The Milky Way Galaxy Once Died… But Then Came Back to Life
The Milky Way Galaxy has experienced a much more dramatic history than originally thought. It appears that our galaxy died once before and we are now in what can be considered its second life, according to Masafumi Noguchi of Tohoku University in Japan. His research shows that stars in our galaxy formed over a 10 billion year period in two different epochs through very different mechanisms, with a long dormant period of 5 billion years in between. During that time, star formation ceased and the galaxy was essentially void of stellar life. The history of our galaxy is inscribed in the elemental composition of its stars… And there are two groups of stars in the solar neighborhood with very different compositions. One group (the first generation) is rich in alpha elements such as oxygen, magnesium, and silicon. The second generation contains a lot of iron. Our Sun is a second generation star. The Hertzsprung-Russell diagram shown here provides a convenient way to shows how the size, color, luminosity, spectral class, and absolute magnitude of stars relate. Each dot on this diagram represents a star in the sky whose absolute magnitude and spectral class have been determined. Stars tend to clump naturally into four groups – Main Sequence stars, Giants, Supergiants, and White Dwarfs. (Credit: Tohoku University, Noguchi) (Image Credit: Royal Astronomical Society of Canada – RASC)
The Milky Way Galaxy Once Died… But Then Came Back to Life
Our home galaxy has turned out to have experienced a more dramatic history than originally thought. The Milky Way galaxy died once before and we are now in what is considered its second life. Calculations by Masafumi Noguchi, of Tohoku University in Japan, published in the July 26, 2018 edition of Nature, reveal previously unknown details about the Milky Way. It appears that stars formed in two different epochs through different mechanisms and there was a long dormant period of 5 billion years in between. During that time, star formation ceased and the galaxy was essentially void of stellar life.
Noguchi has calculated the evolution of the Milky Way over a 10 billion year period, which includes a “Cold Flow Accretion” period -- a new idea proposed by Avishai Dekel of The Hebrew University in Israel for how galaxies collect surrounding gas during their formation. Although the two stage formation was originally suggested for much more massive galaxies by Yuval Birnboim (also of The Hebrew University), Noguchi has been able to confirm that the same concept applies to our own Milky Way.
The history of the Milky Way is inscribed in the elemental composition of stars, because stars inherit the composition of the gas from which they are formed. Namely, stars memorialize the element abundance in gas at the time they are formed.
There are two groups of stars in the solar neighborhood with different compositions. One is rich in alpha elements such as oxygen, magnesium, and silicon. The other group contains a lot of iron. Recent observations by Misha Haywood of the Observatoire de Paris in France revealed that this phenomenon prevails over a vast region of the Milky Way. The origin of this dichotomy was unclear. Noguchi’s model provides an answer to this long standing riddle.
Noguchi’s depiction of the Milky Way’s history begins at the point when cold gas streams flowed into the galaxy (Cold Flow Accretion) and stars formed from this gas. During this period the gas quickly began to contain alpha elements released by explosions of short-lived type II supernovae. These first generation stars are therefore rich in alpha elements.
When shock waves appeared and heated the gas to high temperatures 7 billion years ago, the gas stopped flowing into the galaxy and stars ceased to form. During this period, retarded explosions of long-lived type Ia supernovae injected iron into the gas and changed the elemental composition of the gas. As the gas cooled by emitting radiation, it began flowing back into the galaxy 5 billion years ago (in a cooling flow) and made the second generation of stars rich in iron, including our Sun.
According to Benjamin Williams of the University of Washington, our neighboring galaxy Andromeda also formed stars in two separate epochs. Noguchi’s model predicts that massive spiral galaxies like the Milky Way and Andromeda experienced a gap in star formation, whereas smaller galaxies made stars continuously. Noguchi expects that future observations of nearby galaxies may revolutionize our view about galaxy formation.
