Posted on by Matt Williams

Stars at the Edge of our Galaxy May Have Been Stolen

Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA

Our Milky Way is a pretty vast and highly-populated space. All told, its stars number between 100 and 400 billion, with some estimates saying that it may have as many as 1 trillion. But just where did all these stars come from? Well, as it turns out, in addition to forming many of its own and merging with other galaxies, the Milky Way may have stolen some of its stars from other galaxies.

Such is the argument made by two astronomers from Harvard-Smithsonian Center for Astrophysics. According to their study, which has been accepted for publication in the The Astrophysical Journal, they claim that roughly half of the stars that orbit at the extreme outer edge of the Milky Way were actually stolen from the nearby Sagittarius dwarf galaxy.

At one time, the Sagittarius Dwarf Elliptical Galaxy was thought to be the closest galaxy to our own (a position now held by the Canis Major dwarf galaxy). As one of several dozen dwarf galaxies that surround the Milky Way, it has orbited our galaxy several times in the past. With each passing orbit, it becomes subject to our galaxy’s strong gravity, which has the effect of pulling it apart.

A model of the tidally shredded Sagittarius dwarf galaxy wrapping around a 3-D representation of the Milky Way disk. Credit: UCLA/D.R. Law

The long-term effects of this can be seen by looking to the farthest stars in our galaxy, which consist of the eleven stars that are at a distance of about 300,000 light-years from Earth (well beyond the Milky Way’s spiral disk). According the study produced by Marion Dierickx, a graduate student at Harvard University’s Department of Astronomy, half of these stars were taken from the Sagittarius dwarf galaxy in the past.

Professor Avi Loeb, the Frank B. Baird, Jr. Professor of Science at Harvard and Marion Dierickx PhD advisor, co-authored the study – titled, “Predicted Extension of the Sagittarius Stream to the Milky Way Virial Radius“. As he told Universe Today via email:

“We see evidence for streams of stars connected to the core of the galaxy, and indicating that this dwarf galaxy passed multiple times around the Milky Way center and was ripped apart by the tidal gravitational field of the Milky Way. We are all familiar with the tide in the ocean caused by the gravitational pull of the moon, but if the moon was a much more massive object – it would have pulled the oceans apart from the Earth and we would see a stream of vapor stretched away from the Earth.”

For the sake of their study, Dierickx and Loeb ran computer models to simulate the movements of the Sagittarius dwarf over the past 8 billion years. These simulations reproduced the streams of stars stretching away from the Sagittarius dwarf galaxy to the center of our galaxy. They also varied Sagittarius’ velocity and angle of approach to see if the resulting exchanges would match current observations.

Computer-generated image showing the disc of the Milky Way (red oval) and the Sagittarius dwarf galaxy (red dot). The yellow circles represent stars that have been ripped from the Sagittarius dwarf and flung far across space. Credit: Marion Dierickx / CfA

“We attempted to match the distance and velocity data for the core of the Sagitarrius galaxy, and then compared the resulting prediction for the position and velocity of the streams of stars,” said Loeb. “The results were very encouraging for some particular set of initial conditions regarding the start of the Sagittarius galaxy journey when the universe was roughly half its present age.”

What they found was that over time, the Sagittarius dwarf lost about one-third of its stars and nine-tenths of its dark matter to the Milky Way. The end result of this was the creation of three distinct streams of stars that reach one million light-years from galactic center to the very edge of the Milky Way’s halo. Interestingly enough, one of these streams has been predicted by simulations conducted by projects like the Sloan Digital Survey.

The simulations also showed that five of Sagittarius’ stars would end becoming part of the Milky Way. What’s more,  the positions and velocities of these stars coincided with five of the most distant stars in our galaxy. The other six do not appear to be from Sagittarius dwarf, and may be the result of gravitational interactions with another dwarf galaxy in the past.

“The dynamics of stars in the extended arms we predict (which is the largest Galactic structure on the sky ever predicted) can be used to measure the mass and structure of the Milky Way,” said Loeb. “The outer envelope of the Milky Way was never probed directly, because no other stream was known to extend that far.”

Computer model of the Milky Way, the Sagittarius dwarf galaxy, and the looping stream of material between the two. Credit: Tollerud, Purcell and Bullock/UC Irvine

Given the way the simulations match up with current observations, Dierickx is confident that more Sagittarius dwarf interlopers are out there, just waiting to be found. For instance, future instruments – like the Large Synoptic Survey Telescope (LSST), which is expected to begin full-survey operations by 2022 – may be able to detect the two remaining streams of stars which were predicted by the survey.

