Chapter 21

This is Chapter 21 – Double Stars, Variable Stars and Novae, again if you notice something strange…. let me know! 😀

1. a) What is the difference between an optical double star and a binary?
Optical double stars are just stars that look like they are together. In fact, they could be lightyears apart, but from the angle on Earth, they sit in the same place in the sky. Binary stars are two stars that orbit each other.

1. b) Sirius is only about 2.5 times as massive as its companion, but is 10,000 times as brilliant. Explain this apparent discrepancy.
Sirius is a lot brighter than its companion, however, Sirius’ brightness hides the brightness of the companion. If Sirius was not there, then the companion would be a lot easier to see, as it would then be as bright as it is.

1. c) What are spectroscopic binaries, and how are they identified?
A pair (or more) of stars that are too close together to be seen separately, so they are identified using the Doppler effect. (Moving towards us, a star looks more blue; moving away, a star looks more red)

Variable Stars2. a) Using a diagram, explain why Algol changes in magnitude even though it is not intrinsically variable.
Algol changes because every few hours there is an eclipse. When the companioning star moves in front of Algol, it drops in brightness causing the magnitude to rise. Similarly when Algol moves in front of the companion.

2. b) A and B are stars with magnitudes of 2.6 and 3.8 respectively. A variable star is seen to be midway in brightness between these two. What is the magnitude of the variable?

The magnitude of the variable star would be 3.2, because it is midway.

2. c) What colour are the most of the Mira variables?
Most of the Mira variables are red, because they are red giants.

3. a) Explain why Cepheid variables are so important to astronomers.
These variables are important to astronomers because they are like a standard candle. There is a very tight correlation between their period of variability and absolute luminosity. This makes them good landmarks for working out the distance to its host cluster/galaxy.

3. b) Eta Aquilae and Zeta Geminorum are both Cepheid variables. Eta Aquilae has a period of 7.2 days, while the period of Zeta Geminorum is 10.1 days. Their apparent magnitudes are almost the same. Which star is the more remote, and why? (You may ignore factors such as the absorption of light in space.)
To have a longer period of variability means the star will have a higher luminosity; and to have a higher luminosity, the more remote the star.
Therefore, Zeta Geminorum has the bigger period and therefore higher luminosity. This means this star is more remote.

3. c) Why does R Coronae show sudden, unpredictable drops in brightness?
Because these stars have a large amount of Carbon in the atmosphere, every once in a while, the carbon builds up and causes a sooty blind across the star. This causes the brightness to decline.

4. a) Explain the cause of a nova outburst.
A nova is a binary system. A low density main sequence star has a neighbouring white dwarf, which slowly pulls mNovaeaterial away from the main sequence star. This accumulates around the dwarf as a ring. When enough has been built up, then a nuclear outburst occurs, ejecting gas at high velocity. Afterwards, the binary system returns to its normal state.

4. b) What is the difference between nova and a supernova?

A nova differs from a supernova, because a supernova generally occurs on a much larger scale and completely destroys the star. A type I supernova is similar to a nova, but in a nova, the star is NOT destroyed.

4. c) Why is amateur observation of variable stars of real scientific importance?
Because there are so many variable stars in the sky, and for professionals to look at all of them it would be far too difficult. As long as an amateur knows what they are doing, they are of immense scientific importance.


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