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Ferrebeekeeper is baffled and alarmed by neutron stars (here is a post about them from back in the day).  A factoid from that post summarizes what makes these super-dense stellar remnants so disconcerting: a 1.27 square centimeter cube of neutron star material has approximately the same mass as all of Earth’s 7.7 billion human inhabitants (although the tiny cube of pure neutrons presumably lacks the same lively personality).  It is almost impossible to conceive of such a material…which is why we are reporting today’s space news! Astronomers at the Greenbank Radio telescope in West Virginia (pictured above) have discovered the largest known neutron star 4600 light years from Earth.  The star is known by the unlovely name J0740+6620 and it has 2.14 times the mass of the sun packed into a sphere with a diameter of 25 kilometers (to contextualize in instantly familiar terms, 25 km is the distance from Hell’s Kitchen to JFK airport).  This particular star is a rotating neutron star—a pulsar–which emits two radio beams from its poles as it rotates at hundreds of revolutions per second.  lies at the upper theoretical limit of how large a neutron star can be without collapsing into a black hole.

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The star was discovered by luck as astronomers researched gravitational waves (which are vast invisible ripples in space time).  Because the neutron star has a white dwarf companion, astronomers were able to precisely calculate the star’s mass with some fancy math.   The mass of the white dwarf distorts spacetime around the neutron star to a degree which causes the pulsar’s radio beacons to be delayed by tenths of millionths of a second.  Astronomers measured these delays (the phenomenon is known as “Shapiro Delay”) and calculated the mass of the neutron star accordingly.

A strangely horrifying illustration of the supermassive black hole located in the middle of the very dense miniature galaxy M60-UCD1

A strangely horrifying illustration of the supermassive black hole located in the middle of the very dense miniature galaxy M60-UCD1

Fifty million light years away from Earth is the dwarf galaxy M60-UCD1. This tiny globular galaxy is 300 light years across–whereas our own beloved spiral galaxy, the Milky Way, is 120,000 light years in diameter! Yet within that 300 million light year sphere, M60-UCD1 is a crazy place. Despite its (comparatively) tiny area, the dwarf galaxy is teaming with stars: astronomers estimate it contains 140 million star systems. If Earth were located in M60-UCDI, the night sky would positively glow with millions of visible tars (as opposed to the measly 4000 which are visible to the naked eye in our present location). This is all quite odd, yet only recently did astronomers discover the strangest thing about M60-UCDI. At the center of the tiny galaxy is a supermassive black hole which weighs more than twenty million suns. To quote the European Space Agency’s website, “The supermassive black hole at the centre of M60-UCD1 makes up a huge 15 percent of the galaxy’s total mass, and weighs five times that of the black hole at the center of the Milky Way.”

Messier 60 with M60-UCDI (Composite image from NASA's Hubble & Chandra space telescopes)

Messier 60 with M60-UCDI (Composite image from NASA’s Hubble & Chandra space telescopes)

Astronomers speculate that something went terribly wrong to form this oddball of a galaxy. A prime culprit is Messier 60, a large scary galaxy which lurks near the little dwarf galaxy. The black hole at the center of Messier 60 is 4.5 billion times the size of our Sun! Perhaps once upon a time M60-UCDI was a normal galaxy with billions of stars…till it wandered too close to Messier 60. The larger galaxy tore off the majority of the stars which made up M60-UCDI and added them to itself (while Messier 60’s black hole swallowed up its fair share of star systems). It is a horrifying image of galactic bullying! Why can’t we all get along?

Ebony label depicting the pharaoh Den, found in his tomb in Abydos, circa 3000 BC

Ebony label depicting the pharaoh Den, found in his tomb in Abydos, circa 3000 BC

The English word “ebony” comes from the Ancient Egyptian word “hbny” (well, that is actually an approximation: I cannot find the vulture, asp, or little man characters on my keyboard). An obsession with the dense sable wood has clearly been a long-standing feature of human culture. The Greek word for the trees is “Diospyros” which apparently means something like “God’s wheat” or “fruit of Zeus” (since the Greeks first encountered ebonies in the form of Caucasian Persimmon trees). There are over 700 separate species in the Diospyros Genus—many of these are weird little shrubs or deciduous persimmon trees–but some are evergreen tropical giants.

 

Gabon Ebony for Carving

Gabon Ebony for Carving

Arguably the most famous of all these ebony/persimmon trees is the Gabon ebony (Diospyros crassiflora) which produces a close-grained black wood so dense that it sinks in water. This precious wood is beautiful for carving and cabinet making, but the magnificence of the timber has been the sad downfall of the actual living tree. Diospyros crassiflora lives in West Africa from Nigeria, Cameroon, and Gabon down through the Central African Republic and through the two Congo Republics.

 

Ebony and Ivory Chess Set--East Africa (early 2oth century)

Ebony and Ivory Chess Set–East Africa (early 2oth century)

The tree grows very slowly and to great age. It tends to be solitary—but mature trees can grow to 20 meters (60 feet) in height. Sadly most of the large specimens have been cut down for the exotic timber trade and the tree is now listed as endangered.  Infuriatingly I can’t even find a picture of the living tree–it’s like I wanted to show you a bull, but could only find pictures of hamburger.

 

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)

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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