The stars in the Milky Way have generally circular orbits, and move through the galaxy at a speed of about 200 km/s. This is also true for our Sun. But how do we know this, given that our Sun is in the disk of the Milky Way, and we can only see our galaxy from our Sun’s vantage point? It all has to do with relative motion.

Measuring the motion of a star. Credit: Wikipedia

For stars near the Sun, we can determine their motion through the galaxy by measuring two types of motion relative to the Sun. Proper motion, which is the apparent drift of the star relative to more distant stars, and radial motion, which we measure by looking at the Doppler shift of a star’s spectral lines. This gives us the motion of a star through space, but only relative to the Sun. The Sun itself is moving through the galaxy, and we can only measure the Sun’s speed relative to other stars.

So how do we know the Sun and other stars are orbiting the center of our galaxy? One way is to look at the statistics of stars moving around us. If the stars were stationary, or if the stars in our galaxy rotated as a solid disk, then generally the stars in our galaxy would stay together. An individual star could have some velocity relative to the Sun, but on average all the stars around us would have no net velocity. Statistically the velocities of nearby stars should add up to zero.

On the other hand, if the stars orbit galactic center, we would expect stars closer to the galactic center to speed past the Sun, since they trace a smaller orbit than the Sun. Stars farther away from galactic center would be left behind by the Sun, since they would have a larger orbit. You can imagine this as a group of runners on a circular track. If they had to stay in their own lane, the runners on the inside track would finish the race before those on the outer track, because they would have a shorter distance to run. This is why on actual races the starting positions are staggered to make the distances equal.

The relative motion of stars in our galaxy.

For the galaxy this means that stars in the direction of galactic center have (on average) a speed relative to the Sun in the direction of galactic rotation. Stars in the opposite direction have a net speed opposite to galactic rotation. This is exactly what we observe. Not only that, by comparing the relative speeds of stars with their distance from the Sun, we can determine how stellar speed varies with distance. This is one of the ways we know that stars in our galaxy have roughly the same speed, regardless of their distance from galactic center.

Of course this only works for stars where we know their distance from the Sun, and that’s only for stars out to about 500 parsecs, or 1600 light years. The whole galaxy has a diameter of about 100,000 light years, so stellar motion covers only a small portion of the galaxy. To look at the motion of more distant parts of the galaxy we have to use another set of observations.

But that’s a story for another day.

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Throughout our lives the stars rise and set in a seemingly unchanging pattern. Over the centuries humanity has named constellations and made them legends, and navigated seas by the eternal stars. We now know that stars are not fixed points of light, but rather move ever so slowly across the sky. 

While many early astronomers speculated that the stars might move over time, it wasn’t until 1718 that Edmund Halley confirmed the motion of a few stars. Halley was trying to determine the precession of the equinoxes, which is the overall shift of the Earth’s axis of rotation relative to the stars. At the time it was known that the celestial sphere shifted alignment over time, but the rate of that shift hadn’t been measured. To do this, Halley measured the latitudes and longitudes (we now use right ascension and declination) of about a thousand stars listed in Ptolemy’s Almagest star catalog. Halley compared his measurements with the results listed in the Almagest, as well as those listed in another star catalog by Hipparchus. Ptolemy’s catalog was published around 300 BC, and Hipparchus’ was published about 170 years later, so Halley could compare stellar motion over about 2,000 years. Halley found that overall the stars shifted in longitude by about 50 arcseconds per year, but he noticed that Aldebaran, Sirius and Arcturus shifted in latitude differently than other stars. Over 2,000 years they had shifted relative to other stars.

Credit: ESA/ATG medialab

This relative motion of stars across the sky is known as proper motion. It is this motion that causes the stars to move over thousands of years. The constellations we see today are only temporary. They were not the constellations of our long-ago ancestors, and they will not be the constellations of our far-future descendants. Since Halley’s first observations, astronomers have measured the proper motion of thousands of stars. But in the past couple decades that number as grown exponentially. In 2000, the Hipparcos catalog was released. Based on data gathered by the Hipparcos spacecraft, it contains the positions and proper motions of more than 100,000 stars. This year initial data from the Gaia spacecraft was released, containing the position and proper motion of more than 2 million stars. Since stars basically move freely through space, we can use their proper motions to predict where they will be in the future. You can see this in the video above, which plots the proper motions of  Gaia’s 2 million stars. It gives us a glimpse of how future generations might see the night sky.

