Tauris argues that a lopsided supernova explosion may be the source of certain hypervelocity stars (image credit: IsiacDaGraca).
Hypervelocity stars have been observed traversing the Galaxy at extreme velocities (700 km/s), but the mechanisms that give rise to such phenomena are still debated. Astronomer Thomas M. Tauris argues that lopsided supernova explosions can eject lower-mass Solar stars from the Galaxy at speeds up to 1280 km/s. “[This mechanism] can account for the majority (if not all) of the detected G/K-dwarf hypervelocity candidates,” he said.
Several mechanisms have been proposed as the source for hypervelocity stars, and the hypotheses can vary as a function of stellar type. A simplified summary of the hypothesis Tauris favors begins with a higher-mass star in a tight binary system, which finally undergoes a core-collapse supernova explosion. The close proximity of the stars in the system partly ensures that the orbital velocities are exceedingly large. The binary system is disrupted by the supernova explosion, which is lopsided (asymmetric) and imparts a significant kick to the emerging neutron star. The remnants of supernovae with massive progenitors are neutron stars or potentially a more exotic object (i.e., black hole).
Conversely, Tauris noted that the aforementioned binary origin cannot easily explain the observed velocities of all higher-mass hypervelocity stars, namely the B-stars, which are often linked to an ejection mechanism from a binary interaction with the supermassive black hole at the Milky Way’s center. Others have proposed that interactions between multiple stars near the centers of star clusters can give rise to certain hypervelocity candidates.
There are several potential compact objects (neutron stars) which feature extreme velocities, such as B2011+38, B2224+65, IGR J11014-6103, and B1508+55, with the latter possibly exhibiting a velocity of 1100 km/s. However, Tauris ends by noting that, “a firm identification of a hypervelocity star being ejected from a binary via a supernova is still missing, although a candidate exists (HD 271791) that’s being debated.”
Be it atoms, stars or snowflakes from the latest nor’easter pounding the New England seaboard, anything worth studying involves movement. And as skies and snowbound roads clear, this Wednesday and Thursday evening will give us a reason to brave the January cold, as the waxing gibbous Moon pierces the Hyades star cluster to graze past the bright star Aldebaran.
During Thursday night’s passage, the Moon will be 78% illuminated. In a sort ‘cosmos mimics controversy’ irony, the gibbous Moon is doing its best to mimic a sky bound ‘deflategate’ football just in time for Superbowl XLIX this weekend.
The motion of the Moon this week across the Hyades. Credit: Stellarium.
But the January 29th event also marks the first occultation of Aldebaran for 2015.
Fun fact: At magnitude +0.8, Aldebaran is the only star brighter than +1st magnitude north of the celestial equator that the Moon can currently occult. Regulus, the runner up, shines at magnitude +1.4. Two other second magnitude stars — Antares and Spica — lie along the Moon’s path on occasion, and up until the 2nd century BC, it was possible for the Moon to occult Pollux in the constellation Gemini as well.
There are 13 occultations of Aldebaran in 2015, and the Moon occults the star 49 times overall until the last event in the current cycle on September 3rd, 2018. Aldebaran is also occulted by the Moon more often in the current 2010-2020 decade than any other bright star. You can even spy Aldebaran near the daytime Moon with binoculars, as we did back in 1996 from North Pole, Alaska.
Maps for the 13 occultations of Aldebaran by the Moon in 2015, click to enlarge. solid lines denote regions were the occultation occurs under dark skies. Credit: Occult 4.0.
Of course, the January 29th event is an occultation only for the high Arctic, with only a scattering of villages and distant early warning stations along the northern Nunavut coast welcoming the sequence of 2015 occultations of the bright star.
The rest of us will see a close photogenic pass, as the Moon makes an end run through the Hyades star cluster every 27.3 day sidereal lunar month in 2015. The Moon will thus occult several members of the Hyades on each pass. Our best bet for North America is the occultation of Aldebaran on November 26th, though the Moon will be just 13 hours past Full.
The occultation of 68 Tauri (a member of the Hyades) for January 29th. Credit: Occult 4.0.
Why doesn’t the path of the Moon just stay put with respect to the sky? Because the orbit of our Moon is fixed at an inclination of 5.1 degrees not with respect to our equator, but to the plane of the ecliptic. This means that the Moon’s orbit is in motion as well, and can wander anywhere from declination 28.6 degrees north to south as it cycles from a shallow to steep path every 18.6 years. We’re actually in a shallow year in 2015 (known as a minor lunar standstill) after which the apparent path of the Moon through the sky begins to widen again until April 2025.
An occultation is celestial motion that you can see in real time as a star or planet is photobomb’d by the onrushing Moon like a January snowplow… but those background stars are in motion as well.
