The Astronomy Book

241

See also: Measuring the universe 130–37 ■ The birth of the universe 168–71 ■ Beyond the Milky Way 172–77 ■
Nuclei and radiation 185 ■ The primeval atom 196–97 ■ Dark matter 268–71

NEW WINDOWS ON THE UNIVERSE

evolve over time. The light from
remote galaxies can take millions
or billions of years to reach Earth,
where the galaxies are seen as they
were in the distant past. The farther
away they are, the earlier they
appear in their stage of evolution.
In other words, a distant galaxy
that appears elliptical may be quite
unlike a closer, known elliptical
galaxy. In calculating distances
to remote galaxies, she argued,
corrections needed to be applied,
based on the factors that change
as galaxies evolve—specifically the
abundance of different chemical
elements and the rate of star birth.
Tinsley outlined ways in which
galaxies evolve in terms of their
brightness, shape, and color. The
stars and the non-stellar material
(gas and dust) in galaxies change
over long periods. For example, some
stars eventually turn into giant stars
and get brighter as they age, star
formation rates change as gas and
dust are used up, and the interstellar

medium (the matter between

stars) is enriched with elements

heavier than helium and hydrogen

as old stars die.

Galaxy models

Tinsley’s thesis was described as

“extraordinary and profound” by

her peers at the University of Texas.

For the rest of her short career, she

continued studying the ways

in which different populations

(groups) of stars age and affect

the observable qualities of galaxies.

From this, she developed models of

the ways in which galaxies evolve,

which combined an understanding

of stellar evolution with knowledge

of the motions of stars and nuclear

physics. Today, these models form

the basis for studies of galactic

evolution. They also provide

information about what protogalaxies

(galaxies in their infancy) might

look like. Tinsley’s work also

contributed to research on whether

the universe is open (will expand

forever) or closed (will eventually

stop expanding and contract). One

of the most insightful theoreticians

in astronomy over the past century,

Tinsley has been described as

“opening doors to the future study

of the evolutions of stars, galaxies,

and even the universe itself.” ■

The apparent brightness

of galaxies from Earth

depends on both their

distance and their age.

Remote galaxies differ from closer galaxies in how

they appear from Earth in part because they are being

seen at an earlier stage of evolution.

Distant galaxies are

seen through telescopes

as they were millions or

billions of years ago.

When measuring the distance of galaxies, it is

necessary to take into account how old they are.

This artwork depicts the night sky

from a hypothetical planet within the

Milky Way, when the galaxy was just

3 billion years old. The sky is ablaze with

the hydrogen clouds of new star birth.