This article is a guest post by Anna Ho, who is currently doing research on stars in the Milky Way through a one-year Fulbright Scholarship at the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany.
In the Milky Way, an average of seven new stars are born every year. In the distant galaxy GN20, an astonishing average of 1,850 new stars are born every year. “How,” you might ask, indignant on behalf of our galactic home, “does GN20 manage 1,850 new stars in the time it takes the Milky Way to pull off one?”
To answer this, we would ideally take a detailed look at the stellar nurseries in GN20, and a detailed look at the stellar nurseries in the Milky Way, and see what makes the former so much more productive than the latter.
But GN20 is simply too far away for a detailed look.
This galaxy is so distant that its light took twelve billion years to reach our telescopes. For reference, Earth itself is only 4.5 billion years old and the universe itself is thought to be about 14 billion years old. Since light takes time to travel, looking out across space means looking back across time, so GN20 is not only a distant, but also a very ancient, galaxy. And, until recently, astronomers’ vision of these distant, ancient galaxies has been blurry.
Consider what happens when you try to load a video with a slow Internet connection, or when you download a low-resolution picture and then stretch it. The image is pixelated. What was once a person’s face becomes a few squares: a couple of brown squares for hair, a couple of pink squares for the face. The low-definition picture makes it impossible to see details: the eyes, the nose, the facial expression.
A face has many details and a galaxy has many varied stellar nurseries. But poor resolution, a result simply of the fact that ancient galaxies like GN20 are separated from our telescopes by vast cosmic distances, has forced astronomers to blur together all of this rich information into a single point.
There are over 3,000 stars in this image: The glowing dots are newborn stars that have recently emerged from their cocoons. Stellar cocoons are made of gas: thousands of these gas cocoons sit nestled in immense cosmic nurseries, which are rich with gas and dust. The central region of that Hubble image, encased by what looks like a bubble, is so clear and bright because the massive stars within have blown away the dust and gas they were forged from. Majestic stellar nurseries are scattered all over the Milky Way, and astronomers have been very successful at uncloaking them in order to understand how stars are made.
Observing nurseries both here at home and in relatively nearby galaxies has enabled astronomers to make great leaps in understanding stellar birth in general: and, in particular, what makes one nursery, or one star formation region, “better” at building stars than another. The answer seems to be: how much gas there is in a particular region. More gas, faster rate of star birth. This relationship between the density of gas and the rate of stellar birth is called the Kennicutt-Schmidt Law. In 1959, the Dutch astronomer Maarten Schmidt raised the question of how exactly increasing gas density influences star birth, and forty years later, in an illustration of how scientific dialogues can span decades, his American colleague Robert Kennicutt used data from 97 galaxies to answer him.
Understanding the Kennicutt-Schmidt Law is crucial for determining how stars form and even how galaxies evolve. One fundamental question is whether there is one rule that governs all galaxies, or whether one rule governs our galactic neighborhood, but a different rule governs distant galaxies. In particular, a family of distant galaxies known as “starburst galaxies” seems to contain particularly productive nurseries. Dissecting these distant, highly efficient stellar factories would mean probing galaxies as they used to be, back near the beginning of the universe.
Enter GN20. GN20 is one of the brightest, most productive of these starburst galaxies. Previously a pixelated dot in astronomers’ images, GN20 has become an example of a transformation in technological capability.
In December 2014, an international team of astronomers led by Dr. Jacqueline Hodge of the National Radio Astronomy Observatory in the USA, and comprising astronomers from Germany, the United Kingdom, France, and Austria, were able to construct an unprecedentedly detailed picture of the stellar nurseries in GN20. Their results were published earlier this year.
The key is a technique called interferometry: observing one object with many telescopes, and combining the information from all the telescopes to construct one detailed image. Dr. Hodge’s team used some of the most sophisticated interferometers in the world: the Karl G. Jansky Very Large Array (VLA) in the New Mexico desert, and the Plateau de Bure Interferometer (PdBI) at 2550 meters (8370 feet) above sea level in the French Alps.
With data from these interferometers as well as the Hubble Space Telescope, they turned what used to be one dot into the following composite image:
This is a false color image, and each color stands for a different component of the galaxy. Blue is ultraviolet light, captured by the Hubble Space Telescope. Green is cold molecular gas, imaged by the VLA. And red is warm dust, heated by the star formation it is shrouding, detected by the PdBI.
Unbundling one pixel into many enabled the team to determine that the nurseries in a starburst galaxy like GN20 are fundamentally different from those in a “normal” galaxy like the Milky Way. Given the same amount of gas, GN20 can churn out orders of magnitude more stars than the Milky Way can. It doesn’t simply have more raw material: it is more efficient at fashioning stars out of it.
This kind of study is currently unique to the extreme case of GN20. However, it will be more common with the new generation of interferometers, such as the Atacama Large Millimeter/submillimeter Array (ALMA).
Located 5000 meters (16000 feet) high up in the Chilean Andes, ALMA is poised to transform astronomers’ understanding of stellar birth. State-of-the-art telescopes are enabling astronomers to do the kind of detailed science with distant galaxies – ancient galaxies from the early universe – that was once thought to be possible only for our local neighborhood. This is crucial in the scientific quest for universal physical laws, as astronomers are able to test their theories beyond our neighborhood, out across space and back through time.
