1. Reading
Unit 31, 32, 51

2. Unit 26 problem 5, problem 6, problem 10
Homework 6
Unit 26 problem 5, problem 6, problem 10
Unit 28 problem 4, problem 5
Unit 29 problem 7
Unit 30 problem 7
Unit 31 problem 9
Unit 32 problem 9, problem 10
Unit 51, problem 9

3. Atmospheric Absorption
The Earth’s atmosphere absorbs most of the radiation incident on it from space
This is a good thing for life – high energy photons would sterilize the planet!
This is not a good thing for astronomy, however!
Visible, radio and some infrared wavelengths are not absorbed readily by the atmosphere
Optical and radio telescopes work well from the ground
Gamma Rays, X-rays, and UV photons are absorbed
Observatories for these wavelengths must be kept above the Earth’s atmosphere!

4. Ground- and Space-based Observatories

5. Ambient light from cities are a real problem for optical astronomy.
Light Pollution
Ambient light from cities are a real problem for optical astronomy.
This light pollution washes out images in telescopes.
Research telescopes are built far from cities to reduce the effects of light pollution
It is getting harder to find good locations for telescopes!

6. Air refracts light just like glass or water, but to a lesser degree.
Atmospheric Effects
Air refracts light just like glass or water, but to a lesser degree.
Cool air refracts light more than warm air
Pockets of cool air in the atmosphere create moving lenses in the sky, shifting the light rays randomly
This causes a twinkling effect, called scintillation.
A stable atmosphere causes less scintillation
We say the seeing is good.

7. This technique is called adaptive optics
Some observatories measure the amount of atmospheric turbulence with lasers, and then adjust the mirrors in their telescopes with tiny motors to eliminate the effect
This technique is called adaptive optics

8. Spitzer Space Telescope
James Web Space telescope

9. Edwin Hubble
Hubble Telescope

10. Observatories in Space

11. The Sun is a huge ball of gas at the center of the solar system
1 million Earths would fit inside it!
Releases the equivalent of 100 billion atomic bombs every second!
Exists thanks to a delicate balance of gravity and pressure

12. The immense mass of the Sun generates a huge gravitational force
A delicate balance…
The immense mass of the Sun generates a huge gravitational force
Gravity pulls all of the Sun’s matter toward its center
This crushing force produces a high temperature and pressure on the interior of the Sun
This balance of gravity and pressure will allow a star like the sun to live for billions of years

13. The Solar Interior

14. The photosphere is the visible “surface” of our star
Not really a surface, as the Sun is gaseous throughout
Photosphere is only 500 km thick
Average temperature is 5780 K

15. Energy Transport in the Sun
Just below the photosphere is the convection zone.
Energy is transported from deeper in the Sun by convection, in patterns similar to those found in a pot of boiling water (hot gas rises, dumps its energy into the photosphere, and then sinks)
Energy in the convection zone comes from the radiative zone.
Energy from the core is transported outward by radiation – the transfer of photons.
Takes more than 100,000 years for a single photon to escape the Sun!

16. Just above the photosphere lies the chromosphere
The Solar Atmosphere
Regions of the Sun above the photosphere are called the Sun’s atmosphere
Just above the photosphere lies the chromosphere
Usually invisible, but can be seen during eclipses
Spicules, found here, look like a prairie grass fire!
Above the chromosphere is the corona
Extremely high temperatures (more than 1 million K!)
Rapidly expanding gas forms the solar wind.

17. The surface and atmosphere of the Sun are extremely active
A Very Active Star
The surface and atmosphere of the Sun are extremely active
Solar wind streams out of coronal holes, regions of low magnetic field
Active regions send arcades of plasma shooting from the surface. These are regions of high magnetic field
Coronal mass ejections send large quantities of mass out into space
Solar flares release energy into space

18. Pressure = Constant  Temperature  Density
The Ideal Gas Law
Pressure = Constant  Temperature  Density

19. Stars like the Sun can be seen as having a kind of thermostat
The Solar Thermostat
Stars like the Sun can be seen as having a kind of thermostat
Gravity pulls inward, pressure pushes outward.
If temperature begins to fall, pressure decreases and gravity pulls more mass toward the center
This inward-falling mass increases the temperature and pressure, restoring balance!

20. How do we know all of this?
Naturally, we’ve never seen the inside of the Sun
Computer models suggest the layered structure we’ve discussed
We can probe the interior using helioseismology, the study of sunquakes!

21. Sunspots
Sunspots are highly localized cool regions in the photosphere of the Sun
Discovered by Galileo
Can be many times larger than the Earth!
They contain intense magnetic fields, as evidenced by the Zeeman effect

22. A Sunspot’s Magnetic Field
The intense magnetic fields found in sunspots suppress particle motion
Solar ions cannot leave these regions of high magnetic field, and the field lines are “frozen” to the plasma
This trapped plasma keeps hot material from surfacing below the sunspot, keeping it cool.

23. Prominences
Fields have their “footpoints” in sunspots in the photosphere
These loops are relatively unstable, and can release vast quantities of plasma into space very quickly
Prominences are large loops of glowing solar plasma, trapped by magnetic fields
Coronal Mass Ejections

24. Can damage satellites, spacecraft, and humans in space
Solar Flares
Solar flares are huge eruptions of hot gas and radiation in the photosphere
Can damage satellites, spacecraft, and humans in space
The study of coronal mass ejections and solar flares is called “space weather”

25. A Coronal Mass Ejection

26. The Aurora
When CME material reaches the Earth, it interacts with the Earth’s magnetic field and collides with ionospheric particles
The collision excites ionospheric oxygen, which causes it to emit a photon
We see these emitted photons as the aurora, or Northern Lights