Astronomy Chapter 10
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- Only DIRECT way to measure the distances to Stars
- Parallax
- What is the mathematical equation that determines the distance to a star?
- distance (parasec)= 1/ parallax (in arc sec)
- Star A has a higher parallactic angle than does Star B. Which is closer to earth?
- Star B.
- how is paralax measured?
- the earth's orbit around the sun is taken as the baseline then the paralactic angle up to the Sun is measured. With one distance (1 A.U) and one angle, we can determine the hypotnuse of the triangle (distance to the star).
- If the paralax is .5 arc sec, what is the distance to the star?
- Distance (pc) = 1/ .5 = 2 pc's or about 7 ly
- What is the closest star? next closest star?
-
Alpha Centauri (4.3 ly)
Bernard's Star (6 ly) - Is parallax an actual motion or an apparent motion?
- Parallax is an apparent motion because we are moving, not the stars.
- A star's movement in the sky is based on two motions. What are they?
-
Radial motion- moving toward/away from Earth.
Transverse motion- moving perpendicular to earth's surface. - how is radial motion measured?
- Doppler effect as stars approach us they appear blue and those that are shifting outward are red.
- Recall that bigger stars are brighter/fainter, while closer stars are brighter/fainter.
- brighter; brighter
- A faint close star and a bright far star can thus have the same...
- apparent brightness
- Because of the last card, it becomes important in the calculation of a star's actual luminosity to have what...
- It's brightness and its distance from the observer.
- Mathematically, Luminosity is equal to...
- Luminosity = Flux (brightness)x 4*pi*d^2
- On the magnitude scale what do larger numbers indicate?
- less brightness
- What does the color of a star tell us?
- Temperature
- How do we determine the temperature from color?
- We calculate the flux through a blue filter then trhough a red filter and compare it to a blackbody curve.
- Different spectra result from
- different temperatures
-
Spectral classifications of stars were catagorized according to...
What is the order of star class from hottest stars to coolest stars. -
Surface temperature;
O B A F G K M - Mathematically, what is stefan's law?
- Luminosity= T^4 x r^2
- Luminosity is proportional to
- the square of the star's radius and the fourth of the star's temperature.
- HR diagrams plot stars as a function of their
- Luminosity and temperature
- About 90% of all stars (including the Sun) lie
- on the main sequence
- what general property do all stars on the main sequence posess that those not on the sequence dont?
- Those on the main sequence are in the hydrogen-burning phase of their life.
- From stefan's law, a more luminous star that is the same temparature must...
- be bigger than the other star.
- From steffan's law, if one star is more luminous than the other but the are the same size, what can we tell...
- the brighter star must be hotter. (higher temperature)
- Where are most bright stars found relative to the H-R diagram?
-
Most are found on the main sequence but at the larger and hotter region (top left).
Some of the brightest are classed into red dwarfs (cool and large) and others are in blue giants (Very hot and very large). - which class of stars is barely seen despite being the most abundant?
- Red Dwarfs
- Recall that parlax is the only DIRECT method to calculate the distance of a star. What is spectroscopic paralax?
- Also comes up with distance. But first we obtain the temperature through spectroscopy, then we determine the luminosity, finally we use the inverse square law to determine the distance.
- We determine the star’s Luminosity Class based on it's
- its spectral line widths
- Those stars that have broader spectral lines are
- smaller.
-
The distance ladder:
We get distances to nearby planets from - radar ranging
-
The distance ladder:
Nearby stars (greater than 1 AU) are found using what method... - STELLAR PARALAX (the only direct method)
-
The distance ladder:
What method is used to find the distance to furthest stars? - Spectroscopic parallax
- How do we determine a star’s mass?
- By observing it’s gravitational effect on another star - Binary Stars!
- How do we obtain gravitational effects on orbiting stars?
- They are studied by observing the doppler shifting of their spectral lines as they rotate around each other.
- Eclipsing binary systems are studied to determine what
-
the intensity of a star how much intensity is lost is measured and multiplied by the ratio of area covered to total area..
Also, we can determine the radii of the stars. - Why is mass so important?
- Together with the initial composition, mass defines the entire life cycle and all intrinsic properties of the star! (except distance)
- More mass means
-
⬢ more gravity,
⬢ more pressure on core,
⬢ higher core temperatures,
⬢ faster nuclear reaction rates,
⬢ higher Luminosities! - what is the mathematical relationship between luminosity and mass?
- Luminosity = mass^3
- What is a star's lifetime?
- Lifetime = 1 / Mass^2
- Based on that last information what can be said about BIG stars?
- Big stars live shorter lives and burn their fuel faster⬦.