THE COMPLETE COSMOS Chapter 21: Milky Way Outline Sub-chapters

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1 THE COMPLETE COSMOS Chapter 21: Milky Way Our Galaxy explored, light years explained. The life and death of stars. Supernovae -and the clouds where stars are born. Outline Time-lapse of the Milky Way traversing the night sky - the sideways view of a flattened disk of stars, our galaxy. From above, the Milky Way is a spiral, a family of 150 billion stars. Our Sun is but one of these stars - two-thirds of the way from the center and orbiting the galactic center once every 225 million years. The heart of the Milky Way - choked with gas and dust - seethes with energy. Distances within our galaxy, and beyond, are measured in light years - the distance traveled by light in one year. The disk of the Milky Way is 100,000 light years across. Our Solar System lies 30,000 light years from the galactic center. The distances to some of our nearest stars. A journey from the constellation of Orion, past the Pleiades, Aldebaran, Sirius and Barnard's Star to the Alpha Centauri system, a trio of stars fourand-a-quarter light years away from the Sun. Emission nebulae, where stars are born. The violence of starbirth and the action of stellar winds. The more massive a star, the shorter its life. As they age, some stars swell to become red giants or supergiants. Most end by puffing off their outer layers to form beautiful planetary nebulae and collapsing into white dwarfs. In a binary star system, a white dwarf can provoke periodic nova eruptions. The most devastating stellar explosions are supernovae - when a supermassive star blows itself apart. The Crab Nebula is an example. With a rapidly spinning pulsar at its core, intense radiation causes the gas of the nebula to glow. Eta Carinae is a star with 100 times the mass of the Sun. When it goes supernova, Eta Carinae may collapse beyond the pulsar stage to become - a black hole. Birth, life and death in the Milky Way. Sub-chapters Milky Way Time-lapse from Earth of the Milky Way delicately arching across the sky. In spectacular animation, a sideways look at a flattened disk of stars, our galaxy. From above, the Milky Way is a spiral containing 150-billion stars. Our Sun is one of those stars. It lies two-thirds of the way from the galactic center, towards the edge of the disk - and orbits the center once every 225 million years. The galactic center shines brightest of all. Veiled in gas and dust, it seethes with energy. By contrast, our Sun's neighborhood is suburban - quiet stars leading ordinary lives. The Sun is no exception - yellow, average and middle-aged. Nearest Stars Distances in the Milky Way are too large for kilometers or miles. Measurement is based on the distance traveled by light in a given time. Light travels from the Sun to Earth in eight-and-a-half minutes. A light year is the distance light travels in one year - equal to 9.46 million million kilometers. The Sun's nearest neighbor, the Alpha Centauri system, is four-and-a-quarter light years

2 away - or more than 40 million million kilometers. The Milky Way is 100,000 light years across. The Sun is 30,000 light years from the galactic center. From Earth, stars that seem close to each other - like the familiar constellation of Orion - may, in reality, be hundreds of light years apart, the result of a line-of-sight effect. Relatively nearby stars lie tens or hundreds of light years away. A journey past the stars of the Pleiades light years away - and the older, dimmer Hyades, 150 light years distant. Although it appears to be part of the Hyades, the star Aldebaran is at less than half as far. Sirius, the brightest star in the sky, is closer to home - less than nine light years away. In 10,000 years, the dim red dwarf Barnard's Star will be the Sun's closest companion. But for now, the stars of the Alpha Centauri system are the Sun's nearest neighbors - at a distance of four-and-a-quarter-light years. Young Stars The spiral galaxy M83 reveals regions of starbirth. They glow pink and are called emission nebulae. The Milky Way also has them - clouds of glowing hydrogen streaked with dark dust. Where material clumps, stars fire into life. Stellar birth is violent. Young stars emit powerful winds. In the Rosette Nebula, a hole 12 light years across is the result of powerful stellar winds from young stars. Red Giants, Planetary Nebulae The Sun is an average star, consuming its hydrogen fuel in moderation. Massive stars have shorter lives - voraciously using their fuel and swelling to red supergiants hundreds of times their original size. Antares is surrounded by a reflection nebula - a star so big that its atmosphere leaks into space, a cloud of its own material