The Hyades themselves — along with our own solar system — are moving around the galactic center. The nearest open cluster to us at 153 light years distant, the Hyades provided a unique object of study for 19th century astronomers. Astronomer Lewis Boss of the Dudley observatory spent several decades studying the proper motion — the apparent motion that a star seems to be moving across the sky from our solar system-bound perspective, measured in arc seconds — of the Hyades, and found the entire group was converging on a point in the constellation Orion near 6 hours 7’ right ascension and +7 degrees declination.
The imaginary convergent point of the Hyades in the night sky. Credit: Starry Night Education software.
Of course, this motion is relative and demonstrates a changing perspective, as the Hyades recedes from our solar system like a defensive line rushing to sack a quarterback.
OK, enough with the sports similes. The Hyades are so close that the actual Hyades Stream — often referred to as the Hyades Moving Group — is actually strewn across the constellations Orion, Taurus and Aries and more.
Some stars, such as 20 Arietis in the adjacent constellation Aries and Iota Horologii in the southern hemisphere may actually members as well. There’s always a bit of ongoing controversy when it comes to actual moving group membership, which is usually pegged by determining proper motion, coupled with the age and metallicity of prospective stars. Growing up in the Milky Way galaxy, our Sun was once a member of some unnamed ancient open cluster that has since long dispersed, like the Hyades are in the process of doing now.
The asterism of the Hyades and the ‘eye of the Bull.’ Photo by author.
The Hyades contains hundreds of stars and ironically, Aldebaran is not a member of the cluster, but is merely 65 light years away from us in the foreground. The V-shaped asterism of the Hyades gives the Head of Taurus the Bull its distinctive shape. The Hyades are named after the rain nymph daughters of Atlas from Greek mythology, whose half daughters the Pleiades also adorn the nearby sky.
And as an added bonus, don’t miss comet C/2014 Q2 Lovejoy crossing the constellation Triangulum, also nearby. Q2 Lovejoy reaches perihelion this week on January 30th, and although it’s completing with the evening Moon, it’s still holding out at a respectable magnitude +4.5.
Comet Q2 Lovejoy skirts by the Hyades and the Pleiades. Credit and Copyright: John Chumack.
All reasons to get out these chilly January evenings and ponder a hurried universe continually in motion, both fast and slow.
This artist’s impression shows an eclipsing binary star system. Credit: ESO/L. Calçada.
Update: It’s off. This past weekend, the AAVSO issued Special Notice #395 calling off the campaign to observe Alpha Comae Berenices this month due to “position measurements published a century ago (which) contained errors that affected the predictions for the time of eclipse…”
And the mystery of Alpha Comae Berenices continues. Oh well. Such is the wiles and whims of the universe, and the exciting field of variable star observing!
A truly fascinating event may be in the offing this month.
Picture two distant burning embers (candles, light bulbs, LEDs, what have you) circling each other in the distance. From our far-flung vantage point, the two points of light are too faint to resolve individually, but as they pass in front of each other, a telltale dip in combined brightness occurs as one blocks out the other.
Welcome to the fascinating world of eclipsing binary stars. This week, we’d like to turn our attention towards a special star in the constellation of Coma Berenices which may — or may not — put on such a dimming act later this month.
An Alpha Comae Berenices (Diadem) finder chart, with comparison stars and magnitudes, decimals omitted. Credit: Starry Night Education Software.
The brightest star in the constellation Coma Berenices, Alpha (sometimes referred to as Diadem, or the ‘crown’ of Queen Berenice) shines at an apparent magnitude of +4.3. Located 63 light years distant, the system consists of two +5th magnitude F-type stars each about 3 times more luminous than our Sun locked in a 26 year orbital embrace. The physical separation of the pair is about 10 astronomical units: place Alpha Comae Berenices in our solar system, and the pair would fit nicely between the Sun and Saturn.
The orbital plane of the pair is inclined nearly along our line of sight as seen from the Earth, and it’s long been thought that catching a grazing or central eclipse of the pair might just be possible. No eclipse was recorded last time ‘round back in February 1989, but times have changed lots in observational astronomy. Today, there are enough backyard observers armed with dedicated observatories and rigs that’d be the envy of a small university that documenting such an eclipse might just be possible. In fact, a central eclipse might just dim the star by 0.8 magnitudes, and should be noticeable to the naked eye.
The binary nature of Alpha Comae Berenices was first noted by F. G. W. Struve in 1827, and the split is a challenging one during the best of years with a maximum angular separation of just 0.7 arc seconds. The pair also has a third faint +10th magnitude companion located about 89 arc seconds away.
A simplified diagram depicting an eclipsing binary event along our line of sight. Created by the author.
