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ASTRONOMY Slide 2 THE LIFE OF STARS Slide 3 KEY TERMS Absolute magnitude Apparent magnitude Black hole Bright nebula Dark nebula Emission nebula Eruptive variables H R diagram Hydrogen burning Interstellar dust Magnitude Main sequence stars Nebula Neutron star Nova Planetary nebula Protostar Pulsar Pulsating variables Red giant Supergiant Supernova White dwarf Slide 4 What is a Star? A star is a huge sphere of very hot, glowing gas. Stars produce their own light and energy by a process called nuclear fusion. Fusion happens when hydrogen atoms are forced to become helium atoms. When this happens, a tremendous amount of energy is created causing the star to burn and shine. Slide 5 How are Stars Measured? 1. Magnitude Absolute magnitude The apparent brightness of a star if it were viewed from a distance of 10 parsecs (32.6 light years) A parsec is defined as the distance from the Sun which would result in a parallax of 1 second of arc as seen from Earth. 1 parsec = 3.262 light-years Parallax is an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. Astronomers use the principle of parallax to measure distances to celestial objects including to the Moon, the Sun, and to stars beyond the Solar System. Used to compare the true luminosity or brightness of a star Slide 6 PARALLAX A simplified illustration of the parallax of an object against a distance background due to a perspective shift. When viewed from Viewpoint A, the object appears to be in front of the blue square. When the viewpoint is changed to Viewpoint B, the object appears to have moved in front of the red square. Slide 7 How are Stars Measured? Apparent magnitude The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower the value of its magnitude. Visible to typical human eye Apparent magnitude Brightness relative to Vega Number of stars brighter than apparent magnitude Yes 1250%1 0100%4 140%15 216%48 36.3%171 42.5%513 51.0%1 602 60.40%4 800 No 70.16%14 000 80.063%42 000 90.025%121 000 100.010%340 000 Slide 8 How are Stars Measured? Luminosity, temperature, and size Hertzsprung-Russel Diagram a graphical tool that astronomers use to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage. Stars in the stable phase of hydrogen - burning lie along the Main Sequence according to their mass. After a star uses up all the hydrogen in its core, it leaves the main sequence and moves towards the red giant branch. The most massive stars may also become red supergiants, in the upper right corner of the diagram. The lower left corner is reserved for the white dwarfs. Slide 9 Slide 10 TYPES OF STARS Slide 11 Main Sequence Stars The main sequence is the point in a star's evolution during which it maintains a stable nuclear reaction. It is this stage during which a star will spend most of its life. Our Sun is a main sequence star. A main sequence star will experience only small fluctuations in luminosity and temperature. The amount of time a star spends in this phase depends on its mass. Large, massive stars will have a short main sequence stage while less massive stars will remain in main sequence much longer. Very massive stars will exhaust their fuel in only a few hundred million years. Smaller stars, like the Sun, will burn for several billion years during their main sequence stage. Very massive stars will become blue giants during their main sequence. Slide 12 Main Sequence Stars Core temperature > about 10 million K hot enough that hydrogen nuclei (protons) can overcome fuse together. Through several steps, hydrogen is fused to form helium nuclei. (hydrogen burning) In the process a small amount of mass is converted into energy, released in the form of high-energy gamma photons. This hydrogen fusion provides the radiation pressure that supports main sequence stars against further gravitational collapse. In the Sun On Earth Slide 13 Main Sequence Stars The main factor that determines where a star lays on the main sequence is its mass. A mass = 1/10 that of the Sun enough gravitational force to heat the core to 10 million K (hydrogen fusion) less massive = no fusion brown dwarf or a "failed" star forms Emits only infrared radiation Greater mass = higher core temperature = greater rate of hydrogen fusion. Higher-mass stars more energy than lower mass ones more luminous than lower mass ones. Comes at a cost. High mass stars consume their core hydrogen fuel much faster than lower-mass ones. Our Sun has sufficient hydrogen in its core to last about 10 billion years on the main sequence. A 5 solar-mass star would consume its core hydrogen in about 70 million years An extremely massive star may only last 3 or 4 million years. Slide 14 Mass/M Sun Luminosity/L Sun Effective Temperature (K) Radius/R Sun Main sequence lifespan (yrs) 0.10 310 -3 2,900 0.16 210 12 0.50 0.03 3,800 0.6 210 11 0.75 0.3 5,000 0.8 310 10 1.0 1 6,000 1.0 110 10 1.5 5 7,000 1.4 210 9 3 60 11,000 2.5 210 8 5 600 17,000 3.8 710 7 10 10,000 22,000 5.6 210 7 15 17,000 28,000 6.8 110 7 25 80,000 35,000 8.7 710 6 60 790,000 44,500 15 3.410 6 Key Properties of Main Sequence Stars Slide 15 Slide 16 Spectral Class Type of Stars 1.Class O Main sequence star Hydrogen fusing Very hot and extremely luminous Have between 15 and 90 times the mass of the Sun Surface temperatures between 30,000 and 52,000 K Between 30,000 and 1,000,000 times as luminous as the Sun Very hot cores Burn through hydrogen very quickly First stars to leave the main sequence Bluish in color Output is in the ultraviolet range Over a million times our Sun's output Rarest of all main-sequence stars Most massive Example: Zeta Orionis Slide 17 Class B Main sequence star Hydrogen fusing Very luminous and blue Very hot and powerful Surface temperatures between 10,000 and 30,000 K Short lived Cluster together in what are called OB associations