Mapping the Family Tree of Stars
Astronomers are borrowing principles applied in biology and archaeology to build a family tree of the stars in the galaxy. By studying chemical signatures found in the stars, they are piecing together these evolutionary trees looking at how the stars formed and how they are connected to each other. The signatures act as a proxy for DNA sequences. It's akin to chemical tagging of stars and forms the basis of a discipline astronomers refer to as Galactic Archaeology.
It was Charles Darwin who, in 1859, published his revolutionary theory that all life forms are descended from one common ancestor. This theory has informed evolutionary biology ever since but it was a chance encounter between an astronomer and an biologist over dinner at King's College in Cambridge in the UK that got the astronomer thinking about how it could be applied to stars in the Milky Way.
Dr Paula Jofre, of the University of Cambridge's Institute of Astronomy, describes how she set about creating a phylogenetic "Tree of Life" that connects a number of stars in the galaxy.
"The use of algorithms to identify families of stars is a science that is constantly under development. Phylogenetic trees add an extra dimension to our endeavors, which is why this approach is so special. The branches of the tree serve to inform us about the stars' shared history" she says.
The team picked twenty two stars, including the Sun, to study. The chemical elements have been carefully measured from data coming from ground-based high resolution spectra taken with large telescopes located in the north of Chile. Once the families were identified using the chemical DNA, their evolution was studied with the help of their ages and kinematical properties obtained from the space mission Hipparcos, the precursor of Gaia, the spacecraft orbiting Earth that was launched by the European Space Agency (ESA).
Stars are born from violent explosions in the gas clouds of the galaxy. Two stars with the same chemical compositions are likely to have been born in the same molecular cloud. Some live longer and serve as fossil records of the composition of the gas at the time they were formed. The oldest star in the sample analyzed by the team is estimated to be almost ten billion years old, which is twice as old as the Sun. The youngest is 700 million years old.
In evolution, organisms are linked together by a pattern of descent with modification as they evolve. Stars are very different from living organisms, but they still have a history of shared descent as they are formed from gas clouds, and carry that history in their chemical structure. By applying the same phylogenetic methods that biologists use to trace descent in plants and animals it is possible to explore the evolution of stars in the Galaxy.
"The differences between stars and animals is immense, but they share the property of changing over time, and so both can be analyzed by building trees of their history," says Professor Robert Foley, of the Leverhulme Centre for Human Evolutionary Studies at Cambridge.
With an increasing number of datasets being made available from both Gaia and more advanced telescopes on the ground, and ongoing and future large spectroscopic surveys, astronomers are moving closer to being able to assemble one tree that would connect all the stars in the Milky Way.
So, How Old Are the Stars in the Milky Way?
Using completely new ways of estimating the ages of so-called red giant stars from observational data, astronomers have created the first large-scale map that shows stellar ages in the Milky Way. Determining the ages of nearly 100,000 red giant stars, at distances of up to 50,000 light-years from the galactic center, the astronomers, led by Melissa Ness and Marie Martig of the Max Planck Institute for Astronomy, were able to test key ideas about the growth of the Milky Way. Notably, the map confirms that our home galaxy has grown inside out. In the present epoch, most old stars can be found in the middle and more recently formed ones in the outskirts.
In the past decades, powerful astronomical surveys have provided astronomers with data about millions of astronomical objects, allowing for large-scale statistical analysis. But big data of this kind is only as good as the tools available for analysis. Now, Melissa Ness and Marie Martig of the Max Planck Institute for Astronomy have added two powerful new tools to astronomy's arsenal.
Using sample data from the APOGEE survey (part of the Sloan Digital Sky Survey) and NASA's Kepler Space Telescope, Ness and Martig devised two independent methods for determining the age of a red giant star from its spectrum (that is, from the properties of its light).
Using these methods, the astronomers were able to estimate the ages of nearly 100,000 stars that had been observed with the APOGEE survey. The result is an age map of the Milky Way, showing exactly which regions of our galaxy harbor young, old, or middle-age stars. The map provides a representative cross section from the center of the Milky Way to the outskirts at a distance of 65,000 light-years from the galactic center.
