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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.