The American Association of Variable Star Observers (AAVSO) has an Alert Notice calling for sky watchers worldwide to monitor the star. We also understand the orbit of Alpha Comae Berenices much better in 2015 than back in 1989, and the suspected eclipse should occur somewhere between January 22nd and January 28th and may last anywhere from 28 to 45 hours. This lingering ambiguity means that having a dedicated team of observers worldwide may well be key to nabbing this eclipse.
Alpha Comae Berenices rising. Photo by the author.
The Navy Precision Optical Interferometer (NPOI) has already begun refining measurements of the brightness of the star last month, and professional facilities, to include the Fairborn Observatory atop Mt Hopkins in Arizona and the CHARA (the Center for High Angular Resolution Astronomy) Array at Mount Wilson Observatory in southern California will also be monitoring the event.
Sky and Telescope magazine also has an excellent article in their January 2015 issue on the prospects for catching this eclipse.
Looking eastward past local midnight. Credit: Stellarium.
In late January, the constellation of Coma Berenices rises high to the northeast just after local midnight. It’s worth noting that, if the eclipsing binary nature of Alpha Comae Berenices is confirmed, it would be the longest period known, beating out 14.6 year Gamma Persei discovered in 1990 by more than a decade. A system with as wide a separation as Alpha Comae Berenices would have about a 1 in 1,200 chance in eclipsing along our line of sight due to random chance.
Note: Epsilon Aurigae does have a comparable 27 year period involving a debris disk surrounding its host star. Thanks to sharp-eyed reader Dr. John Barentine for pointing this out!
Of course, the universe does provide us with lots of near misses, allowing for an ‘occasional Diadem’ to indeed occur. Most famous eclipsing variables, such as Algol or Beta Lyrae have periods measured over the span of days or hours. Incidentally, these also make great ‘practice stars’ to test your skills as a visual athlete leading up to the big event next week. A skilled visual observer can note a change as slight as a 0.1 of a magnitude, and it’s a good idea to begin familiarizing yourself with the environs of the star now. The Coma Cluster of galaxies, the globular cluster M53, and the galactic plane crossing intruder Arcturus all lie nearby.
The Coma Cluster as seen by Spitzer Space Telescope and the Sloan Digital Sky Survey. Credit: NASA/Spitzer.
Why study eclipsing binaries? Well, said fleeting mutual events when coupled with spectroscopic measurements and determinations of parallax can tell us a good deal about the astrophysical nature of the stars involved. Eclipsing binary stars have even been used to back up standard candle measurements over extragalactic distances. And of course, orbiting observatories such as Kepler and TESS (to be launched in 2017) look for transiting exoplanets using virtually the same method.
Have a scope+DSLR? Then you can make refined measurements of eclipsing variable stars. Credit: Brad Timerson/IOTA.
But beyond its practical application, we just think that it’s plain cool that you can actually see something out beyond our solar system changing in the span of just a few days or hours.
Observers also still carry out visual observations of variable stars, just like those pipe-smoking, pocket watch carrying astronomers of yore. This involves merely comparing the target star to nearby stars of the same brightness. If you have a DSLR or a CCD rig plus a telescope, the AAVSO also has instructions for how to monitor a star’s brightness as well. No pocket watch required.
A homemade ‘card interferometer’ used to measure the separation of close double stars. Photo by author.
Unless, of course, you want to carry a pocket watch just for good luck. Don’t let the cold January winters keep you from joining the hunt. Let’s make some astrophysical history!
The dark nebula LDN 483 imaged by ESO's La Silla Observatory in Chile (ESO)
What may appear at first glance to be an eerie, empty void in an otherwise star-filled scene is really a cloud of cold, dark dust and molecular gas, so dense and opaque that it obscures the distant stars that lie beyond it from our point of view.
Similar to the more well-known Barnard 68, “dark nebula” LDN 483 is seen above in an image taken by the MPG/ESO 2.2-meter telescope’s Wide Field Imager at the La Silla Observatory in Chile.
While it might seem like a cosmic no-man’s-land, no stars were harmed in the making of this image – on the contrary, dark nebulae like LDN 483 are veritable maternity wards for stars. As their cold gas and dust contracts and collapses new stars form inside them, remaining cool until they build up enough density and gravity to ignite fusion within their cores. Then, shining brightly, the young stars will gradually blast away the remaining material with their outpouring wind and radiation to reveal themselves to the galaxy.
The process may take several million years, but that’s just a brief flash in the age of the Universe. Until then, gestating stars within LDN 483 and many other clouds like it remain dim and hidden but keep growing strong.
