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In 1912 Henrietta Leavitt carefully measured the brightness of variable stars in the Small Magellanic Cloud, a satellite galaxy to the Milky Way.   She noticed that the brightness of one type, Cepheid variables, was linked to their pulsation period.   She had discovered a tool for estimating the distances of objects that were too distant to use any other means.   Her achievement was recognised by a naming of a crater on the Moon in her honour.

Henrietta Leavitt

Back in 18th century William Herschel had mapped the distribution of nearby stars and had determined that they were arranged in a flattened, disc-like shape.   This fitted with the band of dense star clouds that Galileo had discovered  make up the Milky way.   It was thought that the Milky way was the extent of the Universe.

A Cepheid variable star in the galaxy M100

In the 1920s a young astronomer, Edwin Hubble, used the largest telescope in the World at that time, the100inch on Mount Wilson, to work out the distance to faint starry clouds.   One of the largest of these is in the constellation Andromeda.   With the 100 inch, Hubble could just make out Cepheid variables in the cloud.  

Edwin Hubble at the eyepiece of the 100 inch telescope on Mount Wilson

Hubble used the brightness of the Cepheid variables he could see pulsating in the clouds, to estimate the distance of the Andromeda nebula.    He was amazed by his findings.   Their distance was enormous; over two million light years.   This is a hundred thousand times further than the furthest stars you can see in the night sky with the naked eye.  There were other similar clouds but these were far too dim to spot stars in them.   They must be even further away, tens or hundreds of million light years distant.   The Universe, already though by people to be enormous, suddenly became mind numbingly huge to Hubble.   The countless faint clouds that he could see in his telescope were not clouds of gas and stars in our galaxy but galaxies in their own right.   They were island universes, separated from each other and our Milkyway by enormous expanses of almost empty space.  

The spiral galaxy NGC 4414 imaged by the HST

In 1929 Hubble noticed a pattern between the distances he was finding for galaxies and their relative speeds as shown by the redshifts of their specra.   The inevitable conclusion from this relationship is that the Universe is expanding.   Over 20 years before Einstein had worked out that the Universe should be expanding.  He thought the idea so outrageous that he changed his equations to make the Universe stable.  When Hubble came up with his evidence for an expanding universe, Einstein said that changing his findings to make a stable Universe was the biggest mistake he had ever made.

The famous graph from Hubble's paper of 1929 showing the relationships between the distances of galaxies from us and their velocity .

Of course if space is expanding, there must have been a time when everything was together.   This compressed origin of the Universe is known as the Big Bang.

Leavitt and Hubble's pioneering work still underpins the methods that astronomers use to measure the Universe.   However our telescope's, like the Hubble Space Telescope, can see further than those available in the early 20th century.  Other distance measures have joined our tool kit such as observations of exploding stars - super novae.

The Cartwheel galaxy, created by a head-on collision between two galaxies 500 million light years away from Earth.  Imaged by the space telescope named in honour of Edwin Hubble.

LINKS

The European Space Agency and the European Southern Observatory have produced an interesting astronomy exercise using  Cepheid variable stars to find the distance to the galaxy M100.   There are 4 exercises in all, the Cepheids and M100 is exercise 2.