Chapter 12, Problem 1QQ

Choose the best answer to each of the following. Explain your reasoning.

Which of these galaxies is likely to be oldest? (a) a galaxy in the Local Group (b) a galaxy observed at a distance of 5 billion light-years (c) a galaxy observed at a distance of 10 billion light-years

To determine

The oldest galaxy among the given options.

Answer to Problem 1QQ

A galaxy observed at a distance of 10 billion light years will be oldest.

Explanation of Solution

Introduction:

Light-year is the unit for measuring astronomical distance. It is defined as the distance travelled by the light in one year. It is used to measure the distance between two stars or planet.

  $\begin{array}{l}1\text{ light year =3}\times {\text{10}}^8\times 365\times 24\times 3600\\ 1\text{ light year = 9}\text{.46}\times {\text{10}}^{15}\text{m}\end{array}$

Option c is: A galaxy observed at a distance of 10 billion light years.

  $\begin{array}{l}1\text{ light year = 3}\times {\text{10}}^8\times 365\times 24\times 3600\\ 10\times {10}^9\text{ light year = }1\times {10}^9\times \text{9}\text{.46}\times {\text{10}}^{15}\text{m}\\ {10}^{10}\text{ light year = }9.46\times {10}^{25}\text{m}\end{array}$

This means that the light started travelling from these galaxies 10 billion light years ago. It is also possible that these galaxies do not exist now.

Thus, option (c) is correct.

Option a is: A galaxy in the local group.

Milky way galaxy is part of local group. Light from the other galaxies of the local group reach Earth in very few years. This means that these galaxies are not the oldest.

Thus, option (a) is incorrect.

Option b is - A galaxy observed at a distance of 5 billion light years.

  $\begin{array}{l}1\text{ light year = 3}\times {\text{10}}^8\times 365\times 24\times 3600\\ 5\times {10}^9\text{ light year = }5\times {10}^9\times \text{9}\text{.46}\times {\text{10}}^{15}\text{m}\\ 5\times {10}^9\text{ light year = }47.3\times {10}^{24}\text{m}\end{array}$

For a galaxy at 5 billion light years distance, it takes 5 billion years for light to reach us. It might exist or not currently. It would be an old galaxy but not older than the one present 10 billion light years away.

Thus, option (b) is incorrect.

Conclusion:

A galaxy observed at a distance of 10 billion light years would be oldest.