The Sun

1
The Sun

• Dr Bryce

2
Class notices

• Second class exam 21st March
• Wednesday after Spring Break
• Monday 19th March will be review, including Who
  wants to be a millionaire?

3
Solar activity is like weather

• Sunspots
• Solar Flares
• Solar Prominences
• All are related to magnetic fields

4
Sunspots Are cooler than other parts of the
Suns surface (4000 K) Are regions with strong
magnetic fields
5
Zeeman Effect We can measure magnetic fields in
sunspots by observing the splitting of spectral
lines
6
Charged particles spiral along magnetic field
lines
7
Loops of bright gas often connect sunspot pairs
8
Magnetic activity causes solar flares that send
bursts of X-rays and charged particles into space
9
Magnetic activity also causes solar prominences
that erupt high above the Suns surface
10
Corona appears bright in X-ray photos in places
where magnetic fields trap hot gas
11
Coronal mass ejections send bursts of energetic
charged particles out through the solar system
12
Charged particles streaming from Sun can disrupt
electrical power grids and can disable
communications satellites
13
Number of sunspots rises and falls in 11-year
cycle
14
Sunspot cycle has something to do with winding
and twisting of Suns magnetic field
15
Introducing stars

• Chapter 15

16
Luminosity Amount of power a star radiates
(energy per second Watts) Apparent
brightness Amount of starlight that reaches
Earth (energy per second per square meter)
17
Luminosity passing through each sphere is the
same Area of sphere 4p
(radius)2 Divide luminosity by area to get
brightness
18
The relationship between apparent brightness
and luminosity depends on distance
Luminosity Brightness
4p (distance)2
We can determine a stars luminosity if we can
measure its distance and apparent brightness
Luminosity 4p (distance)2 x
(Brightness)
19
Thought Question

• How would the apparent brightness of Alpha
  Centauri change if it were three times farther
  away?
• A. It would be only 1/3 as bright
• B. It would be only 1/6 as bright
• C. It would be only 1/9 as bright
• D. It would be three times brighter

20

Most luminous stars 106 LSun Least
luminous stars 10-4 LSun (LSun is
luminosity of Sun)
21
Properties of Thermal Radiation

• Hotter objects emit more light per unit area at
  all frequencies.
• Hotter objects emit photons with a higher average
  energy.

22

Hottest stars 50,000 K Coolest stars
3,000 K (Suns surface is 5,800 K)
23
Level of ionization also reveals a stars
temperature
106 K
105 K
Ionized Gas (Plasma)
104 K
103 K
Neutral Gas
Molecules
102 K
10 K
Solid
24
Absorption lines in stars spectrum tell us
ionization level
25
Lines in a stars spectrum correspond to a
spectral type that reveals its temperature
(Hottest) O B A F G K M
(Coolest)
26
Thought Question

• Which kind of star is hottest?
• A. M star
• B. F star
• C. A star
• D. K star