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Chapter 20: Problem 10

Why do we not observe planetary nebulae that are more than about 50,000 years old?

Short Answer

Expert verified
Planetary nebulae are not observed beyond about 50,000 years because by this time the gas shell of the nebula has expanded and thinned out to such an extent that it is no longer ionized by the uv radiation from the central white dwarf, ceasing to glow and hence, not visible.

Step by step solution

01

Understanding the formation of Planetary nebulae

Planetary nebulae are formed when ageing low-to-mid-mass stars (like Sun) shed off their outer layers. This results in a glowing shell of gas and dust. Inside the nebula, the core of the star remains as a dense and hot white dwarf.
02

Discussing life cycle of a Planetary nebula

Once the planetary nebula is formed, the glowing gas expands and cools, while the embedded white dwarf emits ultraviolet (UV) radiation. This UV radiation ionizes the expanding gas, causing it to glow and create the visible nebula.
03

Age of a Planetary nebula

Over time, the nebula expands away progressively, thinning out the gas shell until it becomes so thinly scattered that it is no longer ionized by the UV radiation of the white dwarf and stops glowing, thus ceases to be visible. This process takes about 50,000 years, which is why we don't observe planetary nebulae more than this age.

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Most popular questions from this chapter

. Use a telescope to observe the remarkable triple star 40 Eridani, whose coordinates are R.A. \(=4^{\mathrm{h}} 15.3^{\mathrm{m}}\) and Decl. \(=-7^{\circ} 39^{\prime}\). The primary, a 4.4-magnitude yellowish star like the Sun, has a 9.6-magnitude white dwarf companion, the most easily seen white dwarf in the sky. On a clear, dark night with a moderately large telescope, you should also see that the white dwarf has an 11 th-magnitude companion, which completes this most interesting trio.

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