associated with giant molecular clouds. Example: Pleiades Slide 18 Class A Main sequence star Hydrogen fusing more common naked eye stars white or bluish-white surface temperatures between 7,600 and 10,000 K Example: Vega Slide 19 Class F Main sequence star Hydrogen fusing surface temperatures between 6,000 and 7,600 K Yellow white color Example: Procyon Slide 20 Class G Best known Main sequence star Hydrogen fusing Surface temperature of between 5,300 and 6,000 K Converts H to He in its core by means of nuclear fusion Yellow to white in color Example: Sol (our sun) Slide 21 Class K Main sequence star Hydrogen fusing Orange color Giants and supergiants Surface temperatures between 3,900 and 5,200 K Are of particular interest in the search for extraterrestrial life They are stable on the main sequence for a very long time (15 to 30 billion years, compared to 10 billion for the Sun). May create an opportunity for life to evolve on terrestrial planets orbiting such stars. Example: Epsilon Eridani Slide 22 Class M Most common Late Main sequence Red giants and red dwarfs Red Dwarf Small and relatively cool star Surface temperature of less than 4,000 K Most common star type in the galaxy Less than half the mass of the Sun (down to about 0.075 solar masses) Example: Barnards Star Slide 23 Red giants Evolve from main sequence stars Low to intermediate mass (0.5 10 solar masses) Late phase of stellar evolution Outer atmosphere is inflated and tenuous Immense radius and low surface temperature, (5,000 K and lower) Hydrogen fusing shells Tens to hundreds of times larger than that of the Sun Example: Aldebaran Slide 24 Red Giants A red giant is a large star that is reddish or orange in color. It represents the late phase of development in a star's life, when its supply hydrogen has been exhausted and helium is being fused. This causes the star to collapse, raising the temperature in the core. The outer surface of the star expands and cools, giving it a reddish color. Red giants are very large, reaching sizes of over 100 times the star's original size. Very large stars will form what are called red supergiants. Betelgeuse in Orion is an example of a red supergiant star. Slide 25 Brown Dwarf A failed star Some protostars never reach the critical mass required to ignite the fires of nuclear fusion. If the protostar's mass is only about 1/10 that of the Sun, it will glow only briefly until its energy dies out. What remains is a giant ball of gas that is too massive to be a planet but not massive enough to be a star. They are smaller than the Sun but several times larger than the planet Jupiter They emit no light or heat. Just energy in the infrared wavelength Slide 26 Slide 27 Gliese 229B The best known brown dwarf, and one that we can actually look at through an Earth-bound 60-inch telescope, is Gliese 229B, discovered in 1995. This one is in a binary system with the low-mass red dwarf Gliese 229A, at a distance of just 19 light-years from the Sun. Slide 28 The separation between the brown dwarf and its companion star is about the same as that between the Sun and Pluto. Its luminosity is about one tenth of the faintest star. Its spectrum has large amounts of methane and water vapor. Methane could not exist if the surface temperature were above 1500K. Slide 29 Astronomers consider its temperature to be about 900K (compared to Jupiters 130K), its mass to be between 20 and 55 Jupiters, and the age of the binary system to be between 1 and 5 billion years old. It has a smoggy haze layer deep in its atmosphere, essentially making it "much fainter in visible light than it would otherwise be". Slide 30 Binary Stars A system of two stars that are gravitationally bound to each other. They orbit around a common point, called the center of mass. It is estimated that about half of all the stars in our galaxy are part of a binary system. Slide 31 Visual binaries can be seen as two separate stars through a telescope. Spectroscopic binaries appear as one star and can only be detected by studying the Doppler shifts on the star's spectrum. Eclipsing binaries are binary systems where one star blocks the light from another as it orbits its companion. Slide 32 Types of Binary Systems 3 Types Detached binary: neither star fills its Roche lobe, so that there is no significant mass transfer between the components. Semidetached binary: one of the stars fills its Roche lobe, which results in this star losing material in a matter stream that either falls directly onto its companion, or, as is more usual, that enters an accretion disk. Contact binary both components fill their Roche lobes or, more often, overflow them so that there is a common convective envelope. Slide 33 Roche lobe The volume around a star in a binary system in which, if you were to release a particle, it would fall back onto the surface of that star. A particle released above the Roche lobe of either star will, in general, occupy the circumbinary region that surrounds both stars. The point at which the Roche lobes of the two stars touch is called the inner Lagrangian point. If a star in a close binary system evolves to the point at which it fills its Roche lobe, calculations predict that material from this star will overflow both onto the companion star (via the L1 point) and into the environment around the binary system. Slide 34 Cepheid Variable Stars Stars that changes in brightness. These fluctuations can range from seconds to years depending on the type of variable star. Stars usually change their brightness when they are young and when they are old and dying. They are classified as either intrinsic or extrinsic. Intrinsic variables change their brightness because of conditions within the stars themselves. Extrinsic variables change brightness because of some external factor, like an orbiting companion star. These are also known as eclipsing binaries. Slide 35 White Dwarf A small star composed mostly of electron- degenerate matter. weight of overlying material tries to force all of the electrons surrounding the atomic nucleus into the lowest energy state. The electrons resist and exert a pressure that halts further collapse. Very dense; Comparable to that of the Sun and its volume is comparable to that of the Earth. Slide 36 After the hydrogenfusing lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to carbon and oxygen in its core. