David Block’s discovery sheds new light on how galaxies are formed South African astronomer and a team of international collaborators have recently discovered the missing link in galaxy formation, a revelation that will help to shape human thinking on the fundamental building blocks of our universe.

The astronomer, David Block, professor of Applied Mathematics and Astronomy at the University of the Witwatersrand, led the discovery team. The discovery made this year is special because this year has been designated the International Year of Astronomy by the United Nations. We are commemorating the moment 400 years ago when Galileo Galilei, in 1609, first used what he called a perspicillum to look at the sky, only a year after the telescope was first discovered. (Source: Cape Times, 1 May 2009) Block said: “There has been a missing link in our understanding. We know that massive young stars are born in clusters, but the huge mystery has been how you go from the clusters to the smooth distribution of stars across the galaxy. “When you look at the disks of galaxies, especially in the infrared, they are remarkably smooth. All of the older stars are evenly distributed, as in a mist. But stars aren’t born that way; they’re born in dense clusters like the Pleiades. “So the question is – why are galaxies so smooth?” Astronomers know that clusters begin to disappear when their ages reach several hundred million years. Models suggest some of them simply evaporate after random internal motions kick out stars one by one. Others probably are shredded by collisions with dense gas clouds in space. But on large scales, evaporation and collisions are ineffective at smoothing out initial structures. Such a dramatic transition between young and old stars is something like the dispersal of dandelion seeds over a whole field from a few small flowers that bud in the spring. The next year, dandelions are everywhere! In galaxies the dispersal of stars is similar, but the breeze that carries them is a regular pattern of shearing motions caused by the galaxy’s rotation around its centre, Block told Leadership. A very special – and expensive – telescope The discovery of the missing link was made possible by the observation of star streams, as found in the spiral galaxies Whirlpool and Messier 81 (25 million and 12 million light years away, respectively). The observations indicate that young stars form in their clusters, and then seed out in distinct dispersion patterns, the so-called star streams. It was only with the use of Nasa’s Spitzer Space Telescope, which was launched in August 2003, that the identification of the star-streams had been possible. (Source: The Star, 30 April 2009) The Spitzer telescope is Nasa’s fourth largest space-based infrared observatory and costs $800 million (about R6.7 billion). Explaining the infrared features of the Spitzer telescope and its usefulness, Prof. Robert Gehrz of the University of Minnesota, a member of the team that made the startling discovery, said: “It has the ability to penetrate through obscuring dust in the dense clouds where stars form. “Most of the chemical elements, other than hydrogen and helium, condense into dust particles that have a size comparable to or smaller than the wavelength of visible light. “Dust blocks optical starlight very effectively,” says Gehrz, “but infrared light with its longer wavelength goes right around each particle. “This allows the infrared light from young stars to be seen more clearly. To see rivers of stars emerging from their natal clouds in galaxies millions of light years away was unimaginable a year ago,” he added. Giovanni Fazio, project leader for the Spitzer Infrared Array Camera, the team used to take the pictures, and co-author of the discovery, summed it up in The Astrophysical Journal: “We see before our eyes, streams of stars in other galaxies for the first time. This discovery continues to highlight the enormous potential of the Spitzer Space Telescope to make contributions none of us could have dreamed possible.” Block explained to Leadership: “We did not know what was happening in other galaxies because in the past, optical images from space telescopes have not been able to penetrate thick cosmic dust clouds; we just didn’t know if star streams were present or if they had a role to play in the formation of galaxy discs.” Apart from the Spitzer telescope being able to penetrate dense cosmic dust, it is also able to filter out older stars from newer ones so that observations and analysis such as those made by the team are possible. The infrared star stream images were first received by the team in 2006. Over the past three years they have been carefully scrutinised and analysed. What the scientists first saw were small arcs of lights that on closer investigation turned out to be the once elusive star streams. The infrared images from the Spitzer produced enhanced space pictures that revealed groups of stars transforming themselves from dense, irregular patches into mini-spirals caused by shearing, which is a rotating action that causes the deformation of the star clusters. Two filters, a spatial filter and a time filter, were also used so that the arc patterns could be identified. (The spatial filter comes from the Fourier mathematical technique, and the time filter comes from the infrared wavelengths.) “When I saw the arcs for the first time, I knew we were looking at something special and I was very excited,” Block told The Star. A new understanding of how galaxies are formed Block told the Sunday Times that the findings would help explain how galaxies are formed: “Galaxies are the fundamental building blocks of our universe. Just like in trying to understand life on Earth, we need to examine all the different species. “Until we understand what makes galaxies tick, we can’t understand the dynamics of the universe. It’s as fundamental as that.” The discovery sheds light on our own galaxy, the Milky Way, whose stars – including our sun – were probably distributed in a similar manner. “Using the Spitzer Space Telescope with new analysis techniques, one finds a revolutionary way of moving the study of star streams from our immediate neighbourhood out to galaxies millions of light years away,” Block emphasised. A love affair with the stars Block has always been fascinated by the stars. In 1969, as a teenager, his love affair with astronomy began when he watched a blazing comet with utter awe and personally met a South African astronomer, Jack Bennett, who had a comet named after him. Shortly after meeting Bennett, his father, Leon Block, bought him a four-and-a-half inch reflector telescope, which was no little thing for a teenager. “With that incredible instrument I could start to look at planets like Saturn and at some of the nebulae in which stars are born,” said Block. “I wanted to pursue studies in astronomy and my father was my biggest supporter. Leon Block always encouraged me to question things, to look beyond the ordinary and to make up my own mind. After all, we were Jews and that was part of our tradition as well.” David Block was elected a fellow of the Royal Astronomical Society of London at age 19. His first research paper, on relativistic astrophysics, was published in London, by the Royal Astronomical Society, at age 20. He completed a Master of Science degree in relativistic astrophysics and a PhD degree in astronomy dealing with “The Morphology of Spiral Galaxies”. That is a title which, when decoded for dummies, means the Birth of Stars, of which there are 100 000 million in our Milky Way galaxy. Block has been a visiting astronomer at the European Southern Observatory near Munich, West Germany and also at the Institute of Astronomy, University of Hawaii. Currently, he is also a director of the Anglo American Dust Laboratory at Wits. In 2006, he received the university’s highest research accolade, the vice-chancellor’s research award. s Fanie Heyns