Although it may seem preposterous, it has to get cold in order
for a hot baby star to be born. This is because stars are born tucked
inside relatively dense concentrations of gas and dust embedded within
giant, dark molecular clouds. These regions are extremely frigid, with
temperatures of only 10 to 20 Kelvin--just slightly above absolute zero.
At these temperatures, gases become molecular, which means that atoms
merge together, making the gas clump to high densities--and when the
density reaches a certain point, stars are born. In December 2014, a new
study was announced suggesting that a surge of warm gas into a
neighboring galaxy, left over from the tragic cannibalization of a
separate galaxy, has suffocated star-birth by stirring up the available
chilly gas.
The study is important because it casts an entirely
new light on the way galaxies evolve over time. The findings of this
research are derived from the European Space Agency's (ESA's) Herschel Space Observatory, in which NASA made important contributions, especially with both the Spitzer Space Telescope (SST) and the venerable Hubble Space Telescope (HST).
Astronomers
are trying to understand why galaxies in the local Universe seem to
fall into two major types: relatively youthful spirals like our own
Milky Way, that look like whirling and majestic starlit pin-wheels in
space, and the much more elderly ellipticals, in which baby stars are no
longer forming, and are therefore populated primarily by old stars. The
new study's target galaxy, dubbed NGC 3226, seems to represent
a transitional middle-ground between the two distinct types, and
therefore an understanding of its star-birthing abilities is critical
for those astronomers who are trying to solve this galactic mystery.
"We have explored the fantastic potential of big data archives from NASA's Hubble, Spitzer and ESA's Herschel Observatory
to pull together a picture of an elliptical galaxy that has undergone
huge changes in its recent past due to violent collisions with its
neighbors. These collisions are modifying not only its structure and
color, but also the condition of the gas that resides in it, making it
hard--at the moment--for the galaxy to form many stars," explained Dr.
Philip Appleton in a December 5, 2014 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Appleton is project scientist for the NASA Herschel Science Center of the California Institute of Technology (Caltech) in Pasadena and lead author of a paper appearing in the Astrophysical Journal describing the new results.
A Star Is Born
Stars
are enormous balls of searing-hot, roiling, and glaring gas. The
billions upon billions of sparkling stars in the observable Universe are
all primarily made up of hydrogen--the most abundant atomic element
listed in the familiar Periodic Table, as well as the lightest.
Stars transform hydrogen fuel deep within their hot nuclear-fusing
cores into progressively heavier and heavier atomic elements. The only
elements that were born in the Big Bang birth of the Universe about 13.8
billion years ago, were hydrogen, helium, and small amounts of lithium (Big Bang nucleosynthesis). All of the other elements of the Periodic Table
were formed deep in the secretive, seething hearts of the stars, their
glaring, hot interiors progressively fusing the nuclei of atoms into
heavier and heavier things (stellar nucleosynthesis).
Stars
are born in the billowing depths of very dense, dark, and frigid
molecular clouds, that haunt our Milky Way Galaxy, and likely other
galaxies as well. Even though molecular clouds are primarily composed of
gas and dust, they also contain large populations of glittering stars.
The material within the swirling, ghostly clouds is clumped together in
an assortment of sizes, with the smaller clumps extending approximately
one light-year across. The dense clumps eventually collapse to form protostars--and the entire star-birthing process takes about 10 million years.
The glittering multitude of stars in the Universe are kept bouncy as a result of the energy that is produced by the process of nuclear fusion
that is taking place in their cores. The stars maintain a precious and
delicate equilibrium between their enormous, squeezing, crushing
gravity--which tries to pull everything in--and their huge energy output, which produces radiation pressure, that tries to push everything out. This enormous production of energy is the result of stellar nucleosynthesis,
that manufactures heavier atomic elements out of lighter ones. This
necessary balance between gravity and radiation pressure is maintained
from star-birth to star-death--the entire "lifetime" of the star--which
it spends on the hydrogen-burning main sequence. At last, the
star meets its inevitable doom when it has finally burned its necessary
supply of hydrogen fuel, and gravity wins the war against pressure. At
this point, the star's core collapses, and the star "dies." Small stars,
like our Sun, meet their doom with great beauty and relative
peacefulness, puffing off their multicolored outer gaseous layers into
interstellar space. Larger, heavier stars do not go gently into that
good night. Instead, the larger denizens of the stellar population blast
themselves to pieces in the violent and catastrophic rage of a
supernova explosion, which blasts the tragedy that was once a star to
pieces. The size of a star, therefore, is what determines its ultimate
fate.