Wide-field view of the LDN 483 region. (Credit: ESO and Digitized Sky Survey 2)
To a distant observer, our own Milky Way and the Andromeda galaxy would probably look very similar. Although Andromeda is longer, more massive, and more luminous than the Milky Way, both galaxies are vast spirals composed of hundreds of millions of stars. But new research presented at this week’s AAS conference in Seattle suggests that there are other differences as well – namely, in the movement and behavior of certain stellar age groups. This observation is the first of its kind, and raises new questions about the factors that contribute to the formation of spiral galaxies like our own.
Armed with data from both the Hubble Space Telescope and the Keck Observatory in Hawaii, a group of astronomers from UC Santa Cruz resolved 10,000 tiny points of light in the Andromeda galaxy into individual stars and used their spectra to calculate the stars’ ages and velocities – a feat never before accomplished for a galaxy outside of our own.
Led by Puragra Guhathakurta, a professor of astrophysics, and Claire Dorman, a graduate student, the researchers found that in Andromeda, the behavior of older stars is surprisingly more frazzled than that of their younger counterparts; that is, they have a much wider range of velocities around the galactic center. Meanwhile, in the Milky Way, stars of all ages seem to coexist far more peacefully, moving along at the same speed in a consistent, ordered pack.
The astronomers believe that this asymmetry causes Andromeda to look more distinct from our own galaxy than previously thought. “If you could look at [Andromeda’s] disk edge on, the stars in the well-ordered, coherent population would lie in a very thin plane, whereas the stars in the disordered population would form a much puffier layer,” said Dorman.
What could account for such disorderly conduct among Andromeda’s older generation? It is possible that these more mature stars could have been disturbed long ago, during episodes of the kind of “galactic cannibalism” that is thought to go on among most spiral galaxies. Indeed, trails of stars in its outer halo suggest that Andromeda has collided with and consumed a number of smaller galaxies over the course of its lifetime; however, these effects cannot completely account for the jumbled flow of Andromeda’s most elderly stars.
A few examples of galactic cannibalism. NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University), K. Noll (STScI), and J. Westphal (Caltech)
Astronomers believe that a second explanation could fill in the blanks – one that owes to events occurring far earlier in history, during the birth of the galaxy itself. After all, if Andromeda originated from a lumpy, irregular gas cloud, its oldest stars would naturally appear fairly disordered. Over time, the parent gas would have settled down, giving rise to ever more organized generations of stars.
Guhathakurta, Dorman, and the rest of the team hope that their work will encourage other scientists to create simulations that will better constrain these possibilities. To them, understanding Andromeda is a vital key to learning more about our own galaxy. Guhathakurta explained, “In the Andromeda galaxy we have the unique combination of a global yet detailed view of a galaxy similar to our own. We have lots of detail in our own Milky Way, but not the global, external perspective.”
Now, thanks to this new research, scientists can cite our own galaxy’s comparative orderliness as strong evidence that we live in a quieter, less cannibalistic neighborhood than most other spiral galaxies in the Universe. “Even the most well ordered Andromeda stars are not as well ordered as the stars in the Milky Way’s disk,” said Dorman.
At least until 4 billion years from now, when the Milky Way and Andromeda collide.
We may as well enjoy the A+ for conduct while we can.
A colorful photo of the "Tulip Nebula" taken by Julian Hancock.
We often publish photos from professional observatories, but it’s important to note that amateurs can also do a great job taking pictures of the sky with modest equipment and photo processing software.
On Universe Today’s Flickr pool, we’re proud to showcase the work of all the fans of the cosmos. Included here are some of the best shots of galaxies and nebulas that we’ve seen uploaded to the site in recent days.
The Milky Way shines over Termas de Chillán in this photo taken by “Miss Andrea” on Flickr.The center of the Heart Nebula captured by David Wills on Flickr.Simeis 147, the “Spaghetti Nebula”, shines in hydrogen alpha in this image captured by Rick Stevenson on Flickr.The Tarantula Nebula imaged in Ha, OIII and SII by Alan Tough on Flickr.
Collisions of neutron stars produce powerful gamma-ray bursts – and heavy elements like gold (Credit: Dana Berry, SkyWorks Digital, Inc.)
It’s known as HIP 85605, one of two stars that make up a binary in the Hercules constellation roughly 16 light years away. And if a recent research paper produced by Dr. Coryn Bailer-Jones of the Max Planck Institute for Astronomy in Heidelberg, Germany is correct, it is on a collision course with our Solar System.
Now for the good news: according to Bailer-Jones’ calculations, the star will pass by our Solar System at a distance of 0.04 parsecs, which is equivalent to 8,000 times the distance between the Earth and the Sun (8,000 AUs). In addition, this passage will not affect Earth or any other planet’s orbit around the Sun. And perhaps most importantly of all, none of it will be happening for another 240,000 to 470,000 years from now.
