Milky Way Shakes, Rattles and Rolls…

For decades astronomers have puzzled over the many details concerning the formation of the Milky Way Galaxy. Now a group of scientists headed by Ivan Minchev from the Leibniz Institute for Astrophysics Potsdam (AIP) have managed to retrace our galaxy’s formative periods with more detail than ever before. This newly published information has been gathered through careful observation of stars located near the Sun and points to a rather “moving” history.

To achieve these latest results, astronomers observed stars perpendicular to the galactic disc and their vertical motion. Just to shake things up, these stars also had their ages considered. Because it is nearly impossible to directly determine a star’s true age, they rattled the cage of chemical composition. Stars which show an increase in the ratio of magnesium to iron ([Mg/Fe]) appear to have a greater age. These determinations of stars close to the Sun were made with highly accurate information gathered by the RAdial Velocity Experiment (RAVE). According to previous findings, “the older a star is, the faster it moves up and down through the disc”. This no longer seemed to be true. Apparently the rules were broken by stars with the highest magnesium-to-iron ratios. Despite what astronomers thought would happen, they observed these particular stars slowing their roll… their vertical speed decreasing dramatically.

So what’s going on here? To help figure out these curious findings, the researchers turned to computer modeling. By running a simulation of the Milky Way’s evolutionary patterns, they were able to discern the origin of these older, slower stars. According to the simulation, they came to the conclusion that small galactic collisions might be responsible for the results they had directly observed.

Smashing into, or combining with, a smaller galaxy isn’t new to the Milky Way. It is widely accepted that our galaxy has been the receptor of galactic collisions many times during its course of history. Despite what might appear to be a very violent event, these incidents aren’t very good at shaking up the massive regions near the galactic center. However, they stir things up in the spiral arms! Here star formation is triggered and these stars move away from the core towards our galaxy’s outer edge – and near our Sun.

In a process known as “radial migration”, older stars, ones with high values of magnesium-to-iron ratio, are pushed outward and display low up-and-down velocities. Is this why the elderly, near-by stars have diminished vertical velocities? Were they forced from the galactic center by virtue of a collision event? Astronomers speculate this to be the best answer. By comparison, the differences in speed between stars born near the Sun and those forced away shows just how massive and how many merging galaxies once shook up the Milky Way.

Says AIP scientist Ivan Minchev: “Our results will enable us to trace the history of our home galaxy more accurately than ever before. By looking at the chemical composition of stars around us, and how fast they move, we can deduce the properties of satellite galaxies interacting with the Milky Way throughout its lifetime. This can lead to an improved understanding of how the Milky Way may have evolved into the galaxy we see today.”

Original Story Source: Leibniz Institute for Astrophysics Potsdam News Release. For further reading: A new stellar chemo-kinematic relation reveals the merger history of the Milky Way.

What is the Milky Way?

Artist's conception of the Milky Way galaxy. Credit: Nick Risinger

When you look up at the night sky, assuming conditions are just right, you might just catch a glimpse of a faint, white band reaching across the heavens. This band, upon closer observation, looks speckled and dusty, filled with a million tiny points of light and halos of glowing matter. What you are seeing is the Milky Way, something that astronomers and stargazers alike have been staring up at since the beginning of time.

But just what is the Milky Way? Well, simply put, it is the name of the barred spiral galaxy in which our solar system is located. The Earth orbits the Sun in the Solar System, and the Solar System is embedded within this vast galaxy of stars. It is just one of hundreds of billions of galaxies in the Universe, and ours is called the Milky Way because the disk of the galaxy appears to be spanning the night sky like a hazy band of glowing white light.

Discovery and Naming:

Our galaxy was named because of the way the haze it casts in the night sky resembled spilled milk. This name is also quite ancient. It is translation from the Latin “Via Lactea“, which in turn was translated from the Greek for Galaxias, referring to the pale band of light formed by stars in the galactic plane as seen from Earth.

Persian astronomer Nasir al-Din al-Tusi (1201–1274) even spelled it out in his book Tadhkira: “The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color.”

The Milky Way Galaxy. Astronomer Michael Hart, and cosmologist Frank Tipler propose that extraterrestrials would colonize every available planet. Since they aren't here, they have proposed that extraterrestrials don't exist. Sagan was able to imagine a broader range of possibilities. Credit: NASA
Artist’s impression of the Milky Way Galaxy, as seen from above the galactic “North pole”. Credit: NASA

Astronomers had long suspected the Milky Way was made up of stars, but it wasn’t proven until 1610, when Galileo Galilei turned his rudimentary telescope towards the heavens and resolved individual stars in the band across the sky. With the help of telescopes, astronomers realized that there were many, many more stars in the sky, and that all of the ones that we can see are a part of the Milky Way.

In 1755, Immanuel Kant proposed that the Milky Way was a large collection of stars held together by mutual gravity. Just like the Solar System, this collection of stars would be rotating and flattened out as a disk, with the Solar System embedded within it. Astronomer William Herschel (who discovered Uranus) attempted to actually map out the shape of the Milky Way in 1785, but he didn’t realize that large portions of the galaxy are obscured by gas and dust, which hide its true shape.