A planetary system’s early days readily tell of turmoil. Giant planets are swept from distant birthplaces into sizzling orbits close to their host star. Others are blasted away from their star into the darkness of space. And smaller bodies, like asteroids and comets, are being traded around constantly.
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have seen the latter: swarms of Pluto-size objects turning to dust around a young star. And the image is remarkable.
“This system offers us the chance to study an intriguing time around a young, Sun-like star,” said coauthor Stuartt Corder and ALMA Deputy Director in a news release. “We are possibly looking back in time here, back to when the Sun was about 2 percent of its current age.”
The young star, HD 107146, is located roughly 90 light-years from Earth in the direction of the constellation Coma Berenices. Although the star itself is visible in any small telescope, ALMA can probe the star’s radically faint protoplanetary disk. This is the star’s dusty cocoon that coalesces into planets, comets and asteroids.
ALMA’s image revealed an unexpected bump in the number of millimeter-size dust grains far from the host star. This highly concentrated band spans roughly 30 to 150 astronomical units, the equivalent of Neptune’s orbit around the Sun to four times Pluto’s orbit.
So where is the extra dust coming from?
Typically, dust in the debris disk is simply left over material from the formation of planets. Early on, however, Pluto-size objects (otherwise known as planetesimals) will collide and blast themselves apart, also contributing to the dust. Certain models predict that this leads to a much higher concentration of dust in the most distant regions of the disk.
Although this is the case for HD 107146, “this is the opposite of what we see in younger primordial disks where the dust is denser near the star,” said lead author Luca Ricci from the Harvard-Smithsonian Center for Astrophysics. “It is possible that we caught this particular debris disk at a stage in which Pluto-size planetesimals are forming right now in the outer disk while other Pluto-size bodies have already formed closer to the star.”
Adding to this hypothesis is the fact that there’s a slight depression in the dust at 80 astronomical units, or twice Pluto’s average distance from the Sun. This could be a slight gap in the dust, where an Earth-size planet is sweeping the area clear of a debris disk.
If true, this would be the first observation of an Earth-size planet forming so far from its host star. But for now that’s a big if.
A team of Australian astronomers has been busy utilizing some of the world’s leading radio telescopes located in both Australia and Chile to carve away at the layered remains of a relatively new supernova. Designated as SN1987A, the 28 year-old stellar cataclysm came to Southern Hemisphere observer’s attention when it sprang into action at the edge of the Large Magellanic Cloud some two and a half decades ago. Since then, it has provided researchers around the world with a ongoing source of information about one of the Universe’s “most extreme events”.
Representing the University of Western Australia node of the International Centre for Radio Astronomy Research, PhD Candidate Giovanna Zanardo led the team focusing on the supernova with the Australia Telescope Compact Array (ATCA) in New South Wales. Their observations took in the wavelengths spanning the radio to the far infrared.
“By combining observations from the two telescopes we’ve been able to distinguish radiation being emitted by the supernova’s expanding shock wave from the radiation caused by dust forming in the inner regions of the remnant,” said Giovanna Zanardo of the International Centre for Radio Astronomy Research (ICRAR) in Perth, Western Australia.
“This is important because it means we’re able to separate out the different types of emission we’re seeing and look for signs of a new object which may have formed when the star’s core collapsed. It’s like doing a forensic investigation into the death of a star.”
“Our observations with the ATCA and ALMA radio telescopes have shown signs of something never seen before, located at the centre or the remnant. It could be a pulsar wind nebula, driven by the spinning neutron star, or pulsar, which astronomers have been searching for since 1987. It’s amazing that only now, with large telescopes like ALMA and the upgraded ATCA, we can peek through the bulk of debris ejected when the star exploded and see what’s hiding underneath.”
A video compilation showing Supernova Remnant 1987A as seen by the Hubble Space Telescope in 2010, and by radio telescopes located in Australia and Chile in 2012. The piece ends with a computer generated visualization of the remnant showing the possible location of a Pulsar. Credit: Dr Toby Potter, ICRAR-UWA, Dr Rick Newton, ICRAR-UWA
But, there is more. Not long ago, researchers published another paper which appeared in the Astrophysical Journal. Here they made an effort to solve another unanswered riddle about SN1987A. Since 1992 the supernova appears to be “brighter” on one side than it does the other! Dr. Toby Potter, another researcher from ICRAR’s UWA node took on this curiosity by creating a three-dimensional simulation of the expanding supernova shockwave.
“By introducing asymmetry into the explosion and adjusting the gas properties of the surrounding environment, we were able to reproduce a number of observed features from the real supernova such as the persistent one-sidedness in the radio images”, said Dr. Toby Potter.
So what’s going on? By creating a model which spans over a length of time, researchers were able to emulate an expanding shock front along the eastern edge of the supernova remnant. This region moves away more quickly than its counterpart and generates more radio emissions. When it encounters the equatorial ring – as observed by the Hubble Space Telescope – the effect becomes even more pronounced.
A visualization showing how Supernova1987A evolves between May of 1989 and July of 2014. Credit: Dr Toby Potter, ICRAR-UWA, Dr Rick Newton, ICRAR-UWA
“Our simulation predicts that over time the faster shock will move beyond the ring first. When this happens, the lop-sidedness of radio asymmetry is expected to be reduced and may even swap sides.”
“The fact that the model matches the observations so well means that we now have a good handle on the physics of the expanding remnant and are beginning to understand the composition of the environment surrounding the supernova – which is a big piece of the puzzle solved in terms of how the remnant of SN1987A formed.”