Proper motion only tells us about part of a star’s motion. A star can also move toward us or away from us, following what is known as radial motion. The combination of radial motion and proper motion determines a star’s true motion relative to us. Fortunately the radial motion of a star can be measured by observing the spectrum of light it emits. As a star moves toward or away from us, its spectral pattern is shifted by the Doppler effect, allowing us to determine its radial motion. The Gaia spacecraft is also making radial motion measurements, and that data will be released soon.

In the past it was believed that the stars could determine the future of humanity. But through careful observations, humanity can now tell the future of the stars.

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When you think of a family tree, you probably think of human ancestry, and how we can trace our ancestors back to different geographical regions. All living things have a common family tree, which can be seen in our genetic code. While stars aren’t living things, they have a similar family tree, and we’re starting to gather enough data to piece it together. 

We’ve long known that stars can be categorized into generations depending on their origin. The very first generation of stars formed from the hydrogen and helium formed in the big bang. When the largest of those stars died in supernovae, the remnant gas and dust collapsed to form a new generation of stars. The largest second-generation stars later exploded, a third generation of stars formed, and so on. Since heavier elements such as carbon and iron forms within stars, later generations of stars tend to have a higher metallicity. Our Sun, for example, has a relatively high metallicity, and is therefore a third or fourth generation star.

Diagram showing how our Sun is chemically related to other stars. Credit: Paula Jofré, et al.

As our observations of stars has increased, there have been efforts to find stellar siblings of our Sun.  Stars don’t form on their own, but rather form with other stars in large nebulae known as stellar nurseries. Stars formed in the same stellar nursery would have similar ages and similar chemical compositions. The more similar the composition and age, the more likely the stars are to be related.

With sky survey satellites such as Gaia, we are finally gathering this kind of information on millions of stars, so in principle we should be able to study the connections between stars and groups of stars. A new paper in MNRAS shows how this can be done. By piecing together a stellar family tree, we can better understand the dynamics of stellar evolution within our galaxy.

Paper: Paula Jofré et al. Cosmic phylogeny: reconstructing the chemical history of the solar neighbourhood with an evolutionary tree. MNRAS 467 (1): 1140-1153. (2017)

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A popular dream of science fiction has been the creation of a vast database containing all known information about the galaxy, an Encyclopedia Galactica. It would list the characteristics of stars in the Milky Way, their planets, and even the history of human alien civilizations. While still a dream in most ways, the foundation of such a cosmic Library of Alexandria is being laid with the first release of data from the Gaia spacecraft. 

The initial data release contains information on more than a billion stars. It gives the position and brightness of 1,140,622,719 stars in our galaxy and nearby dwarf galaxies, with distance and motion data for about 2 million stars. If a cosmic encyclopedia is created, it will begin with the data from Gaia. What makes the data so powerful is its combination of size and precision. Other spacecraft such as Hipparcos have mapped about 100,000 stars, but the Gaia mission will precisely determine the position and motion of more than a billion stars. That’s still only about 1% of the total number of stars in our galaxy, but it’s large enough that we can study aspect of the Milky Way like never before.

For example, since stellar motion is determined by gravitational interactions, the data will allow us to determine the overall mass and distribution of mass within the Milky Way. Comparing the mass distribution to the distribution of stars, we can better determine the distribution of dark matter in our galaxy. The data will also allow us to determine the motions of stars within clusters, which will not only help us understand the origin and evolution of star clusters, it will help us learn more about our galactic history.

How Gaia’s accuracy and mission varies with distance. Credit: ESA; Background: Lund Observatory

Measuring the positions and motions of all these stars requires precise observations of the stars’ brightness and spectra over time. This data can also be used to discover exoplanets. It’s expected that Gaia will discover any Jupiter-mass planet orbiting stars within 150 light years of Earth. It could detect nearly 50,000 planets over the course of its mission. The data not only determines the location of stars, but also their size and type.  The initial data contained 3,194 variable stars, which not only help us understand stellar evolution, they are also used to determine galactic distances. Overall the precision of the data is so high it will allow us to test theories such as general relativity in new and subtle ways, possibly leading to the discovery of new physics.

Then there are the secondary discoveries that might occur. While Gaia’s mission is to measure stars, in practice it measures the brightness and location of any object over an apparent magnitude of 20. This includes countless asteroids, Kuiper Belt objects, and any large outer solar system planet within 200 parsecs. It will also observe occultations of stars by solar system bodies, such as the recent occultation of a faint star by Pluto, which helped us study Pluto’s atmosphere.

Perhaps what makes Gaia most exciting is that the data is released publicly. Since anyone can access the data, it can be used in ways the Gaia team hasn’t anticipated. It truly is a great encyclopedia available to all.

