Announcements

• Observing Monday-Thursday of this week counts on the third exam.

• Fourth exam is Wednesday, November 17

• Review will be Monday, November 15

Northern Lights

or

Aurora Borealis

Coronal mass ejection

Movie

Northern lights are caused by particles emitted from the Sun

High energy particles from the Sun are trapped in the Earth’s magnetic field and directed down towards the Earth. Animation

The particles interact with atoms in the atmosphere producing light. The colors are from the spectral lines of atoms in the atmosphere.

3-day Solar-Geophysical Forecast issued Nov 7 at 22:00 UTC

Geophysical Activity Forecast: The geomagnetic field is expected to be at unsettled to major storm levels. A shock passage from the CME associated with the M2/M5 flares from 5 November is expected to arrive early on 8 November, and a shock passage from the CME associated with the M9 flare from 6 November is expected to arrive on 9 November. Today's X2 flare very likely produced a CME that could arrive late on 9 November. These anticipated shock passages, along with the elevation in solar wind speed due to a geoeffective coronal hole, should keep geomagnetic activity at unsettled to minor storm levels all three days, with isolated major storming possible on 8-9 November.

Web sites

Nordlys Northern Lights - http://www.northern-lights.no/

Today’s Space Weather - http://www.sec.noaa.gov/today2.html

Black Holes• Observed properties of black holes

• Gravitational energy

• Rotating black holes

• Eddington luminosity

• Accretion disks

• Reading 24.3

Black Holes

• Fundamental properties of black holes– Mass– Spin– Charge = zero for astrophysical black holes

Observed properties of black holes

Luminosity

Orientation

Jets

Gravitational energy The Hoover dam generates 4 billion kilowatt hours of power per year.

Where does the energy come from?

Gravitational energy Water falling down to the generators at the base of the dam accelerates to 80 mph.

The same water leaving the turbines moves at only 10 mph.

The gravitational energy of the water at the top of the dam is converted to kinetic energy by falling. The turbines convert kinetic energy to electricity.

Gravitational energy

Black holes generate energy from matter falling into them.

Rotating black holesFor non-rotating black holes:

- event horizon is at the Schwarzschild radius

- inner edge of the disk is at 3 Schwarzschild radii

For maximally rotating black holes:

- event horizon is at ½ Schwarzschild radii

- inner edge of the disk is at ½ Schwarzschild radii

Schwarzschild radius = 3 km (M/MSun)

Luminosity• Gravitational energy is converted to kinetic

energy as particles fall towards BH

• Efficiency of generators:– Chemical burning < 0.000001%– Nuclear burning < 1%– Non-rotating black hole = 6%– Rotating black hole = 42%

A quasar varies in brightness by a factor of 2 in 10 days. What does this tell us about the quasar?

1. It has a large magnetic field.

2. It is quite small.

3. It must be highly luminous.

4. It cannot emit radio waves.