B00B7H7M2E EBOK (34 page)

This method doesn’t rely on any other steps in the cosmic distance ladder, even though quasars are some of the most distant objects from Earth. There are, in fact, none near us. Though they aren’t as old as the cosmic microwave background radiation, they nevertheless beam their light to Earth like beacons near the limits of the observable universe. So much time has passed since the light we observe left the quasars that we see them as they existed when the universe was young, only a tiny percentage of its present age. By now, quasars have almost certainly evolved into something quite different from the way they appear to present-day observers. Perhaps into the sort of galaxies now situated closer to Earth.

In 1964, when Refsdal first suggested that gravitational lensing might be a clue to the distance of quasars, they were a very recent discovery and still something of an enigma. They looked more like faint stars than galaxies, but they also in some ways resembled gaseous nebulae. Those who studied these objects concluded that by cosmic standards they were very small but extremely bright, and their brightness varied over days, weeks, even years. All had astoundingly large red shifts.

Sandage was involved in some of the first investigation of these objects in the early 1960s, along with Thomas Matthews and Maarten Schmidt of the California Institute of Technology. At that time, existing physics was inadequate to make sense of them. Three possible ways of explaining their large red shift all presented problems:

First, the red shift might be caused by a gravitational field. We’ve said earlier that red shift is the result of the stretching of light waves as the object from which they are coming accelerates away from us. Gravity, like acceleration, stretches light waves, which isn’t too surprising since even in more familiar situations, gravity and acceleration can feel the same and have the same effect. We experience it when the speeding up and slowing down of a lift makes us feel heavier or lighter, as though the pull of the Earth’s gravity has changed. However, in the case of quasars, if it were a gravitational field causing the red shifts, quasars would have to be so massive and so near that they would disturb the orbits of the planets in the solar system. There was no sign that that was occurring, which seemed to rule out possibility number one.

Second, the objects might be stars in our own galaxy that have been ejected from somewhere with a force powerful enough to drive them away at a speed necessary for the measured red shift. Examination of the objects’ spectra indicated that this was highly unlikely.

The best possibility left was that the objects are actually very
distant
indeed. To cause such a red shift as astronomers were measuring, quasars would have to be receding at as high a rate as 37 per cent of the speed of light. For such a red shift to be caused by the expansion of the universe they would have to be a vast distance away. How is it, then, that observers on Earth are able to see them as we do? For instance, one of the first studied was 3C273. Its red shift indicates that it is about two billion light years away. Yet it appeared as early as 1895 in photographs taken with optical telescopes of modest size. If it really is two billion light years away, it has to be radiating 100 times more power than the most luminous galaxies. That also seemed improbable to those who first struggled to understand these mysterious objects. How large
were
these things?

Their varying brightness was the giveaway. No source of light can flicker faster than radiation can cross it. If it did, the next flicker would begin before the previous one ended and the flicker would appear blurred, not a flicker at all. Radiation can’t cross anything at a speed faster than the speed of light. The light output of 3C273 changes substantially in periods as short as one month, which means that most of the light from it must come from a region no larger than the distance light travels in a month.

Using this line of reasoning, researchers calculated that quasars are tiny by cosmic standards, but many have been discovered with greater red shifts than 3C273. For us to observe such small objects as we do, at such distances, they must be by far the brightest things in the universe, as bright as dozens or even hundreds of galaxies combined. Yet the light from 3C273 comes from a region only one light
month
across, while the light from a galaxy comes from a region of probably some 100,000 light
years
across or more.

Though they are much better understood now than they were in the early 1960s, quasars are still mysterious. The source of their power is probably a black hole at the core. Quasars are indeed all incredibly long ago and far away, some of the most
remote
objects in both space and time, and with their close relatives, violent BL-Lac objects and blazars, more powerful than any other sources of energy yet discovered.

As the century progressed there was a trend towards more comprehensive surveys and mapping of the Galaxy and the cosmos beyond. The first modern ‘census’ of the universe took place in the 1950s. It was the National Geographic Society–Palomar Observatory Survey, which relied on a 48-inch telescope and photographic plates.

By the mid-1950s, researchers were fairly sure that the Milky Way Galaxy was similar to thousands of other galaxies they were able to study at a distance, and that among them it was mid-sized. The closest Earthly observers come to ‘seeing the Galaxy’ with the naked eye is when we look up at the Milky Way in the night sky. That stream of light is part of the disc of the Galaxy, viewed edge-on (along the plane of the Galaxy), and not from outside the Galaxy but from within. Nevertheless, astronomers were able to glean from the study of other galaxies that a galaxy having a mass like the Milky Way has to be one of two types – a gas-free elliptical or a spiral like Andromeda. Ours is certainly not gas-free. That left spiral as the only option, which means – judging from knowledge of spiral galaxies – that it must have a Catherine wheel shape and be comprised of a thin disc of gas, dust and bright, relatively young stars; a central bulge of more densely packed older stars, around which the disc rotates; and a faint halo of even older stars. William Herschel compared the Galaxy to a grindstone. Modern astronomers wax even less poetic and compare it to a giant fried egg. The disc is the white of the egg, the central bulge is the yolk.

Beginning in 1950, American astronomer William Morgan of the Yerkes Observatory near Chicago pioneered the mapping of the Milky Way’s spiral structure. He plotted the positions of two types of brilliant young stars – O and B stars, less than
about
10 million years old – and found that they are arranged in two lines that run parallel to one another. One of the lines marks the arm in which our solar system lies, now called the Local Arm. The other line is the next arm out, the Perseus Arm. Morgan confirmed these findings by searching for nebulae and estimating their distances by studying the stars that light them. This investigation also revealed a third arm closer to the Galactic centre, later dubbed the Sagittarius Arm.

