astr 115 final exam study guide | Detailed and Complete Solutions

Hydrostatic Equilibrium The current distinguishing criterion between dwarf planets and small Solar System bodies Evolution of stars of different mass -Star formation produces a wide variety of stars -Only a few high-mass stars (O, B Type) are produced -The lowest mass stars (eg.M type) are the most common. -If a "star" forms with a mass less than about 1/10 the mass of the Sun, it will not have enough pressure in its core to ignite nuclear fusion. •The lower limit to the mass of a true star is:0.08 Msun •Smaller objects are not true stars, they are called "brown dwarfs" •They don't produce much light, but can sometimes be seen in infrared wavelengths. HENCE, -Stars "live" by fusing Hydrogen into Helium -When they run out of Hydrogen, the begin to "die" -Their temperature and luminosity change, so they "move" off the Main Sequence on the HR diagram. -When a medium-mass star (like the Sun) begins to die, it turns into a Red Giant, a Planetary Nebula, then a White Dwarf. Red Dwarfs A small, dim star with relatively cool surface temperatures, positioned to the lower right on the main sequence in the Hertzsprung-Russell diagram. Red dwarfs, at about 0.1 to 0.5 solar mass, consume their nuclear fuel very slowly and live for about 100 billion years. Although they are difficult to see, they are so long-lived that they are likely the most abundant type of star. Giant Stars A very large, bright non-main-sequence star that burns hydrogen at a much faster rate than a dwarf star. Giant stars are much more luminous and have shorter lifespans than the slower-burning dwarfs. The larger the giant, the shorter its lifespan; the largest stars, with solar mass of around 100, blaze at several hundred thousand times the energy of the Sun and will last only a few million years, a very brief time when compared with the Sun's 10- billion-year lifespan. Supergiants A star that is larger, brighter, and more massive than a giant star, being thousands of times brighter than the Sun and having a relatively short lifespan—only about 10 to 50 million years as opposed to around 5 billion years for the Sun. Cepheid Variable Stars as Standard Candles A class of variable stars with regular cycles of variations in luminosity (most ranging from three to fifty days). There is a relationship between the periods of variation and the absolute magnitudes, which is used for measuring the distance of such stars. Often supergiants. Deaths of StarsThe core runs out of hydrogen & then helium, the core contracts & the outer layers expand, cool, & become less bright -Will eventually collapse & explode -They, assuming they are massive stars can become 1000 times larger than the Sun! *They are called:red supergiants -Lifetime & fate is determined by mass of star White Dwarfs A small very dense star that is typically the size of a planet. A white dwarf is formed when a low-mass star has exhausted all its central nuclear fuel and lost its outer layers as a planetary nebula. Neutron Stars a celestial object of very small radius (typically 18 miles/30 km) and very high density, composed predominantly of closely packed neutrons. Neutron stars are thought to form by the gravitational collapse of the remnant of a massive star after a supernova explosion, provided that the star is insufficiently massive to produce a black hole. Black Holes A black hole is a region of spacetime where gravity is so strong that no particle or light ray entering that region can ever escape from it. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Supernovae A stellar explosion that briefly outshines an entire galaxy, radiating as much energy as the Sun or any ordinary star is expected to emit over its entire life span, before fading from view over several weeks or months. Type Ia Their light curves exhibit sharp maxima & then die away gradually, typically occurs in elliptical galaxies, originate from white dwarfs, astronomers use it as "standard candles" to measure cosmic distances because all are thought to blaze with equal brightness at their peaks Type II -Must be several times more massive than the sun, heavier elements build up at the center it becomes layered like an onion, with elements becoming lighter towards the outside of the star. -Once the star's core surpasses a certain mass it begins implode. Core heats up & becomes denser. Eventually the implosion bounces back off the core, expelling the stellar material into space, forming the supernova Pulsars Supernovae can be triggered in one of two ways: by the sudden reignition of nuclear fusion in a degenerate star; or by the gravitational collapse of the core of a massive star. Discovery of Pulsars Pulsars were discovered by accident in 1967 while Jocelyn Bell and Antony Hewish were looking for twinkling sources of radio radiation. The explanation for the radio pulses proved the existence of neutron stars, incredibly dense remains of massive collapsed stars. Lighthouse Model - ______ for Pulsars= A pulsar is a rotating neutron star. *Its beam is like a ____ beacon. *If the beam shines on Earth, then we see a Pulse of radio waves -explains pulsars as spinning neut