Voyager and Pioneer’s Grand Tour of the Milky Way

During the early 1990s, NASA’s Pioneer 10 and 11 probes became the first robotic missions to venture beyond Neptune. In 2012 and 2018, the Voyager 1 and 2 missions went even farther by crossing the heliopause and entering interstellar space. Eventually, these probes may reach another star system, where their special cargo (the Pioneer Plaques and the Golden Records) could find their way into the hands of another species.

Which raises an important question: where might these spacecraft eventually wander? To address this, Coryn Bailer-Jones of the Max Planck Institute for Astronomy and Davide Farnocchia of NASA’s Jet Propulsion Laboratory recently conducted a study that examined which star systems the Voyager and Pioneer probes will likely encounter as they drift through the Milky Way over the next few million years…

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Thanks to Gaia, We Now Know Exactly When We’ll be Colliding with Andromeda

The trajectories of the Milky Way, Andromeda, and the Triangulam galaxies. Image Credit: E. Patel, G. Besla (University of Arizona), R. van der Marel (STScI)

Astronomers have known for some time that the Milky Way and the Andromeda galaxies will collide on some future date. The best guess for that rendezvous has been about 3.75 billion years from now. But now a new study based on Data Release 2 from the ESA’s Gaia mission is bringing some clarity to this future collision.

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The Milky Way is Actually Warped

For centuries, astronomers have been studying the Milky Way in order to get a better understanding of its size and structure. And while modern instruments have yielded invaluable observations of our galaxy and others (which have allowed astronomers to gain a general picture of what it looks like), a truly accurate model of our galaxy has been elusive.

For example, a recent study by a team of astronomers from National Astronomical Observatories of Chinese Academy of Sciences (NAOC) has shown that the Milky Way’s disk is not flat (as previously thought). Based on their findings, it appears that the Milky Way becomes increasingly warped and twisted the farther away one ventures from the core.

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One of Our Best Views of the Supermassive Black Hole at the Heart of the Milky Way

Top left: simulation of Sgr A* at 86 GHz without interstellar scattering. Top right: simulation with interstellar scattering. Bottom right: observed image of Sgr A*. Bottom left: observed image of Sgr A* after removing the effects of interstellar scattering. Credit: S. Issaoun, M. Mo?cibrodzka, Radboud University/ M. D. Johnson, CfA

An almost unimaginably enormous black hole is situated at the heart of the Milky Way. It’s called a Supermassive Black Hole (SMBH), and astronomers think that almost all massive galaxies have one at their center. But of course, nobody’s ever seen one (sort of, more on that later): It’s all based on evidence other than direct observation.

The Milky Way’s SMBH is called Sagittarius A* (Sgr. A*) and it’s about 4 million times more massive than the Sun. Scientists know it’s there because we can observe the effect it has on matter that gets too close to it. Now, we have one of our best views yet of Sgr. A*, thanks to a team of scientists using a technique called interferometry.

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The Milky Way Could Be Spreading Life From Star to Star

For almost two centuries, scientists have theorized that life may be distributed throughout the Universe by meteoroids, asteroids, planetoids, and other astronomical objects. This theory, known as Panspermia, is based on the idea that microorganisms and the chemical precursors of life are able to survive being transported from one star system to the next.

Expanding on this theory, a team of researchers from the Harvard Smithsonian Center for Astrophysics (CfA) conducted a study that considered whether panspermia could be possible on a galactic scale. According to the model they created, they determined that the entire Milky Way (and even other galaxies) could be exchanging the components necessary for life.

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Narrowing Down the Mass of the Milky Way

Since the birth of modern astronomy, scientists have sought to determine the full extent of the Milky Way galaxy and learn more about its structure, formation and evolution. At present, astronomers estimate that it is 100,000 to 180,000 light-years in diameter and consists of 100 to 400 billion stars – though some estimates say there could be as many as 1 trillion.

And yet, even after decades of research and observations, there is still much about our galaxy astronomers do not know. For example, they are still trying to determine how massive the Milky Way is, and estimates vary widely. In a new study, a team of international scientists presents a new method for weighing the galaxy based the dynamics of the Milky Way’s satellites galaxies.

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We’re in the Milky Way’s Second Life. Star Formation was Shut Down for Billions of Years

Since the birth of modern astronomy, scientists have sought to determine the full extent of the Milky Way galaxy and learn more about its structure, formation and evolution. According to current theories, it is widely believed that the Milky Way formed shortly after the Big Bang (roughly 13.51 billion years ago). This was the result of the first stars and star clusters coming together, as well as the accretion of gas directly from the Galactic halo.