that catches the light of other stars. As hydrogen runs out, stars switch to helium and other elements. Eventually the delicate equilibrium is lost. Radiation pushing outwards beats gravity pulling inwards. The star swells to a bloated red giant. The outer layers are lost to form beautiful planetary nebulae. With the core exposed, the star collapses to a white dwarf. Some white dwarfs are more spectacular. In a binary system, a white dwarf may draw material from its companion. On reaching critical mass, the overloaded outer shell of the white dwarf explodes as a nova. Both stars survive and the process repeats. The Sun's lifespan is ten billion years. A supergiant star, with 30 times the Sun's mass, will survive just one million years. These massive stars explode as supernovae, the fate of any star more than eight times the mass of the Sun. The Crab Pulsar The supernova that created the Crab Nebula was observed from China in Within the nebula, a pulsar survives - a superdense relic of the original star spinning 30 times a second. The pulsar emits shockwaves, particles spinning from the surrounding disk of material at almost the speed of light - seen in time-lapse from the Hubble Space Telescope.

3 Exploding Superstars Eta Carinae, a star with a 100 hundred times the mass of the Sun, is venting great clouds of material - an indication that catastrophe is not far away. When Eta Carinae explodes as a supernova, it will collapse beyond the pulsar stage to become a black hole. Background The Light Year A ray of light travels at almost 300,000 kilometers per second. Consequently, it takes light only about 500 seconds to travel the 150 million kilometers between the Sun and the Earth. If a ray of light traveled non-stop for one year, it would cover a distance of about 9,460,000-million kilometers. This distance is called the light year. The concept confuses some people because a light year is a measure of distance and not of time. Distances to the stars are so vast that measuring them in kilometers or miles (or even millions of kilometers or miles) is not very practical. Even the nearest star system - Alpha Centauri - is about 40 million million kilometers distant. Yet light would travel this distance in only four-and-aquarter years. So astronomers say that the distance to the Alpha Centauri system is about fourand-a-quarter light years. Other well known stars are much farther away. Betelgeuse, the red supergiant star in Orion, is about 590 light years from us. Deneb, the brightest star in Cygnus the Swan - or the Northern Cross - is even more remote, lying 1,800 light years away. The distances to other galaxies is also measured in light years. One of the nearest galaxies is Andromeda - more than two-and-a quarter million light years away. Most galaxies are much farther. Some of the most distant objects known are called quasars. Many have distances of over ten thousand million (ten billion) light years. The Milky Way Galaxy The Milky Way is our galaxy - sometimes known simply as the Galaxy. It contains at least 150 thousand million (150 billion) stars of which our Sun is one. They form a gigantic flattened disk. It is thin at the edges but has a bulge - or nucleus- at the center. Look more closely at the disk and a spiral structure is apparent. Our Sun, which is nowhere near the galactic center, lies close to the edge of one of the spiral arms, about two-thirds of the way out from the center, towards the edge. As a result of the Sun's peripheral position, when we look up into the night sky from Earth, the stars of the Milky Way are not scattered evenly across the heavens. Instead, they appear as the edge of a flattened disk - the galactic plane. It is a luminous band stretching across the sky - fairly narrow and quite beautiful. Within it, stars appear to be crowded together. The galactic center lies in the direction of the constellation of Sagittarius. For observers in Europe and the United States, the band of the Milky Way passes through Aquila, Cygnus, Cassiopeia, Perseus, Auriga and parts of Gemini and Orion. The diameter of the Milky Way is about 100,000 light years. The Sun is just over 30,000 light years from the galactic center. The maximum thickness of the Milky Way through the central bulge - is about 20,000 light years. A "halo" that surrounds the galaxy contains fewer stars than the disk of the galaxy itself. The extent of the halo is uncertain - but all the stars within it are old. They are believed to have formed early in the life of the Milky Way, maybe 12,000 million years ago. Most of the stars in the disk - and probably those in the nucleus - are stars of intermediate age, probably from 3,000 to 5,000 million years old. The youngest stars

