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IB Physics SL: IA
Here is my IA I submitted for my physics subject in which my final grade was a 7. It is a model of an aspect of astronomy.
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How many years will it take for Sirius to disappearfrom sight from the Earth’s surface at night?
1
,Table of contents:
Table of contents: 2
Introduction: 3
Rationale: 3
Aim: 3
Context: 3
Hypothesis: 3
Plan of procedure: 3
Body of exploration: 4
Assumptions: 4
Sirius A’s apparent magnitude: 4
Sirius A’s luminosity: 4
Sirius’ distance from the Earth: 5
Sirius’ apparent magnitude: 6
How apparent magnitude changes with distance: 7
Time it will take for us not to see Sirius anymore: 8
Radial velocity: 8
Closest distance to which Sirius will approach the Sun in 60,000 years before it starts
receding: 11
Sirius’ apparent brightness when it is at its closest to the Sun: 11
Distance that needs to be travelled to reach an apparent brightness of 6.5: 11
Time taken to travel that distance: 11
Total time taken: 12
Sirius’ orbit: 12
Conclusion: 13
Evaluation: 13
Bibliography: 13
2
, Introduction:
Rationale:
I had decided to complete this exploration after watching a video from the Youtube Channel of
Kurzgesagt – In a Nutshell. It was the video explaining “TRUE Limits Of Humanity – The Final
Border We Will Never Cross”. He mentioned in it that there will be a time when we won’t be able to
see the stars from Earth anymore due to the expanding universe. Thus, I decided to do a very
simplified exploration of this subject with the use of the brightest star in the sky: Sirius.
Aim:
I will therefore be calculating the number of years it would take for the star Sirius to disappear from
the naked eye’s sight from Earth.
Context:
Sirius A is the brightest star in the night sky. Its name is derived from the Greek word Σείριος
(Seirios, lit. 'glowing' or 'scorching'). It is designated α Canis Majoris (Alpha Canis Majoris). Sirius is
gradually moving closer to the Solar System, so it is expected to slightly increase in brightness over
the next 60,000 years.1 After that time, its distance will begin to increase, and it will become fainter,
but it will continue to be the brightest star in the Earth's night sky for approximately the next 210,000
years.2
Since this star is the brightest star in the Earth’s night sky, we will infer that its current characteristics
will remain unchanged for the sake of the experiment and we’ll therefore use it as a reference for all
other stars seen in Earth’s night sky.
Hypothesis:
Knowing that the Earth is about 4.5 billion years old and has gone nearly 20 times around the Milky
Way, I can infer that it will take much less time than that for us not to see Sirius anymore. From this
information, we know that it takes approximately 225 million years for the Earth to complete one
orbit around the galaxy. Therefore, I believe that it will probably take less than half that value: 112.5
million years. It was also the first star to be identified since about 120 million years ago when it
became a white dwarf. I will therefore hypothesise that it would take about 10 million years until we
cannot see it anymore as it will start receding from the Sun in 60,000 years.
Plan of procedure:
1) Calculate Sirius A’s apparent magnitude: a numerical value to describe how we see Sirius by
naked eye:
a) Find its luminosity in the solar constant using Stefan’s Law
1
Tomkin, Jocelyn. “Once and Future Celestial Kings.” Free Online Library, Sky and Telescope, 1 Apr. 1998,
www.thefreelibrary.com/Once+and+future+celestial+kings-a020468305.
2
---. “Sirius.” Wikipedia, 8 Dec. 2021, en.wikipedia.org/wiki/Sirius.
3
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