Artist's impression of a white dwarf star in orbit around Sirius (a white supergiant). Credit: NASA, ESA and G. Bacon (STScI)
Stars are beautiful, wondrous things. Much like planets, planetoids and other stellar bodies, they come in many sizes, shapes, and even colors. And over the course of many centuries, astronomers have come to discern several different types of stars based on these fundamental characteristics.
For instance, the color of a star – which varies from bluish-white and yellow to orange and red – is primarily due to its composition and effective temperature. And at all times, stars emit light which is a combination of several different wavelengths. On top of that, the color of a star can change over time.
Composition:
Different elements emit different wavelengths of electromagnetic radiation when heated. In the case of stars, his includes its main constituents (hydrogen and helium), but also the various trace elements that make it up. The color that we see is the combination of these different electromagnetic wavelengths, which are referred to as as a Planck’s curve.
Diagram illustrating Wein’s Law, which describes the emission of radiation from a black body based on its peak wavelength. Credit: Wikipedia Commons/Darth
The wavelength at which a star emits the most light is called the star’s “peak wavelength” (which known as Wien’s Law), which is the peak of its Planck curve. However, how that light appears to the human eye is also mitigated by the contributions of the other parts of its Planck curve.
In short, when the various colors of the spectrum are combined, they appear white to the naked eye. This will make the apparent color of the star appear lighter than where star’s peak wavelength falls on the color spectrum. Consider our Sun. Despite the fact that its peak emission wavelength corresponds to the green part of the spectrum, its color appears pale yellow.
A star’s composition is the result of its formation history. Ever star is born of a nebula made up of gas and dust, and each one is different. While nebulas in the interstellar medium are largely composed of hydrogen, which is the main fuel for star creation, they also carry other elements. The overall mass of the nebula, as well as the various elements that make it up, determine what kind of star will result.
The change in color these elements add to stars is not very obvious, but can be studied thanks to the method known as spectroanalysis. By examining the various wavelengths a star produces using a spectrometer, scientists are able to determine what elements are being burned inside.
Temperature and Distance:
The other major factor effecting a star’s color is its temperature. As stars increase in heat, the overall radiated energy increases, and the peak of the curve moves to shorter wavelengths. In other words, as a star becomes hotter, the light it emits is pushed further and further towards the blue end of the spectrum. As stars grow colder, the situation is reversed (see below).
A third and final factor that will effect what light a star appears to be emitting is known as the Doppler Effect. When it comes to sound, light, and other waves, the frequency can increase or decrease based on the distance between the source and the observer.
When it comes to astronomy, this effect causes the what is known as “redshift” and “blueshift” – where the visible light coming from a distant star is shifted towards the red end of the spectrum if it is moving away, and the blue end if it is moving closer.
Modern Classification:
Modern astronomy classifies stars based on their essential characteristics, which includes their spectral class (i.e. color), temperature, size, and brightness. Most stars are currently classified under the Morgan–Keenan (MK) system, which classifies stars based on temperature using the letters O, B, A, F, G, K, and M, – O being the hottest and M the coolest.
Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. O1 to M9 are the hottest to coldest stars). In the MK system, a luminosity class is added using Roman numerals. These are based on the width of certain absorption lines in the star’s spectrum (which vary with the density of the atmosphere), thus distinguishing giant stars from dwarfs.
Luminosity classes 0 and I apply to hyper- or supergiants; classes II, III and IV apply to bright, regular giants, and subgiants, respectively; class V is for main-sequence stars; and class VI and VII apply to subdwarfs and dwarf stars. There is also the Hertzsprung-Russell diagram, which relates stellar classification to absolute magnitude (i.e. intrinsic brightness), luminosity, and surface temperature.
The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude (expressed as Mv) to place them on a 2-dimensional chart (see below).
The Hertzspirg-Russel diagram, showing the relation between star’s color, AM. luminosity, and temperature. Credit: astronomy.starrynight.com
On average, stars in the O-range are hotter than other classes, reaching effective temperatures of up to 30,000 K. At the same time, they are also larger and more massive, reaching sizes of over 6 and a half solar radii and up to 16 solar masses. At the lower end, K and M type stars (orange and red dwarfs) tend to be cooler (ranging from 2400 to 5700 K), measuring 0.7 to 0.96 times that of our Sun, and being anywhere from 0.08 to 0.8 as massive.
Stellar Evolution:
Stars also go through an evolutionary life cycle, during which time their sizes, temperatures and colors change. For example, when our Sun exhausts all the hydrogen in its the core, it will become unstable and collapse under its own weight. This will cause the core to heat up and get denser, causing the Sun to grow in size.
At this point, it will have left its Main Sequence phase and entered into the Red Giant Phase of its life, which (as the name would suggest) will be characterized by expansion and it becoming a deep red. When this happens, it is theorized that our Sun will expand to encompass the orbits of Mercury and even Venus.
Earth, if it survives this expansion, will be so close that it will be rendered uninhabitable. When our Sun then reaches its post-Red Giant Phase, the Sun will begin to eject mass, leaving an exposed core known as a white dwarf. This remnant will survive for trillions of years before fading to black.
This is believed to be the case with all stars that have between 0.5 to 1 Solar Mass (half, or as much mass of our Sun). The situation is slightly different when it comes to low mass stars (i.e. red dwarfs), which typically have around 0.1 Solar Masses.
It is believed that these stars can remain in their Main Sequence for some six to twelve trillion years and will not experience a Red Giant Phase. However, they will gradually increase in both temperature and luminosity, and will exist for several hundred billion more years before they eventually collapse into a white dwarf.
On the other hand, supergiant stars (up to 100 Solar Masses or more) have so much mass in their cores that they will likely experience helium ignition as soon as they exhaust their supplies of hydrogen. As such, they will likely not survive to become Red Supergiants, and will instead end their lives in a massive supernova.
To break it all down, stars vary in color depending on their chemical compositions, their respective sizes and their temperatures. Over time, as these characteristics change (as a result of them spending their fuel) many will darken and become redder, while others will explode magnificently. The more stars observe, the more we come to know about our Universe and its long, long history!
Copyright: ESA with changes to annotations by the author
This montage of photos of Comet 67P/Churyumov-Gerasimenko was taken by ESA’s Rosetta spacecraft between Jan. 31 and March 25, 2015 and shows increasing activity as the comet approached perihelion. Credit: NAVCAM /CC-BY-SA-IGO-3.0
Rosetta awoke from a decade of deep-space hibernation in January 2014 and immediately got to work photographing, measuring and sampling comet 67P/C-G. On September 30 it will sleep again but this time for eternity. Mission controllers will direct the probe to impact the comet’s dusty-icy nucleus within 20 minutes of 10:40 Greenwich Time (6:40 a.m. EDT) that Friday morning. The high-resolution OSIRIS camera will be snapping pictures on the way down, but once impact occurs, it’s game over, lights out. Rosetta will power down and go silent.
A simplified overview of Rosetta’s last week of maneuvers at Comet 67P/Churyumov–Gerasimenko. Starting today (Sept. 24) the spacecraft will leave the flyover orbits and transfer towards a 16 x 23 km orbit that will be used to prepare for the final descent. The collision course maneuver will take place in the evening Sept. 29 with impact expected to occur at 10:40 GMT (6:40 a.m. EDT), which taking into account the 40 minute signal travel time between Rosetta and Earth on Sept. 30, means the confirmation would be expected at mission control at 11:20 GMT (7:20 a.m. EDT). Copyright: ESA
Nearly three years have passed since Rosetta opened its eyes on 67P, this curious, bi-lobed rubber duck of a comet just 2.5 miles (4 km) across with landscapes ranging from dust dunes to craggy peaks to enigmatic ‘goosebumps’. The mission was the first to orbit a comet and dispatch a probe, Philae, to its surface. I think it’s safe to say we learned more about what makes comets tick during Rosetta’s sojourn than in any previous mission.
