Welcome to Environmental Science!!!
The Sun
Physical Data
• Mass = 2x1030
kg (333,000 time
more massive than the Earth)
• Diameter: 7x105km (about 100
Earth radii)
• Volume: you can fit about 1.3
million earths inside the sun!
• 70% Hydrogen, 28% Helium, 2%
other stuff.
Distance to the sun
• The average earth-sun distance
is called an Astronomical Unit
(AU)
• 92.8 Million Miles
• Keppler knew the distance to
the planets in terms of the Earth-
Sun distance, but not the
distance itself (in meters)
Interior of the
Sun
• VERY hot (27 million F)
and dense (150 times
denser than water) in
the center (core)
• Density & Temp drop
rapidly toward the
outside
• 10,000 F at the surface
the Sun
• The sun is far away (93 million miles)
• And that’s why it looks so small!
• The sun is hot (10,000 F on surface)
• The sun is big (1 million Earth’s fit inside)
• Lots of gravity!
• This adds up to Temperature and pressure
being very high on the sun.
Quick Chem review
Core of the sun
• The temperature and pressure
on the sun is amazingly high.
• How high? High enough to push
hyrdogen protons together.
They fuse to form helium.
• What is this called?
Core of the sun
• Every second on the sun, 600
million tons of hydrogen is
converted into 595 million
tons of helium.
• Huh? You can’t do that!
Why not?
• CONSERVATION OF MASS!
• The law implies that matter
can neither be created nor
destroyed, although it may be
rearranged.
Core of the sun
• If 600 millions tons of hydrogen
is converted into 595 million
tons of helium… what happens
to the lost mass?
Core of the sun
• Einstein’s theory of relativity
states that under enough heat
and pressure (like in the core of
the sun) energy and mass are
interchangeable (E = M)
• Therefore the lost mass is
turned into HUGE amounts of
energy (E=mc2) Thanks Einstein.
• Again, nuclear fusion occurs in
the core of the sun.
Fusion in the sun
The Sun’s Future?
• The sun is currently crushing
hydrogen into helium in the
process of nuclear fusion.
• The sun is currently 70%
hydrogen and 28% helium.
• How will these numbers change
as time goes on?
Photosphere
• Photosphere – brightest in optical –
this is where most of the light from
the sun comes from. The spectrum
is formed here.
Photosphere
• Optical light
comes from
here
• Sunspots are
areas of
intense
magnetic
field
The End
Chromosphere
• Activity starts at sunspots and gas travels
along magnetic field lines
• If the gas loops back – prominence
• If the gas escapes to the corona -- flare
Coron
a• The top picture is in X-
ray
• The bottom two are in
optical from SOHO
• You can see material
leaving the sun
Energy transfer
• The energy created in the center of
the sun has to travel to the outside.
This happens in an orderly fashion
in the interior
• Near the outside, energy is
transferred with convection
Measurements
What we can measure What we can calculate
Distance to Venus (radar)
Apparent mag. of sun (and D)
Period of the Earth’s orbit
(and D)
Spectrum of the sun
Sunspots
Distance to the sun
Absolute mag.
(Luminosity)
Mass of Sun
Temperature, chemical
composition, rotation
Rotation
Rotation, magnetic
field
The sun as a main sequence
star
• The sun is a G2 main sequence
star with an absolute magnitude
of 4.85
• All main sequence stars change
H to He
• All spectra come from the
photospheres of the stars
• We can only detect the
chromosphere and corona of a
few stars besides the sun
Properties of Stars
Properties of Stars
• Brightness
• Luminosity (brightness and
distance)
• Magnitude scale
• Temperature (Color, spectrum)
• Composition (Spectrum)
• Velocity, rotation, magnetic field
(spectrum)
• Distance (parallax, comparison)
Triangulating the Stars
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)
Brightness and
Luminosity
• Apparent magnitude measures
the brightness of a star
• The true property of the star is
luminosity.
• Luminosity, the total power
coming from the surface of a
star, is measured using Absolute
Magnitude
Calculating Magnitude’s
• You need to know the distance
to the star in question (parallax
for the nearest stars – harder for
everything else)
• Then you can calculate the
absolute magnitude, M
Putting it together: an HR
diagram
• Luminosity is measured in Watts
or absolute magnitude
• Brightness is measured in
Watts/m2
or apparent magnitude
• Temperature is measured in
color or spectral type
(OBAFGKM)
Types of HR diagrams
Theorist’s Observer’s Color-magnitude
Here is an HR
diagram for a few
hundred randomly
selected stars
from the HD
catalog
Notice the main
sequence
Absolute Magnitude vs. Spectral Type
-7.00
-5.00
-3.00
-1.00
1.00
3.00
5.00
7.00
9.00
11.00
0 1 2 3 4 5 6 7Spectral type
Ab
so
lute
Ma
gn
itu
de
Again, on the basis of the appearance of the spectra of stars, astronomers discovered that
the density of gas and the strength of gravity at the surface of a star indicate that some
stars are much larger than other, even if their temperatures are the same. This difference
is denoted by the luminosity class of a star.
The sequence of luminosity classes is:
Luminosity Class Name assigned to class:
I or Ia Supergiants
II Bright Giants
III Giants
IV Sub-giants
V Dwarfs
VI Sub-dwarfs
The complete classification of a star is based upon the spectral type and luminosity class of a star.
Thus, it turns out that the sun is classified as a G2V star.
Our old friend Betelgeuse is an M1I star.
“Luminosity Classes” of Stars