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Origins in Astronomy Today
From: Cambridge University Press
| By:
Simon Mitton |
EDITOR'S INTRODUCTION |
These are exciting times for astronomy. The Hubble Space Telescope is offering images of distant space unlike anything seen before. Planetary missions are extending in extraordinary ways our knowledge of our nearest neighbours. In an interview recorded especially for Fathom, Simon Mitton, the executive director of science and publishing at Cambridge University Press, argues that contemporary astronomy can be understood as being organized around three key themes, each of which is a search for origins. |
Fathom: Can you explain your idea of astronomy today being framed or focused by the idea of origins? |
Simon Mitton: Astronomy today is an expensive business. The old funding system that worked well in the nineteenth century and for much of the twentieth century allowed individual astronomers to look through telescopes and work on intellectual puzzles that pleased them. That system served astronomy well until the mid-twentieth century, but today this is a big science. |
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| The planetary nebula NGC 6751, observed by the Hubble Space Telescope. | |
The way in which you have to look at cutting-edge astronomy today is in terms of thematic programmes; thinking in terms of origins is a very useful way of doing that. So there are really three grand overarching themes into which most research slots. |
The key questions of astronomy today
First of all, how did we get from a situation of the Big Bang in which everything began in a moment apparently consisting of radiative energy and matter, to a universe that has tremendously complicated structures that we can investigate today from the subatomic scale right through to huge galaxies like the Milky Way? That's the first theme, the origin of structure in the universe. |
The second origins theme has to do with the birth of stars and of planetary systems. Once the matter in the early universe began to agglomerate into the kinds of things that we now call the galaxies, how did that matter then go on to form the first generations of stars? What has been the history of the matter inside those stars, and then, crucially, what are the stages in star formation and the history of stars which mean that you can have planets, which we now know exist around numerous star systems? |
The third origins theme, which in many ways is rather new, is our understanding of the origin of life in the universe. Put simply, we know that there is life in the universe, because we are here, but what has been learned in the past 20 years, particularly in molecular biology, is that life at the submicroscopic level is a lot more complicated and ubiquitous than we thought. |
The question of the origin of life in the universe has now become quite large. In fact, it is leading to a whole new discipline called astrobiology. And that part of origins attempts to look at what the origin of life was on the earth and what we learn from the evolution of the earth as a planet and the evolution of life on earth. Can we learn general principles that we might then want to apply to other places in the universe, such as Mars, which probably had water in the past? |
Those are three great thematic programmes driving most of the research in universities, research institutes and the observatories. |
History and technology
To one side of that, we have another interesting aspect of the whole of astronomical science, which is history. In many ways astronomy is unlike quite a lot of the other sciences, because it has a huge legacy that chemistry, which basically was invented in the late eighteenth century, doesn't have. Astronomy has the ancient Greek philosophers, it has Babylonian magical philosophers, and it has Copernicus and Newton and the nineteenth-century physicists. So there's a very rich area that can be explored there and quite a lot of those characters were interested either in origins or in the geometry of the universe. |
Then, on the other side of our three themes, there's the technology. Technology is a very powerful driver in modern astronomy, for the simple reason that over the past 60 years astronomy has broadened out from being a subject in which you could look at only a very narrow band of the radiation from the universe, the band that we call visible light. That was the only thing that was really accessible. |
Astronomy has expanded from that to take in areas such as infrared astronomy, which looks at heat radiation in the universe, and radio astronomy, which looks at exploding galaxies, the dying echo of the Big Bang and exotic stars. And as we go up in the frequency range, beyond the visible you come to ultraviolet and on to X-rays and gamma rays. What's exciting there are the conditions under which you can create X-rays and gamma rays, which are extremely energetic and very violent, in the first place. So that part of the spectrum takes us to what some have called the violent universe. |
In order to do these invisible astronomies you need to get away from planet Earth, by putting telescopes and detectors into orbit. And to explore the moons and planets, the solar system, you have to fly missions to them. So, that's another very interesting aspect of the thematic programmes, the way in which they're actually driving completely new technologies; they're often very demanding technologies. |
The origin of structure
Fathom: If we go back to the origin of structure and complexity in the universe, what are the key issues there? |
