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White Dwarfs Sirius, the brightest star in the sky, is a visual binary. Sirius A vastly outshines Sirius B in visible and infrared wavelengths. Sirius A Sirius B Infrared X-ray However, Sirius B outshines Sirius A in X-ray wavelengths. Sirius B is a white dwarf, and therefore much hotter than Sirius A. It is fainter in visible and infrared because it is so much smaller. However, Sirius A is not hot enough to radiate much in X-rays, so even a small star that is hot enough can outshine it in X-rays. So you think you know what a white dwarf is? • Its composition depends on the mass of the star it formed from: • The smallest stars form helium white dwarfs. • Sun-sized stars form carbon white dwarfs. • Larger (but still “low-mass”) stars form white dwarfs with a mix of heavier elements • A teaspoon of this material – held up against gravity by electron degeneracy pressure – would weigh several tons on Earth. More massive white dwarfs are smaller than less massive white dwarfs. (Degeneracy pressure only depends on density) Stronger gravity requires stronger degeneracy pressure to balance it. Which is hotter, a red giant star or a white dwarf? A) The red giant B) The white dwarf C) They are generally the same temperature D) Can’t answer without more information Which is more luminous, a red giant star or a white dwarf? A) The red giant B) The white dwarf C) They are roughly the same luminosity D) Can’t answer without more information Which is hotter, a red giant star or a white dwarf? A) The red giant B) The white dwarf C) They are generally the same temperature D) Can’t answer without more information Which is more luminous, a red giant star or a white dwarf? A) The red giant B) The white dwarf C) They are roughly the same luminosity D) Can’t answer without more information Which is larger, a 0.8 MSun white dwarf or a 1.3 MSun white dwarf? A) The 0.8 MSun white dwarf B) The 1.3 MSun white dwarf C) They are the same size D) Can’t answer without more information Back to strange behavior from close binary stars… Matter spilling off a partner star onto a white dwarf forms an accretion disk Accretion disks generate a lot of heat and can radiate in UV and even X-rays. That material eventually falls all the way to the surface of the white dwarf. Temperature and pressure increase as more and more hydrogen accumulates. When the temperature reaches 10,000,000 K, hydrogen fusion ignites! This burst of fusion is called a nova The nova shines for a few weeks, as brightly as 100,000 Suns at its peak The nova does not destroy the star. In fact, accretion resumes shortly after the nova subsides… A single white dwarf can go nova more than once: T Pyxidis went nova in 1890, 1902, 1920, 1944, 1966 and 2011 HST – VIS mv : 15.5 7.0 (15,600 ly away) Over many cycles of accretion – nova – accretion – nova … the white dwarf’s mass can grow. But there is a limit to how much mass can be supported by electron degeneracy pressure. When the white dwarf reaches about 1.4 MSun (the Chandrasekhar limit) it hits that limit and goes BOOM! This is called a white dwarf supernova When the white dwarf hits the mass limit, it gets hot enough for carbon fusion to start. It undergoes carbon fusion everywhere at once, so it’s a HUGE release of energy. This is called a “light curve” It plots luminosity as a function of time Type II – hydrogen lines present Type I – no hydrogen lines (WD SN = Type 1a) Astro-Cash Cab! Tyler Zach Aaron Katie 1) What is the source of energy in a white dwarf supernova? Nuclear fusion Nuclear fission Gravitational potential energy Chemical potential energy 2) Which supernova shows no hydrogen lines in its spectrum? Type I – White dwarf supernova Type II – Massive star supernova They both show hydrogen lines Neither shows hydrogen lines 3) What is the upper mass limit of a white dwarf? 4) True / False Someday our Sun may end its existence in a white dwarf supernova explosion.