So why end it? One of the big reasons is power. As Rosetta races farther and farther from the Sun, less sunlight falls on its pair of 16-meter-long solar arrays. At mid-month, the probe was over 348 million miles (560 million km) from the Sun and 433 million miles (697 million km) from Earth or nearly as far as Jupiter. With Sun-to-Rosetta mileage increasing nearly 620,000 miles (1 million km) a day, weakening sunlight can’t provide the power needed to keep the instruments running.
Rosetta’s last orbits around the comet
Rosetta’s also showing signs of age after having been in the harsh environment of interplanetary space for more than 12 years, two of them next door to a dust-spitting comet. Both factors contributed to the decision to end the mission rather than put the probe back into an even longer hibernation until the comet’s next perihelion many years away.
Since August 9, Rosetta has been swinging past the comet in a series of ever-tightening loops, providing excellent opportunities for close-up science observations. On September 5, Rosetta swooped within 1.2 miles (1.9 km) of 67P/C-G’s surface. It was hoped the spacecraft would descend as low as a kilometer during one of the later orbits as scientists worked to glean as much as possible before the show ends.
Rosetta is targeted to land at the site within this planned impact ellipse in the Ma’at region on the comet’s smaller lobe. See below for a closer view. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
The final of 15 close flyovers will be completed today (Sept. 24) after which Rosetta will be maneuvered from its current elliptical orbit onto a trajectory that will eventually take it down to the comet’s surface on Sept. 30.
The beginning of the end unfolds on the evening of the 29th when Rosetta spends 14 hours free-falling slowly towards the comet from an altitude of 12.4 miles (20 km) — about 4 miles higher than a typical commercial jet — all the while collecting measurements and photos that will be returned to Earth before impact. The last eye-popping images will be taken from a distance of just tens to a hundred meters away.
The landing will be a soft one, with the spacecraft touching down at walking speed. Like Philae before it, it will probably bounce around before settling into place. Mission control expects parts of the probe to break upon impact.
Taking into account the additional 40 minute signal travel time between Rosetta and Earth on the 30th, confirmation of impact is expected at ESA’s mission control in Darmstadt, Germany, within 20 minutes of 11:20 GMT (7:20 a.m. EDT). The times will be updated as the trajectory is refined. You can watch live coverage of Rosetta’s final hours on ESA TV.
ESAHangout: Preparing for Rosetta’s grand finale
“It’s hard to believe that Rosetta’s incredible 12.5 year odyssey is almost over, and we’re planning the final set of science operations, but we are certainly looking forward to focusing on analyzing the reams of data for many decades to come,” said Matt Taylor, ESA’s Rosetta project scientist.
The spacecraft landing site is shown in red and located next to Deir el-Medina, a large pit (arrowed). Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Plans call for the spacecraft to impact the comet somewhere within an ellipse about 1,300 x 2,000 feet (600 x 400 meters) long on 67P’s smaller lobe in the region known as Ma’at. It’s home to several active pits more than 328 feet (100 meters) in diameter and 160-200 feet (50-60 meters) deep, where a number of the comet’s dust jets originate. The walls of the pits are lined with fascinating meter-sized lumpy structures called ‘goosebumps’, which scientists believe could be early ‘cometesimals’, the icy snowballs that stuck together to create the comet in the early days of our Solar System’s formation.
Close-up of a curious surface texture nicknamed ‘goosebumps’. The bumps are about 9 feet (3 meters) across and seen on very steep slopes and exposed cliff faces. They may represent the original balls of icy dust that glommed together to form comets 4.5 billion years ago. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
During free-fall, the spacecraft will target a point adjacent to a 425-foot (130 m) wide, well-defined pit that the mission team has informally named Deir el-Medina, after a structure with a similar appearance in an ancient Egyptian town of the same name. High resolution images should give us a spectacular view of these enigmatic bumps.
While we hate to see Rosetta’s mission end, it’s been a blast going for a 2-year-plus comet ride-along.
NASA has unveiled a new exercise device that will be used by Orion crews to stay healthy on their mission to Mars. Credit: NASA
On Sept. 15th, the Senate Committee on Commerce, Science, and Transportation met to consider legislation formally introduced by a bipartisan group of senators. Among the bills presented was the NASA Transition Authorization Act of 2016, a measure designed to ensure short-term stability for the agency in the coming year.
And as of Thursday, Sept. 22nd, the Senate Commerce Committee approved the bill, providing $19.5 billion in funding for NASA for fiscal year 2017. This funding was intended for the purpose of advancing the agency’s plans for deep space exploration, the Journey to Mars, and operations aboard the International Space Station.
According to Senator Ted Cruz, the bill’s lead sponsor, the Act was introduced in order to ensure that NASA’s major programs would be stable during the upcoming presidential transition. As Cruz was quoted as saying by SpaceNews:
“The last NASA reauthorization act to pass Congress was in 2010. And we have seen in the past the importance of stability and predictability in NASA and space exploration: that whenever one has a change in administration, we have seen the chaos that can be caused by the cancellation of major programs.”
Graphic shows Block I configuration of NASA’s Space Launch System (SLS). Credits: NASA/MSFC
This last act was known as the “NASA Authorization Act of 2010“, which authorized appropriations for NASA between the years of 2011-2013. In addition to providing a total of $58 billion in funding for those three years, it also defined long-term goals for the space agency, which included expanding human space flight beyond low-Earth orbit and developing technical systems for the “Journey to Mars”.
Intrinsic to this was the creation of the Space Launch System (SLS) as a successor to the Space Shuttle Program, the development of the Orion Multipurpose Crew Vehicle, full utilization of the International Space Station, leveraging international partnerships, and encouraging public participation by investing in education.
“In order to maximize the cost-effectiveness of the long-term exploration and utilization activities of the United States, the Administrator shall take all necessary steps, including engaging international, academic, and industry partners to ensure that activities in the Administration’s human exploration program balance how those activities might also help meet the requirements of future exploration and utilization activities leading to human habitation on the surface of Mars.”
NASA has unveiled a new exercise device that will be used by Orion crews to stay healthy on their mission to Mars. Credit: NASA
While the passage of the bill is certainly good news for NASA’s bugeteers, it contains some provisions which could pose problems. For example, while the bill does provide for continued development of the SLS and Orion capsule, it advised that NASA find alternatives for its Asteroid Robotic Redirect Missions (ARRM), which is currently planned for the 2020s.
This mission, which NASA deemed essential for testing key systems and developing expertise for their eventual crewed mission to Mars, was cited for not falling within original budget constraints. Section 435 (“Asteroid Robotic Redirect Mission“), details these concerns, stating that an initial estimate put the cost of the mission at $1.25 billion, excluding launch and operations.
However, according to a Key Decision Point-B review conducted by NASA on July 15th, 2016, a new estimate put the cost at $1.4 billion (excluding launch and operations). As a result, the bill’s sponsors concluded that ARM is in competition with other programs, and that an independent cost assessment and some hard choices may be necessary.