With an age map of this kind, current models of how our home galaxy come into being and can be put to the test. For instance, such models predict that stellar disks, the dominant stellar component of galaxies like the Milky Way, should have formed from the inside out. So, one would expect to find the older stars closer to the galactic center and the younger stars at the outside. The map confirms this distribution.
Also, at any given radius, the younger stars are typically found closer to the galactic plane than their older cousins. This is also confirmed by the age map.
Techniques such as could help astronomers reconstruct the entire star formation history of our galaxy -- How many stars within our galaxy were formed at different times of galactic history, in which regions, and how these stars have enriched our galaxy's raw material with the various elements they produce via nuclear fusion (thus enabling the production of heavier elements, of planets and, eventually, of living beings).
Planetary Nebulae Also Help Determine the Chemical Composition of the Early Milky Way Galaxy
A planetary nebula is a beautiful object created during the final stages of a star’s life. The wispy, colorful halo of gas making up the nebula and surrounding the dying star is actually material that was originally part of the star itself but has been cast off and is expanding outward into interstellar space. It glows as the result of being heated by the ultraviolet radiation produced by the dying star. The word planetary is really misleading, as these objects have nothing to do with the planets in our solar system. Rather, they acquired the name because when they were first observed in the 19th century their extended appearance (versus the point-like image of a normal star) reminded astronomers of the way planets like Uranus and Neptune appear in a telescope. In a galaxy such as our own Milky Way there are estimated to be several thousand planetary nebulae at any one time. Most of them are concentrated toward the plane of the Milky Way's disk, but a few are also known to exist in the halo and a number have been identified in the bulge of the galaxy as well.
What's so interesting about planetary nebulae? Astronomers are drawn to study these objects because they provide opportunities to analyze material that was once a part of a shining star. For example, by studying the chemical composition of the nebula we can gain an understanding about the material out of which the star originally formed. In addition, the abundances of certain elements such as carbon and nitrogen in the nebula reveal details about the physical processes that occurred within the star during its nuclear fusion lifetime. Studying planetary nebulae helps us to understand how a star changes, or evolves, during its lifetime.
But why and how does a planetary nebula form in the first place? Interestingly enough, it's related to the star's lifelong battle against the relentless force of gravity. In order to keep from collapsing upon itself, a star maintains high internal gas pressure by creating its own energy through nuclear fusion. During most of the star's life the principal fuel for fusion is hydrogen, but as the star exhausts it supply of this element, it will rely increasingly on heavier, more complex elements. Ultimately, however, available fuels run out, the star becomes unstable, and it ejects its outer gaseous layers which expand outward and form a planetary nebula. The lines in each spectrum can be analyzed to determine nebular properties such as chemical composition, temperature, and density.
By studying the distribution of the chemical elements in planetary nebulae, which are shells of gas cast off by sun-like stars as they die, the astronomers can uncover patterns of chemical enrichment that yield clues to how spiral galaxies were formed and how they have developed. A planetary nebula is a kind of "time machine.” Its chemical abundances reveal the composition of the galaxy at the time and place where its ancestral star was formed billions of years in the past.
Carrying out this ambitious project will require, among other efforts, a search for previously undetected planetary nebulae in the outlying regions of the Milky Way and Andromeda Galaxies, followed by observations of their spectra to determine their chemical compositions.
The Milky Way and Andromeda Galaxies – A Pair of Fine Young Cannibals
An international team of astronomers has identified two new tidal streams in the Andromeda galaxy -- the remnants of dwarf galaxies consumed by our large galactic neighbor. Analysis of the stars in Andromeda's tidal streams and other components of its extended halo is yielding new insights into the processes involved in the formation and evolution of massive galaxies according to Puragra Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz.