Mitton: All the evidence suggests the universe began in a singular event, a Big Bang, in which out of nothing all of the present energy and the present matter appeared. We understand most of what took place in a broad-brush way after the first millionth of a millionth of a second. So from 10-12 seconds onward, we have a pretty good understanding of what went on, basically because physics as we know it today applied to the universe after the time it was 10-12 seconds old. |
The intellectual puzzle is how structure arose in a universe that is expanding very rapidly and cooling. So you reach a point about 300,000 years after the original Big Bang, when the universe is sufficiently cold that electrons can for the very first time orbit a proton and form a hydrogen atom. Before that, the temperature of the radiation was higher than the energy you need to eject the electron from the proton: if an atom did form, it would immediately collide with an energetic photon and disintegrate. |
By the time we reach 300,000 years after the Big Bang, which is called the recombination era, the hydrogen nuclei (protons) and the helium nuclei are uniting with electrons to form atoms of hydrogen and helium. What we need to know then is this: starting with a sea in which the matter and the radiation are uniformly distributed, how do we get to the highly asymmetrical distribution we have today? |
Putting it another way, and as simply as possible, the average density of the universe is 1029 times smaller than the average density of water. But most of the matter in the universe today is organized into structures that are denser than water! So an enormous amount of concentration of matter has taken place; the agent for doing that is the force of gravity. |
The way in which gravity does this must be something to do with taking very small inhomogeneities or unevennesses in the early universe and then amplifying them, because gravity is always an attractive force. Therefore, if you have a uniform or almost uniform bag full of matter, if there's any little seed of inhomogeneity in there it's going to grow with time and it will amplify more and more strongly. |
Now, while gravity is trying to pull this stuff together and concentrate it, the momentum of the expanding Big Bang universe is flinging it apart. All of this could be very finely balanced. You could imagine that the universe expanded so fast that gravity never had time to concentrate anything; that would be a universe with all of the matter very thinly spread out and nothing would happen. Equally, if the expansion isn't quick enough, then gravity could quickly overwhelm everything and you'd get implosion. |
Looking at the universe long ago
We're making rapid progress in this topic at the moment, thanks to projects like the Hubble Space Telescope and its programme called the Hubble Deep Field, which concentrated its power on a single tiny area of the sky for hundreds of hours. We're able to see from this programme very faint galaxies that are far away in the universe. In astronomy, the more distant an object is, the more remote it is from you in time. So, by looking at the furthest galaxies, we are looking at the universe long ago. |
What the galaxies in the Hubble Deep Field tell us is that galaxies long ago were smaller than they are now. The first galaxies to form may have been quite numerous, but relatively small. Subsequently the power of gravity has enabled these small galaxies to merge and fold into each other in a cannibalistic process. In the long run, you end up with giant galaxies like the Milky Way; the Milky Way is much bigger than anything we see in the Hubble Deep Field, or the great Andromeda nebula. But how did the first texture arise in the universe 300,000 years after the Big Bang? Radio astronomers are now building experiments that will be able to show us the structure in the universe at the moment when it first became transparent, so we will get a much better handle on the mechanisms that led to the formation of galaxies. |
Fathom: How far away in time from us is that moment of transparency? |
Mitton: The Big Bang was something like 13 to 15 thousand million years ago, so the universe was about 2 percent of its present age when it became transparent. |
The birth of stars
Fathom: Is the process of the formation of stars and of planetary structure just a continuation of that process of the formation of galaxies? |
Mitton:What we need to understand is the chemical evolution of the universe, which proceeds through nuclear reactions and explosions. These take place inside stars, so in a star like the Sun today hydrogen is being processed to form helium. In stars that are considerably more massive than the Sun, there are also reactions going on in which the helium is being processed to form carbon. |
There are also special kinds of nuclear reactions that take place when giant stars (those with a mass 10 times the mass of the sun and above) explode as supernovas. A supernova occurs when nuclear burning has exhausted the fuel. The star is no longer supported by heat pressure; the core collapses under the crush of gravity and this detonates a gigantic explosion in the outer layers of the star. The outer layers have not been through nuclear reactions, so a new round of nuclear explosions is detonated, and in this many of the heavy elements are rapidly synthesized. |
In terms of the history of matter, we have a good understanding of how regular nuclear burning builds some of the chemical elements by processing hydrogen, helium, carbon and so on. We know the physics of how that happened. We have a good handle on the physics of how the elements heavier than iron were formed in stellar explosions. But there's a lot of detailed work to be done in terms of modeling how this chemical evolution continues in subsequent generations of stars. |
