How Do Stars Explode?

   

 

    Supernovas can occur in one of two ways: through a process of runaway nuclear fusion or through a rapid collapse of the star’s core.

   

 The first process occurs in binary star systems where at least one star is a white dwarf, a dense, aging star that can no longer support nuclear fusion. The secondstar can be another white dwarf, a redgiant, or a main sequence star such as our own Sun, that fuses hydrogen atoms to form helium atoms at its core. In either case, the white dwarf siphons off (or collides with) the mass of its companion star, reigniting nuclear fusion. Once the white dwarf reignites, it gets so hot so fast that it blows apart, outshining an entire galaxy and leaving no remnant behind.

     

  Less luminous, though no less spectacular, are core collapse supernovas.Instead of exploding in a runaway fusion reaction, this type of supernova occurs when the star’s fusion reaction grinds to a halt. For most of a star’s life, it burns by fusing hydrogen atoms. This is the sameprocess that ignites thermonuclear weapons. Eventually, the star converts most of its hydrogen into helium. The starthen must fuel itself by fusing helium intocarbon. If the star is heavy enough—about eight times the mass of the Sun—it will then proceed to fuse carbon into neon and helium. The star continues to fuse heavier and heavier elements until it reaches theiron phase.

   

    It’s during the iron phase that things getreally heavy. Fusing iron does not producemore energy—in fact, iron fusion requiresenergy. Without the fusion pressure thatcounteracted the star’s gravity, the core ofthe star, which is approximately the size of Earth, collapses into a space less than 10 miles (16 km) in diameter at about one- quarter light speed. When the stellar massbounces back into space (crashing into theouter shell of the doomed star), theresultant shock wave is what we on Earthwitness as a supernova.

     
 Upon going supernova, the star may tear itself apart entirely or leave behind an extremely dense neutron star. If the core ofthe star is heavy enough, the supernovaleaves behind one of the most mysteriousobjects in the known universe: a blackhole.

How Much of the HumanBody Is Replaceable?

     

    Fans of the old TV shows and saw scientists revive nearly dead human beings, bringing them back to life with high-tech body parts that gave them extraordinary capabilities. Today, replacing parts of the human body using state-of-the-art technology is moving out of the realm of science fiction and into reality.

       

  Replacement of body parts means transplanting organs and tissues from one person to another or using artificial body parts. Organs currently transplanted are the heart, kidneys, liver, lungs, pancreas, and intestines. Tissues and cells include the corneas, cartilage, muscles, tendons, ligaments, skin, and heart valves (mechanical versions of the valves are also used).

     

  Artificial limbs and organs can replace parts throughout the body. Doctors commonly replace knees and hips, along with finger, elbow, and shoulder joints. Cochlear implants are electronic devices that restore hearing, and researchers are currently testing a new brain implant that can help patients who lack functioning auditory nerves. Prosthetic noses, hands,  arms, and legs are available; artificial legs
are among the most sophisticated prosthetics today, and researchers continue to improve “bionic” hands with an almost human sense of touch. One, the bebionic3, has 14 different grip patterns, including  ones that allow users to pick up a coin or write with a pen.

      

  The science of developing artificial body parts is constantly changing. In 2014, hospitals across the United States tested a “bioartificial” liver that combines liver cells and a mechanical device that together perform liver functions outside the body while a patient’s diseased liver regenerates healthy tissue. Researchers in Japan and elsewhere are developing 3-D printers that combine stem cells and artificial materials to custom-make artificial ears. The Japanese team hopes to also create skin and bones using this method.

     

  Scientists are also working to grow real replacement parts in the lab. Doris Taylor of the Texas Heart Institute is one of the pioneers in using stem cells to create such body parts as hearts, livers, and kidneys for transplants. Taylor says, “I absolutely see a day where you’ll walk into a manufacturing facility somewhere, and there will be jars of kidneys, jars of livers, and jars of lungs, whatever it is you need.”