In Section 435, subsection b (parts 1 and 2), its states that:
“[T]he technological and scientific goals of the Asteroid Robotic Redirect Mission may not be commensurate with the cost; and alternative missions may provide a more cost effective and scientifically beneficial means to demonstrate the technologies needed for a human mission to Mars that would otherwise be demonstrated by the Asteroid Robotic Redirect Mission.”
Artist’s impression of NASA’s ARM, which could be threatened by the agency’s new budget. Credit: NASA
The bill was also subject to amendments, which included the approval of funding for the development of satellite servicing technology. Under this arrangement, NASA would have the necessary funds to create spacecraft capable of repairing and providing maintenance to orbiting satellites, thus ensuring long-term functionality.
Also, Cruz and Bill Nelson (D-Fla), the committee ranking member, also supported an amendment that would indemnify companies or third parties executing NASA contracts. In short, companies like SpaceX or Blue Origin would now be entitled to compensation (above a level they are required to insure against) in the event of damages or injuries incurred as a result of launch and reentry services being provided.
According to a Commerce Committee press release, Sen. Bill Nelson had this to say about the bill’s passage:
“I want to thank Chairman Thune and the members of the committee for their continued support of our nation’s space program. Last week marked the 55th anniversary of President Kennedy’s challenge to send a man to the Moon by the end of the decade. The NASA bill we passed today keeps us moving toward a new and even more ambitious goal – sending humans to Mars.”
With the approval of the Commerce Committee, the bill will now be sent to the Senate for approval. It is hoped that the bill will pass through the Senate quickly so it can be passed by the House before the year is over. Its supporters see this as crucial to maintaining NASA’s funding in the coming years, during which time they will be taking several crucial steps towards the proposed crewed mission to Mars.
Artist's concept of Trojan asteroids, small bodies that dominate our solar system. Credit: NASA
The Solar System is filled with what are known as Trojan Asteroids – objects that share the orbit of a planet or larger moon. Whereas the best-known Trojans orbit with Jupiter (over 6000), there are also well-known Trojans orbiting within Saturn’s systems of moons, around Earth, Mars, Uranus, and even Neptune.
Until recently, Neptune was thought to have 12 Trojans. But thanks to a new study by an international team of astronomers – led by Hsing-Wen Lin of the National Central University in Taiwan – five new Neptune Trojans (NTs) have been identified. In addition, the new discoveries raise some interesting questions about where Neptune’s Trojans may come from.
The PS1 telescope at dawn, with the mountain of Mauna Kea visible in the distance. Credit: pan-starrs.ifa.hawaii.edu
The team used data obtained by the PS-1 survey, which ran from 2010 to 2014 and utilized the first Pan-STARR telescope on Mount Haleakala, Hawaii. From this, they observed seven Trojan asteroids around Neptune, five of which were previously undiscovered. Four of the TNs were observed orbiting within Neptune’s L4 point, and one within its L5 point.
The newly detected objects have sizes ranging from 100 to 200 kilometers in diameter, and in the case of the L4 Trojans, the team concluded from the stability of their orbits that they were likely primordial in origin. Meanwhile, the lone L5 Trojan was more unstable than the other four, which led them to hypothesize that it was a recent addition.
As Professor Lin explained to Universe Today via email:
“The 2 of the 4 currently known L5 Neptune Trojans, included the one L5 we found in this work, are dynamically unstable and should be temporary captured into Trojan cloud. On the other hand, the known L4 Neptune Trojans are all stable. Does that mean the L5 has higher faction of temporary captured Trojans? It could be, but we need more evidence.”
In addition, the results of their simulation survey showed that the newly-discovered NT’s had unexpected orbital inclinations. In previous surveys, NTs typically had high inclinations of over 20 degrees. However, in the PS1 survey, only one of the newly discovered NTs did, whereas the others had average inclinations of about 10 degrees.
Animation showing the path of six of Neptune’s L4 trojans in a rotating frame with a period equal to Neptune’s orbital period.. Credit: Tony Dunn/Wikipedia Commons
From this, said Lin, they derived two possible explanations:
“The L4 “Trojan Cloud” is wide in orbital inclination space. If it is not as wide as we thought before, the two observational results are statistically possible to generate from the same intrinsic inclination distribution. The previous study suggested >11 degrees width of inclination, and most likely is ~20 degrees. Our study suggested that it should be 7 to 27 degrees, and the most likely is ~ 10 degrees.”
“[Or], the previous surveys were used larger aperture telescopes and detected fainter NT than we found in PS1. If the fainter (smaller) NTs have wider inclination distribution than the larger ones, which means the smaller NTs are dynamically “hotter” than the larger NTs, the disagreement can be explained.”
According to Lin, this difference is significant because the inclination distribution of NTs is related to their formation mechanism and environment. Those that have low orbital inclinations could have formed at Neptune’s Lagrange Points and eventually grew large enough to become Trojans asteroids.
Illustration of the Sun-Earth Lagrange Points. Credit: NASA
On the other hand, wide inclinations would serve as an indication that the Trojans were captured into the Lagrange Points, most likely during Neptune’s planetary migration when it was still young. And as for those that have wide inclinations, the degree to which they are inclined could indicate how and where they would have been captured.
“If the width is ~ 10 degrees,” he said, “the Trojans can be captured from a thin (dynamically cold) planetesimal disk. On the other hand, if the Trojan cloud is very wide (~ 20 degrees), they have to be captured from a thick (dynamically hot) disk. Therefore, the inclination distribution give us an idea of how early Solar system looks like.”
In the meantime, Li and his research team hope to use the Pan-STARR facility to observe more NTs and hundreds of other Centaurs, Trans-Neptunian Objects (TNOs) and other distant Solar System objects. In time, they hope that further analysis of other Trojans will shed light on whether there truly are two families of Neptune Trojans.
This was all made possible thanks to the PS1 survey. Unlike most of the deep surveys, which are only ale to observe small areas of the sky, the PS1 is able to monitor the whole visible sky in the Northern Hemisphere, and with considerable depth. Because of this, it is expected to help astronomers spot objects that could teach us a great deal about the history of the early Solar System.
The Moon crosses the Hyades on July 29th, 2016. Image credit and copyright: Alan Dyer
This week, we thought we’d try an experiment for tonight’s occultation of Aldebaran by the Moon. As mentioned, we’re expanding the yearly guide for astronomical events for the year in 2017. We’ve done this guide in various iterations since 2009, starting on Astroguyz and then over to Universe Today, and it has grown from a simple Top 10 list, to a full scale preview of what’s on tap for the following year.
You, the reader, have made this guide grow over the years, as we incorporate feedback we’ve received.
Anyhow, we thought we’d lay out this week’s main astro-event in a fashion similar to what we have planned for the guide: each of the top 101 events will have a one page entry (two pages for the top 10 events) with a related graphic, fun facts, etc.
So in guide format, tonight’s occultation of Aldebaran would break down like this:
Wednesday, September 21st: The Moon Occults Aldebaran
The occultation footprint of tonight’s Aldebaran event.
Image credit Occult 4.2
The 67% illuminated waning gibbous Moon occults the +0.9 magnitude star Aldebaran. The Moon is two days prior to Last Quarter phase during the event. Both are located 109 degrees west of the Sun at the time of the event. The central time of conjunction is 22:37 Universal Time (UT). The event occurs during the daylight hours over southeast Asia, China, Japan and the northern Philippines and under darkness for India, Pakistan and the Arabian peninsula and the Horn of Africa. The Moon will next occult Aldebaran on October 19th. This is occultation 23 in the current series of 49 running from January 29th 2015 to September 3rd, 2018. This is one of the more central occultations of Aldebaran by the Moon for 2016.