It wasn’t until the 1920s, when Edwin Hubble provided conclusive evidence that the spiral nebulae in the sky were actually whole other galaxies, that the true shape of our galaxy was known. Thenceforth, astronomers came to understand that the Milky Way is a barred, spiral galaxy, and also came to appreciate how big the Universe truly is.

Structure and Composition:

The Milky Way looks brightest toward the galactic center, in the direction of Sagittarius. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the Solar System lies near the galactic plane. The Milky Way has a relatively low surface brightness due to the gases and dust that fills the galactic disk. That prevents us from seeing the bright galactic center or from observing clearly what is on the other side of it.

a mosaic of the images covering the entire sky as observed by the Wide-field Infrared Survey Explorer (WISE), part of its All-Sky Data Release.
A mosaic of the images covering the entire sky as observed by the Wide-field Infrared Survey Explorer (WISE), part of its All-Sky Data Release. Credit: NASA/JPL

If you could travel outside the galaxy and look down on it from above, you’d see that the Milky Way is a barred spiral galaxy measuring about 120,000 light-years across and about 1,000 light-years thick. For the longest time, the Milky Way was thought to have 4 spiral arms, but newer surveys have determined that it actually seems to just have two spiral arms, called Scutum–Centaurus and Carina–Sagittarius.

The spiral arms are formed from density waves that orbit around the Milky Way. As these density waves move through an area, they compress the gas and dust, leading to a period of active star formation for the region. However, the existence of these arms has been determined from observing parts of the Milky Way – as well as other galaxies in our universe – and are not the result of seeing our galaxy as a whole.

In truth, all pictures that depict our galaxy are either artist’s renditions or pictures of other spiral galaxies. Until recently, it was very difficult for scientists to gauge what the Milky Way looks like, mainly because we’re embedded inside it. If you had never been outside of your own house, you wouldn’t know what it looked like from outside. But you’d get a sense by looking at the interior and comparing it to other houses in the neighborhood.

From ongoing surveys of the night sky with ground-based telescopes, and more recent missions involving space telescopes, astronomers now estimate that there are between 100 and 400 billion stars in the Milky Way. They also think that each star has at least one planet, which means there are likely to be hundreds of billions of planets in the Milky Way – and at least 17 billion of those are believed to be the size and mass of the Earth.

Illustris simulation, showing the distribution of dark matter in 350 million by 300,000 light years. Galaxies are shown as high-density white dots (left) and as normal, baryonic matter (right). Credit: Markus Haider/Illustris
Illustris simulation, showing the distribution of dark matter in 350 million by 300,000 light years. Galaxies are shown as high-density white dots (left) and as normal, baryonic matter (right). Credit: Markus Haider/Illustris

The Milky Way, like all galaxies, is surrounded by a vast halo of dark matter, which accounts for some 90% of its mass. Nobody knows precisely what dark matter is, but its mass has been inferred by observations of how fast the galaxy rotates and other general behaviors. More importantly, it is believed that this mass helps keep the galaxy from tearing itself apart as it rotates.

Location of our Sun:

Our Sun is located in the Orion Arm, a region of space in between the two major arms of the Milky Way, and about 27,000 light years from the galactic core. At the heart of the Milky Way is a super-massive black hole, just like all of the other galaxies, known as Sagittarius A*. This monster is more than 4 million times

Our Sun takes about 240 million years to orbit the Milky Way once, in what is known as a galactic year (or cosmic year). Just imagine, the last time the Sun was at this region of the galaxy, dinosaurs roamed the Earth, and the Sun has only made an estimated 18-20 trips around in its entire life. By this reckoning, the birth of our Sun took place 18.4 galactic years ago, and the Universe itself was created approximately 61 galactic years ago.

Future of the Milky Way:

It is further believed that our galaxy formed through the collisions of smaller galaxies, early in the Universe. These mergers are still going on, and the Milky Way is expected to collide with the Andromeda galaxy in 3-4 billion years. The two galaxies will combine to form a giant elliptical galaxy, and their super-massive black holes might even merge.

A mosaic of telescopic images showing the galaxies of the Virgo Supercluster. Credit: NASA/Rogelio Bernal Andreo
A mosaic of telescopic images showing the galaxies of the Virgo Supercluster. Credit: NASA/Rogelio Bernal Andreo

The Milky Way and Andromeda are part of a larger collection of galaxies known as the Local Group. And these are contained within an even larger region called the Virgo Supercluster – a mass concentration of galaxies that contains at least 100 galaxy groups and clusters within its diameter of 33 megaparsecs (110 million light-years).

You might be amazed to know that dung beetles actually navigate at night using the Milky Way. If you’ve never seen the Milky Way with your own eyes, you should take the chance when you can. Go to a place with nice dark skies, free from light pollution, and look up and appreciate the Milky Way. And be sure to wave hello to all the neighboring stars who share our galaxy with us.

Those are just a few of the interesting facts, figures, and images that you will find among the links below.