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.