Sky & Telescope - July 2023 - 37

LOCKYER: NOTABLES OF BRITAIN: AN ALBUM OF PORTRAITS AND AUTOGRAPHS OF
THE MOST EMINENT SUBJECTS OF HER MAJESTY IN THE 60TH YEAR OF HER REIGN /
WIKIMEDIA COMMONS / PUBLIC DOMAIN; ANTARES / SCORPIUS: ALAN DYER
how stars evolve, " says David DeVorkin
(National Air and Space Museum), author
of Henry Norris Russell: Dean of American
Astronomers. " Russell took an evolutionary
interpretation, whereas Hertzsprung did
not want to make a claim. "
In the late 1800s, the prevailing view
FORCE OF NATURE English astronomer Norman
Lockyer created the journal Nature and served
as its editor for half a century. Late in life he was
delighted when his theory of stellar evolution received
support from Henry Norris Russell.
the star turn from red to yellow to blue.
During his 1913 lecture Russell said,
was that stars started off hot and then
cooled down as they radiated their light
away. They therefore proceeded from
blue to yellow to red. Even today astronomers
refer to hot and cool stars with the
anachronistic terms " early-type " and
" late-type, " respectively.
There was a dissenting voice, however: English astronomer
J. Norman Lockyer. In 1868 he had spectroscopically detected
a new chemical element on the Sun that he named helium. A
year later he founded Nature, which began as a popular science
magazine but soon became a scientific journal. Lockyer
thought a star originated as a cool swarm of meteoritic particles,
which collided with one another and heated up, while
gravity caused the swarm to contract. Then, after reaching a
peak temperature, the star cooled off again.
Russell revived this discredited cool-to-hot-to-cool scenario,
though he thought stars consist of gas, not meteoritic particles.
In Russell's view, a young star's gravitational pull compressed
its gas, which by the laws of physics heated up, making
" As almost everybody will agree that a star
contracts as it grows older, this leads us
to suppose that the giant stars of Class M
represent a very early stage of evolution, the
other giant stars later stages according to
whiteness . . . " Thus, Antares and Betelgeuse
were - in Russell's opinion - stellar
newborns. Russell proposed that a star begins life as a large,
diffuse M-type giant. He said gravity then compresses the star
and heats it up, making it a K-type giant like Arcturus. The
star then shrinks further, becoming a yellow G-type giant like
Capella, until eventually it shines as a hot blue giant. During
its red-to-blue giant transition, the star's luminosity holds
steady: Even though the surface is heating up, the diameter is
decreasing, so the two effects cancel each other out.
As Russell noted, when a star shrinks enough to become
hot and blue, it gets so dense that the ideal gas law - which
says the gas obeys simple relations among temperature, pressure,
and volume - starts to break down. As a result, the star
has a harder time shrinking further, which means gravitaRIVAL
OF MARS - AND BETELGEUSE Marking the heart of Scorpius, Antares is a conspicuous sight on July evenings as it crosses the
meridian due south. In this striking photo, the star appears above Mount Custer in Glacier National Park, Montana. Antares is normally the
second brightest red supergiant in the sky, behind Orion's brilliant Betelgeuse, though both stars vary in brightness.
sk yand tele scope .o r g * JULY 2023 37

http://skyandtelescope.org

Sky & Telescope - July 2023

Table of Contents for the Digital Edition of Sky & Telescope - July 2023