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Astronomers Have Found Grease in Space

Over the course of many centuries, scientists learned a great deal about the types of conditions and elements that make life possible here on Earth. Thanks to the advent of modern astronomy, scientists have since learned that these elements are not only abundant in other star systems and parts of the galaxy, but also in the medium known as interstellar space.

Consider carbon, the element that is essential to all organic matter and life as we know it. This life-bearing element is also present in interstellar dust, though astronomers are not sure how abundant it is. According to new research by a team of astronomers from Australia and Turkey, much of the carbon in our galaxy exists in the form of grease-like molecules.

Their study, “Aliphatic Hydrocarbon Content of Interstellar Dust“, recently appeared in the Monthly Notices of the Royal Astronomical Society. The study was led by Gunay Banihan, a professor from the Department of Astronomy and Space Sciences of Erge University in Turkey, and included members from multiple departments from the University of New South Wales in Sydney (UNSW).

A new study by a team of Turkish and Australian astronomers has found evidence of grease-like molecules in interstellar dust. Credit: ESO/R. Fosbury (ST-ECF)

For the sake of their study, the team sought to determine exactly how much of our galaxy’s carbon is bound up in grease-like molecules. At present, it is believed that half of the interstellar carbon exists in pure form, whereas the rest in bound up in either grease-like aliphatic molecules (carbon atoms that form open chains) and mothball-like aromatic molecules (carbon atoms that form planar unsaturated rings).

To determine how plentiful grease-like molecules are compared to aromatic ones, the team created material with the same properties as interstellar dust in a laboratory. This consisted of recreating the process where aliphatic compounds are synthesized in the outflows of carbon stars. They then followed up on this by expanding the carbon-containing plasma into a vacuum at low temperatures to simulate interstellar space.

As Prof. Tim Schmidt, from the Australian Research Council Centre of Excellence in Exciton Science in the School of Chemistry at UNSW Sydney and a co-author on the paper, explained:

“Combining our lab results with observations from astronomical observatories allows us to measure the amount of aliphatic carbon between us and the stars.”

Using magnetic resonance and spectroscopy, they were then able to determine how strongly the material absorbed light with a certain infrared wavelength. From this, the team found that there are about 100 greasy carbon atoms for every million hydrogen atoms, which works out to about half of the available carbon between stars. Expanding that to include all of the Milky Way, they determined that about 10 billion trillion trillion tonnes of greasy matter exists.

Artist’s impression of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

To put that in perspective, that’s enough grease to fill about 40 trillion trillion trillion packs of butter. But as Schmidt indicated, this grease is far from being edible.

“This space grease is not the kind of thing you’d want to spread on a slice of toast! It’s dirty, likely toxic and only forms in the environment of interstellar space (and our laboratory). It’s also intriguing that organic material of this kind – material that gets incorporated into planetary systems – is so abundant.”

Looking ahead, the team now wants to determine the abundance of the other type of non-pure carbon, which is the mothball-like aromatic molecules. Here too, the team will be recreating the molecules in a laboratory environment using simulations. By establishing the amount of each type of carbon in interstellar dust, they will be able to place constraints on how much of this elements is available in our galaxy.

This in turn will allow astronomers to determine exactly how much of this life-giving element is available, and could also help shed light on how and where life can take hold!

Further Reading: RAS, MNRAS

Globular Clusters Might not be as Old as Astronomers Thought. Like, Billions of Years Younger

Globular clusters have been a source of fascination ever since astronomers first observed them in the 17th century. These spherical collections of stars are among the oldest known stars in the Universe, and can be found in the outer regions of most galaxies. Because of their age and the fact that almost all larger galaxies appear to have them, their role in galactic evolution has remained something of a mystery.

Previously, astronomers were of the opinion that globular clusters were some of the earliest stars to have formed in the Universe, roughly 13 billion years ago. However, new research has indicated that these clusters may actually be about 4 billion years younger, being roughly 9 billion years old. These findings may alter our understanding of how the Milky Way and other galaxies formed, and how the Universe itself came to be.

The study, titled “Reevaluating Old Stellar Populations“, recently appeared online and is being evaluated for publication in The Monthly Notices for the Royal Astronomical Society. The study was led by Dr. Elizabeth Stanway, an Associate Professor in the Astronomy group at the University of Warwick, UK, and was assisted by Dr. J.J. Eldridge, a Senior Lecturer at the University of Auckland, New Zealand.