4 are confined to a layer some 1,500 light years deep - along the disk's central plane. Many of the brightest stars of the Milky Way are in its spiral arms. They wind outwards from the nucleus, close to the central plane of the disk. The Sun is only a few light-years north of the galactic plane, near the inner edge of one of the spiral arms. The youngest stars in the galaxy, together with very bright, short-lived hot blue-white stars, are mainly found in the spiral arms. Close to the Sun, astronomers have identified and traced out parts of three spiral arms. These are the Orion arm, in which the Sun is situated; the Sagittarius arm, which lies about 6,500 light years nearer to the galactic center; and the Perseus arm which lies about 6,500 light years farther out. About one-tenth of the mass of the Milky Way is probably gas and dust, occupying the space between the stars. Huge quantities have been detected within or close to the galactic plane. Vast clouds are also found in the spiral arms where stars are still being formed. But this gas and dust obscures many objects. We cannot directly see the center of the galaxy, because it lies beyond the star-clouds in Sagittarius. The star-count increases rapidly towards the center of the galaxy. A powerful source of radio waves, called Sagittarius A, appears to lie right at the galactic center. The entire Milky Way rotates around its center. The galactic disk rotates more rapidly than the halo. In our part of the galaxy, the disk rotates at about 250 kilometers per second. The Sun takes some 225 million years to complete one revolution - a period known as the cosmic year. The halo rotates at only about 50 kilometers per second. The Birth of Stars Stars are huge balls of gas held together, like everything else in the Universe, by the force of gravity. They differ from planets in more than just size. Stars generate energy in their cores by nuclear fusion - reactions that for all stars begin with the conversion of hydrogen into helium. This process releases energy which makes its way slowly to the surface. From there it pours into space. Planets have no light of their own. They shine only by reflected starlight. Stars are formed from the collapse of giant clouds of interstellar dust and gas. As more and more material falls in, the developing star grows hotter. Eventually, when the temperature in the core exceeds seven million degrees, nuclear fusion reactions begin. In fact, a star is just a pause in a long-term process of collapse. The halt is caused by a balancing act. On the one hand, the star is a naturally expansive body pushing outwards. On the other, huge forces of gravity are pushing inwards. This is how it works. Radiation is generated at the core by nuclear fusion - the combination of hydrogen atoms to make helium. The energy released has to force its way out. But everywhere within the star, gravitational pull exactly balances the outward thrust of the radiation pressure. A star's mass will influence the course of its life. For a star to shine at all requires a mass of at least one-fifteenth that of our Sun - about 80 times the mass of the largest planet, Jupiter. Some stars, however, are much larger and more massive than the Sun, the biggest being around 100 solar masses. The mass of a star also determines its temperature and luminosity. The Evolution and Death of Stars When nuclear fusion fires up, stars have temperatures and luminosities prescribed precisely by their masses - with a very slight variation caused by the chemical mix. It is these factors that define a star's Main Sequence - in other words, the major part of its lifespan. Massive stars are very luminous and hot. They shine blue. Stars in the middle range are less hot and shine yellow - the Sun being a good example. The least massive stars are the coolest and glow a dull red. Again, mass is the vital factor in governing how long a star spends in the Main Sequence. For instance, our Sun's Main Sequence is ten billion years. Roughly half the sequence has already elapsed. For a star like Sirius, which is twice as massive as

