Sky & Telescope - February 2021 - 17

still a protostar. This idea also explains why the star is larger
and more luminous than most other T Tauri stars: because it
hasn't yet contracted much toward the main sequence.
T Tauri North continues to orbit T Tauri South, Reipurth
says, and he thinks that within a few thousand years T Tauri
North will dive back into its birthplace for a family reunion
with its two siblings. He expects chaos, because three stars
close together normally ﬂing one another around and often
eject the least massive one. That's probably what happened
with the Alpha Centauri system, which has two Sun-like stars
and a distant red dwarf they likely kicked away (S&T: Apr.
2019, p. 34). If T Tauri's three stars do something similar,
then T Tauri North and South a will link up to form a closer
binary and cast the red dwarf far away - perhaps out of the
system altogether.
Thus, T Tauri has a storied past whose class played a pivotal
role in revealing that the universe is still an active star maker.
But the star's future story promises to be rich as well - both
for itself, as its three members dance with one another, and
for astronomers, as they discern new clues about this famous
infant in the sky.

W PROTOSTAR A dense core of dusty gas collapses to form a protostar, which draws material onto itself gravitationally. The material strikes
the protostar and creates an accretion shock, and the heated gas that's
produced radiates - the source of the protostar's luminosity. Eventually,
the cloud (here simplified as a sphere) will collapse into a protoplanetary
disk; the protostar will accrete most of its mass during the disk stage.

in the Pleiades star cluster, which is
120 million years old, still shine via gravity power, not nuclear power.
Early in its career, however, an infant
star does get hot enough to stoke one
simple nuclear reaction, which turns
deuterium, or hydrogen-2, into helium.
Unlike ordinary hydrogen, deuterium
burns at a temperature of only 1 million
kelvin, which even protostars attain.
Deuterium-burning gives the star additional energy. But deuterium is rare,
so deuterium-burning doesn't last much
beyond the protostar phase.
As gravity further squeezes and heats
the pre-main-sequence star, its center
gets so hot that enough hydrogen-1
ignites to provide significant additional
energy. Eventually this new energy
source halts the star's contraction, at
which point the star starts its tenure on
the main sequence.
Notably, the rarest and most mas-

Percentage of energy the young
Sun generated via each process

LE AH TISCIONE / S&T, SOURCE: EM A NUELE TOG NELLI (UNIV ERSIT Y OF PISA , ITA LY )

the star's visible light but not the light of its two southern
siblings, whose disk is more edge-on.
In the August 1, 2020, Astrophysical Journal, however,
Christian Flores, Bo Reipurth, and Michael Connelley (all
University of Hawai'i) offer a new interpretation of T Tauri
North. They used high-resolution infrared spectra to uncover
another peculiarity: The star's surface gravity is low, a sign
the star has a large diameter. In fact, the star is so large and
luminous, they say, that it's not a pre-main-sequence star at
all. Instead, it's an even younger type of star: a protostar.
Protostars are stars so young they are still in the process of
being born (see sidebar below). Such stars are normally invisible at optical wavelengths because they are encased in the
dust clouds that gave them birth, and from which material
still rains down upon them.
Why, then, can we see T Tauri North's visible light?
" T Tauri North is an orphaned protostar, " Reipurth says. In
his view, all three stars in the system were born close together
a few hundred thousand years ago. But the other two stars
kicked T Tauri North out of the dust cloud just a few thousand years ago, allowing us to see its light, even though it's

While earning his PhD, KEN CROSWELL observed three
T Tauri stars that had flared up in brightness. He is the author
of eight books, including The Alchemy of the Heavens and The
Lives of Stars, and has also written for Knowable Magazine,
National Geographic, and PNAS.

Gravity
Star is
contracting

50%

Nuclear
Star is
fusing
hydrogen

Age of Sun (millions of years)
S THE SUN'S POWER SOURCE Early on, a pre-main-sequence star generates nearly all
its energy from gravity, which gently squeezes and heats the star. Eventually, however, this
gravitational contraction heats the star's center so much that another power source starts to
contribute: the nuclear burning of hydrogen-1. For a star with the Sun's mass, the star then
continues to contract for millions of years, until the switchover to nuclear is complete.

sive stars, which exceed about six solar
masses, skip the pre-main-sequence
stage altogether. While still protostars,

they begin burning hydrogen-1 and
graduate directly from the protostar
phase to the main sequence.

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Sky & Telescope - February 2021

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