Other optical astronomers, following in the footsteps of Morgan, continued to study O and B stars and nebulae but ran into the problem of blotchy curtains of dust through which radiation in the optical range can’t pass. This barrier is more like a forest in deep fog than an opaque wall, for in some areas it’s possible for optical telescopes to penetrate through lighter dust, between what in a forest would be bushes and in the Galaxy are black molecular clouds. Nevertheless, in the optical range, astronomers hoping to explore nearer the Galactic centre can effectively see little further than 10,000 light years. Though the view is 50 times better looking in the other direction, from the southern hemisphere, curiosity about the centre of the Galaxy ran into frustration only about halfway there.

Meanwhile, Dutch astronomer Jan Oort had joined with Australian astronomers in a project to map the spiral arms using radio telescopes in Holland and Australia. Much of the interstellar matter spread throughout a spiral galaxy consists of hydrogen atoms, and these atoms emit radio noise at the wavelength of 21 centimetres. The study of 21-centimetre emission from hydrogen in the Milky Way Galaxy is one way of finding out how gas is distributed. Oort reasoned that the motion of the gas would show up as a shift in the wavelength it was emitting and would give him information about the way the Galaxy rotates. Plotting the speed of that rotation against the distance from the Galactic centre would enable him to measure the distance to gas clouds and nebulae.

Oort and his colleagues proceeded to map the spiral arms by
graphing
the intensity of the radiation against the speeds measured from the red or blue shifts, reasoning that each peak in the graph was a hydrogen cloud and the shift revealed its distance. The resulting map of concentrations of hydrogen gas indicated a pattern of spiral arms, but the map didn’t much resemble other spiral galaxies that can be seen more directly. It wasn’t until the 1970s that the reason emerged. Hydrogen, it turns out, is not very strongly concentrated in the Galaxy’s spiral arms, and some of the peaks on Oort’s graph represented gas between the arms instead. Furthermore, the way gas moves and changes its speed upon entering a spiral arm leads to other misleading data. Hydrogen is not so effective a spiral arm ‘tracer’ as Oort hoped, though it did reveal the Outer Arm that lies beyond the Perseus Arm.

In 1976, Yvon and Yvonne Georgelin of France, who also had been following up on William Morgan’s work, published a map of the Galaxy based instead on distances to hot nebulae. More recently, their map has been greatly refined by Patrick Thaddeus of Harvard and his colleagues, through the study of molecular clouds, the bushes in the optical forest. As with hydrogen, the motion of such a cloud shows up as a shift in the wavelength of radiation in the radio range of the spectrum – this time from carbon monoxide molecules. Except within a dense molecular ring near the Galactic centre – so crowded as to defy mapping of its overall structure – Thaddeus and his team have managed to separate individual molecular clouds along any line of sight by their different velocities, and from those velocities to calculate their distances from the Sun. The result is the most detailed and accurate map yet produced of the spiral arms.

Oort was on the right track to think that the best way to explore the structure of the Galaxy was to study the way things move in it. Most distant clouds of hydrogen or molecules of carbon monoxide have motion towards us or away from us, because the Galaxy doesn’t rotate in the rigid way a solid structure like a wagon wheel would. What is true in the solar
system
is also true in a spiral galaxy: the influence of the gravity from the concentration of mass at the centre is weaker the further you are from the centre, and that means the outer parts of a spiral galaxy move at slower speeds, just as the outer planets in the solar system do. The result is that distant clouds don’t stay continually the same distance from us. They do have motion – as much as 100 kilometres per second – towards us or away from us, and that motion shows up as a Doppler shift.

In both directions along the Sun’s orbit around the Galaxy – the way we’re headed and the way from which we’re coming – the nearer gas clouds seem to remain always the same distance from us. Only further away in those two directions is there an observable shift. Looking from Earth in a third direction, directly away from the Galactic centre, there is virtually no shift. Looking in the direction directly towards the centre there is also no shift nearby, but in the vicinity of the centre itself there is violent non-circular motion of gas. On the near side of the centre the motion is towards us. On the other side it’s away from us. Astronomers currently disagree about whether this movement indicates colossal explosive motion at the core of the Galaxy or means that the central bulge is bar-shaped.

Figure 7.2

Comparing the centre of a barred spiral galaxy with the centre of a galaxy having a circular central bulge.

Most descriptions of the Galaxy as viewed face on, the direction from which it looks like a Catherine wheel, have the central bulge as circular. It may be necessary to alter that image slightly. Not all spiral galaxies have circular central bulges. Instead, about half appear to have a short bar at the centre with the spiral arms beginning at each end of the bar. (See comparison in
Figure 7.2
.) Observations by a satellite known as the Infrared Astronomical Satellite indicate that stars on one side of the Galactic centre are somewhat closer to Earth than stars on the other side. That would be explained if what is out there is a bar of stars set askew of our line of sight. (
See Figure 7.3.
) Also, the gravitational pull of the rotating bar could be causing the motions of clouds of gas observed near the Galactic centre. It may well be that the central bulge is only a short, fat bar, so that viewed from space beyond the Galaxy it looks more elliptical than round or rectangular.

Figure 7.3

The central bulge of a barred spiral galaxy set askew of our line of sight.