StarS and nebulae When a star goes supernova considerable amounts of its matter equaling the material of several Suns may be blasted into space with such a burst of energy as to enable the exploding star to outshine its entire home galaxy Supernovae are characterized by a tremendous rapid brightening lasting for a few weeks followed by a slow dimming A supernova explosion is a cataclysmic event for a star one that essentially ends its active i e energy-generating lifetime Supernovae release many of the heavier elements that make up the components of the solar system including Earth into the interstellar medium White dwarfs have a mass similar to that of the Sun but with a radius comparable to that of Earth making them extremely dense White dwarfs have average densities densities approaching 1,000,000 times that of water Neutron stars are any of a class of extremely dense compact stars thought to be composed primarily of neutrons neutrons Their masses range between 1 and 2 times that of the Sun Having so much mass packed within a ball on the order of 20 km 12 miles in diameter a neutron star has a density that can reach that of nuclear values which is roughly 100 trillion 1014 times the average density of solar matter or of water This approximates the density inside the atomic nucleus and in some ways a neutron star can be conceived of as a gigantic nucleus 8
IntroductIon A black hole can be formed by the death of a massive star When such a star has exhausted the internal thermonuclear thermonuclear fuels in its core at the end of its life the core becomes unstable and gravitationally collapses inward upon itself and the star’s outer layers are blown away The crushing weight of constituent matter falling in from all sides compresses the dying star to a point of zero volume and infinite density called the singularity around which nothing not even light can escape Nebulae are clouds of gas and dust that occur in the space between the stars A nebula is thus made up of the interstellar medium Some nebulae give birth to new stars and dying stars expel nebulae The Sun was formed roughly 4 5 billion years ago inside a nebula that was produced from a supernova For thousands of years people have gazed at the seemingly seemingly infinite number of stars in the night sky For most of this time they could only guess about the nature of these pinpoints of light often making them objects of wonder worship comfort or fear 9
CHAPTER 1 The NaTure of STarS In the observable universe it’s estimated that there are roughly as many stars as there are grains of sand on all of the beaches on Earth There are all different types of stars of all different sizes and ages Throughout the Milky Way Galaxy and even near the Sun itself astronomers have discovered stars that are well evolved or even approaching extinction or both as well as occasional stars that must be very young or still in the process of formation Evolutionary effects on these stars are not negligible even for a middle-aged star such as the Sun More massive stars must display more spectacular effects because the rate of conversion of mass into energy is higher While the Sun produces energy at the rate of about two ergs per gram per second a more luminous main-sequence star can release energy at a rate some 1,000 times 10
the nature of StarS greater Consequently effects that require billions of years to be easily recognized in the Sun might occur within a few million years in highly luminous and massive stars A supergiant star such as Antares a bright main-sequence star such as Rigel or even a more modest star such as Sirius Sirius cannot have endured as long as the Sun has endured These stars must have been formed relatively recently While roots can be traced back through Arab and Greek contributions modern astronomy started with the work of Nicolaus Copernicus in Poland in the early 16th century Copernicus concluded that the Sun not Earth was the center of the universe and that Earth was a planet Left the Sun Hotter areas of the Sun appear in bright white Right limb darkening on the disk of the Sun Mercury can be seen as a small black dot in the lower middle of the solar disk
