astr100 (spring 2008) introduction to astronomy properties of stars prof. d.c. richardson sections...
Post on 20-Dec-2015
221 views
TRANSCRIPT
![Page 1: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/1.jpg)
ASTR100 (Spring 2008) Introduction to AstronomyProperties of Stars
Prof. D.C. Richardson
Sections 0101-0106
![Page 2: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/2.jpg)
![Page 3: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/3.jpg)
How do we measure stellar luminosities?
![Page 4: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/4.jpg)
The brightness of a star depends on both distance and luminosity.
![Page 5: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/5.jpg)
Luminosity:
Amount of power a star radiates.
(Energy per second, or Watts.)
Apparent brightness:
Amount of starlight that reaches Earth.
(Energy per second per square meter.)
![Page 6: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/6.jpg)
Thought Question
These two stars have about the same luminosity. Which one appears brighter to us?
A. Alpha Centauri.B. The Sun.
![Page 7: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/7.jpg)
Thought Question
These two stars have about the same luminosity. Which one appears brighter to us?
A. Alpha Centauri.B. The Sun.
![Page 8: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/8.jpg)
Luminosity passing through each sphere is the same.
Area of sphere:
4 (radius)2
Divide luminosity by area to get brightness.
![Page 9: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/9.jpg)
The relationship between apparent brightness and luminosity depends on distance…
We can determine a star’s luminosity if we can measure its distance and apparent brightness…
Measuring Luminosity
€
brightness =luminosity
4π (distance)2
€
luminosity = 4π (distance)2 × (brightness)
![Page 10: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/10.jpg)
Thought Question
How would the apparent brightness of Alpha Centauri change if it were three times farther away?
A. It would be only 1/3 as bright.B. It would be only 1/6 as bright.C. It would be only 1/9 as bright.D. It would be three times
brighter.
![Page 11: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/11.jpg)
Thought Question
How would the apparent brightness of Alpha Centauri change if it were three times farther away?
A. It would be only 1/3 as bright.B. It would be only 1/6 as bright.C. It would be only 1/9 as bright.D. It would be three times
brighter.
![Page 12: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/12.jpg)
So how far away are these stars?
![Page 13: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/13.jpg)
Parallaxis the apparent shift in position of a nearby object against a background of more distant objects.
![Page 14: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/14.jpg)
Parallax is measured by comparing snapshots taken at different times and measuring the shift in angle to the star.
![Page 15: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/15.jpg)
Apparent positions of nearest stars shift by about an arcsecond as Earth orbits the Sun.
![Page 16: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/16.jpg)
The farther the distance, the smaller the angle.
![Page 17: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/17.jpg)
Parallax and Distance
p = parallax angle
d (in parsecs) = 1
p (in arcseconds)
d (in light-years) = 3.26 ×1
p (in arcseconds)
![Page 18: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/18.jpg)
Most luminous stars:
106 LSun.
Least luminous stars:
10-4 LSun.
(LSun is the luminosity of the Sun.)
![Page 19: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/19.jpg)
The Magnitude Scale
m = apparent magnitude M = absolute magnitude
apparent brightness of Star 1apparent brightness of Star 2
=(1001/5)m1−m2
luminosity of Star 1luminosity of Star 2
=(1001/5)M1−M2
![Page 20: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/20.jpg)
How do we measure stellar temperatures?
![Page 21: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/21.jpg)
Recall: every object emits thermal radiation with a spectrum that depends on its temperature.
![Page 22: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/22.jpg)
An object of fixed size grows more luminous as its temperature rises.
Relationship Between Temperature and Luminosity
![Page 23: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/23.jpg)
Properties of Thermal Radiation
1. Hotter objects emit more light per unit area at all wavelengths (more luminous).
2. Hotter objects emit photons with a higher average energy (bluer).
![Page 24: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/24.jpg)
Hottest stars:
50,000 K.
Coolest stars:
3,000 K.
(Sun’s surface is 5,800 K.)
![Page 25: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/25.jpg)
Solid
Molecules
Neutral Gas
IonizedGas(Plasma)
Level of ionization also reveals star’s temperature.
10 K
102 K
103 K
104 K
105 K
106 K
![Page 26: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/26.jpg)
Absorption lines in a star’s spectrum tell us its ionization level.
![Page 27: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/27.jpg)
Lines in a star’s spectrum correspond to a spectral type that reveals its temperature.
(Hottest) O B A F G K M (Coolest)
Fewer lines
More lines
![Page 28: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/28.jpg)
(Hottest) O B A F G K M (Coolest)
Remembering Spectral Types
“Oh Boy An F Grade Kills Me…”
Also subdivide each type:
e.g., (hottest) A0, A1, …, A9 (coolest).
The Sun is type G2.
![Page 29: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/29.jpg)
Thought Question
Which of the stars below is hottest?
A. M starB. F starC. A starD. K star
![Page 30: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/30.jpg)
Thought Question
Which of the stars below is hottest?
A. M starB. F starC. A starD. K star
![Page 31: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/31.jpg)
Pioneers of Stellar Classification
Annie Jump Cannon and the “calculators” at Harvard laid the foundation of modern stellar classification.
![Page 32: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/32.jpg)
How do we measure stellar masses?
![Page 33: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/33.jpg)
The orbit of a binary star system depends on strength of gravity.
![Page 34: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/34.jpg)
Types of Binary Star Systems
Visual binary. Eclipsing binary. Spectroscopic binary.
About half of all stars are in binary systems.
![Page 35: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/35.jpg)
Visual Binary
We can observe the orbital motions of these stars directly.
![Page 36: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/36.jpg)
Eclipsing Binary
We can measure periodic eclipses.
![Page 37: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/37.jpg)
Spectroscopic Binary
We can measure Doppler shifts.
![Page 38: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/38.jpg)
Isaac Newton
We measure mass using gravity.
Direct mass measurements are possible only for stars in binary star systems.
p = period
a = average separation
p2 = a3 4π2
G (M1 + M2)
![Page 39: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/39.jpg)
r M
v
Need 2 out of 3 observables to measure mass…
1. Orbital period (p).2. Orbital separation (a or r =
radius).3. Orbital speed (v).
4. For circular orbits, v = 2r / p.
![Page 40: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/40.jpg)
Most massive stars:
100 MSun.
Least massive stars:
0.08 MSun.
(MSun is the mass of the Sun.)
![Page 41: ASTR100 (Spring 2008) Introduction to Astronomy Properties of Stars Prof. D.C. Richardson Sections 0101-0106](https://reader034.vdocuments.site/reader034/viewer/2022052603/56649d435503460f94a1f73c/html5/thumbnails/41.jpg)
Summary
We have discussed 5 ways of determining stellar properties:1. Distance – use stellar parallax.2. Luminosity – use distance and apparent
brightness (inverse square law).3. Temperature – use color (thermal
radiation law; red = cool, blue = hot).4. Spectral type (OBAFGKM) – use
absorption spectrum (ionization level).5. Mass – use binary stars.
Visual, eclipsing, and/or spectroscopic binaries.