way's central stars seem to be in stable orbits. If you are interested in reading a (short and fairly uncomplicated) paper about S2, the star closest to our supermassive black hole, you can find it here.

When there is gas or dust in the vicinity of the black hole, it will tend to spiral inwards until it eventually passes the event horizon*. As this occurs, huge amounts of energy are released, outshining all the stars in the galaxy. This is what is called an active galactic nucleus. It is also, more or less, what causes quasars, the most distant objects we observe (because they're so bright we can see them very far away, you see).

Here is a nice NASA / ESA Hubble Space Telescope picture of jets coming off the nearby AGN, M87.

 * The point of no return.

Active life?

So the next question, the crux of this post, is can we have life in a galaxy with an AGN? The short answer is maybe. Of course, we have no concrete proof either way. Sagittarius A* (yes, that asterisk is part of the name), our central black hole, is not currently active** and we have even less evidence for life in other galaxies than we do for life on other planets within the Milky Way. The slightly longer answer is, it depends. There is evidence to suggest that the Milky Way was active in the past few million years and since there is still life on Earth, we can suppose that an AGN doesn't necessarily sterilise a galaxy.

Some months ago, some colleagues and I got into a discussion regarding whether a really bright AGN (even one unrealistically bright for the size of our galaxy and Sag A*) could wipe out life. We came to the conclusion that it would only do so if you were standing close enough to it. From memory, we estimated that if a planet somehow managed to find itself*** in an orbit a parsec from the active black hole, the black hole would through about as much light at it as the sun does. However, AGN emit much harder radiation than stars, meaning that a larger proportion of the energy would be at X-ray and gamma ray frequencies, both unfavourable to life. If we put the planet where Earth is, then even with nothing blocking the way we don't have a very high increase in dangerous radiation.

However, something is blocking the way: dust. As far as we know, dust near the black hole is requisite for turning on an AGN. But even ignoring that, there are many clumps of dust in the disc of the Milky Way. So many that we are unable to see through it all if we look along the disc. (Schlegel et al surveyed the dust in the galaxy and came up with this map. White bits have more dust, black bits have less.) In essence, as well as making it hard for us to notice supernovae near the centre of the galaxy, this would help shield us from AGN light. I wouldn't be surprised if we didn't immediately notice the AGN. Of course, closer in to the centre of the galaxy you have more problems and it starts to depend more on exact placement. Also, the dust actually only shields visible and UV light, so once you get too close the more concentrated X-rays and gamma rays become more of a problem.

On the other hand, a planet is less likely to form in the path of an AGN jet, simply because there are fewer stars in that direction. If it did, however, it would definitely not survive the experience.

Elliptical galaxies have significantly less gas in them (some might say no gas, but there would have to be some in the centre for the AGN to turn on, not to mention dust created by dying stars).  This would mean less shielding, making the AGN more noticeable. The bigger barrier to surviving the experience, however, would be the fact that elliptical galaxies are larger with with more massive central black hole which would generate a more energetic AGN (with more detrimental radiation). The final point to consider is that theories suggest AGN in elliptical galaxies are turned on thanks to dust being stirred up (into the black hole) from a merger. So the merger event could have some impact (see last week's post) on continued habitability. The dearth of dust also means that new planets would not be able to form in an elliptical galaxy.

So to summarise, an AGN wouldn't necessarily sterilise a galaxy, but might kill off life that had set up too close to the centre. Depending on an inhabited planet's placement in a galaxy, an AGN might not have a very large effect on daily life. There are a few additional problems for elliptical galaxies, but again, so long as the planet isn't too close to the centre (and its sun doesn't migrate to the centre of the galaxy too quickly), there's no reason for life to automatically be extinguished. Score 2 for extragalactic life.

Next time: the habitability of galaxies in different environments in the universe.


** Probably.
*** I personally really don't think a planet would be able to form in that region thanks to the density of stars and subsequent gravitational forces. I don't have any hard evidence to support this, but to me it makes sense.

