Episode 16: Plumbing the Depths - The Birth of Astrophysics.

The stars are so distant, it seems amazing that we can know anything about them. Yet, over the last 300 years, a new science has emerged: astrophysics. It began with the first attempts to measure the distances of the stars but took off in late Victorian times with the invention of the spectrascope: a device to split starlight up into its component wavelengths.

The spectra contained dark lines which corresponded to bright lines given off when particular substances were heated in a flame. The dark absorption lines revealed the composition of stars and led to the discovery of a new element on the Sun.

Episode 17: Who discovered the Galaxy? - The Nature of the Milky Way.

Before the invention of the telescope, no one new the nature of the Milky Way. Galileo realised that it was a glowing band of stars across the sky and the philosopher Immanuel Kant recognised that our Sun was just one member of a vast star system. But for centuries, few recognised the true nature of our Galaxy.

In the late 19th century, William Herschel realised that the Milky Way was a giant disk of stars, but still his discovery was ignored. It was not until early in the 20th century that Harlow Shapley began to estimate the distances of the stars and hence map out the Milky Way in three-dimensional detail. Finally came radio astronomy and the ability to penetrate the dark clouds towards the galactic centre and see the true distribution of matter in our Galaxy.

Episode 18: Deep Space - Nebulae Are Island Universes..

In 1845 the third Earl of Rosse completed the biggest telescope in the world at his castle in the centre of Ireland. In spite of cloudy skies, this leviathan enabled him to see spectacular detail in the sky, including a spiral structure in certain fuzzy patches known as nebulae. Some of these seemed so big that astronomers thought they must be nearby; perhaps the rotating clouds of gas out of which planetary systems are born. A meticulous American lady-astronomer was to change that. Henrietta Swan Leavitt studied variable stars and realised that a certain type of star varied at a rate that was linked to its actual brightness. Thus, these so-called Cepheid variables could be used as standard candles to estimate the vast distances of space.

In 1919, a new telescope in California took over from Lord Rosse's leviathan to become the biggest in the world. A young astronomer, Edwin Hubble, used it to search for Miss Leavitt's Cepheids in spiral nebulae and discovered, to his astonishment, that the nearest, the Andromeda nebula, lay well outside our own Galaxy and is an island universe in its own right.

Episode 19: Einstein's Biggest Blunder and the expanding, accelerating universe.

In 1915, Albert Einstein found that he had to introduce a new factor into his equations of gravity to prevent the stars from falling in on one another. He called it the cosmological constant. But then, astronomers discovered that the light from galaxies was stretched or red-shifted in a way that could only be explained if the galaxies were flying apart from each other and the universe was expanding. That removed the need for a cosmological constant and Einstein is said to have described it as his biggest blunder.

A recent twist to this tale came in the year 2000 when estimates of the rate of the expansion revealed that the most distant galaxies are in fact accelerating. The only way to explain that was to introduce something called dark energy: in effect, a cosmological constant.

Episode 20: How the Universe Began.

If the universe is expanding, it seems reasonable to try to draw those lines of expansion back and form a theory about how the universe began. That line of reasoning led cosmologists to suggest that the origin lay in a compact, dense, hot fireball. The Cambridge astronomer Fred Hoyle thought this so ridiculous that he disparagingly called it the big bang. The name stuck and there followed intense arguments between supporters of the big bang theory and Fred Hoyle and his colleagues who favoured a steady state universe.

A decisive blow to the steady state model came from a radio telescope in New Jersey in the early 1960s. That detected a gentle background glow at microwave radio frequencies that theorists had predicted as the dying embers of the big bang itself.

Friday 13 June 9.00pm - Omnibus edition

A special edition based on the this week's programmes.