Question

Astronomy Ranking Task: Star Evolution & Lookback Time Exercise #1 Description: Imagine that the four stars listed below all became Main Sequence (MS) stars at exactly the same time 10 billion years ago but in different locations of the universe. Cosmo Star is an O spectral class star with a MS lifetime of 3 million years. Its life will eventually end as a SN type II and become a black hole. Cosmo Star is located in a galaxy 10 billion light years (ly) from Earth. Ollie Star is a K spectral class star with a MS lifetime of 30 billion years. Its life will eventually end as a cooling white dwarf. Ollie Star is located in the MW at a distance of 10,000 ly from Earth. Sullivan Star is an F spectral class star that is part of a binary star system. It has a MS lifetime of 5 billion years. Its life will eventually end in a SN type I that completely destroys Sullivan Star. Sullivan Star is located in a galaxy 6 billion ly from Earth. Sliver Star is a B spectral class star with a MS lifetime of 20 million years. Its life will eventually end as a SN type II and become a neutron star. Sliver is located in the MW at a distance of 40,000 ly from Earth. A. Ranking Instructions Rank the stars (from first to last) based on when they became main sequence stars. Ranking Order: First 1_ _2_ _3_ _4_ Or, all the stars became main sequence stars at the same time. _ _ Last _ (indicate with check mark). Carefully explain your reasoning for ranking this way: B. Ranking Instructions Rank the stars (from first to last) based on when they reach(ed) their final end state. Ranking Order: First 1_ 2 _3___ 4 _ Last Or, all the stars reach(ed) their final end state at the same time. (indicate with check mark). Carefully explain your reasoning for ranking this way:

Answer 1

A) the Ranking is as follows:

1. Cosmo star , 2. Sullivan star , 3. Silver star , 4. Ollie star.

The reasoning behind this is as follows:

As seen from the Earth,these stars became main sequence stars at the same time. But they were formed in different regions of the universe ,at different distances. So the light that emerged from stars when they entered the main sequence,must have taken a long time to reach Earth. For a star like Cosmo star,10 billion lightyears away, this time is 10 billion years. So even though from Earth it appears like this star entered main sequence now,it actually happened 10 billion years ago. So we have established that the farther the star, the sooner it entered the main sequence. The stars like Ollie star which is only 10000lightyears away, this happened 10000 years ago. So we can rank them in the descending order of their distances from Earth.

B) 1. Cosmo star , 2. Sullivan star , 3. Silver star , 4. Ollie star.

Now we know that these stars were at different times. The Cosmo star was formed 10 billion years ago,but it's lifespan is only 3 million years. So it has already perished and turned to a white dwarf. Same argument can be made for Sullivan star. It has a lifespan of 5 billion years,but it was formed 6 billion years ago. So this star reached an end about 1 billion years ago.

The two stars that are still on the main sequence are silver star and Ollie. Silver star was formed 40000 years ago and will last for another 20 million years. Ollie was formed 10000 years ago and it will last another 30 billion years.