Given the time scales and the distances involved, it is rather difficult to probe our galaxy (and by extension, the Universe) to see exactly how it evolved over time. Pairing observational data with computer models, however, has been proven to test our best theories of how things came to be. In the future, thanks to improved instruments and more detailed surveys, we just might know for certain!

And sure to check out this animation of the computer simulation, which shows the effects on the Milky Way’s gravity on the Sagittarius dwarf galaxy’s stars and dark matter.

Further Reading: CfA

Posted on by Matt Williams

Turns out Proxima Centauri is Strikingly Similar to our Sun

Artist's depiction of the interior of a low-mass star, such as the one seen in an X-ray image from Chandra in the inset. Credit: NASA/CXC/M.Weiss

In August of 2016, the European Southern Observatory announced that the nearest star to our own – Proxima Centauri – had an exoplanet. Since that time, considerable attention has been focused on this world (Proxima b) in the hopes of determining just how “Earth-like” it really is. Despite all indications of it being terrestrial and similar in mass to Earth, there are some lingering doubts about its ability to support life.

This is largely due to the fact that Proxima b orbits a red dwarf. Typically, these low mass, low temperature, slow fusion stars are not known for being as bright and warm as our Sun. However, a new study produced by researchers at the Harvard Smithsonian Center for Astrophysics (CfA) has indicated that Proxima Centauri might be more like our star than we thought.

For instance, our Sun has what is known as a “Solar Cycle“, an 11-year period in which it experiences changes in the levels of radiation it emits. This cycle is driven by changes in the Sun’s own magnetic field, and corresponds to the appearance of Sunspots on its surface. During a “solar minimum”, the Sun’s surface is clear of spots, while at a solar maximum, one hundred sunspots can appear on an area the size of 1% the Sun’s surface area.

This image is a composite of 25 separate images spanning the period of April 16, 2012, to April 15, 2013. It uses the SDO AIA wavelength of 171 angstroms and reveals the zones on the sun where active regions are most common during this part of the solar cycle. Credit: NASA/SDO/AIA/S. Wiessinger
Composite of 25 separate images spanning the period of April 16, 2012, to April 15, 2013, revealing active regions during this part of the Solar Cycle. Credit: NASA/SDO/AIA/S. Wiessinger

For the sake of their research, the Harvard Smithsonian team examined Proxima Centauri over the course of several years to see if it too had a cycle. As they explain in their research paper, titled “Optical, UV, and X-Ray Evidence for a 7-Year Stellar Cycle in Proxima Centauri” they relied on several years worth optical, UV, and X-ray observations made of the star.

This included 15 years of visual data and 3 years of infrared data from the All Sky Automated Survey (ASAS), 4 years of x-ray and UV data from the Swift x-ray telescope (XRT), and 22 years worth of x-ray observations taken by the Advanced Satellite for Cosmology and Astrophysics (ASCA), the XXM-Newton mission and the Chandra X-ray Observatory.

What they found was that Proxima Centauri does indeed have a cycle that involves changes in its minimum and maximum amount of emitting radiation, which corresponds to “starspots” on its surface. As Dr. Wargelin told Universe Today via email:

“The optical/ASAS data showed a nice 7-year cycle, as well as an 83-day rotation period. When we broke down that data by year we saw the period vary from around 77 to 90 days. We interpret that as ‘differential rotation’ like that found on the Sun. The rotation rate differs at different latitudes; on the Sun it’s around 35 days at the poles and 24.5 at the equator. The “average” rotation is usually given as 27.3 days.”

In essence, Proxima Centauri has its own cycle, but one that is a lot more dramatic than our Sun’s. Besides lasting 7 years from peak to peak, it involves spots covering over 20% of its surface at one time. These spots are apparently much bigger than the ones we regularly observe on our Sun as well.

X-Ray image of Proxima Centauri. Image credit: Chandra
An X-Ray image of Proxima Centauri. Credit: Chandra/Harvard/NASA

This was surprising, given that Proxima’s interior is very different from our Sun’s. Because of its low mass, the interior of Proxima Centauri is convective, where material in the core is transferred outward. In contrast, only the outer layer of our Sun undergoes convection while the core remains relatively still. This means that, unlike our Sun, energy is transferred to the surface through physical movement, and not radiative processes.

