Sky & Telescope - February 2021 - 14

Baby Star

Introducing T Tauri
On October 11, 1852, English astronomer John Russell Hind
spotted a " very small " and " very faint " nebula north of the
Hyades star cluster. Near the nebula he also discovered a
10th-magnitude star. The star did not appear on star charts:
" Possibly it may be variable, " he wrote, inferring that the star
had earlier been too dim to see.
Incredibly, the nebula is also variable (S&T: Feb. 2018,
p. 22). It faded and in 1868 vanished. It later returned, but its
position had changed. When Hind made his find, the nebula
lay south-southwest of the star; now, it's west of the star. The
nebula came to be called Hind's Variable Nebula and the star
T Tauri, the third variable star found in Taurus.
Over the following century, astronomers discovered other
stars like T Tauri. In 1945, Mount Wilson astronomer Alfred
Joy grouped 11 stars together and named the class after the
alliterative member in Taurus, in part because T Tauri was
the best known and also one of the brightest. Seven of the 11
stars lay in Taurus or neighboring Auriga. All the stars, Joy
said, flickered irregularly by about three magnitudes; ranged
in spectral type from F5 to G5; had luminosities like the
Sun's; and lay near nebulae, as T Tauri itself does - because,
as we now know, they are shining on the same clouds that
gave them birth. But Joy's lengthy paper makes no mention
of star formation. Ongoing star formation was then considered anathema.
Instead, other astronomers soon interpreted T Tauri stars
14

FEBRUARY 2 021 * SK Y & TELESCOPE

A Newborn Star
In 1947, however, Soviet astronomer Viktor Ambartsumian
saw the truth - and soon after took a jab at astronomers
elsewhere. " Our experience shows that Western astronomers
often display an inconsistency when trying to give cosmogonic interpretation of results of astrophysical observations, "
he wrote. " Doubtless, the progressive Soviet science, inspired
by the genius of great Stalin, will overcome all difficulties on
the way of cosmogonic investigations and clarify the main
laws of origin and evolution of stars. "
Ambartsumian's praise of a brutal tyrant notwithstanding,
his astronomical reasoning was sound. He noted that T Tauri
stars occur in groups, which he called T-associations; he cited
the one in Taurus and Auriga as an example. The Milky Way's
tidal forces should have torn these groups apart, he said -
unless they formed just a few million years ago. He made a
similar argument about O and B stars, which also reside in
associations. Thus, he reasoned, the Milky Way still mints
new stars today.
Unfortunately, because Ambartsumian's works appeared in
Russian or his native Armenian, most astronomers in America and Europe didn't hear the news until the mid-1950s. By
that time, other evidence was also suggesting that T Tauri
stars could hardly be normal stars passing through nebulae.

2Msun
10

Disk
disappears

0.6Msun

0.3Msun

1

0.1
6500

Ma
in

3x105yr

se
qu
en
ce

106yr
3x106yr

T Tauri stars with disks
Transition
T Tauri stars without disks
6000

5500

5000

107yr
3x107yr
4500

4000

3500

3000

Temperature (kelvin)
S T TAURI EVOLUTION Infant stars contract and heat up as they form
from collapsing clouds and eventually start hydrogen fusion. Because
they are larger than the main-sequence stars they will later become,
T Tauri stars appear more luminous, lying above the main sequence on
the Hertzsprung-Russell diagram. With time, the stars travel down and
left, following the solid gray lines according to their masses. The blue
lines that cut through the gray lines show stellar ages, and the black dotted line marks when the young stars lose their disks. The underlying data
points are T Tauri stars in the Taurus-Auriga T association and are sorted
by their evolutionary stage.

C. BERTOU T E T A L. / ASTR ONOMY & ASTR OPHYSICS 20 07 (473:L 21),
REPRODUCED WITH PER MISSION © ESO

A

as normal suns that by chance were passing through interstellar clouds. When material either fell onto the stars or
obscured them, they brightened or faded, explaining their
variability.

Luminosity (Suns)

century ago, the universe seemed a static place. No
one knew it was expanding. Indeed, Albert Einstein
even temporarily added a fudge factor to his equations in order to keep his model of the cosmos stationary.
But surprisingly, even after recognizing the universe's
expansion, astronomers failed to appreciate another fundamental feature of the cosmos. It's something so commonsensical that both professional and amateur astronomers take it
for granted today: The universe still creates new stars.
" At that time it was assumed that all stars were formed
in some massive catastrophe in the early universe, " says Bo
Reipurth (University of Hawai'i). Thus, no stars form now.
Today we know our galaxy is actually a prolific star creator,
spawning new stars in clouds of gas and dust such as the
Orion Nebula. Each year the Milky Way converts about two
solar masses of its gas and dust into new stars. Most are
much less massive than the Sun, so this figure equates to the
birth of about five new stars a year throughout our galaxy.
But the harbinger of the revolutionary revelation that the
universe still makes new stars was hardly a spectacle. Instead,
it was a dim star in Taurus named T Tauri, about 470 lightyears from Earth.
A century elapsed between this star's discovery and the
recognition of its youthful nature. Nor have its surprises
stopped: In just the past year, astronomers have reported new
features about the star that suggest it may be even younger
than they had thought.



Sky & Telescope - February 2021

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

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