You are currently browsing the tag archive for the ‘Stellar’ tag.

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Sometimes if you aren’t watching the heavens (or the news) closely enough, you miss a major astronomical discovery.  For example last summer, astronomers discovered a galaxy which formed only one or two billion years after the Big Bang (so I guess it is unclear whethter I missed this story by one year or by 12 billion).  At any rate, the galaxy hunters used the Hubble space telescope to peer through a powerful gravitational lense far away in space.  Gravitational lenses are areas where timespace is warped like a huge lense by high-gravity phenomena, and a viewer can use them like a huge lense to see far-away objects.  By using the Hubble telescope together with the gravitational lense they were able to see back a dozen billion years in time to the edge of the universe…as it once was not long after creation.  What they saw perplexed them.

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There is a fundamental difference between galaxies.  Galaxies where stars are being formed tend to be blue and spiral shaped (like our own beloved Milky Way!).  Galaxies where stars have largely stopped forming are “red and dead” since the remaining stars tend to be long lived red dwarf stars and the bright young (short-lived) blue stars are mostly gone.  These red galaxies are not shaped like spirals, but tend to be elliptical shaped (like an egg or a football, not like one of those evil gym machines).

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The ancient galaxy at the edge of the universe was neither of those colors or shapes. It was a dense yellow disk.  Stars formed in an (enormous) accretion disk but then, for some reason, new star formation stopped.  The blue stars burned out (“the light that shines twice as bright etc, etc..”), but the yellow middle aged stars were still burning.   The galaxy had three times the mass of the Milky Way but scrunched into a pancake of much smaller area.

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So do galaxies always form as disks and then either become self-renewing blue spirals (maybe by colliding with other galaxies or clouds of dust)or dead red footballs?  Or was this early yellow disk galaxy an abberation? Or is our own galaxy truly new (well…newish…being only a few billion years old)?  I do not understand astrophysics well enough to answer these questions or even formulate them properly (although I get the sense some of these questions may not yet be answered by anyone in any comprehensive way), but I would love to hear what people can add to this rudimentary yet compelling story of shapes and colors.

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OK, yesterday I promised we would get to the space news.  Clearly the real story is the earthlike planet right in our backyard (erm, relatively speaking). However it isn’t going anywhere right now so I am going to blog about it later when we have all had a moment to think about the real implications.  The space story I am looking at today is closer to home, but still takes place out there in the black: back in October of 2014, NASA lost communication with Stereo B one of two paired spacecraft which orbited the sun from the distance of Earth.

The solar observatory spacecraft allow stereoscopic viewing of the sun.  One spacecraft Stereo A was ahead of Earth on its orbit, whereas Stereo B trailed behind us.  The two observatories allow us to study coronal mass ejections and other stellar phenomena.  In 2011, the craft were 180 degrees apart from each other—allowing humankind to view the entire sun at once for the very first time (a truly remarkable milestone, when you think about it, which I heard nothing about at the time).

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Sadly, however, in 2014, as part of an automation and attitude test, Stereo B began to spin.  Mission controllers then lost contact with the craft which (because of the nature of its work) was on the other side of the sun!  NASA has patiently waited till the orbital path of Stereo B carried it further towards Earth and has used the Deep Space Network, a networked array of radio telescopes to find the errant craft.

We are still working on figuring out what sort of shape the poor guy is in (and maybe rehabilitating the spinning observatory), however I feel the story is worth telling as a sort of reminder of the fleet of crafts we have up there, which we don’t think about very often.

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Andromeda (Credits: NASA/JPL-Caltech/GSFC)

While everyone else was making popcorn garlands and giving sentimental presents and eating marshmallow candies shaped like Santa, astronomers were busy too…busy scanning the Andromeda galaxy with a super powerful x-ray telescope array in space! (Yeah, that’s right, astronomers are no joke, boy!)  According to NASA’s mission overview, “The NuSTAR instrument consists of two co-aligned grazing incidence telescopes with specially coated optics and newly developed detectors that extend sensitivity to higher energies as compared to previous missions such as Chandra and XMM. After launching into orbit on a small rocket, the NuSTAR telescope extends to achieve a 10-meter focal length. The observatory will provide a combination of sensitivity, spatial, and spectral resolution factors of 10 to 100 improved over previous missions that have operated at these X-ray energies.”