“Even though the galaxy contains very many stars,” Bailer-Jones told Universe Today via email, “the spaces between them are huge. So even over the (long) life of our galaxy so far, the probability of any two stars have actually collided — as opposed to just coming close — is extremely small.”
However, in astronomical terms, that still counts as a near-miss. In a universe that is 46 billion light years in any direction – and that’s just the observable part of it – an event that is expected to take place just 50 light days away is considered to be pretty close. And in the context of space and time, a quarter of a million to half a million years is the very near future.
The real concern is the effect that the passage of HIP 85605 could have on the Oort Cloud – the massive cloud of icy planetesimals that surrounds the Solar System. Given that it’s distance is between 20,000 and 50,000 AU from our Sun, HIP 85605 would actually move through the Oort cloud and cause serious disruption.
The layout of the Solar System, including the Oort Cloud, which lies 50,000 AU from our Sun. Credit: NASA
Many of these planetesimals could be blown off into space, but others could be sent hurtling towards Earth. Assuming humanity is still around at this point in time, this could present a bit of an inconvenience, even if it is spread over the course of a million years.
As it stands, such “close encounters” between stars are quite rare. Stellar collisions usually only occur within binaries, where white dwarfs or neutron stars are concerned. “The exception to this is physically bound binary stars in a tight orbit,” said Bailer-Jones. “It can and does happen that one star expands during its evolution and will then interfere with the evolution of the other star. Neutron-neutron star pairs can even merge.”
But of course, on an astronomical timescale, stars passing each other by as they perform their cosmic dance is actually a pretty common occurrence. As part of Bailer-Jones larger study of over 50,000 stars within our galaxy, this “close encounter” is one of several predicted to take place in the coming years.
Of all of them, only HIP 85605 is expected to come within a single parsec between 240 and 470 thousand years from now. He also indicates with (90% confidence) that the last time such an encounter took place was 3.8 million years ago when gamma Microscopii – a G7 giant which has two and a half times the mass of our Sun – came within 0.35-1.34 pc of our system, which may have caused a large perturbation in the Oort cloud.
Tightly bound binary stars, like the ones illustrated here, sometimes result in stellar collisions. Credit: Chandra
On his MPIA webpage, in the study’s FAQ section, Bailer-Jones claims that his research into stellar close encounters was motivated by a desire to study the potential impacts of astronomical phenomena on Earth, and is part of a larger program named “astroimpacts”.
“I am interested in the history of the Earth,” he says, “and astronomical phenomena have clearly played a role in this. But what role precisely, how significant, and what can we expect to happen in the future?” Whereas several studies have been conducted in the past, he feels that the methods – which include assuming a linear relative motion of stars – produces inaccurate results.”
In contrast, Bailer-Jones study relies on “more recent data or re-analyses of data to produce hopefully more accurate results, and then compensate more rigorously for the uncertainties in the data, so that I can attach probabilities to my statements.”
As a result of this, he predicts that HIP 85605 has a 90% chance of passing within a single parsec of our Sun in the next 240 to 470 thousands years. However, he also admits that if the astronomy is incorrect, the next closest encounter won’t be happening for another 1.3 million years, when a K7 dwarf known as GL 710 is predicted to pass within 0.10 – 0.44 parsecs.
Bailer-Jones also believes that the European Space Agency’s Gaia spacecraft will help make more accurate predictions in the future. By understanding and mapping the environment of the Milky Way Galaxy, measuring the gravitational potential and determining the velocity of stars, scientists will be able to see how their various orbits around the galaxy’s center could cause them to intersect.
It is likely that passing stars have a system of exoplanets (like Kepler-186f pictured here), which would place them within a few parsecs of Earth. Image Credit: NASA Ames/SETI Institute/JPL-CalTech
But perhaps the most interesting question explored on his webpage is the possibility of using stellar close encounters as a shortcut for exploring exoplanets. According to current cosmological models, the majority of stars within our galaxy are believed to host exoplanets.
So if a star is passing us at just a few parsecs (or even with a single parsec) why not hop on over and investigate its planets? Well, as Bailer-Jones indicates, that’s not really a practical idea: “Traveling to a star passing our solar system at a distance of around 1 pc with a relative speed of 30 km/s is no easier than traveling the the nearby stars (the nearest of which is just over 1 pc away). And we would have to wait 10s of thousands of years for the next encounter. If we can ever achieve interstellar travel, I don’t suppose it would take that long to achieve, so why wait?”
Darn. Still, if there’s one thing this phenomena and Bailer-Jones study reminds us, it is that in the course of dancing around the center of the Milky Way, stars are not fixed in a single point in space. Not only do they periodically move within reach of each other, they can also have an affect on life within them.
