Stars
Physics 360 - Astrophysics
Brightness
• Different
brightness.
• Different color.
• How bright are
they really?
• What is due to
distance?
• What is due to
luminosity?
Spectral
Classification
Mass
NPOI Observations of Mizar A
(1 Ursa Majoris)
0.005 arcsec
Orbital Phase: 000o
Mizar, 88 light years distant, is the middle star in the handle of
the Big Dipper. It was the first binary star system to be imaged
with a telescope. Spectroscopic observations show periodic
Doppler shifts in the spectra of Mizar A and B, indicating that they
are each binary stars. But they were too close to be directly
imaged - until 2 May 1996, when the NPOI produced the first
image of Mizar A. That image was the highest angular resolution
image ever made in optical astronomy. Since then, the NPOI has
observed Mizar A in 23 different positions over half the binary
orbit. These images have been combined here to make a movie
of the orbit. As a reference point, one component has been fixed
at the map center; in reality, the two stars are of comparable size
and revolve about a common central position.
Stellar Radii
How big are stars?
• Stars have different sizes.
• If you know:
– Distance
– Angular size
• Learn real size.
50 mas
Sizes of Stars
Supergiants, Giants and Dwarfs
The Sun, Our Star
• The Sun is an average star.
• From the Sun, we base our understanding of
all stars in the Universe.
• No solid surface.
Vital Statistics
•
•
•
•
•
•
Radius = 100 x Earth (696,000 km)
Mass = 300,000 x Earth (1.99 x 1030 kg)
Surface temp = 5,800 K
Core temp = 15,000,000 K
Luminosity = 4 x 1026 Watts
Solar “Day” =
– 24.9 Earth days (equator)
– 29.8 Earth days (poles)
Interior Properties
• Core = 20 x density of iron
• Surface = 10,000 x less
dense than air
• Average density = Jupiter
• Core = 15,000,000 K
• Surface = 5800 K
1. The Core
• Scientific Method:
– Observations
– Make hypothesis (a model)
• Models make predictions
– Test predictions
• Compare results of predictions with observations
– Revise model if necessary.
Testing the Core
• Observe Sun’s:
– Mass (how?)
– Composition (how?)
– Radius
• Use physics to make a model Sun.
• Predict:
– Surface temp/density (how do you test?)
– Surface Luminosity (how do you test?)
– Core temp/density  Fusion Rate  neutrino rate
(test?)
In The Core
• Density = 20 x
density of Iron
• Temperature =
15,000,000 K
• Hydrogen
atoms fuse
together.
• Create Helium
atoms.
Nuclear Fusion
• 4H  He
• The mass of 4 H nuclei (4 protons):
4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg
• The mass of He nuclei: = 6.643 x 10-27 kg
• Where does the extra 4.7 x 10-29 kg go?
• ENERGY!  E = mc2
• E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2
• E = hc/l  l = 4.6 x 10-14 m (gamma rays)
• So: 4H  He + light
2. Helioseismology
• Continuous monitoring
of Sun.
– Ground based
observatories
– One spacecraft (SOHO)
• Surface of the Sun is
‘ringing’
• Sound waves cross the
the solar interior and
reflect off of the surface
(photosphere).
Solar Interior
• Core
– Only place with
fusion
• Radiation Zone
– Transparent
• Convections Zone
– Boiling hot
Convection
• A pot of boiling
water:
• Hot material rises.
• Cooler material sinks.
• The energy from the
pot’s hot bottom is
physically carried by
the convection cells in
the water to the
surface.
• Same for the Sun.
Solar Cross-Section
• Progressively smaller
convection cells carry the
energy towards surface.
• See tops of these cells as
granules.
The Photosphere
• This is the origin of the 5,800 K thermal radiation
we see.
l = k/T = k/(5800 K)  l=480 nm (visible light)
• This is the light we see.
• That’s why we see this as the surface.
•
•
•
•
11-year sunspot cycle.
Center – Umbra: 4500 K
Edge – Penumbra: 5500 K
Photosphere: 5800 K
Sunspots
Magnetic fields and Sunspots
• At kinks, disruption in convection cells.
• Sunspots form.
Magnetic fields and Sunspots
• Where
magnetic
fields “pop
out” of Sun,
form sunspots.
• Sunspots
come in pairs.
Prominences
Hot low density gas = emission lines