When Dead Stars Collide

Before the first stars were born, the Universe was an expanse of featureless darkness--devoid of light and life. The first generation of stars to shatter this swath of blackness, with their fierce stellar fires, were not like the stars we see today, because they were not born the same way. The first stars were giants, that formed directly from the lightest of all atomic gases--mostly hydrogen, with a smaller amount of helium, both of which formed in the wildly expanding fireball of the Universe's mysterious Big Bang birth almost 14 billion years ago. The first stars are responsible for changing the Universe from what it was then, to what it now is. This is because they created, in their nuclear-fusing hearts, all of the atomic elements heavier than helium, thus "polluting" the Cosmos with the atomic elements that made planets, moons, and people possible. In June 2018, scientists at the California Institute of Technology (Caltech) in Pasadena, announced that they have found, for the first time, that colliding and merging duos of dense neutron stars are responsible for creating most of the heavy atomic elements present in small dwarf galaxies, shedding new light on yet another mystery of star-birth.
Heavy atomic elements, such as gold and silver, are called "metals" by astronomers, and they are of critical importance for planet formation, as well as the emergence of life itself. By observing these relatively tiny dwarf galaxies, the scientists hope to learn more about the primary sources of "metals" for the entire Universe.
The origin of most of the heaviest atomic elements listed in the familiar Periodic Table, including 95% of all the gold present on Earth, has been a subject of debate among astronomers for decades. However, it is now understood that the heavy "metals" are created when the nuclei of atoms within stars snare elementary particles called neutrons. In the case of most elderly stars, including those inhabiting the dwarf galaxies observed in this study, the process happens very rapidly--and, as a result, is termed an r-process, where the r signifies "rapid."
There are currently two proposed potential sites where the r-process is theorized to take place. The first possible site is a rare form of supernova called a magnetorotational supernova, which is a type of stellar explosion that can create large magnetic fields. The second proposed site involves two neutron stars that collide and then merge. In August 2017, the National Science Foundation (NSF)-funded Interferometry Gravitational-wave Observatory (LIGO), along with other ground-based telescopes, spotted just such a neutron star collision that was in the midst of creating a treasure trove composed of the heaviest atomic elements. However, observing only one event is not sufficient to tell astronomers where most of these heavy "metals" are created within galaxies.
Stellar Generations
Astronomers classify stars as either Population I (metal-rich) or Population II (metal-poor). However, even the most metal-poor stars belonging to Population II contain small amounts of metals. This means that these metal-poor ancient stars are composed of more than only the pristine hydrogen and helium gas that was produced in the Big Bang (Big Bang Nucleosynthesis). For this reason, there had to exist an earlier population of stars to manufacture these heavy metals.
Therefore, astronomers were forced to propose the existence of a third stellar population--the very ancient Population III stars that were composed entirely of ancient primordial gas that had been churned out in the Big Bang. Big Bang Nucleosynthesis produced only hydrogen, helium, and trace quantities of lithium or beryllium. The first stars produced the first batch of metals that "polluted" younger generations of stars. Population III stars served as the source of the small amount of metals observed in the metal-poor Population II stars.
The more massive the star, the shorter its hydrogen burning "life." Massive stars burn their necessary supply of nuclear-fusing hydrogen fuel in their cores much more rapidly than smaller stars, thus manufacturing increasingly heavier and heavier atomic elements out of lighter ones. The end comes when the massive star finally has managed to fuse for itself a core of iron that cannot be used for fuel. At this terrible grand finale of a massive star's "life", it collapses and then blows itself to smithereens in a fatal supernova blast. In contrast, relatively small stars like our Sun--which is a metal-rich Population I star--blissfully burn their hydrogen fuel for about 10 billion years. More massive stars, however, "live" for mere millions, as opposed to billions, of years and do not die quietly. When small stars like our Sun reach the end of the stellar road they first become swollen Red Giant stars that eventually puff off their outer gaseous layers. The relic core of a small Sun-like star becomes a dense dead stellar corpse, called a white dwarf, that is surrounded by a beautiful, multicolored, sparkling shroud of what was once the dead progenitor star's outer gases.
Therefore, massive stars, like Population III stars--as well as younger generations of massive stars--do not die in peace. They go out with a bang. When a massive star dies, it explodes as a supernova--a brilliant fatal blast that causes the erstwhile star to either leave a relic neutron star behind, or a stellar mass black hole. The core-collapse (Type II) supernova blasts out into space a good-bye gift to the Universe--its freshly forged batch of metals. These metals will ultimately be incorporated into Populations I and II stars--with all of their beautiful life-sustaining possibilities. We are here because the stars are here.
Stellar Burn-Out
Neutron stars are both the smallest and densest of known stellar objects. Usually, a neutron star will sport a radius of about 6.2 miles and a mass that ranges between 1.4 and 3 times that of our Sun. They are the end-product of a supernova that has compressed the core of the massive progenitor star to the density of an atomic nucleus. Once born, neutron stars can no longer generate heat, and they cool off as time goes by--but, it is still possible for them to evolve further as a result of collisions or accretion.
