Reading Unit 31, 32, 51

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

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!

Ground- and Space-based Observatories

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!

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.

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

Spitzer Space Telescope James Web Space telescope

Edwin Hubble Hubble Telescope

Observatories in Space

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

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

The Solar Interior

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

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!

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.

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

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

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!

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!

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

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.

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

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”

A Coronal Mass Ejection

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