ASTRO Midterm Exam #2: VOCAB (ch. 16-21)
For stars, the Sun is of average luminosity. For the most part, very dim stars are most common, stars of medium L like the sun are fairly common, and highly luminous stars are very rare.
If a star is approaching us, the wavelength decreases (blue shift)
If a star is moving away from us, the wavelength increases (red shift)
Parallax only works for stars that are relatively nearby, and the majority of stars in our galaxy are much too far away to exhibit any apparent shift in position as Earth orbits the Sun.
Parallax angles are smallest for the most distant stars, so if a parallax angle was measured to be too small then the astronomer would assume the star is farther away than it actually is
D =distance in parsecs,
p = parallax angle in arcseconds
Satellite created by the European Space Agency that measures parallax angles with an accuracy of up to .001 arcseconds
Expressed as Watts of power.
For example, if the distance between an observer and a star doubles, that is increase by 2 x, the apparent brightness decreases by a factor of 4 (radiation spread over an area that is 4 times larger); If you increase the distance by 3 times your brightness decreases by a factor of 9; 4, 16; 5, 25; you get the idea
Relates a star’s luminosity, distance, and apparent brightness to the corresponding quantities for the Sun
Based on this relationship, we are able to determine the luminosity of a star by comparing the distance of that star and the brightness of that star to the distance of earth to the sun and the apparent brightness of the sun.
A very nearby flashlight will appear much brighter than a very distant spotlight.
Although a large spotlight will emit more light, the light that appears brighter depends on their distances away
The Sun’s absolute magnitude is 14.8
The greater the value for distance modulus, the greater distance that star is from earth.
Large discrepancies in absolute and apparent tend to mean the star is comparatively very far from earth
The intensity of light from a relative hot star peaks at short wavelengths, making it look blue.
The shorter the peak wavelength, the higher the temperature, and the higher the frequency.
Blue is always hotter than red
Red stars are relatively cold, with low surface temps; Blue stars are relatively hot, with high surface temps
-Absorption lines are created when the light flows outward through the upper layers of the star’s atmosphere
-Atoms absorb wavelengths of the radiation at specific levels depending on the specific atom present (hydrogen, helium, etc) and the pattern of each atom’s spectral lines serve as the comparison for new observations (though they vary a lot)
-Due to the large amount of spectra that comes form the stars, similar looking stellar spectra are placed into spectral classes (OBAFGKM; each letter representing a different class)
– O is most similar to B, B is more similar to F than it is to G
-Within a particular spectral class, the larger numbers correspond to cooler stars. For example, a G2 star is hotter than a G8 star because a G2 star is closer to the hotter F-spectral class in the sequence OBAFGKM
-*For a given luminosity, the greater the surface temperature, the smaller the radius will be.
-*For a given surface temperature, the greater the luminosity the larger the radius must be
– increasing luminosity on the vertical axis and increasing temperature on the horizontal axis.
The data points are grouped into a few regions throughout the graph showing that luminosity and surface temperature are correlated
Most stars cluster around the red curve called the main sequence
Makes sense if we remember that the majority of stars have comparatively cooler surface temperatures
For a given Stellar Radius:
As the surface temperature increases, the star glows more intensely and the luminosity of that star increases
As the surface temperature decreases, the star will glow less with a much smaller luminosity
As you move to the left of the graph, surface temperature increases
As you move up the graph on the y axis, luminosity increases
Span from the top left of the HR graph to the bottom right
Gain their energy through the conversion of hydrogen to helium in their cores
The Sun is a main sequence star
Stars like our Sun derive their energy from nuclear reactions in the core, whereas a brown dwarf derives its energy from gravitational contraction.
Since they are cool, they are much less bright (emit less energy). So, in order for these cool stars to be so luminous, they must be incredibly large (Giants)
10 – 100x larger than our Sun.
Between 3000 and 6000K surface Temperature
The cooler giants (3000-4000K) are called Red Giants as they appear red
Together, giants and super-giants make up about 1% of the stars in the sky
This means they are extremely hot, but also have very low luminosity, meaning they must be very small aka dwarfs
About the same size as earth
No thermonuclear reactions occur in their cores
Glowing remnants of what used to be giant stars
Cool with low luminosity
Never will become stars, not massive enough for significant fusing of hydrogen in their core
Gets energy from gravitational contraction, not a true star
The higher the density and pressure, the more hydrogen collision, the broader the hydrogen spectral lines are
Typically, the smaller the star (comparatively), the higher the density and pressure is, meaning broad and clear absorption lines
Luminosity broken down into 5 classes I – V. The higher the class, the lower the average luminosity for a given surface temperature
If 2 stars have identical surface temperatures, but 1 has a much larger luminosity, you can assume that that star must be of a much more massive size to emit its higher levels of luminosity.
Deduce using the formula for the relationship between luminosity, distance, and apparent brightness
No actual parallax involved, just similar because it answers a question of distance
Helpful to make claims about stars that are simply too far to observe true parallax.
H-R diagram plays a big role in determining the luminosity of a star from its spectral type and luminosity class
Can then deduce property of distance from that information
L = 4pid^2b
L = 4piR^2T^4
Helps us determine the weight of these stars using Kepler’s 3rd law
Visual Binaries – Binary pairing that you can actually see orbiting each other
Have extremely long orbital periods
The center of mass in a binary star system is always nearer to the more massive star
The two stars are always on opposite ends of the center of mass, so they never collide
If two stars orbiting a common center of mass were moved farther apart, their masses would not change, but the period would increase following Kepler’s 3rd law
No motion is involved
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