Hubble’s Use of Cepheids

Meet Edwin Hubble

Contrast/conflict with Shapley!

Rhodes Scholar

war hero (?) super athlete (?) Hollywood celebrity (?)…

As much myth as reality

A Dose of Reality

Hubble as a high-school basketball coach (1914)

Hubble’s Discovery of Cepheids

n  Using the 100-inch Hooker telescope, he took a series of photographs of nearby spiral nebulae (like M31)

n  On the plates, he identified any objects that varied in brightness

n  He discovered some that varied in the periodic way characteristic of Cepheids

A Cepheid in M31

The Implications

1. The nebulae contain stars. In particular, we have identified Cepheids, which we know to be bright supergiants.

2. But these Cepheids are scarcely detectable,

at the limits of the world’s largest telescopes! So the nebulae must be very far away – and thus very large, comparable to our Milky Way.

A Tragic Irony

Milton Humason, a former mule-driver turned astronomer, had earlier noticed the variability of these stars on photographs and had described that to Shapley, when he was still working at Mount Wilson. (Shapley later moved to Harvard.)

Shapley had dismissed this as impossible – he ‘knew’

that the spiral nebulae were small, nearby objects. It was galling for him to see Hubble credited with this

profound discovery.

One Important Limitation

Cepheids are already faint in M31. In somewhat more remote galaxies, only the very brightest Cepheids will be detectable. But many galaxies are very far away.

Indeed, there is a limit beyond which we can’t see Cepheids, or

any individual stars, at all. We see only their combined luminosity, merged together,and can only study the integrated (summed) light.

Out of the billions of galaxies we know, individual Cepheids have

been studied within only several hundred of the nearest of them.

Meet M100 Quite nearby! It is ‘only’ 30 million light years away

Cepheids Studied by

the Hubble Space Telescope - working at its very limits!

Remember the Leavitt Law

In galaxies at the limit, we detect only the very brightest Cepheids: those with extremely long periods. This also means that studying them (to determine the periods) takes a long time.

How Far Can We Reach?

Cepheids can now be studied in galaxies out to a distance of about fifty million light years. (New bigger telescopes will reach somewhat farther.)

This sounds huge, but it’s really just a tiny fraction of the

observable universe! There are literally hundreds of billions of galaxies that are much more remote, at distances of billions of light years.

In those galaxies, we cannot distinguish individual stars at

all (except rarely, when one explodes as a supernova).

How Do We Go Beyond That?

We determine the distances of even more remote galaxies in other ways, including:

n  study bright supernovae within them

n  study the brightest globular clusters within them (my PhD thesis work!)

n  compare the properties of a whole galaxy to a similar galaxy closer to us, whose distance is known already

n  and so on – in short, a chain of reasoning, built up step-by-step.

Distance Determinations

Anyway, Hubble’s Crucial Discovery: Other Galaxies Exist!

Having made this discovery, Hubble naturally wanted to learn as much about them as he could, to advance our further understanding.