The Astronomy Book

137

was also employed by Hubble in
his discovery of the relationship
between the distance and
recessional velocity of galaxies,
leading to confirmation that the
universe is expanding.

Revising the scale
In the 1940s, the German
astronomer Walter Baade was
working at the Mount Wilson
Observatory in California. Baade
made observations of the stars
at the center of the Andromeda
galaxy during the enhanced
viewing conditions afforded
by the wartime blackout. He
distinguished two separate
populations, or groups, of Cepheid
variables that have different
period–luminosity relationships.
This led to a dramatic revision in
the extragalactic distance scale—
for example, the Andromeda
galaxy was found to be double
the distance from the Milky
Way that Hubble had calculated.
Baade announced his findings
at the International Astronomical
Union in 1952. The two groups
of Cepheids became known as

classical and Type II Cepheids,

and started to be used for different

purposes in distance measuring.

Today, classical Cepheids are

used to measure the distance of

galaxies out to about 100 million

light-years—well beyond the

local group of galaxies. Classical

Cepheids have also been used to

clarify many characteristics of

the Milky Way galaxy, such as its

local spiral structure and the sun’s

distance from the plane of the

galaxy. Type II Cepheids have been

used to measure distances to the

galactic center and globular clusters.

THE RISE OF ASTROPHYSICS

The measurement of distances

to Cepheid variables for more

accurate calibration of period–

luminosity relationships is still

considered extremely important,

and it was one of the primary

missions of the Hubble Space

Telescope project when it was

launched in 1990. A better

calibration is crucial, among

other things, to calculate the age

of the universe. Leavitt’s findings

from over a century ago are still

having significant repercussions

in terms of truly understanding

the scale of the cosmos. ■

A simplified version

of the mechanisms that

cause Cepheid variables

to fluctuate in size is

shown here. The pressure

forces inside a star include

gas pressure, maintained

by heat output from the

star’s core, and radiation

pressure. Another

mechanism that may

be involved is a cyclical

change in the opacity

(resistance to the

transmission of radiation)

in gas within the star’s

outer layers.

Pressure forces exceed

gravity. The star begins

to expand.

Pressure and gravity are now

in balance but inertia causes

the star to expand further.

A B

C

With continued expansion,

the pressure forces decrease,

as does the gravity, though

to a lesser extent. Eventually,

gravity exceeds the pressure

from the pressure forces, and

the star stops expanding

and begins to shrink.

As the star contracts, the

pressure forces increase until

they exceed the inward-

pulling gravity. The star

stops shrinking and begins

to expand again, starting

a new pulsation cycle.

Pressure and gravity

are in balance again

but inertia causes the

star to shrink further.

D E

Hubble’s underwhelming

acknowledgment of Leavitt

is an example of the ongoing

denial and lack of professional

and public recognition that

she suffers from, despite her

landmark discovery.

Pangratios Papacosta

Science historian

Gravity Pressure

forces