The view from India tonight, just before the occultation begins. Image credit: Stellarium
Fun Fact-In the current century, (2001-2100 AD) the Moon occults Aldebaran 247 times, topped only by Antares (386 times) and barely beating out Spica (220 times).
Or maybe, another fun fact could be: A frequent setting for science fiction sagas, Aldebaran is now also often confused in popular culture with Alderaan, Princess Leia’s late homeworld from the Star Wars saga.
Like it? Thoughts, suggestions, complaints?
Now for the Wow! Factor for tonight’s occultation. Aldebaran is 65 light years distant, meaning the light we’re seeing left the star in 1951 before getting photobombed by the Moon just over one second before reaching the Earth.
There are also lots of other occultations of fainter stars worldwide over the next 24 hours, as the Moon crosses the Hyades.
And follow that Moon, as a series of 20 occultations of the bright star Regulus during every lunation begins later this year on December 18th.
Gadi Eidelheit managed to catch the March 14th, 2016 daytime occultation of Aldebaran from Israel:
And also in the ‘Moon passing in front of things’ department, here’s a noble attempt at capturing a difficult occultation of Neptune by the Moon last week on September 15th, courtesy of Veijo Timonen based in Hämeenlinna Finland:
Lets see, that’s a +8th magnitude planet next to a brilliant -13th magnitude Moon, one million (15 magnitudes) times brighter… it’s amazing you can see Neptune at all!
Last item: tomorrow marks the September (southward) equinox, ushering in the start of astronomical fall in the northern hemisphere, and the beginning of Spring in the southern. The precise minute of equinoctial crossing is 14:21 UT. In the 21st century, the September equinox can fall anywhere from September 21st to September 23rd. Bob King has a great recent write-up on the equinox and the Moon.
Here’s EVERY occultation of Aldebaran from 2015 through 2018. (Click to enlarge) Credit: Occult 4.2.
Don’t miss tonight’s passage of Aldebaran through the Hyades, and there’s lots more where that came from headed into 2017!
Eta Carinae, one of the most massive stars known. Credit: NASA
The Crab Nebula; at its core is a long dead star… Image credit: NASA, ESA, J. Hester and A. Loll (Arizona State University)
Our sky is blanketed in a sea of stellar ghosts; all potential phantoms that have been dead for millions of years and yet we don’t know it yet. That is what we will be discussing today. What happens to the largest of our stars, and how that influences the very makeup of the universe we reside in.
We begin this journey by observing the Crab Nebula. Its beautiful colors extend outward into the dark void; a celestial tomb containing a violent event that occurred a millennia ago. You reach out and with the flick of your wrist, begin rewinding time and watch this beautiful nebulae begin to shrink. As the clock winds backwards, the colors of the nebula begin to change, and you notice that they are shrinking to a single point. As the calendar approaches July 5, 1054, the gaseous cloud brightens and settles onto a single point in the sky that is as bright as the full moon and is visible during the day. The brightness fades and eventually there lay a pinpoint of light; a star that we don’t see today. This star has died, however at this moment in time we wouldn’t have known that. To an observer before this date, this star appeared eternal, as all the other stars did. Yet, as we know from our privileged vantage point, this star is about to go supernova and birth one of the most spectacular nebulae that we observe today.
Stellar ghosts is an apt way of describing many of the massive stars we see scattered throughout the universe. What many don’t realize is that when we look out deep into the universe, we are not only looking across vast distances, but we are peering back into time. One of the fundamental properties of the universe that we know quite well is that light travels at a finite speed: approximately 300,000,000 m/s (roughly 671,000,000 mph). This speed has been determined through many rigorous tests and physical proofs. In fact, understanding this fundamental constant is a key to much of what we know about the universe, especially in respect to both General Relativity and Quantum Mechanics. Despite this, knowing the speed of light is key to understanding what I mean by stellar ghosts. You see, information moves at the speed of light. We use the light from the stars to observe them and from this understand how they operate.
A decent example of this time lag is our own sun. Our sun is roughly 8 light-minutes away. Meaning that the light we see from our star takes 8 minutes to make the journey from its surface to our eyes on earth. If our sun were to suddenly disappear right now, we wouldn’t know about it for 8 minutes; this doesn’t just include the light we see, but even its gravitational influence that is exerted on us. So if the sun vanished right now, we would continue in our orbital path about our now nonexistent star for 8 more minutes before the gravitational information reached us informing us that we are no longer gravitationally bound to it. This establishes our cosmic speed limit for how fast we can receive information, which means that everything we observe deep into the universe comes to us as it was an ‘x’ amount of years ago, where ‘x’ is its light distance from us. This means we observe a star that is 10 lightyears away from us as it was 10 years ago. If that star died right now, we wouldn’t know about it for another 10 years. Thus, we can define it as a “stellar ghost”; a star that is dead from its perspective at its location, but still alive and well at ours.
As covered in a previous article of mine (Stars: A Day in the Life), the evolution of a star is complex and highly dynamic. Many factors play an important role in everything from determining if the star will even form in the first place, to the size and thus the lifetime of said star. In the previous article mentioned above, I cover the basics of stellar formation and the life of what we call main sequence stars, or rather stars that are very similar to our own sun. Whereas the formation process and life of a main sequence star and the stars we will be discussing are fairly similar, there are important differences in the way the stars we will be investigating die. Main sequence star deaths are interesting, but they hardly compare to the spacetime-bending ways that these larger stars terminate.
As mentioned above, when we were observing the long gone star that lay at the center of the Crab Nebula, there was a point in which this object glowed as bright as the full moon and could be seen during the day. What could cause something to become so bright that it would be comparable to our nearest celestial neighbor? Considering the Crab Nebula is 6,523 lightyears away, that meant that something that is roughly 153 billion times farther away than our moon was shining as bright as the moon. This was because the star went supernova when it died, which is the fate of stars that are much larger than our sun. Stars larger than our sun will end up in two very extreme states upon its death: neutron stars and black holes. Both are worthy topics that could span weeks in an astrophysics course, but for us today, we will simply go over how these gravitational monsters form and what that means for us.
Inward force of gravity versus the outward pressure of fusion within a star (hydrostatic equilibrium) Credit: NASA
A star’s life is a story of near runaway fusion contained by the grip of its own gravitational presence. We call this hydrostatic equilibrium, in which the outward pressure from the fusing elements in the core of a star equals that of the inward gravitational pressure being applied due to the star’s mass. In the core of all stars, hydrogen is being fused into helium (at first). This hydrogen came from the nebula that the star was born from, that coalesced and collapsed, giving the star its first chance at life. Throughout the lifetime of the star, the hydrogen will be used up, and more and more helium “ash” will condense down in the center of the star. Eventually, the star will run out of hydrogen, and the fusion will briefly stop. This lack of outward pressure due to no fusion taking place temporarily allows gravity to win and it crushes the star downwards. As the star shrinks, the density, and thus the temperature in the core of the star increases. Eventually, it reaches a certain temperature and the helium ash begins to fuse. This is how all stars proceed throughout the main portion of its life and into the first stages of its death. However, this is where sun-sized stars and the massive stars we are discussing part ways.
The core and subsequent layers of a dying star. Each layer has been left over from millions of years of fusing each subsequent element into the next one. This is a snapshot of a massive star about to erupt. Credit: Wikimedia
A star that is roughly near the size of our own sun will go through this process until it reaches carbon. Stars that are this size simply aren’t big enough to fuse carbon. Thus, when all the helium has been fused into oxygen and carbon (via two processes that are too complex to cover here), the star cannot “crush” the oxygen and carbon enough to start fusion, gravity wins and the star dies. But stars that have sufficiently more mass than our sun (about 7x the mass) can continue on past these elements and keep shining. They have enough mass to continue this “crush and fuse” process that is the dynamic interactions at the hearts of these celestial furnaces.
