Download Which property of a star would not change if we could observe it

Exam #1 is in class next monday
25 multiple-choice questions
50 minutes
Similar to questions asked in class
Review sheet to be posted this week.
We will have two 1-hour review sessions
Friday 5-6pm (with me) VanVleck B130 (here!)
Today 6-7pm (with Ella) 3425 Sterling (the
classroom where we had lecture on the very first day
of class)
Which property of a star would not change if we
could observe it from twice as far away?
a) Angular size
b) Color
c) Flux
d) Parallax
e) Proper Motion
1
Light and Distance
• Brighter objects are not
necessarily the closer
objects
– Comet Halley, to the
upper left, is within our
Solar System
– The background stars
are just as bright, but
tens, hundreds or
thousands of light years
more distant
• The total amount of
energy a star emits to
space is its luminosity,
measured in Watts.
• The amount of light
reaching us from a star
is its brightness
The Inverse-Square Law
• A star emits light in all directions,
like a light bulb. We see the photons
that are heading in our direction
• As you move away from the star,
fewer and fewer photons are heading
directly for us, so the star seems to
dim – its brightness decreases.
• The brightness decreases with the
square of the distance from the star
– If you move twice as far from the
star, the brightness goes down by a
factor of 22, or 4!
• Luminosity stays the same – the total
number of photons leaving a sphere
surrounding the star is constant.
2
You see this every day!
• More distant streetlights appear
dimmer than ones closer to us.
• It works the same with stars!
• If we know the total energy output of
a star (luminosity), and we can count
the number of photons we receive
from that star (brightness), we can
calculate its distance
L
d=
4!B
• Some types of stars have a known
luminosity, and we can use this
standard candle to calculate the
distance to the neighborhoods these
stars live in.
The Magnitude System
• We can quantify the brightness of a
star by assigning it an apparent
magnitude
– Brighter stars have lower
magnitudes, possibly negative
numbers
– Dimmer stars have higher
positive numbers
• Differences in magnitudes
correspond to ratios in brightness
– Ex: One star of interest has a
magnitude of 6 (dim), and
another star has a magnitude of
1 (easily seen). The magnitude
difference of 5 means that the
brighter star is 100 times
brighter than the dimmer star…
3
The Magnitude System
• In 129BC, Hipparcos could see
down to 6th magnitude. Today
The Hipparcos sattelite can see
down to 23rd magnitude = 10^9
times dimmer!
• Hubble telescope can see 31st
magnitude= 10^12 times
dimmer!
Absolute Magnitude
• It is easier to compare two
stars’ luminosities if they are
at the same distance from the
Sun
• We can calculate how bright
the stars would appear if they
were all the same distance
from us, say, 10 parsecs
• The magnitude of a star
“moved” to 10 parsecs from
us is its absolute magnitude.
4
Stellar Surface Temperatures
• Remember from Unit 23 that the peak
wavelength emitted by stars shifts with the
star’s surface temperatures
– Hotter stars look blue
– Cooler stars look red
• We can use the star’s color to estimate its
surface temperature
– If a star emits most strongly in a wavelength
λ (in nm), then its surface temperature (T) is:
2.9 #106 K " nm
T=
!
• This is Wien’s Law
Measuring Temperature using
Wein’s Law
2.9 #106 K " nm
T=
!
5
The Stefan-Boltzmann Law
flux = "T 4
Flux is energy / unit area
Where, σ= 5.67×10− 8 W·m-2·K-4
L = flux • Area = "T 4 • 4 # r 2
•
!
The Stefan-Boltzmann Law links a star’s
temperature to the amount of light the
!
star emits
– Hotter stars emit more!
– Larger stars emit more!
•
•
A star’s luminosity is then related to both
a star’s size and a star’s temperature
We need an organizational tool to keep
all of this straight…
Two stars have the same surface temperature, but
the radius of one is 100 times that of the other.
How much more luminous is the larger star?
a) 10 times more luminous.
b) 100 times more luminous.
c) 10,000 times more luminous.
d) 100,000,000 times more luminous.
e) The stars have the same more luminosity.
6
Photons in Stellar Atmospheres
•
•
•
Photons have a difficult time moving through a star’s atmosphere
If the photon has the right energy, it will be absorbed by an atom and raise an electron
to a higher energy level
Creates absorption spectra, a unique “fingerprint” for the star’s composition. The
strength of this spectra is determined by the star’s temperature.
Classify:
1- variation in H line
strengths….
“Spectral types”
based on H lines strength:
A
B
C
D
E
F…
7
1901, Annie Jump Cannon > spectral classification
Spectral Classification
• Spectral
classification system
– Arranges star
classifications by
temperature
• Hotter stars are O
type
• Cooler stars are M
type
• New Types: L and T
– Cooler than M
• From hottest to coldest, they are
O-B-A-F-G-K-M
– Mnemonics: “Oh, Be A Fine Girl/Guy,
Kiss Me
– Or: Only Bad Astronomers Forget
Generally Known Mnemonics
8
A cool star that is very luminous must have :
a) A small radius
b) A large radius
c) A small mass
d) A great distance
e) A low velocity
How big are stars?
How do we know?
9
Interferometry
• Stars are simply too far away
to easily measure their
diameters!
– Atmospheric blurring and
telescope effects smear out the
light
• Can combine the light from
two or more telescopes to pick
out more detail – this is called
interferometry
– Two telescopes separated by a
distance of 300 meters have
almost the same resolution as a
single telescope 300 m across!
• Speckle interferometry uses
multiple images form the same
telescope to increase resolution
Using eclipsing binary systems to measure
stellar diameters
10