lecture15

1. Introduction to modern astronomy15 島袋隼⼠(Hayato Shimabukuro)（云南⼤学、 SWIFAR） ©GETTYIMAGES

2. 8. The interstellar medium （星际介质）

3. Interstellar medium •This is a photography of Milky Way. •You can find black regions in the Milky Way. These regions are called interstellar medium(星际介质). •The starlight is blocked by the interstellar medium. > We cannot study interstellar medium by optical telescope easily!

6. Interstellar medium •Interstellar medium is made up of two components, gas(⽓体) and dust（尘埃） Gas : The gas is made up mainly of individual atoms or molecules. The typical size of gas is roughly 10−10 m(0.1nm) Dust: The dust consists of clumps of atoms and molecules. Typical size is The dust is not very well known. 10−7 m(0.1μm) •The density of interstellar medium is extremely low. ~ 0.01 − 1000/cm3 •For dust, it is even rarer ~10−6 /cm3

7. Why sunset is red?? •We know the sunset seems to be red. •This is because shorter wavelength(bluer) light from sun is scattered by dust in the air and longer wavelength(red) of light can reach us. Same physics is happened in the dust in the universe!

8. Extinction and Reddening •The overall starlight is reduced by dust. This phenomenon is called extinction（消光） •As similar as the case of sunset, long- wavelength(red) light from stars reaches us more than short-wavelength(blue) light.This is called reddening（红化） •(Left) Because of extinction, the interstellar region is dark by visible band. •(Right) But, when we observe interstellar medium by infrared band, it is bright due to reddening.

9. Nebulae •Astronomical objects which consists of cosmic dust and interstellar gas is called nebulae （星云）⻢头星云 •If cloud happens to obscure(遮挡) stars lying behind it, we see the cloud as a dark patch on bright background. In this case, we call it dark nebulae. •But, if there are hot young stars within the cloud, they ionize nebulae and we see bright emission (emission nebulae). LHA120 -N55

10. Emission nebulae Ionized gas Dust lane •At on near the center of nebula, there are hot O or B-type stars. They emit ultraviolet photons and ionize nebula. From emission lines, we can know typical temperature of emission nebula is roughly 8000K. Reflection nebula •Dust lane consists of a relatively dense region where star light is obscured. •Reflection nebula is caused by starlight scattered from dust particles in interstellar clouds.

11. Nebulae structure

12. Nebulae structure

13. Dark dust clouds Dark dust clouds are… •even colder than their surroundings (~ a few tens of K) •1’000- 1’000’000 times denser than surroundings ( ). 107 − 1012 atoms/m3 (*)~8000K for emission nebula •Because dark dust clouds are dense, they obscure visible light. •But, we can observe dark dust clouds by radio wavelength because dark dust clouds emits radio emission(e.g CO).

14. Dark dust clouds Dark dust clouds are… •even colder than their surroundings (~ a few tens of K) •1’000- 1’000’000 times denser than surroundings ( ). 107 − 1012 atoms/m3 (*)~8000K for emission nebula Low temperature and dense number density •Because dark dust clouds are dense, they obscure visible light. •But, we can observe dark dust clouds by radio wavelength because dark dust clouds emits radio emission(e.g CO).

15. Absorption spectra •Since dark dust clouds are high density and low temperature, they generate (narrow) absorption lines. •Absorption lines tell us what kind of elements are contained in the dark interstellar clouds. •We can distinguish absorption lines generated by star itself or generated by cloud because absorption lines generated by stars are broad, while absorption lines generated by clouds are narrow.

16. 21cm radiation •Another probe to interstellar medium is 21cm emission line. •Although it is difficult to observe interstellar medium by optical wavelength, we have another probe to study interstellar medium. •The 21cm emission line is radio wavelength radiation from neutral hydrogen atom. •21cm line emission is generated by energy transition in neutral hydrogen atom High energy Low energy

17. Interstellar molecules（星际分⼦） •In some particular cold (10-20K) interstellar regions, densities can reach as high as . At these regions, material exist in the form of molecule. 1012 particles/m3 •Molecules emit radiation by rotating or violating. We can use molecules as tracer of interstellar medium.

18. Summary • The interstellar medium occupies the space among the stars. • A nebula is a fuzzy bright or dark patch on the sky. • Dark dust clouds are cold and dense. • The interstellar medium also contains many cold, dark molecular clouds. • One of the methods to observe interstellar medium is absorption lines or 21cm emission line.

19. 9.Star formation （恒星的形成）

20. Where do stars form?? (Question ) We know there are many stars in the universe, but where do stars form??

21. Where do stars form?? (Question ) We know there are many stars in the universe, but where do stars form?? (Answer) Stars formed in the interstellar medium(星际介质)

22. Star forming regions •Young blue stars are born in the interstellar medium. •In the interstellar medium, one cold gas begins to collapse to form stars by its gravity. •Let’s see more details how stars form next.

23. F = G Mm r2 Newton’s gravity 1. In the interstellar medium, there are many gas particles such as hydrogen atom and hydrogen molecules. Gravity and heat

24. F = G Mm r2 Newton’s gravity 2. Gas particles come together by their own gravity. Gravity and heat

25. F = G Mm r2 Newton’s gravity 2. Gas particles come together by their own gravity. Gravity and heat

26. F = G Mm r2 Newton’s gravity Gravity and heat 3. But, when gas particles come together, the gas is heated and the temperature increases. This also causes the increase of gas pressure. T

27. F = G Mm r2 Newton’s gravity Gravity and heat 3. But, when gas particles come together, the gas is heated and the temperature increases. This also causes the increase of gas pressure. T

28. F = G Mm r2 Newton’s gravity Gravity and heat 3. But, when gas particles come together, the gas is heated and the temperature increases. This also causes the increase of gas pressure. T T

29. F = G Mm r2 Newton’s gravity Gravity and heat 3. But, when gas particles come together, the gas is heated and the temperature increases. This also causes the increase of gas pressure. T T Gas pressure P ∝ T

30. F = G Mm r2 Newton’s gravity Gravity and heat 3. But, when gas particles come together, the gas is heated and the temperature increases. This also causes the increase of gas pressure. T T Gas pressure P ∝ T

31. F = G Mm r2 Newton’s gravity Gravity and heat 4. Gas pressure prevents gas particle from gravity collapsing T Gas pressure P ∝ T

32. Gravity and heat •We have to consider both gas pressure and gravity for star formation. P F •After gas pressure and gravity are balanced ( ), nuclear fusion starts in the core of star and stars begin to shine on themselves. Star is born! F = P

33. Gravity and heat F = G Mm r2

34. Gravity and heat F = G Mm r2

35. Gravity and heat T F = G Mm r2 P ∝ T

lecture15

lecture15

Recomendados

Más contenido relacionado

Similar a lecture15

Similar a lecture15(20)

Más de Hayato Shimabukuro

Más de Hayato Shimabukuro(20)

Último

Último(20)

lecture15