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Chapter 25: Problem 5

Quasar PC \(1247+3406\) is presently about \(25.9\) billion lightyears from Earth. Explain how it is possible for astronomers to see this quasar, even though light travels at a speed of 1 light-year per year.

Short Answer

Expert verified
We can observe Quasar PC \(1247+3406\), which is \(25.9\) billion light-years away, because the light we are seeing started its journey billions of years ago. While it journeyed towards Earth, the universe continued expanding, moving the quasar further away. This is why current observation distances can exceed the age of the universe measured in light-years.

Step by step solution

01

Understand the Problem

Quasar PC \(1247+3406\) is currently about \(25.9\) billion light-years away from Earth, yet this distance seems impossible as it is greater than the age of the Universe which is approximately 13.8 billion years. This seems paradoxical, as conventional wisdom holds that light travels one light-year distance per year.
02

Consider Light Travel and Cosmic Expansion

While it's true that light travels at a consistent speed, it is also important to note that the universe itself is expanding. This means that while light from the quasar was traveling towards Earth, the universe (and the distance between us and the quasar) continued to increase.
03

Explain Observations

The light from the quasar that we see today actually started its journey billions of years ago. In the meantime, the universe has been expanding, so the quasar is currently much further away than when the observed light began its journey. For this reason, astronomers are able to observe the quasar from such a distant location.

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Most popular questions from this chapter

Use a telescope with an aperture of at least \(20 \mathrm{~cm}\) (8 in.) to observe the Seyfert Galaxy NGC 1068 (also known as M77). Located in the constellation Cetus (the Whale), this galaxy is most easily seen from September through January. The epoch 2000 coordinates are R.A. \(=2^{\mathrm{h}} 2.7^{\mathrm{m}}\) and Decl. \(=-0^{\circ} 01^{\prime}\). Sketch what you see. Is the galaxy's nucleus diffuse or starlike? How does this compare with other galaxies you have observed?

Relativistic Redshift. Access the Active Integrated Media Module "Relativistic Redshift" in Chapter \(2.5\) of the Universe Web site or eBook. Use this to calculate the redshift and recessional velocity of quasar in whose spectrum the \(\mathrm{H}_{\mathrm{\alpha}}\) emission line of hydrogen (unshifted wavelength \(656 \mathrm{~nm}\) ) appears at a wavelength of (a) \(937 \mathrm{~nm}\) and (b) \(5000 \mathrm{~nm}\).

If you have access to a telescope with an aperture of at least \(40 \mathrm{~cm}\) (16 in.), you might try to observe the brightest-appearing quasar, \(3 \mathrm{C} \mathrm{} 273\), which has an apparent magnitude of nearly \(+13\). It is located in Virgo at coordinates R.A. \(=12^{\mathrm{h}} 29^{\mathrm{m}} 07^{\mathrm{s}}\) and Decl. \(=+2^{\circ} 03^{\prime} 07^{\prime \prime}\).

Suppose that an astronomer discovers a quasar with a redshift of \(8.0\). With what speed would this quasar seem to be receding from us? Give your answer in \(\mathrm{km} / \mathrm{s}\) and as a fraction of the speed of light.

What do the brightness fluctuations of a particular active galaxy tell us about the size of the energy-emitting region within that galaxy?
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