These larger stars will continue their fusion process past carbon and oxygen, past silicon, all the way until they reach iron. Iron is the death note sung by these blazing behemoths, as when iron begins to fill their now dying core, the star is in its death throws. But these massive structures of energy do not go quietly into the night. They go out in the most spectacular of ways. When the last of the non-iron elements fuse in their cores, the star begins its decent into oblivion. The star comes crashing in upon itself as it has no way to stave off gravity’s relentless grip, crushing the subsequent layers of left over elements from its lifetime. This inward free-fall is met at a certain size with an impossible force to breach; a neutron degeneracy pressure that forces the star to rebound outwards. This massive amount of gravitational and kinetic energy races back out with a fury that illuminates the universe, outshining entire galaxies in an instant. This fury is the life-blood of the cosmos; the drum beats in the symphony galactic, as this intense energy allows for the fusion of elements heavier than iron, all the way to uranium. These new elements are blasted outwards by this amazing force, riding the waves of energy that casts them deep into the cosmos, seeding the universe with all the elements that we know of.
Artistic impression of a star going supernova, casting its chemically enriched contents into the universe. These new elements are blasted outwards by this amazing force, riding the waves of energy that casts them deep into the cosmos, seeding the universe with all the elements that we know of. Credit: NASA/Swift/Skyworks Digital/Dana Berryto
But what is left? What is there after this spectacular event? That all depends again on the mass of the star. As mentioned earlier, the two forms that a dead massive star takes are either a Neutron Star or a Black Hole. For a Neutron Star, the formation is quite complex. Essentially, the events that I described occurs, except after the supernovae all that is left is a ball of degenerate neutrons. Degenerate is simply a term we apply to a form that matter takes on when it is compressed to the limits allowed by physics. Something that is degenerate is intensely dense, and this holds very true for a neutron star. A number you may have heard tossed around is that a teaspoon of neutron star material would weigh roughly 10 million tons, and have an escape velocity (the speed needed to get away from its gravitational pull) at about .4c, or 40% the speed of light. Sometimes the neutron star is left spinning at incredible velocities, and we label these as pulsars; the name derived from how we detect them.
A pulsar with its magnetic field lines illustrated. The beams emitting from the poles are what washes over our detectors as the dead star spins.
These types of stars generate a LOT of radiation. Neutron stars have an enormous magnetic field. This field accelerates electrons in their stellar atmospheres to incredible velocities. These electrons follow the magnetic field lines of the neutron star to its poles, where they can release radio waves, X-Rays, and gamma rays (depending on what type of neutron star it is). Since this energy is being concentrated to the poles, it creates a sort of lighthouse effect with high energy beams acting like the beams of light out of a lighthouse. As the star rotates, these beams sweep around many times per second. If the Earth, and thus our observation equipment, happens to be oriented favorably with this pulsar, we will register these “pulses” of energy as the stars’ beams wash over us. For all the pulsars we know about, we are much too far away for these beams of energy to hurt us. But if we were close to one of these dead stars, this radiation washing over our planet continuously would spell certain extinction for life as we know it.
What of the other form that a dead star takes; a black hole? How does this occur? If degenerate material is as far as we can crush matter, how does a black hole appear? Simply put, black holes are the result of an unimaginably large star and thus a truly massive amount of matter that is able to “break” this neutron degeneracy pressure upon collapse. The star essentially falls inward with such force that it breaches this seemingly physical limit, turning in upon itself and wrapping up spacetime into a point of infinite density; a singularity. This amazing event occurs when a star has roughly 18x the amount of mass that our sun has, and when it dies, it is truly the epitome of physics gone to the extreme. This “extra bit of mass” is what allows it to collapse this ball of degenerate neutrons and fall towards infinity. It is both terrifying and beautiful to think about; a point in spacetime that is not entirely understood by our physics, and yet something that we know exists. The truly remarkable thing about black holes is that it is like the universe working against us. The information we need to fully understand the processes within a black hole are locked behind a veil that we call the event horizon. This is the point of no return for a black hole, for which anything beyond this point in spacetime has no future paths that lead out of it. Nothing escapes at this distance from the collapsed star at its core, not even light, and thus no information ever leaves this boundary (at least not in a form we can use). The dark heart of this truly astounding object leaves a lot to be desired, and tempts us to cross into its realm in order to try and know the unknowable; to grasp the fruit from the tree of knowledge.
A black hole is the final form a massive star collapses to. The light (and spacetime itself) is warped around the black hole’s event horizon due to extreme gravitational effects. This is as accurate as we can be to visualizing an actual black hole as it was generated with a code that implemented General Relativity accurately. Credit and Copyright: Paramount Pictures/Warner Bros. From “Interstellar” the film. Mathematical model used to create the image developed by Dr. Kip Thorne
Now it must be said, there is much in the way of research with black holes to this day. Physicists such as Professor Stephen Hawking, among others, have been working tirelessly on the theoretical physics behind how a black hole operates, attempting to solve the paradoxes that frequently appear when we try to utilize the best of our physics against them. There are many articles and papers on such research and their subsequent findings, so I will not dive into their intricacies for both wishing to preserve simplicity in understanding, and to also not take away from the amazing minds that are working these issues. Many suggest that the singularity is a mathematical curiosity that does not completely represent what physically happens. That the matter inside an event horizon can take on new and exotic forms. It is also worth noting that in General Relativity, anything with mass can collapse to a black hole, but we generally hold to a range of masses as creating a black hole with anything less than is in that mass range is beyond our understanding of how that could happen. But as someone who studies physics, I would be remiss to not mention that as of now, we are at an interesting cross section of ideas that deal very intimately with what is actually going on within these specters of gravity.
All of this brings me back to a point that needs to be made. A fact that needs to be recognized. As I described the deaths of these massive stars, I touched on something that occurs. As the star is being ripped apart from its own energy and its contents being blown outwards into the universe, something called nucleosynthesis is occurring. This is the fusion of elements to create new elements. From hydrogen up to uranium. These new elements are being blasted outwards an incredible speeds, and thus all of these elements will eventually find their way into molecular clouds. Molecular clouds (Dark Nebulae) are the stellar nurseries of the cosmos. This is where stars begin. And from star formation, we get planetary formation.
Planets coalescing out of the remaining molecular cloud the star formed out of. Within this accretion disk lay the fundamental elements necessary for planet formation and potential life. Credit: NASA/JPL-Caltech/T. Pyle (SSC) – February, 2005
As a star forms, a cloud of debris that is made up of the molecular cloud that birthed said star begins to spin around it. This cloud, as we now know, contains all those elements that were cooked up in our supernovae. The carbon, the oxygen, the silicates, the silver, the gold; all present in this cloud. This accretion disk about this new star is where planets form, coalescing out of this enriched environment. Balls of rock and ice colliding, accreting, being torn apart and then reformed as gravity works its diligent hands to mold these new worlds into islands of possibility. These planets are formed from those very same elements that were synthesized in that cataclysmic eruption. These new worlds contain the blueprints for life as we know it.