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Cepheid variable stars are most commonly known as a standard candle for measuring galactic distances. That’s because they vary in brightness at a rate proportional to their average brightness.  But they can also tell us something about how young stars are distributed within our galaxy, and a recent study raises an interesting mystery. 

There are basically two types of Cepheid variable stars. Classical cepheids are large bright stars, typically with a mass 5 – 20 times that of our Sun. Since larger stars have shorter lifetimes, a classical Cepheid is typically no more than 100 million years old. Type II Cepheids are small, old stars, with masses much less than our Sun. They are typically around 10 billion years old. Distinguishing between these two types of Cepheid variables is straight forward, because they have very different metallicities (traces of elements other than hydrogen and helium). So we can distinguish them by looking at their spectra and the way they brighten and dim (their light curve). Because classical Cepheids are brighter, they are typically used to determine the distances to galaxies, and helped establish the Hubble law for cosmic expansion. The dimmer type II Cepheids are typically used for distances within our galaxy, such as determining the distance to the center of our galaxy.

Since the ages of these different types of Cepheids are very different we can use them as a gauge for the age of surrounding stars. For example, if a globular cluster contains type II Cepheids, we know it is billions of years old. If a star cluster contains a classical cepheid, we know that stars formed there relatively recently. A new paper in MNRAS uses this fact to look at the distribution of young stars in our galaxy, and found a rather puzzling void.

Mapping young stars in our galaxy can be a challenge, particularly in the direction of the center of our galaxy, where high amounts of gas and dust obscure most of the visible light from distant stars. Fortunately infrared light isn’t absorbed as strongly, so an infrared survey of Cepheids gives us a good view of the central region of the Milky Way. This new study found some classical Cepheids clustered very close to the center of our galaxy, but found a region about 8,000 light years in radius where there aren’t any classical Cepheids. This would seem to indicate that this region hasn’t produced stars in at least 100 million years. This is in agreement with infrared and radio surveys of the central region of our galaxy, which also find a lack of star producing regions in that area.

We don’t know why stars don’t form in this region. There is certainly plenty of matter in the region, and older stars are clearly present there. For some reason the conditions for young stars are lacking there, producing a cosmic “get of my lawn” effect.

Paper: Noriyuki Matsunaga, et al. A lack of classical Cepheids in the inner part of the Galactic disc. MNRAS 462 (1): 414-420. (2016) doi: 10.1093/mnras/stw1548

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Our small world orbits a star in a cosmic sea. Our Sun sails through the galaxy with over 200 billion of its siblings. This sea of stars has many names: Silver River, Lugh’s Chain, the Merchant’s Road, but its most common name is the Milky Way. It’s a delicate veil of light, astonishing in its fragility and grandeur. It’s appearance has been central to the folklore of cultures across the globe. That’s because there was a time when the Milky Way could be seen at some point of the year from anywhere in the world. But that’s no longer true. 

While the Milky Way is always there, it’s also rather faint. As humanity has moved from campfires to electric lights the amount of light pollution has increased. In recent years the rise of LEDs has further reduced our view of the night sky, since the bluish color of LEDs is particularly bad in terms of light pollution. We’ve now reached the point where a third of the world can no longer see the Milky Way. In Europe more than 60% are unable to view that starry veil, and in North America it’s hidden from more than 80% of the population. A cosmic phenomena seen for millennia is now being banished from view.

It’s easy to dismiss as a minor loss unless you’ve actually seen the Milky Way with your own eyes. Anyone who has stood under a clear night sky remembers it. Its appearance is transformative. It fills you with wonder, and for years afterward you will tell the story of that time you truly saw the Milky Way. Its loss is not simply a fading of the night, it’s a disconnect from our cultural heritage. No matter where your ancestors hailed, they looked up and the great starry veil in amazement. They told stories of it, and wondered of its origin.

If you haven’t seen the Milky Way, make it a priority. Give your children an experience they will remember for the rest of their lives. Make a connection to the cosmic sky that is your birthright. For if we lose it, we will be a civilization lost at sea.

Paper: Fabio Falchi, et al. The new world atlas of artificial night sky brightness. Science Advances, Vol. 2, no. 6, e1600377 (2016) DOI: 10.1126/sciadv.1600377

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The center of our galaxy is less than 30,000 light years away, but it is notoriously difficult to observe. Because of the gas and dust surrounding the region it can’t be seen directly at visible wavelengths. Instead we have to observe at infrared or radio wavelengths, where the surrounding gas and dust is more transparent. Even then it can be difficult to get high resolution images, particularly at radio wavelengths. But recent upgrades to the Jansky Very Large Array have produced some incredible radio images of the galactic center, such as the image above. 