Within larger galaxies, like the Milky Way, globular clusters are part of the galactic halo. Between 150 and 180 are estimated to be part of the Milky Way alone. Credit: ESO

For the sake of their study, Dr. Stanway and Dr. Eldridge developed a series of new research models designed to reconsider the evolution of stars. These models, known as Binary Population and Spectral Synthesis (BPASS) models, had previously proven effective in exploring the properties of young stellar populations within the Milky Way and throughout the Universe.

Using these same models, Dr. Stanway and Dr. Eldridge studied a sample of globular clusters in the Milky Way and nearby quiescent galaxies. They also took into account the details of binary star evolution within globular clusters and used them to explore the colors of light and spectra from old binary populations. In short, binary star system evolution consists of one star expanding into a giant while the gravitational force of the smaller star strips away the atmosphere of the giant.

What they found was that these binary systems were about 9 billion years old. Since these stars are thought to have formed at the same time as the globular clusters themselves, this demonstrated that globular clusters are not as old as other models have suggested. As Dr. Stanway said of the BPASS models she and Dr. Eldridge developed:

“Determining ages for stars has always depended on comparing observations to the models which encapsulate our understanding of how stars form and evolve. That understanding has changed over time, and we have been increasingly aware of the effects of stellar multiplicity – the interactions between stars and their binary and tertiary companions.

An artist’s impression of a millisecond pulsar and its companion. The presence of binaries in globular clusters is a good means of providing age estimates of those clusters. Credit: ESA & Francesco Ferraro (Bologna Astronomical Observatory)

If correct, this study could open up new pathways of research into how massive galaxies and their stars are formed. However, Dr. Stanway admits that much work still lies ahead, which includes looking at nearby star systems where individual stars can be resolved – rather than considering the integrated light of a cluster. Nevertheless, the study could have immense significant for our understanding of how and when galaxies in our Universe formed.

“If true, it changes our picture of the early stages of galaxy evolution and where the stars that have ended up in today’s massive galaxies, such as the Milky Way, may have formed,” she said. “We aim to follow up this research in the future, exploring both improvements in modelling and the observable predictions which arise from them.”

An integral part of cosmology is understanding when the Universe came to be the way it is, not just how. By determining how old globular clusters are, astronomers will have another crucial piece of the puzzle as to how and when the earliest galaxies formed. And these, combined with observations that look to the earliest epochs of the Universe, could just yield a complete model of cosmology.

Further Reading: University of Warwick, arXiv

Thanks to a Massive Release from Gaia, we now Know Where 1.7 BILLION Stars are in the Milky Way

On December 19th, 2013, the European Space Agency’s (ESA) Gaia spacecraft took to space with one of the most ambitious missions ever. Over the course of its planned 5-year mission (which was recently extended), this space observatory would map over a billion stars, planets, comets, asteroids and quasars in order to create the largest and most precise 3D catalog of the Milky Way ever created.

The first release of Gaia data, which took place in September 2016, contained the distances and motions of over two million stars. But the second data release, which took place on April 25th, 2018, is even more impressive. Included in the release are the positions, distance indicators and motions of more than one billion stars, asteroids within our Solar System, and even stars beyond the Milky Way.

Whereas the first data release was based on just over a year’s worth of observations, the new data release covers a period of about 22 months – which ran from July 25th, 2014, to May 23rd, 2016. Preliminary analysis of this data has revealed fine details about 1.7 billion stars in the Milky Way and how they move, which is essential to understanding how our galaxy evolved over time.

ESA’s Gaia is currently on a five-year mission to map the stars of the Milky Way. Image credit: ESA/ATG medialab; background: ESO/S. Brunier.

As Günther Hasinger, the ESA Director of Science, explained in a recent ESA press release:

The observations collected by Gaia are redefining the foundations of astronomy. Gaia is an ambitious mission that relies on a huge human collaboration to make sense of a large volume of highly complex data. It demonstrates the need for long-term projects to guarantee progress in space science and technology and to implement even more daring scientific missions of the coming decades.

The precision of Gaia‘s instruments has allowed for measurements that are so accurate that it was possible to separate the parallax of stars – the apparent shift caused by the Earth’s orbit around the Sun – from their movements through the galaxy. Of the 1.7 billion stars cataloged, the parallax and velocity (aka. proper motion) of more than 1.3 billion were measured and listed.