Upon one of these worlds, a certain mixture of hydrogen and oxygen occurs. Within this mixture, certain carbon atoms form up to create replicating chains that follow a simple pattern. Perhaps after billions of years, these same elements that were thrust into the universe by that dying star finds itself giving life to something that can look up and appreciate the majesty that is the cosmos. Perhaps that something has the intelligence to realize that the carbon atom within it is the very same carbon atom that was created in a dying star, and that a supernovae occurred that allowed that carbon atom to find its way into the right part of the universe at the right time. The energy that was the last dying breath of a long dead star was the same energy that allowed life to take its first breath and gaze upon the stars. These stellar ghosts are our ancestors. They are gone in form, but yet remain within our chemical memory. They exist within us. We are supernova. We are star dust. We are descended from stellar ghosts…
We are awash in the light from long dead stars, each contributing essential ingredients to the universe that are necessary for life. Image Credit: Hubble
The Soyuz MS-01 spacecraft preparing to launch from the Baikonur Cosmodrome, in Kazakhstan, on Monday, July 4th, 2016. Credit: (NASA/Bill Ingalls)
On Saturday, September 17th, the Russian space agency (Roscosmos) stated that it would be delaying the launch of the crewed spacecraft Soyuz MS-02. The rocket was scheduled to launch on Friday, September 23rd, and would be carrying a crew of three astronauts – two Russia and one American – to the ISS.
After testing revealed technical flaws in the mission (which were apparently due to a short circuit), Rocosmos decided to postpone the launch indefinitely. But after after days of looking over the glitch, the Russians space agency has announced that it is prepared for a renewed launch on Nov. 1st.
The mission crew consists of mission commander Sergey Ryzhikov, flight engineer Andrey Borisenko and NASA astronaut Shane Kimbrough. Originally scheduled to launch on Sept. 23rd, the mission would spend the next two days conducting a rendezvous operation before docking with the International Space Station on Sept. 25th.
The crew of MS-02 (from left to right): Shane Kimgrough, Sergey Ryzhikov and Andrey Borisenko, pictured in Red Square in Moscow. Credit: NASA/Bill Ingalls
The station is currently being staffed by three crew members – MS-01 commander Anatoly Ivanishin, NASA astronaut Kate Rubins and Japanese astronaut Takuya Onish. These astronauts arrived on the station on Sept.6th, and all three were originally scheduled to return to Earth on October 30th.
Meanwhile, three more astronauts – commander Oleg Novitskiy, ESA flight engineer Thomas Pesquet and NASA astronaut Peggy Whitson – were supposed to replace them as part of mission MS-03, which was scheduled to launch on Nov. 15th. But thanks to the technical issue that grounded the MS-02 flight, this schedule appeared to be in question.
However, the news quickly began to improve after it seemed that the mission might be delayed indefinitely. On Sept.18th, a day after the announcement of the delay, the Russian International News Agency (RIA Novosti) cited a source that indicated that the spacecraft could be replaced and the mission could be rescheduled for next month:
“RIA Novosti’s source noted that the mission was postponed indefinitely because of an identified short circuit during the pre-launch checks. It is possible that the faulty ship “MS – 02 Alliance” can be quickly replaced on the existing same rocket, and then the launch to the ISS will be held in late October.”
Three newly arrived crew of Expedition 48 in Soyuz MS-01 open the hatch and enter the International Space Station after docking on July 9, 2016. Credit: NASA TV
Then, on Monday, Sept.19th, another source cited by RIA Novosti said that the State Commission responsible for the approval of a new launch date would be reaching a decision no sooner than Tuesday, Sept. 20th. And as of Tuesday morning, a new launch date appears to have been set.
According to news agency, Roscomos notified NASA this morning that the mission will launch on Nov.1st. Sputnik International confirmed this story, claiming that the source was none other than Alexander Koptev – a NASA representative with the Russian Mission Control Center.
“The Russian side has informed the NASA central office of the preliminary plans to launch the manned Soyuz MS-02 on November 1,” he said.
It still not clear where the technical malfunction took place. Since this past Saturday, Russian engineers have been trying to ascertain if the short circuit occurred in the descent module or the instrument module. However, the Russians are already prepared to substitute the Soyuz spacecraft for the next launch, so there will be plenty of time to locate the source of the problem.
The Soyuz MS-01 spacecraft launches from the Baikonur Cosmodrome on July 7th, 2016. Credit: NASA/Bill
The Soyuz MS is the latest in a long line of revisions to the venerable Soyuz spacecraft, which has been in service with the Russians since the 1960s. It is perhaps the last revision as well, as Roscosmos plans to develop new crewed spacecraft in the coming decades.
The MS is an evolution of the Soyuz TMA-M spacecraft, another modernized version of the old spacecraft. Compared to its predecessor, the MS model’s comes with updated communications and navigation subsystems, but also boasts some thruster replacements.
The first launch of the new spacecraft – Soyuz MS-01 – took place on July 7th, 2016, aboard a Soyuz-FG launch vehicle, which is itself an improvement on the traditional R-7 rockets. Like the MS-02 mission, MS-01 spent two days undergoing a checkout phase in space before rendezvousing with the ISS.
As such, it is understandable why the Russians would like to get this mission underway and ensure that the latest iteration of the Soyuz MS performs well in space. Until such time as the Russians have a new crewed module to deliver astronauts to the ISS, all foreseeable missions will come down to craft like this one.
Ahuna Mons towers over the Cerean landscape in this photo taken by the Dawn spacecraft. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI
Whoa – what a sight! Ceres’ lonely mountain, Ahuna Mons, is seen in this simulated perspective view. The elevation has been exaggerated by a factor of two. The view was made using enhanced-color images from NASA’s Dawn mission in August from an altitude of 240 miles (385 km) in August 2016. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI
An isolated 3-mile-high (5 km) mountain Ahuna Mons on Ceres is likely volcanic in origin, and the dwarf planet may have a weak, temporary atmosphere. These are just two of many new insights about Ceres from NASA’s Dawn mission published this week in six papers in the journal Science.
Ahuna Mons is seen in this mosaic of images from NASA’s Dawn spacecraft. On its steepest side, this mountain is about 3 miles (5 km) high. Its average overall height is 2.5 miles (4 km). The diameter of the mountain is about 12 miles (20 km). Dawn took these images from its low-altitude mapping orbit, 240 miles (385 kilometers) above the surface, in December 2015. Credits: NASA/JPL/Dawn mission
“Dawn has revealed that Ceres is a diverse world that clearly had geological activity in its recent past,” said Chris Russell, principal investigator of the Dawn mission, based at the University of California, Los Angeles.
The Ahuna Mons dome compared to a dome in Russia. The similarity in appearance is striking though the difference in size is large. Credit: NASA
Ahuna Mons is a volcanic dome similar to earthly and lunar volcanic domes but unique in the solar system, according to a new analysis led by Ottaviano Ruesch of NASA’s Goddard Space Flight Center and the Universities Space Research Association. While those on Earth erupt with molten rock, Ceres’ grandest peak likely formed as a salty-mud volcano. Instead of molten rock, salty-mud volcanoes, or “cryovolcanoes,” release frigid, salty water sometimes mixed with mud.
Learn more about Ahuna Mons
“This is the only known example of a cryovolcano that potentially formed from a salty mud mix, and that formed in the geologically recent past,” Ruesch said. Estimates place the mountain formation within the past billion years.
Dawn may also have detected a weak, temporary atmosphere; the probe’s gamma ray and neutron (GRaND) detector observed evidence that Ceres had accelerated electrons from the solar wind to very high energies over a period of about six days. In theory, the interaction between the solar wind’s energetic particles and atmospheric molecules could explain the GRaND observations.