Habitable Galaxies - Part 1: Galaxy types and origins

Hello blog readers, I'm back :-)

I've talked about habitable planets and habitable parts of our galaxy before. Now I am going to talk about the possible habitability of other galaxies. It's a somewhat lengthy topic so it'll be spread out over a few weeks. Today, some background.

Miscellaneous types

There are two main classes of galaxies: spiral and elliptical. (Also known as late type and early type, but those names are silly as will soon become apparent.) These are called morphological types because they relate to the physical shape of the galaxy. You can also get dwarf galaxies, which are very small—on the scale of galaxies, although they're still billions of times the mass of our sun compares with tens of trillions of suns for galaxy like ours.

Once we get past galaxy shapes there are all sorts of interesting things galaxies can be doing. For example, you can have active (or not active) galaxies, galaxies which are merging, galaxies where stars are actively being born, galaxies filled with old, dying stars, galaxies with high or low metallicities, isolated galaxies and galaxies part of giant groups or clusters. There's a lot of variety, even within each of these (not always mutually exclusive) classifications. You think the two hundred billion* or so stars in our galaxy are a lot? There are eighty billion galaxies just in the observable universe. Which ones, other than carbon copies of our own, can we live in? Let's first have a look at where galaxies come from.

* I use billion here in the American sense to denote 1 000 000 000 = 10⁹ for three reasons: 1. it's most commonly used this way in the scientific community, 2. the word "milliard" (as in the British counting convention) has fallen into disuse and 3. "thousand million" (the replacement for milliard) sounds clunky.

Galactogenesis

As Dougalas Adams said, in the beginning there was nothing, and then it exploded. After that, if we fast forward a bit (which, current theories suggest, the universe did), matter which at this stage consisting mostly of hydrogen with a little bit of helium that was fused in the primordial fires of the big bang, starts to clump together.

(At this point I feel the need to mention that I'm more or less ignoring dark matter because in this context it needlessly complicates things. But it's there too. Just so you know.)

These clouds then collapsed into discs and clumpiness within the discs led to star formation (that is, clumps of hydrogen collapsed in on themselves to make the first stars). So the first galaxies were all disc galaxies, another name (more or less) for spiral galaxies. However, not all clumpy areas are the same size, so you might get a clump of galaxy-sized clumps and then get a bunch of galaxies forming close together. Over time, gravity will pull these galaxies closer together and they will eventually merger (I'll get to mergers shortly). On the other hand, you could have an area of the universe where not much matter ended up and there was only enough stuff to make a few small galaxies.

If you look at our Local Group, there are two big spiral galaxies—ours and Andromeda—a medium-sized galaxy—the Large Magellanic Cloud—and a slew of tiny dwarf galaxies. Some of these dwarf galaxies are in the process of raining down onto the Milky Way or Andromeda. Others are further away and are falling into one of the big galaxies much more slowly. Andromeda and the Milky Way will merge on a time-scale of five or so billion years. Eventually, all the galaxies in the Local Group will merge into one giant Local Structure.

But I'm getting ahead of myself.

So current theory has galaxies being born as discs. Where do the elliptical galaxies, the round or oval balls of stars, come from? The currently popular answer is through mergers.

Mergers

On a cosmic time-scale, small galaxies are constantly falling into (accreting onto) larger galaxies. Every so often two medium-sized or larger galaxies might merge. These events are called major mergers. There are two possible outcomes of major mergers between spiral galaxies:

1. A new, larger, spiral galaxy. There will often also be a period of more vigorous star formation after a major merger like this thanks to the gas and dust in the galaxies being shaken up and combined with more gas from the other galaxy. It is thought this outcome happens when there is enough spare gas in the two progenitor galaxies.
2. An elliptical galaxy. When there isn't enough spare gas in the two merging galaxies, the stars don't rearrange themselves in a nice flat pattern (exactly why is unclear, I think) and we're left with a roundish blob of stars.