The astronomers operating this device (devices?) chose to look at Andromeda (AKA M31) the Milky Way’s big sister galaxy which is located relatively close by in galactic terms…a mere 2.5 million light years away.  They wished to observe X-ray binary systems–disturbing star systems where a supermassive star collapsed either into a black hole or a neutron star.  The huge mass left over from such a collapse plays havoc with the remaining living star.  Frequently great plumes of matter are stripped away from the living star into the gravity well of the white dwarf or the black hole.  As star stuff falls into the massive stellar fragment it produces large amounts of exotic radiation like x-rays.

Astronomers hope that by determining which of these systems harbor black holes versus neutron stars, they can find out more about such systems, which are theorized to have played a critical role in heating the interstellar gas nebulae which gave birth to the galaxies.

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NuSTAR X-Ray Observatory (NASA)

A Stellar Nursery in the Carina nebula: pillars of gas and dust three light years tall (NASA, JPL)

A Stellar Nursery in the Carina nebula: pillars of gas and dust three light years tall (NASA, JPL)

Stars develop in vast nebulae of swirling dust and gas that are light years across. Within these giant molecular clouds (GMCs), gravity gathers matter together into an accretion disk which then further compacts until the density and temperature of the central ball of gas reach the extremes necessary for nuclear fusion to begin. Different stars created in different GMCs thus have different spectrographic characteristics depending on the place of their creation, however GMCs tend to be inconceivably vast and multiple stars form in one stellar nursery at the same time. Such stars share similarities of composition.

The Sun

The Sun

For a long time, astronomers have sought the stars which formed at the same time in the same GMC as our beloved sun (which is approximately four and a half billion years old). Now, at long last, it seems we have found one of the sun’s bigger sisters. A yellow star in the constellation Hercules seems to have the same composition as the sun. Using elaborate computer models of stellar drift, scientists have traced the star (which goes by the unlovely name “HD 162826”) and the sun back to the same place of origin. HD 162826 is 15% larger than the sun (which is why I called it a big sister) and although it does not have any “super Jupiter” type planets, there is a possibility it may have some small rocky inner worlds. The sun has grown somewhat distant from its sibling: during the billions of years since their creation the two stars have drifted 110 light years from each other.

HD 162826

HD 162826

The discovery was made by a team of astronomers from around the world (lead by Ivan Ramirez from the University of Texas). The sun’s sister is not visible to the naked eye, but no doubt many telescopes will be trained on Hercules to discover if there is anything we can learn from our sun’s long sundered nursery mate.

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Between giant planets and small stars exists a bizarre class of heavenly objects known as brown dwarfs. Brown dwarfs are not massive enough to fuse hydrogen elements together as do main sequence stars like the sun, however brown dwarfs larger than 13 Jovian masses are believed to fuse deuterium atoms and large brown dwarfs (65 Jovian masses and up) are believed to fuse lithium.  Since brown dwarfs can be very much like planets or like stars, there is a specific definition to describe the objects: a brown dwarf must have experienced some sort of nuclear fusion as a result of mass and temperature, however it cannot have fused all of its lithium (or it is considered a star or stellar fragment).  A stellar physicist reading this blog might object that medium and large stars have some lithium present in their outer atmosphere, or that a very young white dwarf could still have some unused lithium present, or even that an old heavy brown dwarf could have fused all of its lithium.  That physicist would be correct: she deserves some cookies and a pat on the head for poking holes in unnecessarily simple definitions.