Alas, the timescale on which such things happen, not to mention the consequences they entail, are so large that people here on Earth need not worry. By the time HIP 85605 or GL 710 come within a parsec or two of us, we’ll either be long-since dead or too highly evolved to care!
*Update: According to a new study posted by Erick E. Mamajek and associates on arXiv, the passage of the recently-discovered low mass star W0720 (aka. “Scholtz Star”) – roughly 70,000 years ago and at a distance of 0.25 Parsecs from our Sun – was the closest encounter our Solar System has had with another star. They calculate the possibility that it would have penetrated the System’s Outer Oort Cloud at 98%. However, they also estimate that the impact it would have had on the flux of long-period comets was negligible, but that the passage also highlights how “dynamically important Oort Cloud perturbers may be lurking among nearby stars”.
Having read the study, Bailer-Jones claims on the updated FAQ section of his MPIA webpage that their analysis appears to be correct. Based on the assumption that the star was moving on a constant velocity relative to the Sun prior to the encounter, he agrees that the calculations on the distances and timing of the passage are valid. While his own study identified a possible closer encounter (Hip 85605), he reiterates that the data on this star is of poor quality. Meanwhile, another close encounter took place involving Hip 89825; but here, the approach distance is estimated to have been 0.02 Parsecs larger. Hence, W0720 can be said to have been the closest encounter with some degree of certainty at this time.
The west limb of the Sun imaged by NuSTAR and SDO shows areas of high-energy x-rays above particularly active regions (NASA/JPL-Caltech/GSFC)
What if you had x-ray vision like Superman? Or if those funny-looking glasses they advertised in comic books in the 60s actually worked?* Then with those our Sun might look something like this, lighting up with brilliant flares of high-energy x-rays as seen by NASA’s super-sensitive NuSTAR Space Telescope (with a little help from SDO.)
The NuStar Space Telescope launched aboard a Orbital Sciences Pegasus rocket, on June 13, 2012. (Credit: NASA/Caltech-JPL)
Of course NASA’s orbiting NuSTAR x-ray telescope is not like a typical medical imaging system. Instead of looking for broken bones, NuSTAR (short for Nuclear Spectroscopic Telescope Array) is made to detect high-energy particles blasting across the Universe from exotic objects like supermassive black holes, pulsars, and supernovae.
But astronomers suggested turning NuSTAR’s gaze upon our own Sun to see what sorts of x-ray activity may be going on there.
“At first I thought the whole idea was crazy,” said Fiona Harrison, a Professor of Physics and Astronomy at Caltech and PI for the NuSTAR mission. “Why would we have the most sensitive high energy X-ray telescope ever built, designed to peer deep into the universe, look at something in our own back yard?”
As it turns out NuSTAR was able to reveal some very interesting features on the Sun, showing where the corona is being heated to very high temperatures. The image above shows NuSTAR’s first observations, overlaid onto data acquired by NASA’s Solar Dynamics Observatory.
NuSTAR data is shown in green and blue, revealing high-energy emission around – but not exactly aligned with – active regions on the Sun where solar plasma is being heated to more than 3 million degrees. The red represents ultraviolet light captured by SDO and shows material in the solar atmosphere at a slightly cooler 1 million degrees.
The NuSTAR data overlaid on the full disk SDO image, rotated so north on the Sun is up. (NASA/JPL-Caltech/GSFC)
Because the Sun isn’t terribly intense in high energy x-ray output it’s safe to observe it with NuSTAR — it’s not likely to burn out the telescope’s sensors. But what NuSTAR can detect may help astronomers determine the exact mechanisms behind the intense coronal heating that occurs in and above the Sun’s chromosphere. If so-called “nanoflares” — miniature and as-yet-invisible versions of solar flares — are responsible, for instance, NuSTAR might be able to catch them in action for the first time.
“NuSTAR will be exquisitely sensitive to the faintest X-ray activity happening in the solar atmosphere, and that includes possible nanoflares,” said David Smith, solar physicist and member of the NuSTAR team at the University of California, Santa Cruz.
In addition NuSTAR could potentially detect the presence of axions in the Sun’s core — hypothesized particles that may make up dark matter in the Universe.
NuSTAR may not be a “solar telescope” per se, but that won’t stop astronomers from using its unique abilities to learn more about the star we intimately share space with.
“NuSTAR will give us a unique look at the Sun, from the deepest to the highest parts of its atmosphere.”
– David Smith, solar physicist, University of California Santa Cruz
This Chandra image of the Tycho supernova remnant contains new evidence for what triggered the original supernova explosion. Credit: NASA/CXC/Chinese Academy of Sciences/F. Lu et al.
