The Stellar Baby Boom
By Judith E Braffman-Miller
Sparkling in a wild, frenzied, and joyful dance throughout the vast and mysterious expanse of Space and Time, the stars that we see shining and shimmering in our dark clear sky at night were all mostly born in an explosive stellar “baby boom” that occurred about 10 billion years ago. Galaxies, like our own barred-spiral Milky Way–a starlit pin-wheel spinning in intergalactic Space–experienced this dazzling “baby boom” of infant stars when our 13.8 billion year old Universe was still a youngster. These sparkling stellar “baby boomers” were born at a prodigious rate–30 times faster than the way stars are born today–in a wild celestial party in the sky. In December 2016, astronomers announced that they have gotten their first peek at precisely where most of today’s stars were born, using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) in New Mexico, and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, in order to peer at distant galaxies–seeing them as they were 10 billion years ago.
“We knew that galaxies in that era were forming stars prolifically, but we didn’t know what those galaxies looked like, because they are shrouded in so much dust that almost no visible light escapes them,” Dr. Wiphu Rujopakam explained in a December 20, 2016 National Radio Astronomy Observatory (NRAO) Press Release. Dr. Rujopakam is of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo in Japan and the Chutolongkorn University in Bangkok, Thailand. Dr. Rujopakam was lead author on the research paper.
Unlike visible light, radio waves have the ability to cut through dense, obscuring shrouds of dust to see what lies hidden beneath. However, in order to unveil the details of such remote–and dim–galaxies, the team of astronomers had to make the most sensitive images ever made with the VLA.
The new observations, using the VLA and ALMA, have managed to answer some nagging questions about precisely what mechanisms caused most of the star-birth in those distant galaxies dwelling in the early Universe. The astronomers found that bursts of intense star-birth in the faint galaxies they studied most often occurred throughout the galaxies, in contrast to much smaller regions in present-day galaxies with similar high star-formation rates.
In astronomy, long ago is the same as far away.The farther into Space astronomers look, the further back they look in time. This is because light travels at a finite speed, and the starlight streaming to us from long ago and far away is just now arriving at Earth. No known signal in the Universe can travel faster than light in a vacuum, and so light sets something of a universal speed limit.
The team of astronomers used the VLA and ALMA to observe sparkling galaxies bobbing around in the Hubble Ultra Deep Field image, a small region of the entire sky observed since 2003 with NASA’s venerable Hubble Space Telescope (HST). The HST made extremely long exposures of the area in order to spot galaxies in the remote, ancient Universe. Since then, numerous observing programs with other telescopes have conducted follow-up observations based on the HST images.
In one of the most comprehensive multi-observatory galaxy surveys yet accomplished, another team of astronomers found that even though galaxies like our own Milky Way experienced those intense bursts of prodigious star-birth 10 billion years ago, our own Star, the Sun, apparently made it to the sparkling star-party late. Our Sun was not born until almost 5 billion years ago. By that time the stellar birth-rate in our Galaxy had plummeted to a tiny fraction of what it was in its glory days.
However, sometimes fashionably late stars have an advantage over their more punctual kin. This is precisely what happened with our Sun. Our Star’s late arrival on the stellar scene may actually have been responsible for the formation of our Solar System’s planets. This is because elements heavier than hydrogen and helium–metals in the jargon of astronomers–were much more abundant later. The more massive stars of the boomer generation had ended their brilliant lives early, blowing themselves to pieces in powerful supernova blasts that gifted our Galaxy with material that served as the building blocks of planets–and even life on Earth.
All stars, regardless of their mass, are huge fluffy balls of roiling, glaring, searing-hot gas. Stellar nuclesynthesis is the term used to refer to the way that natural abundances of the chemical elements contained within stars experience a remarkable sea-change as a result of the nuclear-fusion reactions that are occurring within their cores and their surrounding mantles. Stars are a lot like people–they change as they age. These stellar changes involve the abundances of the elements that stars hold within their hot nuclear-fusing hearts. The fusion reactions occurring within a star’s core increases the atomic weight of its constituent elements, and therefore reduces the number of particles. This could conceivably result in the loss of pressure. However, the star’s own powerful gravity triggers a contraction that is followed by a rise in temperature. This results in a precious and extremely delicate balance between two perpetually warring forces–gravity and the pressure derived from the star’s radiation.
Stars are primarily composed of hydrogen gas. Hydrogen is both the lightest and most abundant atomic element in the Universe. In their secretive, hot, and roiling, broiling hearts, the stars transform their supply of hydrogen into progressively heavier and heavier atomic elements. All atomic elements heavier than helium are manufactured in the furnaces of stars–or, alternatively, in the supernovae blasts that herald the explosive “deaths” of the most massive stars. The heaviest atomic elements of all–such as gold and uranium–are formed when a massive star goes supernova.
Baby stars are cradled within mysterious, lovely, billowing folds that are swirling within undulating frigid, gigantic, dark molecular clouds that haunt our Milky Way Galaxy in huge numbers. These dark clouds are beautiful, and they glitter with the lovely, fierce fires of newborn stars. Molecular clouds are primarily composed of gas and smaller quantities of dust, and they are the strange nurseries of baby stars. Particularly dense blobs embedded within the swirling folds of these whirling, phantom-like clouds, merge together in an assortment of sizes, with the smaller blobs reaching about one light-year across. The dense blobs ultimately collapse under the merciless, relentless weight of their own gravity to give birth to a new baby star (protostar). The entire episode of baby-star-birth takes about 10 million years.