Various Classifications of Brown Dwarfs

Various Classifications of Brown Dwarfs

Brown dwarfs were theorized to exist in the 1960s, but no astronomer managed to discover one until 1988 when a team of University of California astronomers who were studying white dwarfs found a bizarrely cool red spectral signature for a faint companion to the star GD 165.  Since then many brown dwarfs have been discovered and sorted into the major types M, L, T, and Y.  They occupy a strange ambiguous area at the bottom of the Hertzsprung-Russell diagram—objects which are luminous and massive in comparison to everything else but tiny and dim compared to real stars.

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There are some planets which are known to orbit brown dwarfs and there also brown dwarfs known to orbit true stars.  It is beginning to seem that there a great many brown dwarfs out there: perhaps they are as numerous as true stars (or maybe they are even more common than that).  Since they are hard to detect, scientists do not have a very accurate assay of their frequency in the universe.  The question bears somewhat on our understanding of the universe–since a great deal of matter is  not accounted for.

An artist's conception of a brown dwarf seem from a closely orbiting planet

An artist’s conception of a brown dwarf seem from a closely orbiting planet

My mind keeps returning to the fact that some brown dwarfs have planetary systems.  Imagine these melancholic twilight ice worlds forever orbiting a dim glow which will never blaze into a true sun.   It is a melancholy picture, but not without a certain beauty.

A Brown Dwarf with Planet and Moon (painting by Lynette Cook from fineartamerica.com)

A Brown Dwarf with Planet and Moon (painting by Lynette Cook from fineartamerica.com)

An astronomy story has made big news headlines this week.  Usually most people are not unduly interested in the happenings in the heavens (either because such events are difficult to comprehend, or because they are regarded as remote to human interests), however this story does directly involve matters which humans take great interest in. gold-bar Scientists and theorists working for the Harvard-Smithsonian Center for Astrophysicists have announced a spectacular new theory concerning the origin of gold (and other heavy elements like platinum and uranium): the cosmologists believe that the heaviest natural elements are created when two neutron stars collide or when a neutron star collides with a black hole (here is an easy summary of neutron stars, extremely tiny supernova remnants with a mass greater than the sun).  Elements as complicated as iron are manufactured by normal stars in the course of their lifetime, however the creation of heavier elements is more mysterious.  Until now, chemists and physicists had imagined that gold, platinum, uranium, and what have you, come from supernovae—however computer models of various types of supernova events did not supporting that conjecture. The scientists at the Harvard-Smithsonian Center for Astrophysicists based their hypothesis partly on the massive gamma radiation burst detected on June 3rd, 2013 from 3.9 billion light years away in a galaxy located in the constellation Leo.  Gamma ray bursts tend to be associated with hypernovae/supernovae caused by the collapse of super-giant stars, but the June 3rd burst was different.  In certain rare circumstances, two neutron stars are in a binary system together.  Over time, the orbits decay and the stars come together in a cataclysmic event which releases energy tantamount to that of a supernova.    Based on the unusual exotic energy signatures of the June 3rd gamma ray burst,  it seems that scientists caught a rare peek at such an event.

Neutron Star Collision

Neutron Star Collision

I will confess that I am having trouble imagining two objects the size of small cities (yet each with a mass greater than the sun) slamming into each other at astronomical speeds.  Apparently such events only happen every 100,000 years or so in a galaxy the size of the Milky Way.  When the neutron stars come together, a black hole is ripped in the fabric of spacetime.  Huge parts of the neutron stars fall into the black hole and vanish from this universe, but other portions of the neutron stars (which, as the name hints, are made up largely of neutrons) are jettisoned into space.  Edo Berger, one of the astrophysicists who authored the new theory described the process with an earthy metaphor, saying, “several exciting things happen very quickly…. Most of the material collapses to form a black hole. Some of it is spewed into space. That material is rich in neutrons, which drives the formation of heavier and heavier elements, the way mud piles up on an off-road vehicle.” neutcol The gold, platinum, and heavy elements are created in astonishing mass (like many earths made entirely of gold).  The elements are diffused through the cosmos and become part of newly forming star systems. Gold is strange stuff anyway.  The gold present on Earth during its nebular formation is believed to have sunk deep into the center of planet’s molten core where it is inaccessible.  All the gold that rappers and kings wear (and that Ron Paul and draugers hoard) first began falling to Earth 200 million years after the planet’s final formation on asteroids.  The great gold strikes are well named: gold on the surface of Earth is there because of meteor strikes (although billions of years of geology have buried and twisted and hidden these cosmic remnants).