At one time or another, all science enthusiasts have heard the late Carl Sagan’s infamous words: “We are made of star stuff.” But what does that mean exactly? How could colossal balls of plasma, greedily burning away their nuclear fuel in faraway time and space, play any part in spawning the vast complexity of our Earthly world? How is it that “the nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies” could have been forged so offhandedly deep in the hearts of these massive stellar giants?
Unsurprisingly, the story is both elegant and profoundly awe-inspiring.
All stars come from humble beginnings: namely, a gigantic, rotating clump of gas and dust. Gravity drives the cloud to condense as it spins, swirling into an ever more tightly packed sphere of material. Eventually, the star-to-be becomes so dense and hot that molecules of hydrogen in its core collide and fuse into new molecules of helium. These nuclear reactions release powerful bursts of energy in the form of light. The gas shines brightly; a star is born.
The ultimate fate of our fledgling star depends on its mass. Smaller, lightweight stars burn though the hydrogen in their core more slowly than heavier stars, shining somewhat more dimly but living far longer lives. Over time, however, falling hydrogen levels at the center of the star cause fewer hydrogen fusion reactions; fewer hydrogen fusion reactions mean less energy, and therefore less outward pressure.
At a certain point, the star can no longer maintain the tension its core had been sustaining against the mass of its outer layers. Gravity tips the scale, and the outer layers begin to tumble inward on the core. But their collapse heats things up, increasing the core pressure and reversing the process once again. A new hydrogen burning shell is created just outside the core, reestablishing a buffer against the gravity of the star’s surface layers.
While the core continues conducting lower-energy helium fusion reactions, the force of the new hydrogen burning shell pushes on the star’s exterior, causing the outer layers to swell more and more. The star expands and cools into a red giant. Its outer layers will ultimately escape the pull of gravity altogether, floating off into space and leaving behind a small, dead core – a white dwarf.
Lower-mass stars like our sun eventually enter a swollen, red giant phase. Ultimately, its outer layers will be thrown off altogether, leaving nothing but a small white dwarf star. Image Credit: ESO/S. Steinhofel
Heavier stars also occasionally falter in the fight between pressure and gravity, creating new shells of atoms to fuse in the process; however, unlike smaller stars, their excess mass allows them to keep forming these layers. The result is a series of concentric spheres, each shell containing heavier elements than the one surrounding it. Hydrogen in the core gives rise to helium. Helium atoms fuse together to form carbon. Carbon combines with helium to create oxygen, which fuses into neon, then magnesium, then silicon… all the way across the periodic table to iron, where the chain ends. Such massive stars act like a furnace, driving these reactions by way of sheer available energy.
But this energy is a finite resource. Once the star’s core becomes a solid ball of iron, it can no longer fuse elements to create energy. As was the case for smaller stars, fewer energetic reactions in the core of heavyweight stars mean less outward pressure against the force of gravity. The outer layers of the star will then begin to collapse, hastening the pace of heavy element fusion and further reducing the amount of energy available to hold up those outer layers. Density increases exponentially in the shrinking core, jamming together protons and electrons so tightly that it becomes an entirely new entity: a neutron star.
At this point, the core cannot get any denser. The star’s massive outer shells – still tumbling inward and still chock-full of volatile elements – no longer have anywhere to go. They slam into the core like a speeding oil rig crashing into a brick wall, and erupt into a monstrous explosion: a supernova. The extraordinary energies generated during this blast finally allow the fusion of elements even heavier than iron, from cobalt all the way to uranium.
Periodic Table of Elements. Massive stars can fuse elements up to Iron (Fe), atomic number 26. Elements with atomic numbers 27 through 92 are produced in the aftermath of a massive star’s core collapse.
The energetic shock wave produced by the supernova moves out into the cosmos, disbursing heavy elements in its wake. These atoms can later be incorporated into planetary systems like our own. Given the right conditions – for instance, an appropriately stable star and a position within its Habitable Zone – these elements provide the building blocks for complex life.
Today, our everyday lives are made possible by these very atoms, forged long ago in the life and death throes of massive stars. Our ability to do anything at all – wake up from a deep sleep, enjoy a delicious meal, drive a car, write a sentence, add and subtract, solve a problem, call a friend, laugh, cry, sing, dance, run, jump, and play – is governed mostly by the behavior of tiny chains of hydrogen combined with heavier elements like carbon, nitrogen, oxygen, and phosphorus.
Other heavy elements are present in smaller quantities in the body, but are nonetheless just as vital to proper functioning. For instance, calcium, fluorine, magnesium, and silicon work alongside phosphorus to strengthen and grow our bones and teeth; ionized sodium, potassium, and chlorine play a vital role in maintaining the body’s fluid balance and electrical activity; and iron comprises the key portion of hemoglobin, the protein that equips our red blood cells with the ability to deliver the oxygen we inhale to the rest of our body.
