Sky & Telescope - February 2021 - 16

Baby Star

Even though it is the
prototype for the entire
class, in many ways it is
not a typical T Tauri star.
follows a rather elliptical orbit around its mate. In the May
2020 Research Notes of the AAS, Rainer Köhler (University of
Vienna, Austria) and Karolina Kubiak (University of Lisbon, Portugal) noted an intriguing pattern. The little star
brightens and fades depending on where it is in its orbit. The
astronomers attribute this behavior to a nearly edge-on dust
disk that is known to surround the two infrared stars. The
disk is more tenuous toward its edge, so when the little star
skirts farthest from its partner, less dust obscures it, and so it
brightens. The two stars will be closest together around 2023.
Unfortunately, no one has weighed T Tauri North, the
one star in the system whose visible light we see, and its
exact nature is uncertain. Even though it is the prototype
for the entire class, in many ways it is not a typical T Tauri
star. For one thing, the star outshines most of its peers. The
high luminosity suggests the star is more massive than most
other T Tauri stars, and in the past astronomers have estimated a mass about twice that of the Sun. T Tauri North has
a disk, but it's nearly face-on, which may be why we can see

High-energy
radiation
from shock

Dust
photosphere

r envelope
Oute

t envelope
Dus
p
st va orized
Du

Protostar
Accretion
shock
Partially
ionized gas

Dust
destruction
front

Infrared
radiation

Not to scale

Most T Tauri stars are young pre-mainsequence stars that were born within
the last several million years and are
less than 2.5 times as massive as the
Sun. T Tauri itself, however, may be an
even younger variety of star, called a
protostar.
Despite their similar names, the two
stellar types - protostars and premain-sequence stars - are different
stages of stellar evolution. A protostar
forms after a clump in an interstellar cloud rapidly collapses under the
force of gravity. Gravity causes material to crash down toward the object's
surface, heating it so much that it
begins to emit light. Thus gravity -
and not nuclear fusion - powers the
newborn star.
New material from the birth cloud

FEBRUARY 2 021 * SK Y & TELESCOPE

keeps raining down onto the protostar, increasing its mass. In fact, the
protostar is so shrouded in its dusty
birthplace that no visible light escapes
into space (see diagram above).
The protostar phase is brief. How
brief depends on the star's aspirations:
Just as someone who wants to earn a
PhD has to spend more time in school
acquiring knowledge than someone
who has a high-school diploma, so a
more massive star has to spend more
time as a protostar accreting material
than a less massive star. The Sun's
protostar stage lasted only a few
hundred thousand years. Less massive stars - the vast majority - spend
even less time as protostars.
A powerful wind from the protostar
soon blows away most of its dusty

cocoon, letting the star's visible light
emerge. This metamorphosis marks the
transition from protostar to pre-mainsequence star.
Unlike a protostar, a pre-mainsequence star no longer acquires
much new material. Still, the star keeps
on shining, because gravity gently
squeezes and heats the star. Over
time, thanks to gravity, a pre-mainsequence star shrinks, getting smaller
and smaller.
Most stars spend far more time as
pre-main-sequence stars than protostars. The Sun, for example, was a
pre-main-sequence star for 36 million
years. Less massive stars linger longer,
and the least massive red dwarfs
remain pre-main-sequence stars for
billions of years. Some of the M stars

G REGG DINDER M A N / S&T, SOURCE: S. W. STA HLER / PASP 19 94

Protostar vs. Pre-Main-Sequence Star

Sky & Telescope - February 2021

Table of Contents for the Digital Edition of Sky & Telescope - February 2021