Yeehaw! There's asteroid fragments!

Yeehaw! There’s asteroid fragments!

Mind-blowing diagram comparing Vancouver to a Neutron Star (by Christian Joore)

Most items in the heavens are inconceivably large.  The sun, a fairly ordinary star has a diameter of 1,391,000 kilometers (864,327 miles).  Even a tiny planetoid like the moon has a diameter of 3,474 km (2,159 miles).  However a few noteworthy items in the heavens are so small that we can think of them in human terms—like neutron stars, which are the size of a town or small city with a diameter of only 20 or 30 kilometers (about ten to 15 miles  miles).   But even though they are the size of a small asteroid or Manhattan Island, neutron stars are hardly inconsequential.  These dinky stars can have more mass than the entire glorious sun (which itself is 332,946 times more massive than the Earth and everything on it).  A 1.27 cubic centimeter block of such material (approximately the size of a half an inch sugar cube) would weigh approximately the same as all of the human inhabitants of Earth (give or take).

Neutron stars are left-over fragments of supernovae explosions.  When a star 4 to 8 times more massive than our sun burns through all available fuel, its outer layers blow apart in a supernova which spreads glittering matter across great swaths of space.  The dense remaining portion of the stellar core undergoes a titanic battle between electron degeneracy pressure and gravity.  If the fragment has more than 1.44 stellar masses, gravity wins and the electrons and protons of its constituent matter are crushed into super dense neutrons.  Such explosions are tremendously dynamic and bright.  In 1054 AD, Sung dynasty astronomers recorded such an explosion which outshone the moon.  Contemporary astronomers have determined that the 1054 AD supernova created the Crab Nebula, an oval shaped mass of hydrogen, carbon, oxygen, nitrogen, neon, sulfur, and iron.

The Crab Nebula (which measures 11 light years across and lies 6,500 light-years from Earth) NASA/CXC/SAO/F. Seward

In the center of the Crab Nebula is a spinning neutron star which is emitting jets of particles at a tremendous velocity from its magnetic poles. These jets produce very powerful beams of electromagnetic radiation (which varies in intensity and wavelength according to elaborate nuclear & stellar physics, much of which is not yet understood).  The forces which create neutron stars often leave the stars spinning and pulsing with energy in such a way that they become pulsars.  These pulsars are useful for studying gravity, general relativity, and the behavior of matter at nuclear densities (albeit indirectly).  They also make accurate time measurement devices and useful beacons.  It is strange to think that stars so prominent for vast distances and so useful to astronomers actually have such minimal volume.

A Detailed x-ray image of the pulsar at the center of the Crab Nebula (Chandra)

A day ago an international team of stellar physicists announced that the sun’s surface is covered with thousands of searing hot plasma super tornadoes each of which is the size of a large continent on Earth.  Using a combination of a space telescope and a ground telescope, researchers discovered that each of these plasma vortexes spins at velocities up to 14,500 kilometers (9,000 miles) an hour.

(CREDIT: Wedemeyer-Böhm: Parts of the image produced with VAPOR)

The mystery of why the corona of the sun is 300 times hotter than the star’s surface has long vexed scientists.  The surface of the sun is a balmy 5,526 degrees Celsius (9,980 Fahrenheit), while temperatures in the corona peaks 2 million degrees Celsius (3.5 million Fahrenheit). The discovery of these giant fast-moving storms provides a new mechanism by which heat is transferred through the sun’s atmosphere and ejected into the corona. Energy locked in the powerful magnetic vortexes is effectively self-insulated and does not heat the solar photosphere and chromosphere as much as the corona (where the storms widen and dissipate).