While these findings cannot tell us anything directly about whether or not Proxima b might be habitable, the existence of this solar cycle is an interesting find that might be leading in that general direction. As Dr. Wargelin explained:

“Magnetic fields are what drive high energy emission (UV and X rays) and stellar winds (like the solar wind) in solar-type and smaller stars, AND a stellar cycle (if it has one). That X-ray/UV emission and stellar wind can ionize/evaporate/strip the atmosphere of close-in planets, particularly if the planet doesn’t have a protective magnetic field of its own.

“Therefore….. a necessary but not sufficient requirement for understanding (i.e., modeling) the evolution of a planet’s atmosphere is understanding the magnetic field of the host star.  If you don’t understand why a star has a cycle (and standard theory says fully convective stars like Proxima can NOT have cycles) then you don’t understand its magnetic field.”

As always, further observations and research will be necessary before we can fully understand Proxima Centauri, and whether or not any planets that orbit it could support life. But then again, we’ve only known about Proxima b for a short time, and the rate at which we are learning new things about it is quite impressive!

Further Reading: CfA, arXiv

Posted on by Matt Williams

Venus-like Exoplanet 39 Light Years Distant Is Probably Baked & Sterile

Artist's impression of the "Venus-like" exoplanet GJ 1132b. Credit: cfa.harvard.edu

Last year, astronomers discovered a terrestrial exoplanet orbiting GJ 1132, a red dwarf star located just 12 parsecs (39 light years) away from Earth. Though too close to its parent star to be anything other than extremely hot, astronomers were intrigued to note that it appeared to still be cool enough to have an atmosphere. This was quite exciting, as it represented numerous opportunities for research.

In essence, the planet appeared to be “Venus-like” – i.e. very hot, but still in possession of an atmosphere. What’s more, it was close enough to our Solar System that its atmosphere could be studied in detail. However, a debate began over whether its atmosphere would be hot and wet, or thin and tenuous. And after a year of study, a team of astronomers from the CfA believe they have unlocked that mystery.

In addition to being relatively close to our own Solar System in astronomical terms, the Venus-like exoplanet GJ 1132b also has a relatively small orbital period around its star. This means that opportunities to spot it as it passes in front of its star (i.e. the Transit Method), occur quite often.

Artist's concept of exoplanets orbiting a young, red dwarf star. Credit: NASA/JPL-Caltec
Artist’s concept of exoplanets orbiting a young, red dwarf star. Credit: NASA/JPL-Caltech

This makes it an excellent target for detailed observation and study, which in turn will help astronomers to learn more about terrestrial exoplanets that orbit close to red dwarf stars. But as noted already, astronomers were divided on the issue of GJ 1132b’s atmosphere.

Thanks to the research efforts of Laura Schaefer and her colleagues from the Harvard-Smithsonian Center for Astrophysics (CfA), it now appears that the case for a thin atmosphere is the far more likely. Interestingly enough, this was confirmed by determining just how much oxygen the planet has in its atmosphere.

For the sake of their study, which was outlined in a paper that approved for publication in The Astrophysical Journal – titled “Predictions of the atmospheric composition of GJ 1132b” – they explain how they used a “magma ocean-atmosphere” model to determine what would happen to GJ 1132b over time if it began with a water-rich atmosphere.

They began with the knowledge that a planet like GJ 1132b – which orbits its star at a distance of 2.25 million km (1.4 million mi) – would be subjected to intense amounts of ultraviolet light. This would result in any water vapor in the atmosphere being broken down into hydrogen and oxygen (a process known as photolysis), with the hydrogen escaping into space and the oxygen being retained.

Comparison of best-fit size of the exoplanet GJ 1132 b with the Solar System planet Earth, as reported in the Open Exoplanet Catalogue[1] as of 2015-11-14. Open Exoplanet Catalogue (2015-11-14). Retrieved on 2015-11-14. Aldaron, a.k.a. Aldaro
Size comparison of the exoplanet GJ 1132 b with Earth, as reported in the Open Exoplanet Catalogue as of 2015-11-14. Credit: Open Exoplanet Catalogue/Aldaron
At the same time, they determined that the planet’s atmosphere and proximity to its star would lead to a severe greenhouse effect that would leave the surface molten for a long time. This “magma ocean” would likely interact with the atmosphere by absorbing some of the oxygen. How much would be absorbed and how much would be retained was the big question.

They concluded that the planet’s magma ocean would absorb about one-tenth of the oxygen in the atmosphere. The majority of the remaining 90 percent, according to their model, would be lost to space while a small margin would linger around the planet. This proved to be very much consistent with measurements made of the planet thus far.