The myriad of sparkling stellar denizens of the Cosmos are all kept bouncy and fluffy as a result of the energy that is manufactured by the nuclear fusion occurring in their cores. All stars maintain a necessary–and precarious–balance between the crushing force of gravity that tries to pull everything in, and the radiation pressure that tries to push everything out. This very delicate balancing act between these two stellar enemies continues on and on from star-birth to star-death. The struggle continues throughout the entire “lifetime” of a star–which it spends on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution.
Alas, all parties come to an end. The inevitable occurs when the doomed old star has finally managed to consume its entire necessary supply of hydrogen fuel in its nuclear-fusing heart. At this fatal point, gravity wins the war against pressure, and the star’s core collapses–heralding stellar death. A star dies with either a bang or a whimper–depending on its mass. Small stars, like our own Sun, go gentle into that good night, and with relative peace puff their varicolored, beautiful outer gaseous layers into interstellar space–leaving only a small, dense core behind as its legacy to the Universe. The dense core that lingers, after the death of its star, is termed a white dwarf, and it is encircled by a shimmering shroud of colorful, glimmering gases termed a planetary nebula.
However, more massive stars die with a bigger bang than their smaller stellar kin. The more massive stars blow themselves to smithereens in the catastrophic, explosive rage of a supernova blast. Massive stars leave behind either a neutron star or a black hole of stellar mass to haunt the space between stars, telling the sad story about a star that was, that is no more.
The mass of a star determines how and when it will perish. Small stars live longer than more massive ones because they are relatively cool and burn their supply of fuel much more slowly than their hotter massive kin. The more massive stars live fast and die young–exploding at tender ages of mere millions of years. Stars like our Sun live for approximately 10 billion years, and stars that are even smaller than our own small Star–called red dwarfs–can theoreticaly live for trillions of years. Since our Universe is “only” about 13.8 billion years old, it is generally thought that there are no red dwarf ghosts around. This is because they have not had sufficient time for their progenitor red dwarf stars to die, and leave their relics behind to tell their story.
Our Fashionably Late Star
Since astronomers do not have baby pictures of our Milky Way Galaxy’s youthful years, they must study galaxies that are similar in mass to our Milky Way so that they can trace the history of its resident stars’ growth. In order to accomplish this, they go on a treasure hunt, using clues found in deep surveys of the Cosmos.
In one of the most comprehensive multi-galaxy surveys yet–reported in the spring of 2015–astronomers announced their finding of the stellar baby boom that rocked our Galaxy 10 billion years ago. The farther back in Space a celestial object is, the more ancient it is. Using surveys, such as this one–stretching back in time more than 10 billion years–astronomers were able to put together an album of almost 2,000 pictures of galaxies resembling our own Milky Way.
This census provides the most complete portrait yet of how Milky Way-like galaxies grew and evolved over the passage of 10 billion years to become the majestic spiral galaxies that we see today–including our own. The multi-wavelength observations use ultraviolet to far-infrared light, combining observations from NASA’s HST and Spitzer Space Telescopes, the European Space Agency’s (ESA’s) Herschel Space Observatory, and ground-based telescopes, including the Magellan Baade Telescope at their Las Campanas Observatory in Chile.
“This study allows us to see what the Milky Way may have looked like in the past. It shows that these galaxies underwent a big change in the mass of its stars over the past 10 billion years, bulking up by a factor of 10, which confirms theories about their growth. And most of that stellar-mass growth happened within the first 5 billion years of their birth,” Dr. Casey Papovich explained in an April 9, 2015 NASA Press Release. Dr. Papovich is of Texas A & M University in College Station, and lead author on the paper that describes the study’s findings published in the April 9, 2015 issue of The Astrophysical Journal.
The study showed a strong correlation between the observed galaxies rate of star-birth and their growth in stellar mass. When galaxies start to slow down in their birthing of new baby stars, their growth slows down as well. “I think the evidence suggests that we can account for the majority of the buildup of a Milky Way-like galaxy through its star formation. When we calculate the star-formation rate of a Milky Way-like galaxy in the past and add up all the stars it would have produced, it is pretty consistent with the mass growth we expected. To me, that means we’re able to understand the growth of the ‘average’ galaxy with the mass of a Milky Way galaxy,” Dr. Papovich continued to note.
Even though our Sun missed the big, wild star-party, because it was born too late to be a stellar boomer, it was not born all alone and bereft of companionship. Our Star might have been born as a member of a dense open cluster of stars along with thousands of other glittering sister stars. The stellar baby boom accounts for the peak rate of brilliant stellar birth, but later generations of lovely stars made their glittering mark on the Universe, as well.
The Stellar Baby Boom
Dr. Kristina Nyland, of the NRAO, noted in the December 20, 2016 NRAO Press Release that “We used the VLA and ALMA to see deeply into these galaxies, beyond the dust that obscured their innards from Hubble. The VLA showed us where star formation was occurring, and ALMA revealed the cold gas that is the fuel for star formation.” Dr. Nyland is a co-author on the 2016 paper with lead author Dr. Rujopakam.
“In this study, we made the most sensitive image ever made with the VLA. If you took your cellphone, which transmits a weak radio signal, and put it at more than twice the distance to Pluto, near the outer edge of the Solar System, its signal would be roughly as strong as what we detected from these galaxies,” Dr. Preshanth Jagannathan, who is also of the NRAO, explained in the same NRAO Press Release.
This study was conducted by an international team of astronomers. Others involved include Dr. James Dunlop of the University of Edinburgh (UK); Dr. Robert Ivison, also of the University of Edinburgh; and the European Southern Observatory.
The study was published in the December 1, 2016 issue of The Astrophysical Journal.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, newspapers, and magazines. 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.