Astrophysics Flashcards
What is one light year
The distance a photon will travel in one year
What is the size of the astronomical unit equal to
The mean distance from the earth to the sun
What is one arcsecond equal to
1 arcsecond = 1/3600 degree
Parsec def
The distance at which the radius of the Earth’s orbit (1 AU) around the Sun subtends at an angle of 1 arcsecond
How to derive a parsec
Arc Length equation
Parallax error def (for astronomy)
Half of the angle between the Earth at one time of year, and the Earth six months later, as measured from a nearby star.
what can the spectral characteristics of a star tell us
- precise surface temperature
- composition of the star
- physical conditions within the star
what three types of spectra does spectroscopy give us
- an emission line spectrum
- an emission continuous spectrum
- an absorption spectrum (like an emission line spectrum but reverse)
what is the photosphere
the area of a star where light is radiated
describe the three types of line spectra
Continuous spectrum:
created when photons of all wavelengths are emitted
Appearance: a broad range of colours (depending on a star’s temperature)
Produced by: hot, dense sources, such as the cores of stars
Emission spectrum:
created when photons are emitted by excited electrons in a hot gas
Appearance: discrete wavelengths represented by coloured lines on a black background
Produced by: hot, low-pressure gases, such as a nebula surrounding a star
Absorption spectrum:
created when photons are absorbed by electrons in a cool gas
Appearance: discrete wavelengths represented by dark lines on a continuous spectrum
Produced by: light passing through cool, low-pressure gases, such as the photosphere of a star
how does an absorption spectrum and emission spectrum relate
the lines in an absorption spectrum correspond to the same lines in the emission spectrum of the same element
how are stellar spectral lines formed
- Stellar spectral lines are caused by the interactions between photons and the atoms present in gaseous layers of stars
- Photons produced by fusion reactions in a star’s core move towards the layers of gas in the outer atmosphere of the star
- The photons produced in the core form a continuous spectrum
- Photons are absorbed by the gas atoms, which excite and re-emit other photons of various frequencies in random directions
- Each gas produces a unique pattern of spectral lines due to the specific transition between the element’s energy levels
- The pattern can be used to determine the presence of a certain element within the star
- The chemical composition of a star can be investigated even when extremely distant
- If the element is present in the star, its characteristic pattern of spectral lines will appear as dark lines in the absorption line spectrum of the star
e.g The Sun is predominantly made up of hydrogen and helium gas. This can be verified by comparing the emission line spectra of hydrogen and helium with the absorption line spectrum of the Sun
what is the name of the series which give rise to the visible region of the hydrogen spectrum
the Balmer series
Resolving power def
The ability to produce separate images of closely spaced objects
What is an airy disk
The disk formed when a telescope cannot focus a star to a perfect point
Apparent magnitude def
The perceived brightness of a star as seen from Earth
How much brighter is a magnitude 1 star in comparison to a magnitude 6 star
100 times brighter
What does a larger/ lower apparent magnitude mean
- The more negative the apparent magnitude, the brighter an object appears
- The more positive the apparent magnitude, the fainter an object appears
Luminosity def
The total power output of radiation emitted by a star
What does a higher/lower apparent magnitude mean
- The more negative the apparent magnitude, the brighter an object appears
- The more positive the apparent magnitude, the fainter an object appears
What is luminosity measured in
Watts
Brightness of a star def
The intensity of radiation received on Earth from a star
Brightness is equivalent to power per unit area, or light intensity, and is measured in watts per metre squared (W m−2)
What does the brightness of a star depend on
How much light the star emits (i.e. its luminosity)
How far away the star is (more distant stars are usually fainter than nearby stars)
What is the difference between star luminosity and star brightness
The luminosity is the total power output of the star, whereas the brightness is the power as measured on Earth
Why is knowing the luminosity and brightness of an object useful
it allows us to determine how far away it is from the Earth, as:
- Luminosity tells us how bright the star is at its surface
- Brightness tells us how bright the star is as observed from the Earth