The Sun photographed by the Atmospheric Imaging Assembly (AIA 304) of NASA’s Solar Dynamics Observatory

Sven Wedemeyer-Böhm, a Norwegian scientist working on the team was quick to stress that the tornadoes are likely one of several complicated energy transfer mechanisms by which heat reaches the solar corona. However it seems that there are more than 11,000 of these huge plasma tornadoes on the solar surface at any given time.

A Comparison of the relative sizes of the Fomalhaut system and the solar system (image created by NASA and ESA)

Fomalhaut is a star with twice the mass of the sun located approximately 25 light-years from Earth in the constellation Piscis Austrinus.  It is a bright young star 100 to 300 million years old (out of a projected lifespan of 1 billion years). Coinicientally  the name Fomalhaut is Arabic and means “mouth of the Southern fish.”  Fomalhaut has at least one planet—Fomalhaut b, which is believed to be approximately the same size as Jupiter (but could be anywhere from the size of Neptune to 3 times as large as Jupiter).  Just as Saturn is surrounded by a ring of debris, the entire star system of Fomalhaut is surrounded by a giant toroidal circumstellar disk.   This torus is vastly greater in diameter than our entire solar system (including the Oort Belt) and is made up of somewhere between 260 billion and 83 trillion comets which are constantly colliding and annihilating each other!  The Herschel Space Observatory recently captured an infrared image of this immense comet storm.

An infrared image of the Fomalhaut system--and its huge cloud of disintegrating comets) captured by the Herschel Space Observatory (credit: ESA)

Sirius, located in the constellation Canis Majoris

The brightest star in the night sky is Sirius.  Only 8.6 million light years from Earth, Sirius A is 25 times more luminous than the sun.  Because of its brightness, the star was well known in ancient times—it was named Sopdet in Ancient Egypt and it was the basis of the Egyptian calendar.  After a 70-day absence from the skies Sirius (or Sopdet) first became visible just before sunrise near the Summer solstice—just prior to the annual Nile floods.   Greek and Roman astronomers philosophized and speculated about Sirius, (which they called “the dog star” because of its closeness to the constellation Canis Majoris).  Arabs knew the star as Aschere “the leader”.  Polynesians used it as a principle focus of their astonishing oceanic navigation.  Over countless millennia, Sirius has worked its way deep into human consciousness as one of the immutable landmarks of the night sky.

Hieroglyph of Sirius/Sopdet

So imagine the shock when it was discovered that Sirius is not alone.  The bright star we know is actually Sirius A, a star with twice the mass of the sun. In 1844 the German astronomer Friedrich Bessel hypothesized a tiny companion for Sirius based on the irregular proper motion of Sirius.  Then, in 1862, as the American Civil war was being fought an American astronomer in Chicago first observed the tiny companion, Sirius B (thereafter affectionately known as “the Pup”). Sirius B has nearly the same mass as the sun (.98 solar mass) but it is only 12,000 kilometers (7,500 mi) in diameter—nearly the same distance around as Earth.

An Artist's Conception of Sirius A and Sirius B

Today Sirius B is the closest white dwarf star to planet Earth.  However it has not always been so, Sirius B began its life as a luminous blue B-type main sequence star with a mass five times that of the sun. About 124 million years ago—as the dinosaurs grazed on the first magnolias—Sirius B fused its way through the hydrogen and helium in its mass.  As Sirius B began to fuse together larger elements like oxygen and carbon it expanded into a red giant star with a diameter 10 to 100 times that of the sun. Then Sirius B ran out of nuclear fuel.  Without the heat generated by nuclear fusion to support it, the star underwent gravitational collapse and shrank into a hyper dense white dwarf star.  These tiny stars are extremely dense and hot when they are formed, but since they generate no new energy their heat and radiance gradually radiate away over billions of years until the stars are completely black and dead.

Not this sort of "White Dwarf"--Curse you Google Image Search!

Although Sirius B is largely composed of a carbon-oxygen mixture, its core is overlaid by an envelope of lighter elements. Hydrogen, being lightest, forms the outermost layer (which is why the little star currently appears uniformly white).

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