For about 10% of these, the parallax measurements were so accurate that astronomers can directly estimate distances to the individual stars. As Anthony Brown of Leiden University, who is also the chair of the Gaia Data Processing and Analysis Consortium Executive Board, explained:

The second Gaia data release represents a huge leap forward with respect to ESAs Hipparcos satellite, Gaias predecessor and the first space mission for astrometry, which surveyed some 118 000 stars almost thirty years agoThe sheer number of stars alone, with their positions and motions, would make Gaias new catalogue already quite astonishing. But there is more: this unique scientific catalogue includes many other data types, with information about the properties of the stars and other celestial objects, making this release truly exceptional.

In addition to the proper motions of stars, the catalog provides information on a wide range of topics that will be of interest to astronomers and astrophysicists. These include brightness and color measurements of nearly all of the 1.7 billion stars cataloged, as well as information on how the brightness and color change for half a million variable stars over time.

It also contains the velocities along the line of sight of seven million stars, the surface temperatures of about 100 million, and the effect interstellar dust has on 87 million. The Gaia data also contains information on objects in our Solar System, which includes the positions of 14,000 known asteroids (which will allow for the precise determination of their orbits).

Beyond the Milky Way, Gaia obtained more accurate measurements of the positions of half a million distant quasars – bright galaxies that emit massive amounts of energy due to the presence of a supermassive black hole at their centers. In the past, quasars have been used as a reference frame for the celestial coordinates of all objects in the Gaia catalogue based on radio waves.

However, this information will now be available at optical wavelengths for the first time. This, and other developments made possible by Gaia, could revolutionize how we study our galaxy and the Universe. As Antonella Vallenari, from the Istituto Nazionale di Astrofisica (INAF), the Astronomical Observatory of Padua, Italy, and the deputy chair of the Data Processing Consortium Executive Board, indicated:

The new Gaia data are so powerful that exciting results are just jumping at us. For example, we have built the most detailed Hertzsprung-Russell diagram of stars ever made on the full sky and we can already spot some interesting trends. It feels like we are inaugurating a new era of Galactic archaeology.

The Hertzsprung-Russell diagram, which is named after the two astronomers who devised it in the early 20th century, is fundamental to the study of stellar populations and their evolution. Based on four million stars that were selected from the catalog (all of which are withing five thousand light-years from the Sun), scientist were able to reveal many fine details about stars beyond our Solar System for the first time.

Along with measurements of their velocities, the Gaia Hertzsprung-Russell diagram enables astronomers to distinguish between populations of stars that are of different ages, are located in different regions of the Milky Way (i.e. the disk and the halo), and that formed in different ways. These include fast moving stars that were previously thought to belong to the halo, but are actually part of two stellar populations.

“Gaia will greatly advance our understanding of the Universe on all cosmic scales,” said Timo Prusti, a Gaia project scientist at ESA. “Even in the neighborhood of the Sun, which is the region we thought we understood best, Gaia is revealing new and exciting features.”

For instance, for a subset of stars within a few thousand light-years of the Sun, Gaia measured their velocity in all three dimensions. From this, it has been determined that they follow a similar pattern to stars that are orbiting the galaxy at similar speeds. The cause of these patterns will be the subject of future research, as it is unclear whether its caused by our galaxy itself or are the result of interactions with smaller galaxies that merged with us in the past.

Last, but not least, Gaia data will be used to learn more about the orbits of 75 globular clusters and 12 dwarf galaxies that revolve around the Milky Way. This information will shed further light on the evolution of our galaxy, the gravitational forces affecting it, and the role played by dark matter. As Fred Jansen, the Gaia mission manager at ESA, put it:

Gaia is astronomy at its finest. Scientists will be busy with this data for many years, and we are ready to be surprised by the avalanche of discoveries that will unlock the secrets of our Galaxy.

The third release of Gaia data is scheduled to take place in late 2020, with the final catalog being published in the 2020s. Meanwhile, an extension has already been approved for the Gaia mission, which will now remain in operation until the end of 2020 (to be confirmed at the end of this year). A series of scientific papers describing what has been learned from this latest release will also appear in a special issue of Astronomy & Astrophysics.

From the evolution of stars to the evolution of our galaxy, the second Gaia data release is already proving to be a boon for astronomers and astrophysicists. Even after the mission concludes, we can expect scientists will still be analyzing the data and learning a great deal more about the structure and evolution of our Universe.

Further Reading: ESA