B00B7H7M2E EBOK (33 page)

Sandage and Tammann also picked up on a new scheme from Canadian Sidney van der Bergh for classifying spiral galaxies. Van der Bergh estimated their luminosity from the clarity and contrast of the spiral arms. Here was yet another standard candle to gauge distances of even more remote galaxies. By 1975, Sandage and Tammann had concluded that the universe was possibly as old as 18 billion years – a considerable increase over the 13 billion years Sandage had estimated in 1958 and a highly satisfactory result, because it allowed the universe to be sufficiently old to encompass the age of its oldest stars, even the most ancient globular clusters.

Sandage and others also found supernovae increasingly useful as yardsticks. Supernovae are exploding stars and they are extremely bright, which makes them the easiest stars to observe at great distances. One estimate has it that in a one-minute interval, a supernova can put out more energy than all the ‘normal’ stars in the observable universe. Only a small part of this energy is in the visible part of the spectrum, but
even
that can be sufficient to outshine the entire galaxy in which the supernova occurs. It goes without saying that such events could be remarkable standard candles – if they are in any way standard. If, for instance, all supernovae reach the same maximum brightness, that would mean that the differences in maximum observed brightness are attributable only to distance, providing an excellent yardstick for finding out how the distance to one supernova compares with the distance to another. It might also become possible to understand individual explosions sufficiently well to estimate their distances even if each is unique and comparisons among them are unhelpful. There has been work on both these fronts, and the search for and study of supernovae far beyond the Galaxy is currently one of the ways researchers are most successfully pushing back the frontiers of astronomy and astrophysics.

It turns out that all supernovae do not reach the same brightness, but for a while astronomers thought one particular class, Type Ia, did. It was a setback in the 1990s to find there were brightness differences among Type Ia’s. Fortunately those differences soon became well enough understood to restore Type Ia’s to their status as standard candles. Finding the distance of other types of supernovae is still problematical, but analysis of emissions in different wavelengths enables experts to find what type of star has exploded and its mass, as well as to study detailed properties of the aftermath of the explosion. The blast typically produces radiation at many wavelengths as it expands and meets surrounding matter.

Supernovae fall into two broad groups called Type I and Type II. Type I supernovae are exploding ‘white dwarf’ stars. The story of such an event begins with an elderly star that has exhausted its nuclear fuel and collapsed to a sphere about the size of the Earth, with a mass probably close to or less than the mass of the Sun. When a star is that small and has that mass, the matter of which it is composed is packed to almost inconceivable density.

Many white dwarf stars don’t lead solitary lives but are part of ‘binary systems’ in which two stars circle one other, orbiting their common centre of mass. Often a dwarf star’s partner is a large, far less dense star. As they circle, the gravitational pull of the denser dwarf star cannibalizes matter from its companion and the dwarf gradually puts on weight (mass). Eventually it tips the scales at 1.4 times the mass of the Sun. That mass is called the Chandrasekhar limit, after Subrahmanyan Chandrasekhar. As a young theoretical physicist from India continuing his work in England, he calculated that limit in the early 1930s.

The dwarf star, having no more nuclear fuel to burn and having boosted its mass over the Chandrasekhar limit, collapses under the pull of its own gravity and rips apart in a titanic nuclear explosion. That cataclysm, observable far across the cosmos, is a Type I supernova. Since all white dwarf stars have approximately the same mass when they explode, astronomers reasoned that all these supernovae should have about the same absolute magnitude, which should make them good standard candles. They are that, in spite of some brightness differences between them.

Type I supernovae are relatively rare. In our Galaxy there was one in 1006, another in 1572 that Tycho Brahe saw, and another in 1604, observed by Johannes Kepler. Not a large sample. In order to find out whether Type I supernovae make good standard candles and the best ways to use them, it has been necessary to observe them in other galaxies whose distances are known. Fortunately, in terms of their energy output within the visible part of the spectrum, Type I supernovae are the brightest supernovae. Light from the most distant ones observed in the late 1990s has taken more than seven billion years to reach the Earth.

Type II supernovae, on the other hand, have never been dwarfs. They are exploding giants. The star that explodes in a Type II supernova is definitely well above the Chandrasekhar
limit
without having had to cannibalize a companion star. When an extremely massive star has used up all its nuclear fuel and can no longer support itself against the pull of its own gravity, it collapses and the resulting explosion is a Type II supernova. Though these are more powerful than Type Is, and can be detected at least a third of the way across the observable universe, they tend not to look as bright to the eye, because so little of their energy comes in the form of visible light. Since they range rather widely in their absolute magnitudes, Type IIs are unreliable as standard candles. However, the hope is that measuring the radiation from them, their temperature, and the velocity at which the stars’ debris moves apart may provide a way to estimate their individual distances.

What is needed is the discovery of a great many more supernovae in order to put them to optimum use as distance indicators and perhaps find a way to measure their distances independently. Today, there are teams carrying on this search using the Hubble Space Telescope as well as earthbound telescopes in many parts of the world. Their work involves taking photographs of a large section of sky, away from the light of the Milky Way and nearby galaxies, then comparing these pictures with earlier ones of the same region. Computers scan the photographs, subtracting known galactic light, searching for any new light source. If a likely candidate shows up, researchers photograph the same area of sky later to see whether the light has moved. If it has, that probably indicates it came from a cosmic ray or asteroid. Examining genuinely new light sources in detail, astronomers look particularly for spectral patterns identifying those that are Type Ia supernovae – the type most useful as standard candles.