So, the next time you are having a bad day, try this: close your eyes, take a deep breath, and contemplate the chain of events that connects your body and mind to a place billions of lightyears away, deep in the distant reaches of space and time. Recall that massive stars, many times larger than our sun, spent millions of years turning energy into matter, creating the atoms that make up every part of you, the Earth, and everyone you have ever known and loved.
We human beings are so small; and yet, the delicate dance of molecules made from this star stuff gives rise to a biology that enables us to ponder our wider Universe and how we came to exist at all. Carl Sagan himself explained it best: “Some part of our being knows this is where we came from. We long to return; and we can, because the cosmos is also within us. We’re made of star stuff. We are a way for the cosmos to know itself.”
Look high in the southern sky at nightfall to find the familiar giant square that forms part of the body of Pegasus the Flying Horse. The map shows the sky around 6:30 p.m. local time. Source: Stellarium
Cast your gaze up, up, up on the next dark, moonless night and stare into the Great Square of Pegasus. How many stars do you see? Zero? Two? Twenty? If you’d like to find out how dark your sky is, read on.
The Great Square, one of the fall sky’s best known star patterns, rides high in the south at nightfall in mid-December. It forms part of the larger figure of Pegasus the Winged Horse. For our purposes today, we’re going to concentrate on what’s inside the square.
Bounded by Alpheratz (officially belonging to adjacent Andromeda), Scheat, Markab and Algenib, the Great Square is about 15° on a side or one-and-a-half balled fists held at arm’s length.
At first glance, the space appears empty, but a closer look from all but the most light polluted skies will reveal a pair 4th magnitude stars in the upper right quadrant of the square. Fourth magnitude is about the viewing limit from a bright suburban location.
Astronomers use the magnitude scale to measure star and planet brightness. Each magnitude is 2.5 times brighter than the one below it. Aldebaran, which shines at 1st magnitude, is 2.5 times brighter than a 2nd magnitude star, which in turn is 2.5 times brighter than a 3rd magnitude star and so on.
Moonlight and especially light pollution reduce the number of stars we can see in the night sky. This specially prepared map shows slices of sky based on amateur astronomer and author John Bortle’s Dark Sky Scale. Classes range from 1 (excellent with stars fainter than 7th magnitude visible) to 9 (inner city with a limiting magnitude of 4). Click for more detailed descriptions of each class and rate your own sky. Credit: International Dark Sky Association
A first magnitude star is 2.5 x 2.5 x 2.5 x 2.5 x 2.5 (about 100) times brighter than a 6th magnitude star. The bigger the magnitude number, the fainter the star. From cities, you might see 3rd magnitude stars if you can block out stray lighting, but a dark country sky will deliver the Holy Grail naked eye limit of magnitude 6. Skywatchers with utterly dark conditions might glimpse stars as faint 7.5. My own personal best is 6.5.
With each drop in magnitude the number of stars you can see increases exponentially. There are only 22 first magnitude or brighter stars compared to 5,946 stars down to magnitude 6.
What appears blank at first is filled with stars — 26 of them down to magnitude 6.3 are visible inside the Great Square from a dark sky site. How many can you see? Click for a larger version. Source: Stellarium
Ready to stretch your sight and rate your night sky? Step outside at nightfall and allow your eyes to dark-adapt for 20 minutes. With a copy of the map (above) in hand, start with the brightest stars and work your way to the faintest. Each every small step down the magnitude ladder prepares your eyes the next.
With a little effort you should be able to spot the four 4th magnitude range stars. At magnitude 5, you’ll work harder. Moving beyond 5.5 can be very challenging. I revert to averted vision to corral these fainties. Instead of staring directly at the star, play your eye around it. Look a bit to this side and that. This allows a rod-rich part of the retina that’s excellent at seeing faint stuff play through the scene and snatch up the faintest possible stars.
Magnitude scale showing the limits of the eye, binoculars and telescopes. Credit: Dr. Michael Bolte, UCO/Lick Observatory
From my house I can pick out about dozen points of light inside the Square on a moonless night. How many will you see? Once you know your magnitude limit, compare your result to John Bortle’s Dark Sky Scale … and weep. No, just kidding. But his Class 1 excellent sky includes a description of seeing stars down to magnitude 8 and the summer Milky Way casting shadows.
Hard to believe that before about 1790, when gas lighting was introduced in England, Class 1 skies were the norm across virtually the entire planet. Nowadays, most of us have to drive a hundred miles or more to experience true, untrammeled darkness.
Have fun with the challenge and let us know in the comments area how you do. Here’s hoping you find the Great Square far from vacant.