In the early galaxies, were there mechanisms whereby star formation was a lot more rapid than it is today, and were there perhaps more massive stars? Were there starbursts in which the matter was being recycled very rapidly and fed back into the interstellar medium, then reconcentrated by gravity into the next generations of stars? |
The formation of planets
Answers to these questions tell us how the universe created a portfolio of heavier elements that could do interesting things like form planets with rocky cores. You can't form a habitable planet like the Earth until you've built up iron, nickel, carbon and other heavy elements. The universe was already two-thirds its present age before the Earth had formed from these heavy elements. Once the basic understanding of the chemical evolution is in place, there's a lot to be done in understanding the mechanisms by which planetary systems may form. |
Fathom: How do planets form, then? |
Mitton: That's a new area of vigorous research with challenging questions. For instance, most of the planets we've discovered around other stars are unlike anything in our solar system. We keep finding "the wrong kind of planet." They're more massive than Jupiter, and they orbit much closer in than Jupiter. Now, is that because our solar system is actually very unusual in having many planets? Or is it that our present detection systems are only able to find giant planets orbiting close in? And the answer is that, at the moment, that's all our detection systems can do, and it's going to be another few years before we can detect Earth-like planets. |
There are also puzzles to do with when star formation takes place. Quite often, star formation creates a binary star, in which one star is orbiting another; there are no possibilities for having planets there. So you've got questions about the frequency of the formation of these different kinds of systems. How often do you get a planetary system? How often do those planets form themselves into things like Jupiters, Saturns and Earths? How stable are such systems when formed? Our solar system has been around for a very long time. Is that always the case or is that unique? So there are plenty of intellectual puzzles to keep everyone occupied. |
Life on other planets
Fathom: The third origins question: the origin of life. What is the crossover between the astronomical disciplines and the biological disciplines? |
Mitton: There's been a lot of progress in this area recently. Perhaps I should begin by saying that the whole idea of scientifically considering the origins of life, and the place of life in the universe, was regarded as being flaky and not the kind of thing that respectable astronomers spent their time doing. Indeed, it is still the case that if you wish to embark on experiments, such as the search for extraterrestrial intelligence, or SETI, you will need to be funded privately, rather than at the national or federal government level. The fact that tax dollars aren't going into it tells you something about the standing in the field. But there have been a lot of developments that seem to be separate and then they all connect, if you say these are to do with the origin of life. Let me explain what some of those are and why astronomers are playing a large role. |
The idea that there might be life on Mars has been a science-fiction concept since H.G. Wells and The War of the Worlds. In the Viking mission of 1976, a fairly crude chemical experiment sought any chemical traces for biological activity. The outcome was at best ambiguous, and was probably negative. But subsequently we've come to understand a great deal about what's called the hydrological history of Mars. It does appear that at some points in the past there must have been warm wet water at the surface. |
If you are considering the origin of life, then it seems the minimum configuration for this is that you need liquid water and you need an environment in which you can extract chemical energy. Those seem to be your absolutely basic requirements. Well, we've learned recently that the Martian environment would have had the water and chemical energy in the past. We've also found that one of the moons of Jupiter, Europa, which is very strongly tidally compressed, probably has a warm liquid water ocean beneath a crust of ice probably 20 kilometres thick. |
Mars and Europa are two examples of solar-system objects about which planetary astronomers are saying, "For a long time we dismissed the idea that there could be any liquid water anywhere else in the solar system, but actually Mars and Europa look intriguing, and there may be more sites within reach." |
The new science of astrobiology
To respond to your point about crossover, let me mention geology and evolutionary biology. The geologists and biologists are finding explosive evolution that seems to take place after a huge environmental catastrophe has hit the Earth. The geologists can now point in the fossil record to at least six, probably eight and maybe even 10 environmental catastrophes in which most of the life forms were wiped out. With up to 90 percent of the species gone, subsequent evolution was very rapid, leading to new classes of plants and animals. |
Marine biologists famously discovered very exotic life forms at the deep ocean vents where superheated water is flowing out of the Earth's interior. There are new classes of microbial life called extremophiles living in these environments. |