This particular image shows a 100 light-year wide around galactic center. The supermassive black hole at the heart of our galaxy is in the middle of the bright region. What’s amazing about this image is how much structure we can see. The dynamic range of these new images is 100,000:1, which is a big improvement over the images we’ve had. You can see the mini-spiral structure surrounding the supermassive black hole, which is an interesting find.  Clearly there are some complex interactions in the region.

Analysis of this and other images have already found evidence of a supernova remnant, as well as evidence of gas and dust motion that may be driven by bright star clusters. The data is still young, and it’s clear we still have a great deal to learn about the heart of our galaxy.

Paper: Jun-Hui Zhao, et al. A New Perspective of the Radio Bright Zone at The Galactic Center: Feedback from Nuclear Activities. The Astrophysical Journal, Volume 817, Number 2 (2016) doi:/10.3847/0004-637X/817/2/171

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The Milky Way is visible from anywhere on Earth. While it makes for a lovely milk-like glow across the sky, it is also filled with gas and dust that can limit our view of certain parts of the sky. While we can map out where certain gas and dust is through the Milky Way, making an accurate map is challenging because much of it is cold and diffuse, making it difficult to observe. But with the rise of submillimeter radio astronomy, that’s changing. 

Cold gas and dust emits faint light in the submillimeter range, so to study this material we need good radio telescopes. Unfortunately our atmosphere (mainly water vapor) absorbs these wavelengths, so the radio telescopes need to be located in a dry region at high elevations, such as at Chajnantor plateau in northern Chile. The most famous telescope at Chajnantor is the Atacama Large Millimeter/submillimeter Array (ALMA), which is an array of about 60 antennas. But just down the way from ALMA is the Atacama Pathfinder Experiment (APEX), which has been mapping the gas and dust of our galaxy.

The APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) has been scanning the Milky Way at submillimeter wavelengths, and has just released a full map of our galaxy. You can see one of their images above, which shows the wonderful complexity of the Milky Way, with fine tendrils of gas and dust. Creating a map such as this will not only help astronomers better understand our own galaxy, but also allow astronomers to better take the effects of our galaxy into account when looking beyond our neighborhood of stars.

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There’s an interstellar cloud of gas heading for the Milky Way. It’s about 10,000 light years across, has a mass of a million Suns, and impact our galaxy in about 30 million years. Until now, it’s origin has been a mystery. 

First discovered in 1963 by Gail Smith, the faint cloud of gas was thought to be a failed dwarf galaxy, or simply a large cloud from intergalactic space. We’ve known its trajectory for some time, and found that it was likely in our galaxy about 70 million years ago. For a cloud of its size to pass through our galaxy relatively intact, it would need to have a much higher mass that it seems to have, leading some to speculate that it might be a dark matter galaxy.

But recently a team of astronomers studied the composition of Smith’s cloud by observing ultraviolet light passing through the cloud, they could determine which wavelengths were most absorbed. From this they determined that it has quantities of sulfur at levels similar to that of the outer region of the Milky Way. This means it likely originated from our galaxy about 70 million years ago.

Just how the large cloud was ejected from the Milky Way is still a mystery. One possibility is that dark matter has played a role, capturing material from our galaxy as it passed through. To test that idea, however, we’ll need to study Smith’s cloud a bit more.

Paper: Andrew J. Fox, et al. On the Metallicity and Origin of the Smith High-velocity Cloud.  The Astrophysical Journal Letters, Volume 816, Number 1 (2016)

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If you’re a native of the northern hemisphere, the first thing you notice about the night sky in the southern hemisphere is that things are … off. Constellations are missing, the lunar phases seem backwards, and the constellations you do recognize are upside down. It’s very disorientating until you get used to it. But the one thing you do recognize early on is the Milky Way.

Despite claims to the contrary, our solar system is not aligned with the plane of our galaxy. Earth is also tilted about 23 degrees from the axis of the solar system. As a result, the plane of the Milky Way is tilted about 60 degrees relative to the rotational axis of Earth. Because our solar system is about 3/5 of the distance from galactic center, the Milky Way completely encircles our view of the heavens. As a result, no matter where you are on the Earth, there is at least some part of the year when the Milky Way is visible.

Dark cloud constellations of indigenous Chileans.

This doesn’t mean that all views are equal. The central region of the Milky Way is in the southern region of our sky, which means views in the southern hemisphere tend to be brighter and have more clear contrast between stars and dust clouds. This is likely the reason why many cultures in the southern hemisphere have traditionally created “constellations” out of the shadow regions caused by dust clouds within the Milky Way, such as the Emu in the sky of Australia, or the Llama and Serpent of South America.