Timothy Ferris, in his book
The Whole Shebang
, tells of a far more grass-roots supernova search. The Reverend Robert Evans of the Uniting Church in Australia compares what he sees nightly through his telescope with his remarkable visual memory of the skies. Evans had discovered 27 supernovae by
1995
, a record in the history of astronomy.

Studies of the still-expanding debris of old supernovae whose light reached Earth before our time indicate that the 1006 supernova was approximately 5,000 light years from Earth, and the 1572 supernova about 7,000 light years away. Tycho Brahe was right in insisting that this one was further away than the Moon. He called the phenomenon a ‘nova’, but as we now make the distinction a nova is a less drastic flare-up, usually of a white dwarf star, not an explosion that vaporizes an entire star once and for all. Astronomer Fritz Zwicky coined the name ‘supernova’ in the 1930s.

Among the new techniques that have emerged in the last quarter of a century, there are several that measure the absolute magnitude of other galaxies, and some of these actually bypass the cosmic distance ladder. One is the Tully-Fisher Method, coming from American astronomers R. Brent Tully and U. Richard Fisher. They discovered in 1977 that the absolute magnitude of a spiral galaxy is related to something called its ‘21-centimetre line width’. Most 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. As a distant galaxy rotates (with some parts of the interstellar matter in it approaching us and other parts of it receding from us), the Doppler shift causes this spectral line to be blurred. How much it is blurred is directly related to the speed at which the galaxy is rotating. That speed is, in turn, related to the galaxy’s brightness. It is possible to study the radio spectra at this wavelength from extremely faint sources.

Another new technique is the ‘brightness fluctuation method’, which consists of measuring the unevenness in the brightness of the surface of the central bulge of a spiral galaxy, or near the centre if the galaxy is elliptical. The reasoning is that there should be more apparent unevenness with nearby galaxies than with distant galaxies, because the nearer the galaxy the
more
easily it is resolved into stars and the less likely it is to appear as a smooth body of light.

Astrophysicists Zel’dovich and Rashid Sunyaev have discovered a third approach which measures the distance to faraway clusters of galaxies. They measure the intensity of the cosmic microwave background radiation
through
clusters of galaxies that are emitting X-ray radiation, and they find that the background radiation is heated up as it passes through such a cluster. The effect is a ‘hot spot’ in the background radiation. This radiation started out in the early universe with an extremely high temperature, and the more distant the intervening galaxy cluster, the more dense and overheated the ‘hot spot’ should be. By studying the hot spot, researchers estimate the distance to the galaxy cluster.

Understanding a fourth method, first suggested by the Norwegian Sjur Refsdal in 1964, requires some background information about ‘gravitational lensing’, a phenomenon that was predicted in theory early in the 20th century but not observed until much more recently:

According to Einstein, the presence of mass warps spacetime, and the greater the concentration of mass the greater the warp. Hence, when light travels through space, its path bends as it passes near massive bodies such as planets and stars, or concentrations of mass such as galaxies and galaxy clusters. There is a close-to-home example: the path of light from a star is bent as it passes near the Sun. If we were unaware of the effect, the position where the star appears (detectable during a solar eclipse) would cause us to estimate incorrectly the actual position of the star in the sky.
See Figure 7.1
.

It might seem that such distortion would more likely hinder than help measurement of distances to faraway galaxies. Not so. Take for example a situation in which light from a distant quasar is travelling across the universe towards Earth. Somewhere between the quasar and Earth is a cluster of galaxies, and the cluster is warping spacetime around it as such concentrations of
mass
are wont to do. The warp acts as a lens, bending the path of the light from the quasar so that it reaches Earth via not one but two or even several paths. Observers on the Earth see two or more images of the quasar – or on occasion a ring of light – instead of a single point of light.

Figure 7.1

A beam of light travels from a distant star, passing near the Sun. The warping of spacetime near the Sun causes the path of the light to bend slightly inward towards the Sun. The Sun’s brightness doesn’t allow us to see such starlight in everyday circumstances, but during an eclipse, if we don’t take into account the way the Sun bends the paths of light, it is possible to get a false impression about which direction such a beam of light is coming from and what the distant star’s position is in the sky.

If the cluster responsible for the bending is precisely centred on a direct line between the quasar and the Earth, then the light passing around one side of the cluster will travel the same distance as the light passing around the other. But if the cluster isn’t sitting dead centre, then the light coming around one side must travel further than the light coming around the other and it won’t reach Earth as soon, since light nearly always travels at the same speed no matter what distance it must travel.

When light travelling from a quasar is lensed by a cluster of galaxies, studying the angles at which the light reaches Earth allows experts to calculate the extremely tiny fractional difference between the lengths of the paths. However, knowing, for example, that one path is longer than the other by one part in five billion doesn’t give the actual length of the paths. Quasars have a characteristic that helps at this impasse. They flicker, flaring and fading irregularly over a span of days, weeks, or even years. When the quasar flares, observers on Earth see one image flare before the other, because that particular light has taken the shorter path. From the time delay between the flares, and how much the paths differ in length, it’s possible to calculate the distance to the quasar. If, for instance, the time delay is one year and the paths differ in length by one part in five billion, the quasar is five billion light years away.