There is no question that minor mergers—small galaxies accreting onto larger ones—don't pose much of a threat for life. After all, small galaxies have been accreting onto the Milky Way far longer than we've been around. Whether planets will survive major mergers is another question. In principle, most planets should be more or less OK. Despite the billions or trillions of stars involved, they are actually quite spaced out within galaxies (for example, the nearest stars to our sun are more than four light years away). Therefore, the chances of stars colliding are very small, at least at the sort of distances from the centre of the galaxy where we find ourselves. If the stars don't pass too closely together, their planets will also remain unharmed.

Of course, it's possible that a particular planetary system could be unlucky, but then it could also be unlucky enough to have a supernova explode nearby. In fact, if enough gas nearby is stimulated into forming stars during and after the merger, it could be that a future supernova (from a newly formed massive star) is more of a threat than the actual merger itself.

Finally, will the Earth survive Andromeda and the Milky Way merging? Not exactly, but not because of the actual merger. You see, the sun is scheduled to turn into a red giant before we get to the merger stage, so Earth as we know it will be long gone already.

But wait, there's more

This isn't the full story, of course. Mergers also lead to increased activity in galactic nuclei, but that's a topic I'll be covering next week. Watch this space!

The Galactic Habitable Zone

In the past, I've talked about the habitable zone around stars where water can exist as a liquid. This week, I'm going to talk about the galactic habitable zone which is the area in a galaxy where conditions are sufficiently hospitable for life to develop on planets which themselves are in an appropriate stellar habitable zone. Unlike the stellar habitable zone (also called circumstellar habitable zone), some aspects of the galactic habitable zone apply more broadly to theoretical life forms which might be completely different to the type of life we've encountered on Earth.

This post has been inspired by an article I came across on arXiv.org: A Model of Habitability Within the Milky Way Galaxy by Gowanlock, Patton & McConnell, which I will henceforth refer to as GPM. They constructed a few models of our galaxy and ran simulations to see which regions could be habitable.

What aspects should we care about?

The parent star
The sun of a potentially habitable planet orbits needs to be small enough that it survives for long enough for life to develop. Remember that more massive stars have shorter lifetimes and die explosive, sterilising deaths. The general consensus is that large, blue stars don't last long enough for complex life to develop. Even if a planet survives the actual supernova, its atmosphere would have been obliterated in the explosion and the corpse of the star—either a neutron star or a black hole, depending—wouldn't be very hospitable either.

Nearby stars
By a similar token, we don't want there to be a large short-lived star too close by either. A nearby star going supernova would also be quite bad for potential life harbouring planets. You don't have to be right next to a supernova for the gamma rays (and X-rays and cosmic rays) to do some serious molecule-killing sterilisation. However, if nearby supernovae happen early in the planet's history, there shouldn't be a problem with life developing later on (after the ozone layer heals).

GPM come to the conclusion that, depending on the type of supernova, it could sterilise planets within a range of 2-27 parsecs (6.5-88 light-years). The range is so broad because supernovae come in a variety if flavours from the dying stars I mentioned earlier, which can be of all different masses, to binary stars where the larger one throws off its outer layers first, turns into a white dwarf, cannibalises its partner and then explodes from over-eating. The latter are more bright and will on-average sterilise planets within 18 parsecs (59 light-years) whereas an average star-dying supernova will sterilise within 8 parsecs (26 light years). For a bit of perspective, the our galaxy is about 30 000 parsecs or 100 000 light-years in diameter.

As an aside, I should also mention that there is a theory that some past mass extinction events on Earth were caused by supernovae. Googling "extinction supernova" brought up a lot of hits for different extinctions. Here is one of the top hits, chosen a bit arbitrarily.

Finally, GPM conclude that there is no where outside of the central 2500 parsecs of the galaxy (which they didn't consider in detail) where there are always going to be too many supernovae for life to develop, where they've defined the time taken for complex life to develop as four billion years. That's four billion years either from the time the planet forms or from the time it gets sterilised by a nearby supernova.

