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ASTR 1020 * Spring 2017
MW 5:30-6:45 pm
Stellar and Galactic Astronomy
• Your instructor: Ryan Norris
– Switched from Dr. Doug Gies
• Email: [email protected]
• Remember labs start week of Jan 23
• Please get a syllabus from me if you don’t have
one
• For now slides will be on
www.astro.gsu.edu/~norris/ASTR1020
– Up by tomorrow at 12PM
Chapter 13
Taking the Measure of Stars
Why do stars look different?
Bryce Canyon National Park
Properties of Stars
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Distances
Luminosities
Temperatures
Radii
Masses
Astrometry
•Measurement of position
and movement of stars
and other celestial bodies
•Uses
•Shape and formation
of galaxies
•Exoplanet detection
Gaia, the newest European Space
Agency astrometry mission
•Dark matter
distribution
•Detection of near
Earth objects
Distance
We use the effect known as (ASTR1010) parallax: the
apparent shift of a nearby object against a background
of more distant objects
 As Earth orbits the Sun, nearby stars change their
positions slightly against the background.
 Comparing the position six months apart yields
the distance.
The greater the parallax (or parallactic angle), the smaller
the distance.
The only direct way to measure the distance to a star is from
the parallax
By definition, a star with a parallax of 1 arcsecond (arcsec)
is at a distance of 1 parsec (pc). 1 arcsec = 1/3,600 degree.
1 parsec = 206,265 AU =3.26 light-years
Parallax and Distance
• Parallax angle and distance are inversely proportional: a
small parallax angle means the star is far away
• Distances measured from parallax angles are expressed
in parsecs
• Nearest star: Alpha Centauri system at 1.3 parsecs,
which includes three stars (closest = Proxima Centauri).
• Using the motion of stars to determine their parallax and
to measure distance works well out to 200 parsecs (pc)
• The local 10 pc neighborhood (Adric Riedel, GSU):
https://www.youtube.com/watch?v=up_MqNBv0FE
Thought Question
The parallax of Proxima Centauri (the closest star to
the Sun) is roughly twice that of Sirius (the
brightest star in the Earth’s night sky). This means
that Sirius is:
A. Twice as close as Proxima Centauri
B. Almost the same distance but moving faster
C. Twice as far as Proxima Centauri
D. None of the above
Thought Question
The parallax of Proxima Centauri (the closest star to
the Sun) is roughly twice that of Sirius (the
brightest star in the Earth’s night sky). This means
that Sirius is:
A. Twice as close as Proxima Centauri
B. Almost the same distance but moving faster
C. Twice as far as Proxima Centauri
D. None of the above
Luminosity (L)
Total energy radiated by star
each second
L = surface area x
rate/unit area
L = 4πR 2 σ T 4
where
R = stellar radius
T = stellar effective
temperature (sun~5800K)
σ = Stefan-Boltzmann
constant
Apparent brightness:
Rate at which we receive
that energy
With increasing distance, luminosity is spread over a larger area
Area of sphere = 4π (distance)2
Divide luminosity by area to get brightness
Inverse-Square Law: brightness proportional to 1/distance2
Thought Question
How would the apparent brightness of Alpha
Centauri change if it were three times
farther away?
A.
B.
C.
D.
It would be only 1/3 as bright
It would be only 1/6 as bright
It would be only 1/9 as bright
It would be three times brighter
Thought Question
How would the apparent brightness of Alpha
Centauri change if it were three times
farther away?
A.
B.
C.
D.
It would be only 1/3 as bright
It would be only 1/6 as bright
It would be only 1/9 as bright
It would be three times brighter
Stellar luminosities can be
hugely different, ranging
from 1/10,000th to
1,000,000 times that of the
Sun
The Sun is in the middle
of the luminosity range,
but there are way more
faint stars than bright ones
in the Universe
The Magnitude system
(Hipparcus, ~150BC):
- express this range with
friendlier numbers (-30 to ~30
currently)
- low magnitudes = bright stars
- high magnitudes = faint stars
+5 magnitudes → 100 x fainter
+1 magnitude → ~2.5 x fainter
-1 magnitude → ~2.5 x brighter
-5 magnitudes → 100 x brighter
The apparent magnitude
= what we measure on Earth
→ measures the brightness of a
star
Sun apparent magnitude =
-26.7
The absolute magnitude
= apparent magnitude if it were
10 pc away from the Sun
→ measure luminosity of a star
Sun absolute magnitude = 4.8
Thought Question
How would the absolute magnitude of Alpha
Centauri change if it were three times
farther away?
A. It would be +3 magnitudes fainter
B. It would be -3 magnitudes brighter
C. It would stay the same
Thought Question
How would the absolute magnitude of Alpha
Centauri change if it were three times
farther away?
A. It would be +3 magnitudes fainter
B. It would be -3 magnitudes brighter
C. It would stay the same (its absolute
magnitude is an intrinsic property of a star)
Thought Question
How would the absolute magnitude of Alpha
Centauri change if it suddenly got 100x fainter ?
A. It would gain +1 magnitude
B. It would gain +100 magnitudes
C. It would stay the same
D. It would gain +5 magnitudes
Thought Question
How would the absolute magnitude of Alpha
Centauri change if it suddenly got 100x fainter ?
A. It would gain +1 magnitude
B. It would gain +100 magnitudes
C. It would stay the same
D. It would gain +5 magnitudes
Thought Question
Star A and star B appear equally bright but star A is
twice as far away from us as star B. Which is true?
A. Star A is twice as luminous as star B
B. Star B is twice as luminous as star A
C. They are the same luminosity
D. Star A is four times as luminous as star B
Thought Question
Star A and star B appear equally bright but star A is
twice as far away from us as star B. Which is true?
A. Star A is twice as luminous as star B
B. Star B is twice as luminous as star A
C. They are the same luminosity
D. Star A is four times as luminous as star B
Listen to the history of
astrometry!
Sound file and explanation of sounds available at: http://sci.esa.int/gaia/58311-fromhipparchus-to-hipparcos-a-sonification-of-stellar-catalogues/
Temperature: Thermal Radiation
1. Hotter objects emit more light per unit area at all frequencies.
2. Hotter objects emit photons with a higher average energy.
The color of a star is indicative of its temperature. To
estimate it we can e.g. compare blue B and visual
(yellow-green) V magnitudes
Red stars are
cooler than the
Sun, while blue
stars are hotter.
Photometry
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Measurement of flux (amount of light)
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Generally measured through different filters (or “passbands”)
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Example B (Blue) and V (“visual”, yellow-green)
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B-V→ color index
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Measurements are in magnitudes
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Thus a smaller number means more blue and hotter
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Example
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Sun: B-V: 0.656, Teff: 5772 K
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Vega: B-V: 0.0, Teff: 9602 K
Spectroscopy
Study of the
components of an
object’s
electromagnetic
radiation split into
components
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Emission and
absorption lines
provide information
about composition,
density, temperature,
even motion (via
Doppler effect)
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Kirchoff’s Laws 1: A continuous
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The spectrum of an incandescent light is
an example of 1st law
spectrum is
produced by a hot,
glowing solid
Emission lines in heated hydrogen are
examples of the 2nd law
2: Bright line or
emission spectrum is
produced by a hot,
low density gas
The lines observed by Fraunhofer in the
Sun are examples of the 3rd law
3: If light from a
continous spectrum
passes through a
cool, low density gas
an absorption
spectrum is produced
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