Astrophysics Flashcards
Lens formula
1/u + 1/v = 1/f
u = distance of object from centre of lens
v = distance of image from centre of lens
f = focal length of a thin lens
If diverging: f and v is negative
When is a virtual image produced
When the object is closer than the focus point
AU
Mean orbital radius of the earth around the sun
Parsec
Distance to a star which subtends an angle of 1 arcsecond to the line from the centre of the Earth to the centre of the Sun.
1 arc second
4.85x10^-6 radians
Parallax angle
Parsecs = 1/parallax angle in arc seconds
Parallax method acceptable up to 100pc
What unit of distance is used for the absolute magnitude equation
m - M = 5log(d/10) uses parsecs
Where is our sun in the Hertzsprung-Russel diagram
Absolute magnitude 5
Temperature just over 5000 / right side of class G
Visible spectral classes
O: 50-25 Strong: He+, He. Weak: H
B: 25-11 Strong: He, H
A: 11-7.5 Strongest H lines weak: metal ion
F: 7.5-6 Strong: metal ions
G: 6-5 metal ion and metal atom
K: 5-3.5 Mostly neutral metal atoms
M: less than 3.5 Neutral atoms and compounds like TiO
Luminosity and intensity
Luminosity= power output of a star
Intensity = power of radiation/area
Aperture
Diameter of objective lens / mirror
Difference and similarities in telescopes that pick up different wavelengths
Optical telescopes are similar to infrared UV and radio, however instead of an eyepiece lens a CCD is used and an Ariel for radio. These 3 telescopes a cassegrain reflecting telescope is often used.
X-ray telescopes are usually in space due to the atmosphere preventing the majority of X-radiation reaching the Earths surface. They are very penetrating and not easily reflected off metal surfaces , some reflected and some transmitted. Iridium is used and they are reflected off a series of mirrors at a very shallow angle and focus some 10m away. Due to their short wavelength the telescopes can have a small diameter and still produce well resolved images.
Infrared and UV telescope
Due to atmosphere UV telescopes usually orbit around Earth, some infrared penetrates through, so possible to have on top of mountains, others orbit the Earth to detect all infrared.
Due to being on either end of the wavelength spectrum their collecting powers are similar to optical as diameter similar, however due to UV’s smaller wavelength it has a better resolving power at same diameter. This is opposite for infrared.
Radio telescope
Their mirrors/dishes are very large due to larger wavelengths, meaning collecting power is very high.
They are built so big due to ∅ ≈ lambda/diameter and therefore able to resolve to close radio sources
Compare reflecting Vs refracting telescope
Reflecting pros:
•tend to be cheaper
•Easier to make concave mirror of large diameter than lens
•No chromatic abberation
•Easier to reduce spherical abberation
•Possible to make them with larger diameters as a lens with diameter of over 1m begins to sag under its own weight.
Refracting pros:
•Less maintenance is required, mirror in reflecting is exposed to air and therefore may need recoating..
•Secondary mirror in refracting may block light
Construction of ray diagram
1) ray parallel to the principal axis is refracted so that it passes through the focal point
2) a ray that passes through the optical centre of the lens is undeviated
3) a ray that passes through the focal point is refracted so it travels parallel to the principal axis
When comparing telescopes mention…
Magnifying power M = fo/fe = beta/alpha
Resolving power = ∅ = lambda/d
Collecting power = D1^2/D2^2
Rayleigh Criterion
The minimum subtended angle between two objects
whose (images) can be resolved. ✔1
(Minimum angle is when) the central maximum of (the diffraction pattern of light
from) one object coincides with the first minimum of (the diffraction pattern) of
the second object. ✔2
What to compare when comparing stars
Colour (temp)
Brightness (magnitude)
Power
What happens at less than and more than 1.4 solar masses of a red super giant
What if the supernova mass is less than or more than 3 solar masses
White dwarf
Supernova
Neutron star
Black hole
Electron degeneracy pressure
When the core has shrunk to about Earth-size electrons exert enough pressure to stop it collapsing
(White dwarfs)
Absolute magnitude of a supernova at its peak
-19.3 +/- 0.3
When mentioning balmer lines remember to say whether weak or strong
The defining characteristics of a supernova
Rapid, massive increase in brightness
What type of supernova has hydrogen balmer lines
Type 2 supernova
How is a type 1a (a subset of type 1) supernova formed
When a white dwarf core absorbs matter from a nearby binary partner
Pulsing neutron stars are called…
Pulsars due to them spinning fast and emitting radio waves
Doppler effect definition
The apparent change in the frequency/wavelength of a wave due to the relative motion of the source and the absorber
Exoplanet def
A planet found outside our Solar System, in orbit around another star
Why is it hard to find exoplanets?
Light from the host star is much brighter than the reflected light from the planet
They subtend extremely small angles compared to the resolution of telescopes
Radial velocity method/Doppler shift effect
As a planet orbits its host star, they both orbit around a common centre of mass
During the orbit, the star will move slightly towards, or away from the Earth as the planet moves to different positions in the orbit
The line spectrum of the star will showblueshiftwhen it moves towards the Earth, thenredshiftwhen it moves away
How to determine the orbital period of an exoplanet using the radial velocity method
This causes very small, but measurable, periodic shifts in the wavelength of the light received from the star
The time period of the planet’s orbit is equal to the time period of the Doppler shift
Light Curve using the Transit Method
The dip in brightness can be used to determine the size of the planet
The duration of the dip can be used to determine the orbital period of the planet
Transit method limitations
The accuracy can be reduced if the Earth, planet and star are not aligned in the same plane
Only planets with a short orbital period can be detected
Radial velocity limitation
Low-mass, or Earth-like, planets do not cause as much ‘wobble’ as high-mass planets since they have a greater gravitational pull on the star
Quasar
The quasar features a black hole surrounded by an accretion disk and emits jets of radiation
As matter falls into the black hole,jetsof radiation are emitted from the poles
The equivalent of 100 solar masses of matter can fall into a quasar each year
The gravitational potential energy of infalling matter is transferred to electromagnetic radiation
Now it is known that quasars are strong emitters ofallwavelengths, not just radio waves
Quasar characteristics
Extremely luminous star-like sources of radiation with very high redshifts
Quasars are thought to be some of the most disttant measurable objects in the known universe
This is evidenced by the extremely large redshifts they show
