And so, without further ado, the trailer:
Very cool.
Sunday, September 11, 2011
Winds of Change - Now with its very own book trailer
The wonderfully talented Nicole Murphy has made a book trailer for the upcoming Canberra Science Fiction Guild anthology Winds of Change. (I posted the table of contents here.)
And so, without further ado, the trailer:
And so, without further ado, the trailer:
Wednesday, September 7, 2011
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.
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.
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?
[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?
Friday, September 2, 2011
Some Links!
Some interesting sciencey links, presented in the order in which I came across them (which I think is the order they were published anyway).
First up! This New Scientist article is about how the wakes of ships (the white "trail" they leave behind as they plough through the ocean) temporarily increases the albedo of the Earth—how much light from the sun it reflects. This would be good news for climate change (that is, enacting a more positive change and less the "oh gods we're all going to die in a tropical hell" sort) if not for all the greenhouse gases those ships also produce...
Next! This arXiv paper, "Kepler Exoplanet Candidate Host Stars are Preferentially Metal Rich", seems to back up some of the assumptions made in the paper I discussed a short while ago about the galactic habitable zone. In that paper they assumed that sufficient metallicity would be required to form planets and it looks like Kepler has confirmed that.
Penultimately! Forget about Martian meteorites containing fossilised life, simulations have shown that life from Earth may have made it off world and could be set to land on another planet. Could it be that when we finally get around to drilling through the Europan ice, we find Earthly extremophiles instead of aliens? Cosmos magazine have the full story.
Finally! National Geographic have an article about a newly discovered planet which has the potential to be Earth-like. Seems that it gets less energy from its sun than Venus does from Sol, but more than Earth receives. It's habitability thus depends on the appropriate weather conditions but with surface gravity 1.4 times Earth's it should be more or less possible to walk on the surface.
First up! This New Scientist article is about how the wakes of ships (the white "trail" they leave behind as they plough through the ocean) temporarily increases the albedo of the Earth—how much light from the sun it reflects. This would be good news for climate change (that is, enacting a more positive change and less the "oh gods we're all going to die in a tropical hell" sort) if not for all the greenhouse gases those ships also produce...
Next! This arXiv paper, "Kepler Exoplanet Candidate Host Stars are Preferentially Metal Rich", seems to back up some of the assumptions made in the paper I discussed a short while ago about the galactic habitable zone. In that paper they assumed that sufficient metallicity would be required to form planets and it looks like Kepler has confirmed that.
Penultimately! Forget about Martian meteorites containing fossilised life, simulations have shown that life from Earth may have made it off world and could be set to land on another planet. Could it be that when we finally get around to drilling through the Europan ice, we find Earthly extremophiles instead of aliens? Cosmos magazine have the full story.
Finally! National Geographic have an article about a newly discovered planet which has the potential to be Earth-like. Seems that it gets less energy from its sun than Venus does from Sol, but more than Earth receives. It's habitability thus depends on the appropriate weather conditions but with surface gravity 1.4 times Earth's it should be more or less possible to walk on the surface.
Wednesday, August 31, 2011
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:
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.
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.
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.
Saturday, August 27, 2011
Winds of Change ToC
And now for something completely different.
Rather than a science post, this is a quick note to tell you about the upcoming anthology from the Canberra Science Fiction Guild which includes one of my stories. The anthology is called Winds of Change and is edited by Elizabeth Fitzgerald. The table of contents is at the end of this post.
(Yes, I am a bit late with this announcement, but my excuse is I was on holiday, so shhh.)
My story, "Time Capsule" is third, which, I have decided, is a golly good place to be. And despite all my spacey writing on this blog, the story is set entirely on Earth, so make of that what you will.
Winds of Change is going to be launched at Conflux, the annual Canberran science fiction and fantasy convention, on Friday 30 September. I will post more info about getting your hands on a copy (other than if you're at Conflux, of course) when it's available.
The table of contents:
Rather than a science post, this is a quick note to tell you about the upcoming anthology from the Canberra Science Fiction Guild which includes one of my stories. The anthology is called Winds of Change and is edited by Elizabeth Fitzgerald. The table of contents is at the end of this post.
(Yes, I am a bit late with this announcement, but my excuse is I was on holiday, so shhh.)
My story, "Time Capsule" is third, which, I have decided, is a golly good place to be. And despite all my spacey writing on this blog, the story is set entirely on Earth, so make of that what you will.
Winds of Change is going to be launched at Conflux, the annual Canberran science fiction and fantasy convention, on Friday 30 September. I will post more info about getting your hands on a copy (other than if you're at Conflux, of course) when it's available.
The table of contents:
Winds of Change
Edited by Elizabeth Fitzgerald
Stories and authors:
Wraiths by Jason Nahrung
Gravity Express by Naomi Mondello
Time Capsule by Tsana Dolichva
The Tether of Time by Leife Shallcross
Trigger by Zena Shapter
Babel by Robin Shortt
Saint Olivia's Light by Carol Ryles
In Need of Assistance by Chris Andrews
After the Bombs by Adam Tucker
The Horns of Elfland by Crisetta MacLeod
Time Spent by David Coleman
Soul of the Machine by Maxine McArthur
Dream Shadow by Alan Baxter
Giant by Annelise Roberts
Evolution Baby by Lesley Boland
The Princess by Valerie Y.L. Toh
Children of the Ashes by Greg Mellor
By Watcher's Pool by James Goodrum
Turning the Blood by Donna Maree Hanson
Watching by Nicole R Murphy
The Stormchilds by Helen Stubbs
The Fool by Jane Virgo
Dragonfly by Cat Sheely
Stone-singer by Joanna Fay
Wednesday, August 24, 2011
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:
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!
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:
- 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.
- 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.
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!
Wednesday, August 3, 2011
Blog hiatus
Hi all. Unfortunately, I ended up being too busy to write sufficient posts to cover the "I'm on holiday" gap. This means, no new blog posts for a couple of weeks. I'm sure you'll all cope ;-)
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