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After shedding its outer layers to form a planetary nebula, it will leave behind this core, which forms the remnant white dwarf. Usually, therefore, white dwarfs are composed of carbon and oxygen. The material in a white dwarf no longer undergoes fusion reactions, so the star has no source of energy, nor is it supported by the heat generated by fusion against gravitational collapse. It is supported only by electron degeneracy pressure, causing it to be extremely dense Slide 37 Size Comparison of White Dwarf to Earth Slide 38 Neutron Star A type of stellar remnant that can result from the gravitational collapse of a massive star during a supernova event. Such stars are composed almost entirely of neutrons. Neutron stars are very hot and are supported against further collapse by electron degeneracy pressure. No two subatomic particles can occupy the same place simultaneously. Slide 39 A typical neutron star has a mass between 1.35 and about 2.0 solar masses A radius of about 12 km Density is 2.610 14 to 4.110 14 times the density of the Sun A pulsar is a neutron star that emits beams of radiation that sweep through Earth's line of sight Slide 40 Pulsar A rotating neutron star that emits beams of radiation that sweep through Earth's line of sight The "pulses" of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star's rotation axis and its magnetic axis. Pulsars seem to pulse from our perspective because the rotation of the neutron star causes the beam of radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period, somewhat like the beam of light from a lighthouse. The stream of light is, in reality, continuous, but to a distant observer, it seems to wink on and off at regular intervals. Slide 41 Slide 42 Slide 43 Black Hole Black holes are remnants of stellar masses that form when heavy stars (> 10 solar masses) collapse in a supernova at the end of their life cycle. Creating a region of spacetime from which nothing, not even light, can escape. Due to extreme gravitational forces 2 Types Stellar: formed from collapsed massive star about 30 km in diameter Supermassive : found in galactic cores - about 10 AU Slide 44 Black Hole Anatomy Slide 45 Black Hole Structure A black hole's entire mass is concentrated in an almost infinitely small and dense point called a singularity. This point is surrounded by the event horizon - the distance from the singularity at which its escape velocity exceeds the speed of light. A rotating black hole is surrounded by the ergosphere, a region in which the black hole drags space itself. Slide 46 Black Hole Structure The singularity forms when matter is compressed so tightly that no other force of nature can balance it. In a "normal" star, like the Sun, the inward pull of gravity is balanced by the outward pressure of the nuclear reactions in its core. In the collapsed stars known as white dwarfs or neutron stars, other forces prevent the ultimate collapse. If there is too much mass in a given volume, though, the object reaches a critical density where nothing can prevent its ultimate collapse to form a black hole. Because gravity overcomes the other forces of nature, a singularity follows its own bizarre rules of physics. Time and space as we know them are crushed out of existence, and gravity becomes infinitely strong. As the distance from the singularity increases, the escape velocity decreases. Escape velocity is the speed at which an object must move to get away. For Earth, the escape velocity is around seven miles (11 km) per second. In other words, a spacecraft must go at least that fast to escape Earth's gravitational pull and travel to another planet. At a certain distance from the singularity, the escape velocity drops to the speed of light (about 186,000 miles/300,000 km per second). This distance is known as the Schwarzschild radius, in honor of Karl Schwarzschild, who first defined it. This radius depends on the mass of the black hole. For a black hole as massive as the Sun, the radius is about two miles (3 km). For every extra solar mass, the radius increases by two miles. Slide 47 Black Hole Structure This radius enfolds the singularity in a zone of blackness - in other words, it makes a black hole black. It gives the black hole a visible surface, which is known as the event horizon. This is not a solid surface, though. It is simply the "point of no return" for anything that approaches the black hole. Once any object - from a starship to a particle of light - crosses inside this horizon, it cannot get back out. It is trapped inside the black hole. Anything that enters the black hole increases its mass. And as the mass goes up, the size of the event horizon gets bigger, too. So if you feed a black hole, it gets fatter! If the black hole doesn't rotate, then its gravitational influence on its environment is straightforward. If the black hole is spinning, though, then its gravitational effects are more complicated. It actually pulls the fabric of spacetime along with it - an effect called frame dragging. This area is known as the ergosphere. Seen in cross-section, it is oval-shaped, with the region of influence extending farther into space at the black hole's equator than at its poles. Slide 48 Slide 49 Stellar Black Hole Slide 50 Supermassive Black Hole Slide 51 Slide 52 Slide 53