# lecture22

Associate professor um Yunnan university
15. May 2023
1 von 29

### lecture22

• 1. Introduction to modern astronomy 22 島袋隼⼠(Hayato Shimabukuro)（云南⼤学、 SWIFAR） ©GETTYIMAGES
• 2. Galactic spiral arms •The milky way galaxy has “spiral arms”. ~30kpc
• 3. Galactic spiral arms What do 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 How does galactic spiral arms form? •At first, let’s first see how traffic jam happens!
• 5. Galactic spiral arms How does galactic spiral arms form? •At first, let’s first see how traffic jam happens!
• 6. Galactic spiral arms How does galactic spiral arms form? •At first, let’s first see how traffic jam happens!
• 7. How does galactic spiral 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 How does galactic spiral arms form?
• 9. Galactic spiral arms How does galactic spiral arms form?
• 10. Galactic spiral arms How does galactic spiral arms form?
• 11. Galactic spiral arms How does galactic spiral arms form?
• 12. The mass of milky 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 milky way 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 of motion 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 What exists 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 It may 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 It may 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 •In 2022, EHT directly observed the Sgr A* located in the center of the Milky Way galaxy.
• 21. Summary • The Milky Way 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.
• 23. Is the Milky Way galaxy the only galaxy??
• 24. No! Even children know that there are many galaxies in the universe. Is the Milky Way galaxy the only galaxy??
• 25. No! Even children know that there are many galaxies in the universe. Is the Milky Way galaxy the only galaxy??
• 26. No! Even children know 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 Milky Way 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 Milky Way 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 observed variable 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” !!