Most models indicate that neutron stars are almost entirely made up of neutrons, which are subatomic particles with no net electrical charge and with a slightly larger mass than protons. Protons and neutrons form the nuclei of atoms. Electrons and protons present in normal atomic matter combine to create neutrons at the conditions of neutron stars.
Neutron stars that have been observed are extremely hot, with a surface temperature of approximately 600,000 Kelvin. They are so extremely dense that a teaspoon full of neutron star stuff would have a mass of about 3 billion tons. Their magnetic fields are between 100 million to 1 quadrillion times as powerful as that of our planet. The gravitational field at a neutron star's surface is about 200 billion times that of Earth.
As the massive progenitor star's core collapses, its rotation rate increases due to the conservation of angular momentum. As a result, newborn neutron stars spin at up to several hundred times per second. Some neutron stars emit regular beams of electromagnetic radiation that make them detectable as pulsars. Indeed, these emitted beams are so extremely regular that they are frequently compared to lighthouse beacons on Earth. The 1967 discovery of pulsars by Dr. Jocelyn Bell Burnell provided the first observational evidence that neutron stars really exist in nature.
Astronomers think that there are approximately 100 million neutron stars inhabiting our Milky Way Galaxy. This number has been obtained by scientists calculating the number of stars that have gone supernova in our Galaxy. However, even though the neutron stars that have been observed so far are searing-hot, most neutron stars are old, cold, and difficult to find--unless they are in their neonatal pulsar stage or are members of a tattle-tale binary system. Lazily-rotating and non-accreting neutron stars are almost undetectable. However, thanks to the highly successful Hubble Space Telescope, some neutron stars that apparently emit only thermal radiation have been spotted. Neutron stars in binary systems can experience accretion which makes the system bright in X-rays while the material is tumbling onto the neutron star, thus forming hotspots that rotate in and out of view in identified X-ray pulsar systems. Such accretion can "rejuvenate" elderly pulsarsand potentially cause them acquire more mass and spin-up to extremely rapid rotation rates, thus forming what are termed millisecond pulsars. These binaries will continue to evolve, and ultimately the companion stars can also become compact stellar relics, such as white dwarfs and neutron stars themselves--although some other possibilities include the total destruction of the luckless companion through either merger or ablation. The merger of binary neutron stars may be the source of what are called short-duration gamma-ray bursts--the strong sources of ripples in Spacetime termed gravitational waves. In 2017, just such a direct detection of gravitational waves from this type of event was made, and gravitational waves have also been indirectly spotted in a system where a duo of neutron stars orbit each other.
When Burnt-Out Stars Collide
The team of Caltech astronomers studied several dwarf galaxies in order to observe the production of atomic elements in galaxies as a whole. For this purpose, the researchers used the W.M. Keck Observatory in Maunakea, Hawaii. Our own Milky Way, although quite large, is generally considered to be about average in size--at least, as far as galaxies go. However, these relatively tiny dwarf galaxies, which are in orbit around our Milky Way, contain a puny 100,000 times less mass in stars than does our Galaxy. The astronomers were on the hunt for when the heaviest metals in the small galaxies were made. This is because, magnetorotational supernovae tend to occur in the ancient Universe, while neutron star mergers happen later in the Universe's history.
The results of the Caltech study, submitted for publication in The Astrophysical Journal, were presented at the 232nd meeting of the American Astronomical Society (AAS) held in June 2018 in Denver, Colorado. The Caltech astronomers' study provides evidence that the main sources of the r-process in dwarf galaxies occurs over the passage of a relatively long timescale. This means that the heavy atomic elements were manufactured later in the history of element production in galaxies. By measuring the ratio of elements in stars of differing ages, the scientists were able to say when these elements were created in our Milky Way.
"This study is based on the concept of galactic archaeology, which uses the elements present in stars today to 'dig up' evidence of the history of element production in galaxies. Specifically, by measuring the ratio of elements in stars with different ages we are able to say when these elements were created in the Galaxy," Dr. Evan Kirby explained to the press on June 5, 2018. Dr. Kirby is an assistant professor of astronomy at Caltech.
Astronomers frequently study dwarf galaxies as a method of learning about galaxies in general. Because these galaxies are relatively small, they possess less complicated histories that are easier to read than those of their larger galactic kin.
Gina Duggan, a Caltech graduate student and lead author of the new research, commented that "Unlike the Milky Way, which has grabbed stars from other galaxies throughout its history, these dwarf galaxies were isolated when their stars were born, allowing galactic archaeology to clearly track the buildup of r-process elements over time. This provides an important clue for the timescale of the dominant source of r-process production across the Universe for the first time."
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to explain to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke." will be published soon.
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