Antarctic biologists surprised themselves after slicing open rocks and then conducting measurements that physicists and chemists invented. The biologists are finding that the signatures of microbial life were buried inside these rocks. There is also microbial activity in certain basalt rocks that are one kilometre below the surface of the Earth and probably haven't been exposed to the surface for at least 300 million years. These are reproducing organisms that are cut off from light, and yet they have created a rich biota down there. So that's been another strand, and when you start to put these scientists together--the astronomers, the evolutionary biologists, the geologists--there's a lot of cross-talk, cross-fertilization, and they've got a lot to say to each other. |
Fathom: Are the astronomers setting the pace, then? |
Mitton: The role of the astronomical community is clearly very great in terms of driving this ahead, because it's within the astronomy community that you've got the expertise to do the following: |
One, to detect liquid water by remote sensing on planets round other stars; two, to detect Earth-like planets going round other stars; three, to launch missions within the solar system to do remote sensing; and four, to do sample return. The astronomy community is now taking the lead in this new area, which incidentally is beginning to acquire a discipline name of its own: astrobiology. |
The practice of astronomy today
Fathom: What is the practice of astronomy today? |
Mitton: Astronomy today is big science, so therefore the main observatories and missions have to be organized at the national or supranational level in order to share the costs. In the case of optical astronomy, for example, the costs are now sufficiently great that the optical telescopes are concentrated in Hawaii, Chile and the Canary Islands, where there are superb seeing conditions. At these large observatories, the telescopes are in the four-metre, eight-metre and 10-metre class--big instruments for collecting a lot of light. |
The modern technology is aimed at canceling out all the disadvantages of observing through the atmosphere. The atmosphere causes the twinkling or scintillation of the stars, and all of that can now be taken care of through technology, so that a ground-based optical astronomer today will typically be using a telescope that is highly efficient at gathering the light and recording all the data electronically and able to strip out, if you will, the environmental pollution caused by observing through the Earth's weather. |
The instruments for space-based astronomy are generally international collaborations, because of the expense, and typically it will have taken years to have planned the mission and to get the telescope or the detectors up in space. And the space telescopes are generally run from a particular ground-based institute, with a skilled team of specialists running it. The obvious example is the Space Telescope Science Institute in Baltimore, which is responsible for the operation and the data downloading from the Hubble Space Telescope. |
Optical astronomy, of course, isn't the only kind of astronomy that's done from the ground. There's also radio astronomy, which also is now concentrated into a few centres and uses expensive equipment. You need to use international teams, because one type of radio astronomy called interferometry involves making a connection between telescopes that are very widely spaced, preferably on different continents, to give a long baseline. In that technique you are able to look at fine structure in distant galaxies. Interferometers can see zones of activity as small as our solar system in galaxies that may be billions of light years away. |
Another important area in radio astronomy is in looking at the distribution of molecules in space. Whereas atoms emit energy that we see in the visible region and in the ultraviolet, molecules tend to have their emission concentrated in the microwave region of the spectrum. To look at the distribution of, let's say, carbon dioxide in our galaxy, you would have to do that using radio telescopes. But it's the same basic picture: that people work in teams, they think internationally and they use modern technology a lot to work together and to get the best out of the data. |
Astronomy and space science
Fathom: Is the Hubble the only space telescope? |
Mitton: The Hubble Space Telescope, now 10 years old, is the only telescope in space operating in the optical regime. The Hubble Space Telescope won't last forever, and there are ambitious plans to have the next-generation space telescope in orbit by about 2010. This is a telescope that will operate also in the near infrared, just a little bit outside the visible region, because that's where you can do a lot of interesting work with the formation of planets and the formation of disks around stars. Moving beyond the visible, you've got missions at present like the Chandra X-ray Observatory. X-rays don't penetrate our atmosphere at all, and that's an important mission that has been running for a year and a half now. |
Fathom: Can you speak about the relationships that exist between astronomy and the whole space programme, which clearly isn't driven just by astronomy? |
Mitton: Space science is a lot more than just astronomy, even though astronomers were, arguably, quite important in the birth of the subject, half a century ago. Space science encompasses things that are of no interest to astronomers. It encompasses human biology in space, working in space, everything to do with telecommunications and the exploitation of the near-space environment and all of that kind of thing. Where astronomers today are exploiting space science is in two areas. One is sending things to planets and the other is having telescopes and detectors orbit the Earth. Those are our two applications of space science. |
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