In the currently favored "Lambda Cold Dark Matter" paradigm of structure formation in the universe, the outer halos of large galaxies like our Milky Way Galaxy and neighboring Andromeda are built up through the merger and dissolution of smaller "dwarf" satellite galaxies. "This process of galactic cannibalism is an integral part of the growth of galaxies," Guhathakurta said.
The smooth, well-mixed population of halo stars in these large galaxies represents the aggregate of the dwarf galaxy victims of this cannibalism process, while the dwarf galaxies that are still intact as they orbit their large parent galaxy are the survivors of this process.
"The merging and dissolution of a dwarf galaxy typically lasts for a couple billion years, so one occasionally catches a large galaxy in the act of cannibalizing one of its dwarf galaxy satellites," Guhathakurta said. "The characteristic signature of such an event is a tidal stream -- an enhancement in the density of stars, localized in space, and moving as a coherent group through the parent galaxy."
Tidal streams are important because they represent a link between the victims and survivors of galactic cannibalism -- an intermediate stage between the population of intact dwarf galaxies and the well-mixed stars dissolved in the halo.
The Andromeda galaxy is a unique test bed for studying the formation and evolution of a large galaxy, said Guhathakurta, who lead the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) collaboration, a large survey of red giant stars in Andromeda. "Our external vantage point gives us a global perspective of the galaxy, and yet the galaxy is close enough for us to obtain detailed measurements of individual red giant stars within it," he said.
In a project, the researchers used the Subaru 8-meter telescope and Suprime-Cam camera to map the density of red giant stars in large portions of the Andromeda galaxy, including the hitherto uncharted north side. This led to the discovery of two tidal streams to the northwest (streams E and F) at projected distances of 200,000 and 300,000 light years from Andromeda's center.
The study also confirmed a few previously known streams, including the little-studied diffuse stream to the southwest (stream SW), which lies at a projected distance of 200,000 to 300,000 light years from Andromeda's center.
The SPLASH team followed up with a spectroscopic survey of several hundred red giant stars in Streams E, F, and SW, using the Keck II 10-meter telescope and DEIMOS spectrograph at the W. M. Keck Observatory in Hawaii. The spectrograph spreads out the light from each star into a spectrum, which allows astronomers to measure the velocity of the star and distinguish Andromeda red giant stars from foreground stars in the Milky Way. The spectral data confirmed the presence of coherent groups of Andromeda red giant stars moving with a common velocity.
Complex elements such as iron, magnesium, and calcium in the outer layers of a red giant star were produced within previous generations of massive stars that ended their lives as supernova explosions, spewing out newly forged elements into the interstellar medium. Thus, the fraction of complex elements found in stars indicates the degree to which the gas from the host galaxy (the raw material from which new stars are formed) was enriched by supernova explosions from successive generations of massive stars.
"Massive galaxies like the Milky Way and Andromeda are very effective at recycling chemicals and therefore contain stars like our Sun that are relatively rich in complex elements -- rich enough for rocky planets to have formed and for those planets to contain complex molecules such as proteins," Guhathakurta said.
Dwarf galaxies are less effective at recycling chemicals than massive galaxies. This is partly because the weaker gravity of a dwarf galaxy makes it harder for it to retain the chemically enriched gas that is blown out of massive stars during supernova explosions. As a result, stars in dwarf galaxies are more anemic (have a smaller fraction of complex elements) than those in the interior of massive galaxies. Moreover, the action of merging with a larger galaxy causes a dwarf galaxy to lose its gas, breaking the chemical cycle altogether.
"Dwarf galaxy cannibalism victims have had less time to recycle their chemicals than dwarf galaxy survivors, and this should be reflected as a difference between their chemical properties," Guhathakurta said. "Tidal streams should be somewhere between the victims and the survivors in terms of their chemical properties."
At the present time, detailed studies of the chemical properties of tidal streams, intact dwarf satellites, and smooth stellar halos are possible only in the Milky Way and Andromeda galaxies and their immediate surroundings. Existing telescopes and instruments are simply not powerful enough for astronomers to carry out such studies in more distant galaxies.
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