Dwarf planet Ceres is located in the asteroid belt, between the orbits of Mars and Jupiter. Observations by ESA’s Herschel Space Observatory between 2011 and 2013 found that the dwarf planet has a thin water-vapor atmosphere, the first detection ever of water vapor around an asteroid in the asteroid belt. Copyright ESA/ATG medialab/Küppers et al.
A temporary atmosphere would confirm the water vapor the Herschel Space Observatory detected at Ceres in 2012-2013. The electrons that GRaND detected could have been produced by the solar wind hitting the water molecules that Herschel observed, but scientists are also looking into alternative explanations.
While Ahuna Mons may have erupted liquid water in the not-too-distant past, Dawn found probable water ice right now in the mid-latitude Oxo Crater using its visible and infrared mapping spectrometer (VIR).
The small, bright crater Oxo (6 miles / 10 km wide) on Ceres is seen in this perspective view. The elevation has been exaggerated by a factor of two. The view was made using enhanced-color images from NASA’s Dawn mission. Dawn’s visible and infrared mapping spectrometer (VIR) has found evidence of water ice at this crater. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Exposed water-ice is rare on the dwarf planet, but the low density of Ceres — 2.08 grams/cm3 vs. 5.5 for Earth — the impact-generated ice detection and the the existence of Ahuna Mons suggest that Ceres’ crust does contain a significant amount of water ice.
Impact craters are clearly the most abundant geological feature on Ceres, and their different shapes help tell the complex story of Ceres’ past. Craters that are roughly polygonal — shapes bounded by straight lines — hint that Ceres’ crust is heavily fractured. In addition, several Cerean craters display fractures on their floors. There are craters with flow-like features. Bright areas are peppered across Ceres, with the most reflective ones in Occator Crater. Some crater shapes could indicate water-ice in the subsurface.
In this illustration, a mud slurry rises up through Ceres’ crust to build a dome like Ahuna Mons. Click to see the animation. Credit: Goddard Media Studios
All these crater forms imply an outer shell for Ceres that is not purely ice or rock, but rather a mixture of both. Scientists also calculated the ratio of various craters’ depths to diameters, and found that some amount of crater relaxation must have occurred as icy walls gradually slump.
“The uneven distribution of craters indicates that the crust is not uniform, and that Ceres has gone through a complex geological evolution,” Hiesinger said.
The rim of Hamori Crater on Ceres is seen in the upper left portion of this image, which was taken by NASA’s Dawn spacecraft. Clay is found at many locations on the dwarf planet. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Ceres’ crust also appears loaded with clay-forming minerals called phyllosilicates. These phyllosilicates are rich in magnesium and also have some ammonium embedded in their crystalline structure. Their distribution throughout the dwarf planet’s crust indicates Ceres’ surface material has been altered by a global process involving water.
Now in its extended mission, the Dawn spacecraft has been increasing its altitude since Sept. 2 as scientists stand back once again for a broader look at Ceres under different lighting conditions now compared to earlier in the mission.
The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
As you may recall learning in geology class, the Earth is made up of distinct layers. The further one goes towards the center of the planet, the more intense the heat and pressure becomes. Luckily, for those of us living on the crust (the outermost layer, where all life lives) the temperature is relatively steady and pleasant.
In fact, one of the things that makes planet Earth habitable is the fact that the planet is close enough to our Sun to receive enough energy to stay warm. What’s more, its “surface temperatures” are warm enough to sustain liquid water, the key to life as we know it. But the temperature of Earth’s crust also varies considerably depending on where and when you are measuring it.
Earth’s Structure:
As a terrestrial planet, Earth is composed of silicate rocks and metals which are differentiated between a solid metal core, a molten outer core, and a silicate mantle and crust. The inner core has an estimated radius of 1,220 km, while the outer core extends beyond it to a radius of about 3,400 km.
The layers of the Earth, a differentiated planetary body. Credit: Wikipedia Commons/Surachit
Extending outwards from the core are the mantle and the crust. Earth’s mantle extends to a depth of 2,890 km beneath the surface, making it the thickest layer of Earth. This layer is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales.
The upper layer of the mantle is divided into the lithospheric mantle (aka. the lithosphere) and the asthenosphere. The former consists of the crust and the cold, rigid, top part of the upper mantle (which the tectonic plates are composed of) while the asthenosphere is the relatively low-viscosity layer on which the lithosphere rides.
Earth’s Crust:
The crust is the absolute outermost layer of the Earth, which constitutes just 1% of the Earth’s total mass. The thickness of the crust varies depending on where the measurements are taken, ranging from 30 km thick where there are continents to just 5 km thick beneath the oceans.
The crust is composed of a variety of igneous, metamorphic and sedimentary rocks and is arranged in a series of tectonic plates. These plates float above the Earth’s mantle, and it’s believed that convection in the mantle causes the plates to be in constant motion.
Sometimes these plates collide, pull apart, or slide alongside each other; resulting in convergent boundaries, divergent boundaries, and transform boundaries. In the case of convergent boundaries, subduction zones are often the result, where the heavier plate slips under the lighter plate – forming a deep trench.
In the case of divergent boundaries, these are formed when tectonic plates pull apart, forming rift valleys on the seafloor. When this happens, magma wells up in the rift as the old crust pulls itself in opposite directions, where it is cooled by seawater to form new crust.
A transform boundary is formed when tectonic plates slide horizontally and parts get stuck at points of contact. Stress builds in these areas as the rest of the plates continue to move, which causes the rock to break or slip, suddenly lurching the plates forward and causing earthquakes. These areas of breakage or slippage are called faults.
Illustration of the Earth’s Tectonic Plates and the plate boundaries. Credit: msnucleus.org
Taken together, these three types of tectonic plate action are what is responsible for shaping the Earth’s crust and leading to periodic renewal of its surface over the course of millions of years.
Temperature Range:
The temperature of the Earth’s crust ranges considerably. At its outer edge, where it meets the atmosphere, the crust’s temperature is the same temperature as that of the air. So, it might be as hot as 35 °C in the desert and below freezing in Antarctica. On average, the surface of the Earth’s crust experiences temperatures of about 14°C.
However, the hottest temperature ever recorded was 70.7°C (159°F), which was taken in the Lut Desert of Iran as part of a global temperature survey conducted by scientists at NASA’s Earth Observatory. Meanwhile, the coldest temperature ever recorded on Earth was measured at the Soviet Vostok Station on the Antarctic Plateau – which reached an historic low of -89.2°C (-129°F) on July 21st, 1983.
That’s quite the range already. But consider the fact that the majority of the Earth’s crust lies beneath the oceans. Far from the Sun, temperatures can reach as low as 0-3° C (32-37.5° F) where the water reaches the crust. Still, a lot balmier than a cold night in Antarctica!
And as geologists have known for some time, if you dig down into the continental crust, temperatures will go up. For example, the deepest mine in the world is currently the TauTona gold mine in South Africa, measuring 3.9 km deep. At the bottom of the mine, temperatures reach a sweltering 55 °C, which requires that air conditioning be provided so that it’s comfortable for the miners to work all day.
So in the end, the temperature of Earth’s crust varies considerably. It’s average surface temperature which depends on whether it is being taken on dry land or beneath the sea. And depending on the location, seasons, and time of day, it can range from sweltering to freezing cold!
And yet, Earth’s crust remains the only place in the Solar System where temperatures are stable enough that life can continue to thrive on it. Add to that our viable atmosphere and protective magnetosphere, and we really should consider ourselves to be the lucky ones!
