Unit 1 - Observational Astronomy Flashcards
What is an astronomical unit?
the distance between the earth and the sun
How far is 1 parsec?
3.26 light years
What are the 2 perpendicular components of light (and all EM radiation)?
an oscillating electric field
an oscillating magnetic field
What wavelengths of radiation are absorbed and scattered the most by clouds / dust?
gamma rays and x-rays
shorter wavelengths scattered the most
What is a black body?
an astronomical body that when cool absorbs all the incoming radiation of all wavelengths and when hot emits radiation with 100% efficiency
What is the relationship between the peak wavelength (colour) emitted and temperature of the black body?
as peak wavelength increases temperature decreases (inversely proportional)
What is the relationship between power of radiation emitted and temperature?
power is directly proportional to T^4
Why are planets not black bodies?
they generate little radiative energy and instead reflect sunlight so the observed colour of a planet doesn’t give a true picture of temperature or power output
Where are electrons found in the atom?
energy levels
the energy level closest to the nucleus is known as the ground state and has the smallest energy value
What happens when electrons absorb a photon of light?
they ‘jump’ to a higher energy level as long as the photon corresponds to a specific energy gap
What happens when an electron emits a photon of light?
it drops down an energy level and releases a photon of light corresponding to the specific energy gap (colour)
What do the emission and absorption spectrum allow us to identify?
identify the elements present within a star or galaxy since each element has a unique spectrum which can be matched up
What is the Doppler Effect?
when radiation emitted by astronomical bodies is effected by the relative speed at which the object travels
Explain the Doppler Effect
a source approaching the observer will have a shorter wavelength and higher frequency
a source moving away from the observer will have a longer wavelength and lower frequency
What significance is the + / - before the v in the Doppler Effect equation?
+v means distance is increasing, source is moving away
-v means distance is decreasing, source is moving closer
What is redshift?
a shift in the spectral lines when all lines move to the right (red end of the spectrum) seen for objects moving away from the source
What are the 3 main functions of telescopes?
1) maximise image brightness (light gathering)
2) resolve images that are separated by small angles
3) magnify distant objects
What is the relationship between aperture and light gathered?
the larger the aperture the more light is gathered
What else does the aperture size of a telescope determine?
the resolving power of the telescope
larger aperture = higher resolving power = smaller angle resolution
What does light diffracted by a circular aperture produce?
a bright disk (Airy disk) with circular fringes
What does Rayleigh’s Criterion state?
2 sources of light are said to be resolved if their Airy disks can be distinguished from each other
What are the 2 different types of telescopes?
refractor telescopes - lenses
reflector telescopes - mirrors
What is chromatic aberration in refractor telescopes?
lenses refract different wavelengths by different amounts, the focal length increases with wavelength, the lens therefore focuses the light of different wavelengths in different positions
What is spherical aberration in reflector telescopes?
when you move away from the centre of the mirror the image becomes blurred because the focal length of the mirror decreases
How can spherical aberration be fixed?
by using a parabolic mirror since it focuses every ray to the same point
What is astronomical seeing?
the blurring and twinkling of astronomical objects caused by turbulence in the atmosphere
What does astronomical seeing cause?
1) point sources to break up into speckles
2) long exposure images of these patterns blur into a disk
3) the brightness of stars to fluctuate known as scintillation or twinkling
How can astronomical seeing be fixed?
by deforming the telescope optics, known as adaptive optics
light from a ‘beacon star’ or laser is analysed and the shape of the mirror deformation is calculated, the mirror is deformed by a series of pistons
What is the altitude of a star?
it’s angular distance relative to the horizon
What is the zenith?
the point on the sky directly above the observer
What is the azimuth?
horizontal component of a stars position above us, partnered with altitude
measured relative to the observers meridian (line from N-S), always measured in a N-E direction
What is declination?
analogous to latitude (+N and -S) and measured relative to the celestial equator
What is right ascension?
the longitudinal component, partnered with declination
measured eastwards from the vernal equinox (zero point where the sun crosses the from the S-N hemisphere) along the equator
measured in time
What do right ascension and declination define?
a star’s position on the celestial sphere
they are fixed points
What is the relationship between the altitude of the celestial pole and the observer’s latitude?
in general altitude of celestial pole = observer’s latitude
What is meant by upper culmination?
when a star is at the top of a circular orbit around celestial pole where altitude = max altitude
What is meant by lower culmination?
when a star is at the bottom of a circular orbit around a celestial pole where altitude = min altitude
What are circumpolar stars?
stars that do not rise or set
What is sidereal time?
the time based on the movement of the Earth relative to the stars
What is solar time?
the time based on the movement of the Earth relative to the Sun, based on the rising and setting of the sun
How do sidereal and solar time compare?
there is 1 more sidereal day than solar days in a year, a solar day is 0.28% longer
How is sidereal time measured?
by measuring the passage of the vernal equinox across the sky relative to the observers meridian
In relation to time, what is right ascension?
the time between the vernal equinox crossing the observer’s meridian and the object of interest doing the same
also an angle but measured in units of time
How do you locate a celestial object in the sky?
1) the observers position on Earth
2) the star’s coordinates (right ascension and declination)
3) local sidereal time
How do you calculate the local hour time?
local sidereal time - right ascension
