6. Analysing Starlight Flashcards
Stars in the sky are at varying distances. How does that affect how bright they appear to us?
The apparent brightness of a star is proportional to 1 divided by its distance squared. That is, if you took a star and moved it twice as far away, it would appear 1/4 as bright; if you moved it four times the distance, it would appear 1/16 as bright
The magnitude scale reflects the apparent brightness of objects in the sky
The magnitude scale: negative magnitudes are brighter, larger positive magnitudes are dimmer.
What is the difference between luminosity and apparent brightness?
Luminosity is the rate at which a star radiates energy into space
Apparent brightness is the rate at which a star’s radiated energy reaches an observer on Earth.
Compare the relative temperatures of stars based on their colours
Explain how the colour of a star relates to its temperature via Wien’s law
Wien’s law states that the temperature of an object is inversely proportional to its wavelength. In other words, as the temperature increases, the wavelength decreases
useful device to remember the order of the spectral types:
Hottest to coolest
O - Oh B - Boy! A - Another F - Fucks G - Gonna K - Kill M - Me
Spectral classes are defined based on the appearance (or lack thereof) of absorption lines
Absorption lines are usually seen as dark lines, or lines of reduced intensity, on a continuous spectrum. This is seen in the spectra of stars, where gas (mostly hydrogen) in the outer layers of the star absorbs some of the light from the underlying thermal blackbody spectrum
Explain how spectral classes do not depend on composition
The spectrum given out by a star depends on the elements in its atmosphere and its temperature. Astronomers have grouped similar stellar spectra into spectral classes. A spectral class is the star’s position in a temperature classification scheme based on the appearance of absorption lines in its spectrum
Use the Stefan-Boltzmann relationship to relate luminosity and surface temperature to stellar radius.
The Luminosity of a star is proportional to its Effective Temperature to the 4th power and its Radius squared
Example 1: Two stars are the same size, (RA=RB), but star A is 2x hotter than star B (TA=2TB): Therefore: Star A is 2^4 or 16x brighter than Star B. (If two stars are the same size, the hotter star is brighter)
Example 2: Two stars have the same effective temperature, (TA=TB), but star A is 2x bigger than star B (RA=2RB):
Therefore, Star A is 2^2 or 4x brighter than Star B. (If two stars have the same effective temperature, the larger star is brighter)
Note: The effective temperature of a star is the temperature of a black body with the same luminosity per surface area as the star and is defined according to the Stefan–Boltzmann law
Describe how starlight can be used to determine intrinsic properties of stars including their size, rate of rotation, radial velocity (into our line of sight), and proper motion (across our line of sight).
Size: giant stars have large, extended photospheres. Density of particles in the star’s photosphere is low. The pressure in a giant star’s photosphere is also low. First: shows narrower spectral lines than a star of the same temperature with a higher-pressure photosphere. Second: more atoms are ionized in a giant star than in a star like the Sun with the same temperature.
Rate of rotation: Doppler effect, the lines in the light that come from the side of the star rotating toward us are shifted to shorter wavelengths and the lines in the light from the opposite edge of the star are shifted to longer wavelengths
Radial Velocity: Doppler Effect. Spectral lines of moving stars shifted toward the red end of the spectrum if the star is moving away from us, or toward the blue (violet) end if it is moving toward us. The greater the shift, the faster the star is moving
Proper Motion: We see it as a change in the relative positions of the stars on the celestial sphere
Identify the physical characteristics of stars that are used to create an H-R diagram, and describe how those characteristics vary among groups of stars
The Hertzsprung–Russell diagram, or H–R diagram, is a plot of stellar luminosity against surface temperature. Most stars lie on the main sequence, which extends diagonally across the H–R diagram from high temperature and high luminosity to low temperature and low luminosity. The position of a star along the main sequence is determined by its mass. High-mass stars emit more energy and are hotter than low-mass stars on the main sequence. Main-sequence stars derive their energy from the fusion of protons to helium. About 90% of the stars lie on the main sequence. Only about 10% of the stars are white dwarfs, and fewer than 1% are giants or supergiants
Describe what a brown dwarf is and how it is different to a star, and different to a planet.
Brown dwarf: celestial object that is much smaller than a normal star and has insufficient mass to sustain nuclear fusion but that is hot enough to radiate energy especially at infrared wavelengths.
International Astronomical Union considers any object with enough mass to fuse deuterium to be a brown dwarf, while objects with less than that — approximately 13 Jupiter masses — are considered planets
State and illustrate what is meant by parallax
Aapparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object especially : the angular difference in direction of a celestial body as measured from two points on the earth’s orbit.
