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lecture22
1. The document discusses the Milky Way Galaxy and other galaxies. 2. It describes how the existence of spiral galaxies was debated until Hubble observed variable stars in the Andromeda Nebula and measured its distance, finding it was outside the Milky Way Galaxy. 3. This supported Curtis's model that spiral nebulae exist outside the Milky Way and opened up the view that there are multiple galaxies in the universe beyond just the Milky Way.
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lecture22
- 1. Introduction to modernastronomy 22 島袋隼⼠(Hayato Shimabukuro)（云南⼤学、 SWIFAR） ©GETTYIMAGES
- 2. Galactic spiral arms •Themilky way galaxy has “spiral arms”. ~30kpc
- 3. Galactic spiral arms Whatdo spiral arms contain? •When we observe the galactic arm by radio wavelength, we clearly see its existence of molecular gas. The galactic spiral arm contains cold gas. •The spiral arms contain HII regions and dark nebulae, which are the main regions of star formation in spiral galaxies.
- 4. Galactic spiral arms Howdoes galactic spiral arms form? •At first, let’s first see how traffic jam happens!
- 5. Galactic spiral arms Howdoes galactic spiral arms form? •At first, let’s first see how traffic jam happens!
- 6. Galactic spiral arms Howdoes galactic spiral arms form? •At first, let’s first see how traffic jam happens!
- 7. How does galacticspiral arms form? Galactic spiral arms •If the speed of cars is decreased on a sloping road, the velocity fluctuations are propagated backward and the fluctuations are enhanced. •Then, the density of cars become large and cause traffic jam. Speed down
- 8. Galactic spiral arms Howdoes galactic spiral arms form?
- 9. Galactic spiral arms Howdoes galactic spiral arms form?
- 10. Galactic spiral arms Howdoes galactic spiral arms form?
- 11. Galactic spiral arms Howdoes galactic spiral arms form?
- 12. The mass ofmilky way galaxy m v2 r = G Mm r2 M = rv2 G The milky way galaxy and the sun Equation of motion for the sun : the mass of milky way galaxy (inside sun orbit). M : the mass of the sun. m : the distance between the galactic center and the sun. r M m r We can estimate the mass of the milky way galaxy by this formula
- 13. M = rv2 G r ∼8kpc ∼ 2.37 × 1017 km G = 6.67 × 10−20 km3 kg−1 s−2 v = 240km/s M = 2.05 × 1041 kg ∼ 1011 M⊙ The mass of milky way galaxy The mass of the milky way galaxy is roughly 1011 M⊙
- 14. Galactic rotation •The milkyway galaxy is rotating. We can measure galactic rotation speed by observing the Doppler shift of a 21cm emission line. m v2 r = G Mm r2 Equation of motion for each star r v = GM r ∝ r−1/2 r v
- 15. Galactic rotation v = GM r ∝r−1/2 •The observational galactic rotation curve data suggest that the rotation curve cannot be explained by Kepler motion!! Observational data Theoretical prediction Why??
- 16. Galactic rotation Equation ofmotion for each star v = GM r ∝ r−1/2 m v2 r = G Mm r2 •Except for stars, we may miss some missing mass. There is something that has gravity, but it is not visible in the Milky Way galaxy. Dark matter（暗物质） •But, we have not known what dark matter is. Unknown particle? Blackhole?… It still remains a mystery in the universe.
- 17. The galactic center Whatexists in the center of the Milky Way galaxy? Supermassive black hole (SMBH) called Sgr A* But, how did we find a supermassive black hole in the center of the Milky Way galaxy? Astronomers found a regular orbit of stars in the center of the Milky Way galaxy
- 18. The galactic center Itmay be a supermassive black hole (SMBH)! We need a massive object in the center of the Milky Way to explain this regular orbit •The Nobel Prize was awarded in 2020 for this discovery.
- 19. The galactic center Itmay be a supermassive black hole (SMBH)! We need a massive object in the center of the Milky Way to explain this regular orbit •The Nobel Prize was awarded in 2020 for this discovery.
- 20. The galactic center •In2022, EHT directly observed the Sgr A* located in the center of the Milky Way galaxy.
- 21. Summary • The MilkyWay Galaxy is a huge collection of stellar and interstellar matters. • In order to make a map of the Milky Way Galaxy, we need to measure the distance to the stars. (e.g. stellar parallax, variable stars) . • The galactic spiral arms formed by density wave theory • The rotation curve of stars suggests the existence of dark matter.
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- 23. Is the MilkyWay galaxy the only galaxy??
- 24. No! Even childrenknow that there are many galaxies in the universe. Is the Milky Way galaxy the only galaxy??
- 25. No! Even childrenknow that there are many galaxies in the universe. Is the Milky Way galaxy the only galaxy??
- 26. No! Even childrenknow that there are many galaxies in the universe. But, we did not know whether the Milky Way galaxy was the only galaxy or whether other galaxies existed in the universe. Is the Milky Way galaxy the only galaxy??
- 27. Is the MilkyWay galaxy the only galaxy?? •In 1920, Shapley and Curtis debated the following topics. 1. How large is the Milky Way galaxy? 2. Where is the spiral nebulae? Shapley Curtis These topics are related to the question “Is the Milky Way galaxy the only galaxy?”
- 28. Is the MilkyWay galaxy the only galaxy?? Milky Way galaxy Spiral nebula Sun Globular cluster Shapley’s model Curtis’s model Milky Way galaxy Spiral nebula •Shapley: The size of the Milky Way Galaxy is ~100 kpc and spiral nebulae exist in the Milky Way Galaxy. •Curtis: The size of the Milky Way Galaxy is ~10 kpc and spiral nebulae exist outside the Milky Way Galaxy.
- 29. Hubble’s answer •Hubble observedvariable stars in the Andromeda nebula and measured its distance. •He found that the Andromeda nebula is located outside the Milky Way Galaxy. Supporting Curtis’s model •Currently Andromeda nebula is known as “Andromeda galaxy” Hubble’s discovery revealed there are galaxies other than the Milky Way Galaxy!! It opens “the galaxy universe view” !!