5 the Sun and 20 times as luminous, the Main Sequence is one billion years. For a star of 30 solar masses with a luminosity 100,000 times that of the Sun, the Main Sequence drops dramatically to a mere one million years. So, massive stars have a much shorter Main Sequence than the Sun, despite having more fuel at the outset. That is because they consume it so much faster in order to supply their greater luminosities. Ructions ensue when a star grows old. As hydrogen fuel is consumed, hot but inert helium grows within the core. Meanwhile, on the outside of the core, hydrogen fusion proceeds. The core grows hotter and hotter - to a point where helium can also participate in nuclear fusion. This produces carbon and oxygen - and a sudden change called the helium flash. The inner core shrinks and becomes enormously dense, while the outer parts of the star expand. Luminosity increases - more for smaller stars than for larger ones and the temperature falls rapidly. The star becomes a giant, perhaps 100 times its original diameter. Its outer layers cool so much that the star dims to red or orange-red. Slowly, the giant grows brighter and cooler. Eventually, it becomes a variable star, oscillating in size and fluctuating in brightness. Finally, the outer layers are shed, forming a bubble around the dying star called a "planetary nebula". Beneath, the hot core is revealed - a white dwarf of extreme density. The white dwarf cools - ending as a black dwarf, the cinder of a star. This cycle applies only to stars like our Sun. If the star is much bigger, with a core whose mass exceeds 1.4 solar masses - the Chandrasekhar limit - the story is different. Such stars exhaust their hydrogen quickly and usually progress to the red giant stage in a more convulsive way. Indeed they will become much larger and more luminous than a conventional red giant. They become supergiants - so luminous that they can be easily seen right across the Milky Way galaxy, as well as in other galaxies. In a big star, its great mass compresses and heats the core so much that carbon ignites. This keeps the star "burning" when its helium is gone. There follows a succession of nuclear reactions that create heavier and heavier elements which continue to fuel the star. Oxygen, neon, magnesium and silicon are formed. But, since higher and higher temperatures are needed at every stage, each new shell is confined to a smaller and hotter region around the core. Eventually, the star begins to burn silicon into iron. This signals the end. Nuclear fusion stops with iron. A star with an iron core is out of fuel - and out of time. The supergiant is blown to pieces in a spectacular and catastrophic supernova. As we have seen, stable stars are a delicate balance between their internal radiation pressure and their own force of gravity. The failure of this balance is the underlying cause of a supernova. When fusion reactions cease, the internal radiation pressure disappears and the interior of the star begins to collapse. The effect of this "implosion" is to crush the superdense core, forcing protons and electrons together to produce a ball of neutrons. The star's plunging outer layers strike the neutron core, crushing it still more. The infalling gas is heated to billions of degrees. Finally, the pressure blasts away the outer layers in a titanic supernova. This cataclysm is not exactly at the center of the star, but on the exterior of the dense core. The explosion not only throws off several solar masses of gas at speeds of well over 10,000 kilometers per second, but it crushes the core with unimaginable force. At the very least, a superdense neutron star is formed, about ten kilometers in diameter. In some case, the compression may be so great that a black hole is formed.

6 Links for Further Information The Milky Way - images and text on our galaxy with links to specific features. Good image of the Milky Way's flattened disk acquired by the COBE spacecraft with introductory text. Origins of the Milky Way. Lecture by Barbara Ryden on the theories of formation of our galaxy. Constellation homepage with photographs of the Milky Way, the constellations, the brightest and nearest stars, plus supplementary information. Informative page on starbirth with color photographs and detailed descriptions of features in the images. Includes photographs of the "Pillars of Creation" and the Orion Nebula. The death of stars. Detailed descriptions of novae and supernovae with illustrations. Explains the difference between the two phenomena. Accompanying diagrams. Links to supernova remnants, including the Crab Nebula and Cygnus Loop, and a special link to Eta Carinae with a photograph of this unstable supermassive star. Questions and Activities for the Curious 1. What is a light year? Give some examples of its use in defining astronomical distances. 2. Describe the size, shape and structure of our Milky Way galaxy. 3. Imagine you are flying a spaceship from the Pleiades star cluster to our Sun. Describe the star systems you pass. 4. What do we know of fierce stellar winds blowing from young stars? 5. Discuss the life and times of a star like our Sun - from birth to death. 6. Explain the difference between a nova and a supernova. 7. Outline the events leading to the destruction of a massive star in a supernova explosion. 8. What is the Crab Nebula and what would you find at the heart of this nebula?