Comet 332P breakup. Credit: NASA, ESA, and D. Jewitt (UCLA)
This Hubble Space Telescope image reveals the ancient Comet 332P/Ikeya-Murakami disintegrating as it approaches the sun. The comet debris consists of a cluster of building-size chunks near the center of the image. They form a trail larger than the width of the continental U.S. The fragments are drifting away from the comet at a leisurely pace of just a few miles an hour. The main nucleus of Comet 332P is the bright object at lower left. It measures 1,600 feet across, about the length of five football fields. Credit: NASA, ESA, and D. Jewitt (UCLA)
Breaking up isn’t hard to do if you’re a comet. They’re fragile creatures subject to splitting, cracking and vaporizing when heated by the Sun and yanked on by its powerful gravitational pull.
Recently, the Hubble Space Telescope captured one of the sharpest, most detailed observations of a comet breaking apart, which occurred 67 million miles from Earth. In a series of images taken over a three-day span in January 2016, Hubble revealed 25 building-size blocks made of a mixture of ice and dust that are drifting away from the main nucleus of the periodic comet 332P/Ikeya-Murakami at a leisurely pace, about the walking speed of an adult.
This animation shows the movement of individual comet fragments relative to the parent nucleus, the bright object at lower left, on January 26, 27 and 28 UT. The true motions are very slow, on the order of several miles an hour, and show a fan-like divergence from the parent. Look closely and you’ll see that some of the fragments change in brightness and even shape from day to day. Researcher David Jewitt thinks this is due to continuing outgassing, rotation and breakup of the fragments. Credit: NASA, ESA, and D. Jewitt (UCLA)
The observations suggest that the comet may be spinning so fast that material is ejected from its surface. The resulting debris is now scattered along a 3,000-mile-long trail, larger than the width of the continental U.S. Much the same happens with small asteroids, when sunlight absorbed unequally across an asteroid’s surface spins up its rotation rate, either causing it to fall apart or fling hunks of itself into space.
Being made of loosely bound frothy ice, comets may be even more volatile compared to the dense rocky composition of many asteroids. The research team suggests that sunlight heated up the comet, causing jets of gas and dust to erupt from its surface. We see this all the time in comets in dramatic images taken by the Rosetta spacecraft of Comet 67P/Churyumov-Gerasimenko. Because the nucleus is so small, these jets act like rocket engines, spinning up the comet’s rotation. The faster spin rate loosened chunks of material, which are drifting off into space.
Comet 168P/Hergenrother was photographed by the Gemini telescope on Nov. 2, 2012 and shows three fragments that broke away from the nucleus streaming from the coma down the tail. Credit: NASA/JPL-Caltech/Gemini
“We know that comets sometimes disintegrate, but we don’t know much about why or how they come apart,” explained lead researcher David Jewitt of the University of California at Los Angeles. “The trouble is that it happens quickly and without warning, and so we don’t have much chance to get useful data. With Hubble’s fantastic resolution, not only do we see really tiny, faint bits of the comet, but we can watch them change from day to day. And that has allowed us to make the best measurements ever obtained on such an object.”
In the animation you can see the comet splinters brighten and fade as icy patches on their surfaces rotate in and out of sunlight. Their shapes even change! Being made of ice and crumbly as a peanut butter cookie, they continue to break apart to spawn a host of smaller cometary bits. The icy relics comprise about 4% of the parent comet and range in size from roughly 65 feet wide to 200 feet wide (20-60 meters). They are moving away from each other at a few miles per hour.
The European Space Agency’s Rosetta probe photographed this big crack in the neck region of the double-lobed comer 67P. It’s several feet wide and about 700 feet long. Could it be an indicator that the comet will break into two in the future? Credit: ESA/Rosetta
Comet 332P was slightly beyond the orbit of Mars when Hubble spotted the breakup. The surviving bright nucleus completes a rotation every 2-4 hours, about four times as fast as Comet 67P/Churyumov-Gerasimenko (a.k.a. “Rosetta’s Comet”). Standing on its surface you’d see the sun rise and set in about an hour, akin to how frequently astronauts aboard the International Space Station see sunsets and sunrises orbiting at over 17,000 mph.
Don’t jump for joy though. Since the comet’s just 1,600 feet (488 meters) across, its gravitational powers are too meek to allow visitors the freedom of hopping about lest they find themselves hovering helplessly in space above the icy nucleus.
This illustration shows one possible explanation for the disintegration of asteroids and comets. The effects of sunlight can cause an asteroid to slowly increase its rotation rate until the loosely bound fragments drift apart due to centrifugal forces. In the case of comets, jets of vaporizing ice have a rocket-like effect that can spin up a nucleus to speeds fast enough for the comet to eject pieces of itself. Credit: NASA, ESA, D. Jewitt (UCLA), and A. Feild (STScI)
Comet 332P was discovered in November 2010, after it surged in brightness and was spotted by two Japanese amateur astronomers, Kaoru Ikeya and Shigeki Murakami. Based on the Hubble data, the team calculated that the comet probably began shedding material between October and December 2015. From the rapid changes seen in the shards over the three days captured in the animation, they probably won’t be around for long.
Spectacular breakup of Comet 73P in 2006
More changes may be in the works. Hubble’s sharp vision also spied a chunk of material close to the comet, which may be the first salvo of another outburst. The remnant from still another flare-up, which may have occurred in 2012, is also visible. The fragment may be as large as Comet 332P, suggesting the comet split in two.
“In the past, astronomers thought that comets die when they are warmed by sunlight, causing their ices to simply vaporize away,” Jewitt said. “Either nothing would be left over or there would be a dead hulk of material where an active comet used to be. But it’s starting to look like fragmentation may be more important. In Comet 332P we may be seeing a comet fragmenting itself into oblivion.”
During its closest approach to the Sun on November 28, 2013, Comet ISON’s nucleus broke apart and soon vaporized away, leaving little more than a ghostly head and fading tail.
Astronomers using the Hubble and other telescopes have seen breakups before, most notably in April 2006 when 73P/Schwassmann-Wachmann 3, which crumbled into more than 60 pieces. Unlike 332P, the comet wasn’t observed long enough to track the evolution of the fragments, but the images are spectacular!
The researchers estimate that Comet 332P contains enough mass to endure another 25 outbursts. “If the comet has an episode every six years, the equivalent of one orbit around the sun, then it will be gone in 150 years,” Jewitt said. “It’s the blink of an eye, astronomically speaking. The trip to the inner Solar System has doomed it.”
This annotated image shows the fragments measured by Jewitt and team and their direction of movement. Credit: NASA, ESA, and D. Jewitt (UCLA)
332P/Ikeya-Murakami hails from the Kuiper Belt, a vast swarm of icy asteroids and comets beyond Neptune. Leftover building blocks from early Solar System and stuck in a deep freeze in the Kuiper Belt, you’d think they’d be left alone to live their solitary, chilly lives but no. After nearly 4.5 billion years in this icy deep freeze, chaotic gravitational perturbations from Neptune kicked Comet 332P out of the Kuiper Belt.
As the comet traveled across the solar system, it was deflected by the planets, like a ball bouncing around in a pinball machine, until Jupiter’s gravity set its current orbit. Jewitt estimates that a comet from the Kuiper Belt gets tossed into the inner solar system every 40 to 100 years.
I wish I could tell you to grab your scope for a look, but 332P is currently fainter than 15th magnitude and located in Libra low in the southwestern sky at nightfall. Hopefully, we’ll see more images in the coming weeks and months as Jewitt and the team continue to follow the evolution of its icy scraps.