As Dr. Laura Schaefer explained to Universe Today via email:

“We determined that the planet would likely have a thin atmosphere by doing a suite of models looking at atmospheric loss and interaction with a surface magma ocean. For the allowable composition range (esp. the abundance of water) based on the current mass measurement, nearly all of the allowed compositions resulted in thin atmospheres, except at the very extreme upper end of the range.”

This magma ocean-atmosphere model could not only help scientists to study terrestrial exoplanets that orbit close to their parent stars, but also to understand how our own planet Venus came to be. For some time, scientists have theorized that Venus began with significant amounts of water on its surface, but that it then underwent a significant change.

Artist's impression of three newly-discovered exoplanets orbiting an ultracool dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).
Artist’s impression of three newly-discovered exoplanets orbiting an ultracool dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).

This ocean is believed to have evaporated due to Venus’ closer proximity to the Sun, with the ensuing water vapor triggering a runaway greenhouse effect. Over time, ultraviolet radiation from the Sun broke apart the water molecules, resulting in the hot, virtually waterless atmosphere we see today. However, what happened to all the oxygen has remained a mystery.

“We also have plans to use this model in the future to study Venus, which may have once had about the same amount of water as the Earth but is now very dry,” said Schaefer. “There is very little O2 left in Venus’ atmosphere, so this model would help us understand what happened to that oxygen (whether it was lost to space or absorbed by the planet’s mantle).”

Schaefer predicts that their model will also assist researchers with the study of other, similar exoplanets. One example is the TRAPPIST-1 system, which contains three planets that may lie with the star’s the habitable zone. But as Schaefer put it, the real value lies in the fact that we are more likely to find “Venus-like” worlds down the road:

“Most of the rocky planets that we know of and will discover in the near future will likely be hotter than the Earth or even Venus, just because it is easier to detect hotter planets. So there are a lot of planets out there similar to GJ 1132b just waiting to be studied!”

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. It’s scientists are dedicated to studying the origin, evolution and future of the universe.

And be sure to check out this video, courtesy of MIT news:

Further Reading: CfA, arXiv

Posted on by Nancy Atkinson

Using 19th Century Technology to Time Travel to the Stars

This spiral galaxy image was taken on glass photographic plate is one in a series of photos taken over decades. From the Harvard Plate collection. Image courtesy American Museum of Natural History.

In the late 19th century, astronomers developed the technique of capturing telescopic images of stars and galaxies on glass photographic plates. This allowed them to study the night sky in detail. Over 500,000 glass plate images taken from 1885 to 1992 are part of the Plate Stacks Collection of the Harvard-Smithsonian Center for Astrophysics (CfA), and is is the largest of its kind in the world.

“The images captured on these plates remain incredibly valuable to science, representing a century of data on stars and galaxies that can never be replaced,” writes astronomer Michael Shara, who is Curator in the Department of Astrophysics at the American Museum of Natural History in New York City, who discussed the plates and their significance in a new episode of AMNH’s video series, “Shelf Life.”

These plates provide a chance to travel back in time, to see how stars and galaxies appeared over the past 130 years, allowing astronomers to do what’s called “time domain astronomy”: studying the changes and variability of objects over time. These include stars, galaxies, and jets from stars or galactic nuclei.

But viewing these plates is difficult. The glass plates can still be viewed on a rather archaic plate viewer—a device that’s like an X-ray light box in a doctor’s office. But those aren’t readily available, and Harvard is hesitant about shipping the 100-plus-year-old glass plates around the world. If astronomers travel to Cambridge to dig through the archives, they can spend hours poring over logbooks or just looking for the right plate. Plus, there’s not an easy way to compare these plates to today’s digital imagery.

AMNH is helping CfA to digitize the glass plates, which is discussed in the video. There’s also a citizen science project called DASCH to help digitize the telescope logbooks record that hold vital information associated with a 100-year-long effort to record images of the sky. By transcribing logbook text to put those historical observations in context, volunteers can help to unlock hidden discoveries.

Find out more about DASCH here, and you can read the news release from last year about it here.

Find out more about AMNH’s digitization project here, where you can also see more episodes of “Shelf Life.”

Past episodes usually focus on the “squishy/hold-in-your-hand side of natural history collections,” said Kendra Snyder from AMNH’s communications department, adding that this latest episode about astronomy offers a different take on what people think is in museum collections.