Midterm next Friday in Discussion
Bring
- red "ParSCORE" scantron to discussion (F-288-PAR-L)
- calculator + #2 pencils
Exam will cover Lectures up to Feb 8 , homework, & reading
A review guide is posted on the website.
- know relative timelines
- relative and approximate sizes of things
- know the observations and reasoning that went
in to key advances in understanding.
Busting an online myth
Busting an online myth
Busting an online myth
Apparent brightness of stars (anything that
shines) depends on distance to them.
•  A star’s apparent brightness b will depend on intrinsic
luminosity L and distance to the star R, as:
Units are power per area
L = Luminosity (same as power)
= “energy per second”
- usually measured in Watts
Brightness =“Power per area”
- usually measured in Watts/m2
Apparent Brightness
The same as power per unit
area collected by your eye (or
by a solar panel).
As viewed from Earth, the Sun has an apparent brightness of
b = 1370 Joules/s/m2 =1370 Watts/m2.
•  We see Sun’s apparent brightness: b = 1370 Joules/s/m2 =1370 Watts/m2.
Problem: A typical household needs about 1000 watts of
power continuously averaged over a year to operate its lights,
heating, cooling etc.
How big of a solar panel would you need to power your house
if the solar panel could convert 100% of the sun’s energy into
usable energy?
Sun’s apparent brightness:
b =1370 Watts/m2.
Problem: A typical household needs about 1000 watts of power continuously
averaged over a year to operate its lights, heating, cooling etc.
How big of a solar panel would you need to power your house if the solar
panel could convert 100% of the sun’s energy into usable energy?
Power collected = b x Area
Collecting lots of power
Collecting less power
b =1370 Watts/m2
Problem: A typical household needs about 1000 watts of power
continuously averaged over a year to operate its lights, heating, cooling
etc.
How big of a solar panel would you need to power your house if the solar
panel could convert 100% of the sun’s energy into usable energy?
Power = b x A = 1000 Watts
Problem: A typical household needs about 1000 Watts of power
continuously averaged over a year to operate its lights, heating, cooling
etc.
How big of a solar panel would you need to power your house if the solar
panel could convert 100% of the sun’s energy into usable energy?
b x A = 1000 Watts
Know Sun’s apparent brightness:
b =1370 Watts/m2.
Want to know Area, A
Problem: A typical household needs about 1000 watts of power
continuously averaged over a year to operate its lights, heating, cooling
etc.
How big of a solar panel would you need to power your house if the solar
panel could convert 100% of the sun’s energy into usable energy?
b x A = 1000 Watts
(1370 Watts/m2) x A = 1000 Watts
A = 1000 Watts/(1370 Watts/m2)
A = 0.73 m2
(that’s about 0.85m on a side ~ 2.7 feet)
•  We see Sun’s apparent brightness: b = 1370 Joules/s/m2 =1370 Watts/m2.
Problem: A typical household needs about 1000 watts of power
continuously averaged over a year to operate its lights, heating, cooling
etc.
How big of a solar panel would you need to power your house if the solar
panel could convert 100% of the sun’s energy into usable energy?
Power = b x A = 1000 Watts
(1370 Watts/m2) x A = 1000 Watts
A = 1000 Watts/(1370 Watts/m2)
A = 0.73 m2
Current solar cell efficiencies are less than 20%. Plus it’s dark a lot,
so you’d really need something ~> 10 times larger, A>~10m2
•  We see Sun’s apparent brightness: b = 1370 Joules/s/m2 =1370 Watts/m2.
Problem: What is the apparent brightness of a
nearby star with the same intrinsic luminosity
L, of the sun, but which is 3 lt yrs away?
Use: dsun = 1.5 x 1011 m
1 lt yr = 9.5 x 1015 m
(1.5/(3*9.5))^2 =2.77x10-3
Apparent Brightness
•  b = 1370 Joules/s/m2 = 1370
•
watts/m2.
