astro2 Flashcards
what is a blackbody
a hypothetical perfect radiator of light that absorbs and re-emits all radiation incident upon it. It’s light output depends only on temperature and this output acts as to maintain its temperature.
What is a blackbody spectrum
The continuous spectrum emitted by a blackbody; the flux at each wavelength is given by the formula known as planck’s law.
How do blackbody spectrums vary with temperature?
blackbodies emit light at all wavelengths but hotter blackbodies emit more light at all wavelengths and a greater proportion of this light is at shorter wavelengths than a cooler blackbody.
Explain how the perfectly insulted box is a good model for a blackbody radiator?
incident radiation through the hole is absorbed by the walls and none escapes through the hole (as it is small) so all radiation is absorbed and the walls emit this radiation so that they are in equilibrium with the radiation.
What does the graphs look like if we plot flux against max frequency
they look like this
What is Wein’s law and what is it used to determine?
it is used to determine surface temperature of a blackbody|λ(max)×T=2.9×10^-3
What is Stephan Boltzman’s Law and what can we use it to determine?
It is used to determine the amount of power emitted for every square metre of a blackbody at temperature T in kelvin. This quantity is referred to as the flux.|F=σT⁴
Define luminosity and state the equation relating it to flux.
luminosity is the total power output of a star.|L= surface area × flux |L= 4πr²F|L=4πr²σT⁴
Name 3 methods of determining a stars temperature?
- observing its blackbody spectrum and using weins law to determine surface temperature.|2. looking at the spectrum of the star and observing the strength of absorption lines and what absoprtion lines are present |3. measure the flux here on earth and then using the distance and radius of the star, calculated the flux on the surface of the star.|luminosity of the star in the same in all spheres concentric with the star so if we compare L star = L on earth we get F(on surface of sun)=4πd²/r²F(on earth)|note: only works for close stars due to uncertainty in radius
What do we look at to determine a stars chemical composition?
absorption and emission lines
Why do a stars absorption and emission lines help determine its chemical composition?
- stars continuous thermal spectrum is often punctuated with absorption lines. This happens as if light is behind a thin gas (photosphere) which absorbs specific wavelengths of light according to the elements there.|2.each element produces its own unique set of absorption and emission lines (due to energy levels)|3. we can therefore look at the strength/darkness of absorption lines to see what elements are present. strong absorption line –> lots of element
What do we have to correct for when looking at absorption line in the spectra of stars.
we need to correct for the doppler shift of the star
why is looking at absorption lines not always representative of the stars composition?
many other factors affect absorption lines such as temperature affects whether they are present or not.
How is trigonometric parallax used to measure the distance to nearby stars?
- star is viewed from two positions at 6 month intervals |2. the change in angular position is measured relative to fixed distant background stars|3. diameter of the earths orbit about the sun known so we use trigonometry to calculate a distance.
how to convert between arcseconds and degrees?
1°=3600 arcseconds
