5. The elderly Sun At some stage in its life, the Sun will become a red giant. A

Question

5. The elderly Sun At some stage in its life, the Sun will become a red giant. At that point, its angular size as seen from Earth will be about 30 degrees. Remember that right now its angular size is 30 arcminutes. In this problem we will be eventually calculating how much time a sunset will take when the Sun is a red giant For all parts in this problem, show your work. (a) Objects in the sky, including the Sun, appear to move because the Earth rotates. Knowing this, and using the length of one day, how many degrees will the objects in the sky appear to move in one hour (b) Define the length of a sunset to be the time between when the bottom edge of the Sun first touches the horizon until the time when the top edge of the Sun touches the horizon. How long does a sunset last now? c) How long will a sunset be when the Sun is a red giant? 6. Escape speed from a red giant Remember that the escape speed, v, from an object can be calculated by starting with U U is the gravitational potential energy and K is the kinetic energy. Solving for v gives K, where 2GM When the Sun reaches its red giant phase, it will have a radius that is approximately 100 times as large as the current radius of the Sun (a) Calculate the ratio of the escape speed from the Sun in its red giant phase to the escape speed from the Sun as it is today. In other words, find RG/Vo (b) The actual escape speed from the Sun today is around ve =620 km/s. Combine this value with the answer that you found for part (a) to find what RG is in km/s. 7. The stellar sequence [5pts Come up with your own mnemonic to remember the stellar sequence: OBAFGKMLT. Make it clean enough so that you could read it on the radio and avoid a fine from the FCC

Explanation / Answer

5.
a) In 24 hours the object moves 3600
So in one hour the object moves
1 hour = (360 /24)0 = 150
b) The angular size of the sun is 30 arcminutes
We can find the time required for the earth to rotate 1 arcminute
For 10 = (1 /15) hours
1 arcminute = (60 / (15 x 60)) minutes
30 arcminute = (60 / (15 x 60)) x 30 = 2 minutes
So it takes 2 minutes for the sunset
c) The angular size of the sun after it becomes red giant as seen from earth is 300
We know that for 150 rotation,the earth takes one hour  
So the sunst will last 2 hours

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