Edwin Powell Hubble Biography

Edwin Powell Hubble, biographical essentials: born in 1889 in Missouri.

• Edwin Powell Hubble is famous for showing that many nebulae are external galaxies and for providing the first evidence for the Big Bang.

Edwin Hubble's grandfather introduced him to astronomy by building him a telescope for his 8th birthday. Hubble's enthusiasm for astronomy grew, and he planned to study it in Chicago. But his father compelled him to take a law degree. 

Hubble supplemented his law studies with physics courses from such notables as Albert Michelson and Robert Millikan. The latter employed the undergraduate Edwin Hubble as his part-time lab assistant and endorsed his Rhodes scholarship at Oxford University in 1910. Hubble returned to Missouri in 1913 when his father died and the family's finances collapsed. He worked as schoolteacher and part-time lawyer for eighteen months until the family finances were back on a firm footing. Then he completed a fairly uninspired survey of nebulae to gain his Ph.D. at the Yerkes observatory.

Nebula is Latin for "mist" or "cloud", and nebulae are smudges of light in the night sky that look very different from point-like stars. A handful of nebulae were spotted by ancient astronomers, but many were only discovered after the invention of the telescope. In 1781 Charles Messier published a catalogue of of 103 nebulae, now known as Messier objects. William Herschel  produced an expanded catalogue of 2500 nebulae, and discerned a single star in some. He suggested all nebulae were young stars coalescing out of gas and dust. 

Immanuel Kant disagreed with Herschel It seemed absurd to Kant that the universe should be limited to our galaxy. He wrote, "Eternity is not sufficient to embrace the manifestations of the Supreme Being, if it is not combined with the infinitude of space." Quoted in Big Bang by Simon Singh p.180. This sparked the Great Debate, which raged throughout the 19th century. It revolved around one question: are nebulae collections of stars external to our Milky Way galaxy, or clouds of matter within it?

William Parsons, Third Earl of Rosse in Ireland, built the largest telescope of the 1850s. This showed that M51 (the Whirlpool Nebula) had a spiral structure containing stars. But "Herschelians" argued that relatively few stars were visible, and M51 could be in our galaxy.

In 1885 a nova had appeared in the Andromeda nebula which was one-tenth as bright as the entire nebula. Howard Shapley, in 1920, argued that this showed the Andromeda nebula must be in our Milky Way—no nova could be so bright if the Andromeda nebula were beyond our galaxy. We now know that supernovae can be this bright. Anyway, Edwin Hubble was about to provide winning evidence for Kant.

Edwin Hubble realised that to do the best work he needed the best telescope, so (after serving in World War 1) he moved to Mount Wilson. In 1923 Hubble compared several photographs of the Andromeda galaxy (M31) taken with the 100 inch telescope and spotted a Cephedi variable.

Cepheid variable stars attain peak brightness quickly, and then slowly sink to a minimum brightness. For instance, Delta Cephei takes one day to achieve maximum brightness, before falling to its dimmest in four days. When a Cepheid variable star is cool it contracts under gravity. This causes an increase in nuclear fusion reactions: it heats up, expands, and cools. The process repeats (contraction causes the outer layer to become more opaque, leading to dimming).

Henrietta Leavitt, in the 1890s, identified 25 Cepheid variable stars in the Small Magellanic Cloud. She suspected the Small Magellanic Cloud was distant, therefore these Cepheid variable stars could be assumed to be roughly the same distance from Earth. She found that the period of a Cepheid variable star was proportional to its brightness. This meant the relative distance of any Cepheid variable star could now be determined. Shortly after this, a team of astronomers measured the distance to a nearby Cepheid variable using parallax. To this day, Cepheid variable stars are used as yardsticks for the universe.

n 1923, Edwin Hubble measured the distance to a Cepheid variable star he had observed in the Andromeda nebula. This settled the Great Debate. After Hubble, most nebulae were reclassified as galaxies. The term nebulae is still used for "mere" dust clouds within the Milky Way.

In 1929 Hubble measured the Doppler redshifts of a few dozen galaxies, and created Hubble's law.

In 1929 Hubble, after measuring the redshifts of a few dozen galaxies, discovered a law relating the distance of a typical galaxy to its velocity. This is now known as Hubble's law. In discovering this he also found a constant (Hubble's constant) which gave the age of the Universe.  

• Hubbles law: the velocity at which a typical galaxy recedes (v) is proportional to its distance (d). This is Hubble's law, v = Hd, where H is Hubble's constant. 

The light from distant galaxies is shifted towards the red end of the spectrum. This redshift is explained by the Doppler effect, and means the galaxies are moving away from us and the universe is expanding.

Big Bang by Simon Singh (p.253) shows the plot of Hubble's first set of galactic Doppler shifts which led to Hubble's law. Drawing a line back to the origin showed that when the galaxies had zero velocity they also had zero distance, which was the first evidence for the Big Bang. 

The age of the Universe is the inverse of Hubble's constant, i.e. 1/H (Big Bang by Simon Singh,p.258, spells out the proof of this). Hubble's observations implied the Universe was 2 billion years old, but geological research showed the Earth was roughly twice that old. Fritz Zwicky suggested a tired light theory might explain Galactic redshifts, but this idea was shown to be incorrect.