But regardless of your place of origin, the Milky Way is a part of your sky. No matter where you roam on Earth, you can always find the Milky Way in the night sky and know that you are home.

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Yesterday I talked about how we can measure the size and shape of our galaxy even though much of it is obscured by gas and dust. As I pointed out, we can only observe a small fraction of the stars in the Milky Way. But then how do we know how many stars there are in our galaxy? The short answer is we don’t, but we can get an idea by measuring the mass of our galaxy.

Typically the way the mass of our galaxy is measured is to measure the speed of stars and other objects in the Milky Way. This gives us a mass of about 100 – 400 billion solar masses. Not all of that is due to stars, but it gives us a basic idea of the number of stars in our galaxy. But recently a more accurate measure has been determined, and it was done by observing globular clusters. Globular clusters are dense, spherical cluster of stars. Their shape comes from the fact that they are clustered under their own gravity. They are not located in the plane of our galaxy, but instead are found in all directions in a kind of halo around our galaxy. They were first used by Harlow Shapley to determine the basic structure of our galaxy in the early 1900s.

The thing about globular clusters is that they aren’t completely gravitationally bound. Through various gravitational interactions, a few stars escape a globular cluster at a regular basis (a process called evaporation). As a result, these clusters leave a trail of stars behind as they slowly orbit our galaxy. In this recent work the team looked at these trails and found that they had variations of density within them. This means the rate of evaporation varied over time. When they analyzed these variations, they found a pattern, and this pattern is dependent upon the overall mass of our galaxy. Basically that means the evaporation is affected not only by gravitational interactions within the globular cluster, but also interactions between the cluster and our galaxy. So the team used this data to determine the mass of our galaxy. What they found was that within a radius of 60,000 light years  the mass is about 210 billion solar masses.

It should be emphasized that this is only the mass of our galaxy within a 120,000 light year diameter, which is about the diameter of the visible galaxy. Beyond this region our galaxy is dominated by dark matter, which makes up most of the total mass of our galaxy. So we can say that the Milky Way has roughly 200 billion stars.

Paper: Andreas H. W. Küpper et al. Globular Cluster Streams as Galactic High-Precision Scales – The Poster Child Palomar 5. ApJ 803 80 doi:10.1088/0004-637X/803/2/80 (2015)

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The image above shows a representation of our Milky Way galaxy as we think it appears. The blue region indicates the range of stars visible with the naked eye, and the the white dots represent the location of the 118,200 stars precisely measured by the Hipparcos spacecraft. The points represent the best data we have on stars in our galaxy. Since Hipparcos there has been a less precise Tycho catalog of about 2.5 million stars, and the recently launched Gaia spacecraft will precisely map about a billion stars. But even Gaia will map a small fraction of the estimated 100 – 400 billion stars in the Milky Way.  Most of the stars are simply too dim to be seen at galactic distances, or are hidden by the cloud-obscured zone of avoidance toward galactic center. So how is it that we are able to know what our galaxy looks like?

A real map of the Milky Way based largely on H II regions. Credit: Emily Freeland

Although we can only observe a small fraction of the stars in our galaxy, there are other objects such as nebulae that we can observe. In particular, there are star-forming nebulae known as H II regions, which contain large amounts of ionized hydrogen. In the visible spectrum these regions are often seen as red nebulae, but the hydrogen also gives off radio signals at a particular wavelength of 21cm, which is why its often called the 21cm line. Since radio wavelengths aren’t absorbed significantly by the gas and dust in the Milky Way, much of this 21cm radiation is visible to us across the galaxy. As a result, we can map the locations of these H II regions. From this we find that our galaxy has a clear spiral structure to it. In particular, it is a type of spiral galaxy known as a barred spiral.

While hydrogen is by far the most abundant element in our galaxy, the 21cm line hydrogen emits is not particularly strong, so it can be difficult to observe at times. Fortunately, there are other wavelengths we can observe, such as carbon monoxide emission lines, which are much brighter. From all of our observations, we have a pretty good idea of the size and structure of our galaxy.

The artistic renderings of the Milky Way you often see are inspired by images we have of other similar galaxies we can observe such as Andromeda. We know what barred spiral galaxies look like, and our own galaxy likely looks pretty similar. It’s actually amazing how far we’ve come in only a century, given that it wasn’t until 1918 that Harlow Shapley was able to determine the basic structure of our galaxy.

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