That is, if you had a perfectly efficient solar panel one meter on a
side, if you held it perpendicular to the Sun's rays, it would
generate 1370 watts of electricity
Problem: What is the apparent brightness of a
nearby star with the same intrinsic luminosity
L, of the sun, but which is 3 lt yrs away?
Use: dsun = 1.5 x 1011 m
1 lt yr = 9.5 x 1015 m
(1.5/(3*9.5))^2 =2.77x10-3
Show:
bstar = 3.8 x 10-8 watts/m2
“Betelgeuse” comes
from an Arabic phrase
usually translated as
“The Armpit of the
Giant.”
A plume on Betelgeuse
(artist’s impression)
Betelgeuse is a red supergiant
star (set to go supernova!). 8th
brightest star in the sky.
It’s about 100,000 times more
luminous than the Sun.
How do we know that?
How do we know the overall luminosities of stars?
Betelgeuse is 100,000 times more
luminous than the Sun.
How do we know that?
dbetelgeuse = 430 light years
How do we know the overall luminosities of stars?
Need to know their distances!
Want to know their
luminosity
We measure brightness
(how bright they look to
the eye, or a telescope
on Earth)
Need to know
their distance
How do we know the overall luminosities of stars?
Need to know their distances!
•  Astronomers can determine the
distance to the nearest stars by
measuring the “parallax” angle.
•  This method relies on basic
trigonometry.
How do we know the overall luminosities of stars?
Need to know their distances!
•  Astronomers can determine the
distance to the nearest stars by
measuring the “parallax” angle.
•  This method relies on basic
trigonometry.
How do we know the overall luminosities of stars?
Need to know their distances!
•  Astronomers can determine the
distance to the nearest stars by
measuring the “parallax” angle.
•  This method relies on basic
trigonometry.
Measured
angle
Infer
distance
to star
Known
distance
Parallax angle measurements allow us to determine distances to many
stars and this is what allows us to determine their intrinsic luminosities.
Betelgeuse: Blue Supergiant
dbetelgeuse = 430 light years away
100,000 times more luminous
than Sun
Rigel: Blue Supergiant
dRigel = 770 light years away
About 100,000 times more luminous
than Sun.
Betelgeuse is expected to continue to fuse elements until its
core is iron, at which point it will explode as a supernova.
May explode within the next ~1,000 years. (More likely, next
million years, it’s hard to know with current data and models)
What if Betelgeuse explodes as a super nova:
Bright as the Sun?
Sun is this bright:
Super nova: LSN = 1010 Lsun
d = 430 light yrs
= 2.7e7 AU = 2.7e7 dsun
What if Betelgeuse explodes as a super nova:
Bright as the Sun?
Sun is this bright:
Super nova: LSN = 1010 Lsun
d = 430 light yrs
= 2.7e7 AU = 2.7e7 dsun
What if Betelgeuse explodes as a super nova:
Bright as the Sun?
Sun is this bright:
Sun will be 100,000
times brighter than
betelgeuse supernova
Super nova: LSN = 1010 Lsun
d = 430 light yrs
= 2.7e7 AU = 2.7e7 dsun
What if Betelgeuse explodes as a super nova:
Bright as the Sun?
Betelgeuse SN will be 100,000 times fainter than sun
That’s still pretty bright!
Full moon is 400,000 times fainter than the sun!
Betelgeuse SN could be
~4 times brighter than full
moon!
Artists conception: would remain visible for ~2-3 months
Next Up
Einstein
Much of the information given in this lecture is from: “Subtle is the Lord…” by A. Pais
Einstein’s Legacy
•  Reformulated concept of time & space:
–  Special Relativity: E = m c2
–  time is not an absolute quantity, but appears to flow at a different
rate to depending on relative motion
•  He opened the road to Quantum Mechanics
–  Light “hits” like a particle
–  Light waves have “quantized”, “discrete” energies, depending on
their wavelengths.
•  He presented a revised theory of gravity
–  General relativity: space is curved
–  Provides the foundation of modern cosmology