Metallicity
This is sort of a less obvious one. Most of the universe is made of hydrogen and helium and a little bit of other rubbish. Although chemists define metals in a fairly specific way, astrophysicists tend to lump anything heavier than helium (that is, elements whose atoms have more than two protons in their nuclei) into the “metal” category.

Rocky planets are made out of, well, rocks rather than hydrogen or helium and if there are no heavier elements around, we'll only get gas giants forming. Heavier elements are produced when stars die, either in a supernova or in the more mundane red giant phase that our sun will eventually go through. Therefore, rocky planets can only form in areas where there have been enough stellar deaths to seed the interstellar medium with heavier elements. How big stars are depends mainly on how much gas there was around when they formed. Consequently, bigger stars are able to form closer to the centre of the galaxy (in the most dense environment) earlier, die explosively and leave metal-enriched dust behind. Then, when later generations of stars form, there is more chance of rocky planets forming around them.

The most metal-poor areas of the galaxy are the outer edge and the halo which is the spherical and sparsely populated area surrounding the disk of our galaxy. The spiral arms, where we are (if you're wondering, we're about two-thirds of the way out from the centre, close to the middle of the stellar disk). The other thing to note about metallicity is that it increases over the lifetime of the galaxy.

GPM looked at stars with lifetimes longer than four billion years whose planets escaped being irradiated by a nearby supernova for at least that period of time as well. Most of the habitable planets exist close to the centre of the galaxy, with half of them between 2500 parsecs and about 4000 parsecs, but life was still possible (though much sparser) up to the edge of the galaxy. (Remember, Earth is around 8500 parsecs from the centre.)

Other planets
I'm only going to cover this one briefly. The presence of other planets in any given system with a habitable planet could stuff around with our habitable planet. Our searches for extrasolar planets have found a lot of “hot Jupiters”—gas giants very close to the star—and our current theories of planetary formation suggest that these got there by migrating in after forming much further out. Such a migration would almost certainly spell terminal trouble for the previously habitable planet.

So where is this galactic habitable zone of which you speak?

Previous studies had defined the galactic habitable zone as an annulus (or flat doughnut, for those of you more culinarily and less mathematically inclined), with the inner rim defined by the radius at which there are too many hazards to life (for example from supernovae in the densely starred inner regions), and the outer rim determined by metallicity or lack thereof. In general, this region has in the past been calculated to be centred on our location in the galaxy, extending inwards and outwards by only 1000 or so parsecs.

On the other hand, GPM found that the whole galaxy (minus the inner region which they ignored but will study in a later paper) was habitable but the areas most amenable to life were close to the centre and a bit above and below the main concentration of stars in disk. The former for reasons of metallicity and the latter because those areas had the same metallicity as the main disk but there were fewer nearby stars to go boom and sterilise them. Our Earth, for comparison, is fairly close to the centre of the disk.

There is a bit more I'd like to say about galactic habitability, but I think I'll leave it for a future blog post. This post only covers habitability without our own galaxy, but stay tuned for more!

The Evolution of a Science

As you may have gathered from the intro of my previous post, I've been at a conference, preceded by a winter school, this week. I had planned to have a post prepared before I left, but it was not to be. Instead you get a post inspired by said winter school. (And incidentally, this was written entirely on an iPad soft keyboard. It wasn't a bad experience, if you're wondering.)

One of the talks I attended spoke about where we are today in understanding galaxies. As part of the talk the presenter also went over the history of the scientific study of galaxies, which got me thinking about how a scientific field evolves in time and what we need to consider when we're inventing a fictional scientific field. Then, obviously, I decided this would make a good blog post and here we are.

This isn't really going to take many societal effects into account, which could be very important in some fields, especially when religion disagrees with scientific discoveries. What I am instead going to talk about is how the scientific progress gets made using examples from galaxy astronomy/astrophysics and a few other fields.