Baby Stars Need A Cool Stellar Cradle
The galaxy NGC 3226 is
in our Milky Way's own galactic neighborhood, being a "mere" 50 million
light-years distant. There are several starry, gaseous loops that
extend out from NGC 3226, and filaments also emanate from it and between a companion galaxy dubbed NGC 3227.
These filamentary streamers of material indicate that there was once a
third galaxy that probably dwelled there until recently--that is, until NGC 3226 devoured it, messily hurling off chunks of the shredded, cannibalized galaxy all over the area.
A
particularly prominent chunk of the leftovers from this ghastly cosmic
feast extends 100,000 light-years and reaches directly into the core of NGC 3226.
This long streamer finally ends in the form of a curved plume in a disk
of warm hydrogen gas and a ring of dust. The streamer is believed to be
made up of the debris left over from the tragically cannibalized third
galaxy, and the contents are tumbling into NGC 3226, hoisted in
by its irresistible gravitational pull. In numerous cases, adding
material to galaxies in this particular way rejuvenates them, and
triggers new bursts of sparkling stellar birth as a result of the gas
and dust that are swirling around together. However, the data derived
from the three telescopes show that NGC 3226 has a very low rate of star birth. It therefore appears that, in this particular case, the material tumbling into NGC 3226 is
becoming hotter and hotter as it crashes into other pockets of galactic
gas and dust--and this heat effectively extinguishes star birth, rather
than fueling it.
The strange story might have ended differently. This is because NGC 3226
hosts a supermassive black hole in its secretive heart. The infalling
gas and dust might have wound up as the black hole's dinner, triggering
energetic emissions as the material violently collided together while
swirling in towards its unfortunate doom. However, the supermassive
black hole lurking in the heart of NGC 3226 was content just to
merely take a few nibbles, rather than feast on the material. The
material is spread throughout the galaxy's central regions.
"We
are discovering that gas does not simply funnel down into the center of a
galaxy and feed the supermassive black hole known to be lurking there.
Rather, it gets hung up in a warm disk, shutting down star formation and
probably frustrating the black hole's growth by being too turbulent at
this point in time," Dr. Appleton explained in the December 5, 2014 JPL Press Release.
NGC 3226
is believed to occupy a transitional phase between a young "blue"
galaxy and an elderly "red" galaxy. The colors "blue" and "red" refer to
the galactic blue light that is radiated by ferociously hot, giant,
brilliant baby stars--a tattletale clue that recent star birth has
occurred--while the reddish light is radiated out by mature stars in the
absence of new, fiery, blue ones.
This intermediary galaxy sheds
light on how galaxies acquiring fresh gas and dust can flourish and
blossom with new stellar birth or, alternatively, have their star
factories shut down--at least for a time. Actually, as the warm gas
flowing into NGC 3226 cools off to star-birthing temperatures, the galaxy should get a second chance at producing new baby stars.
It is interesting that ultraviolet and optical light observations indicate that NGC 3226
may have formed more stars in the past, resulting in its current
intermediate hue, which is somewhere between blue and red. The new study
suggests that those lingering traces of lost youth must indeed reflect
earlier, higher levels of stellar birth that occurred before the warm
infalling gas invaded the scene.
"NGC 3226 will continue
to evolve and may hatch abundant new stars in the future. We're learning
that the transition from young-to-old-looking galaxies is not a
one-way, but a two-way street," Dr. Appleton added.
Judith E. Braffman-Miller is a writer and astronomer whose
articles have been published since 1981 in various newspapers,
magazines, and journals. Although she has written on a variety of
topics, she particularly loves writing about astronomy because it gives
her the opportunity to communicate to others the many wonders of her
field. Her first book, "Wisps, Ashes, and Smoke," will be published
soon.
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