Breaking it down

The way I see it, the development of a scientific field can be broken down into four stages:

1. Discovery
The field first has to be discovered. This is a pretty basic requirement. In the case of galaxies, it was thought a hundred or so years ago that the Milky Way was the entire universe and contained everything we could see in the night sky. Then other galaxies outside of our own were discovered or, more accurately, it was realised that that "spiral nebula" in Andromeda was not actually within the Milk Way) and a field was born.

2. Classification
Once a bunch of galaxies had been discovered, Hubble and others started classifying them based on obvious characteristics of appearance. We actually still use a classification system based on Hubble's. However useful it is to be able to say, "Well, that galaxy there is elliptical, that one is a late-type* spiral," it wasn't quite giving us more information just yet.

3. Analysis
This is the part where instead of just collecting things, we start analysing them in different ways. While Hubble was looking at galaxies with optical telescopes, he also took their spectra. It was at approximately this point when he noticed that all the far away galaxies were moving away from us (looping back to the discovery point) and the field of modern cosmology was born.

Since Hubble, of course, many other people have studied galaxies. As new data became available, thanks to the progression of technology, we discovered dark matter (from studying the dynamic properties of galaxies), we learnt that galaxies can interact and merge and we have been able to observe them at all sorts of different wavelengths leading to the discoveries of a variety of properties of galaxies and other things. We have started mapping the universe (which, if you hadn't guessed, is significantly larger than just the Milky Way), inventing models like hierarchical assembly (sorry, I couldn't find a sufficiently lay link for this one) to fit our data and we are now much better equipped to study the evolution of galaxies.

4. Understanding
The last few points I made in the previous section are tied in with starting to really understand galaxies. This is the stage when we start to understand what's going on and become able to make predictions. As technology develops further, we can test our predictions more and more precisely and, sometimes this leads to discoveries of discrepancies and, again, we loop back to analysing and trying to explain these.

As hinted above, these aren't distinct stages. There is almost always going to be some overlap and a considerable amount of looping as the field progresses. And during the development of the field of galaxies, a whole lot of other (sub-) fields were born such black hole physics (well, more specifically, AGN), dark matter and dark energy (which are actually completely unrelated to each other).


*Don't get me started on why Hubble's ideas of "late-type" and "early-type" galaxies irritate me greatly.


Technology-driven advances

I mentioned above that some of the new discoveries were made when new technology made new data available. In the absence of new data what sometimes happens is that more and more elaborate theories are invented to explain bits of observations that we just don't have enough information to address.

An obvious example that springs to mind is the celestial spheres rotating in the sky which were once used to explain orbital mechanics. The idea was that the stars were embedded in a sphere made of ether or quintessence (or insert fifth element of choice here), surrounding the Earth which rotated around the Earth, accounting for the motions of the stars across the night sky. Then more spheres, with each of the planets, sun and moon embedded in one each, were added to explain the motions of the nearer celestial objects. As observations and measurements improved, more spheres were added to account for things like the precession of the equinoxes/solstices. Even Copernicus, when he came along, kept the celestial spheres and just changed them so that, other than the moon, they rotated around the sun rather than the Earth.

It wasn't until Kepler came along and developed his laws of planetary motion that we moved from celestial spheres to orbits and then, shortly after, Newton came up with gravity and proved Kepler's laws. Kepler was able to do this thanks to the more precise measurements of planetary motions made by Tycho Brahe. New observations made it possible to move forward and, indirectly, contributed to a new field (Newtonian gravity) to be born.

Moving forward

Now we have absolute proof of a lot of things in astronomy and astrophysics (and many other areas of science). Basically, we know stuff now. But we don't know everything, not by a long shot. Remember, just over a hundred years ago, scientists thought that we knew almost everything and only a few small details were left to be filled in. Then quantum mechanics was discovered.

I like to think of the pool of human knowledge as fractal; the more we know, the greater the area of the fractal and the more branches of knowledge we develop, the larger and more visible the infinite perimeter between knowledge and known unknowns becomes.