Introduction to Cosmology Flashcards
visible milky way
our galaxy seen ‘edge on’
how to determine distances to stars in the galaxy?
using distance indicators such as types of variable stars or the annual parallax of stars.
or by measuring apparent magnitudes of standard candles.
If we know luminosity and measure its flux from Earth, can estimate distance because…
the flux drops off as the inverse-square of the distance.
the distance modulus formula
expressing idea of distance indicators in terms of magnitudes.
relates apparent and absolute magnitude with a star or galaxy’s distance modulus.
distance modulus
a simple function of its distance in parsecs
parsec
parallax arc second
standard candle
a class of object assumed to have a predictable intrinsic luminosity
commonly used variable star distance indicators
- RR Lyrae stars
- Cepheid variable stars.
RR Lyrae stars
(A and F type giants which are pulsating. Often found in globular clusters)
Cepheid variable stars
F and G type supergiants
pulsate with a period around 1 to 50 days
absolute magnitude can be accurately estimated from their period.
extinction
absorption of starlight by interstellar dust grains which makes stars appear dimmer.
apparent density drop-off was due to extinction and was not a real effect.
how does a galaxy rotate?
not as a rigid body but differentially.
the angular speed of stars around the galactic centre depends on their distances from it.
Inner part rotates like a rigid body. (Think ice skaters holding hands)
Keplerian part
outer part of the disc
called keplerian since orbits approximately obey Kepler’s laws
rotation curve
plot of rotation speed as a function of distance from the centre of the galactic disc
rigid body rotation from x=0 (looks like /)
then kepler rotation after (——-)
estimation of the total mass of the galaxy interior to the Sun’s distance from the galactic centre
using Kepler’s 3rd law
GMgalP^2=4pi^2a^3
Mgal is mass of galaxy interior to a
P is suns orbital period
spiral structure of the galaxy
stars in the disc of the Milky Way are not uniformly distributed.
They lie along spiral arms wound tightly around the galactic bulge.
how can the spiral structure be mapped?
measuring the emission of neutral hydrogen.
Peak at 21cm line, if peak shifted, moving toward/away
the galactic halo
by plotting the rotation curve from radio observations, deduced that galactic disc appears to be embedded in roughly spherical halo of dark matter.
evidence for galactic halo
rotation curve does not drop off as rapidly as expected if only luminous stars in the disc were contributing to gravity.
equating grav force with cent forces, we get v is proportional to r^-1/2 so speed should fall of inversely with the square root of the distance.
dark matter
interacts gravitationally because it has mass but doesn’t electromagnetically.
density wave
spiral shaped wave pattern of high and low density regions.
cause gas to pile up in regions of higher density (like traffic jam)
what does the density wave theory predict?
inside edge of spiral arms are the most active star forming regions
without density wave
structure would be much more chaotic and disordered.
messier catalgue
contains many galaxies eg: andromeda and M31.
In M31, M stands for Messier and then galaxies are numbered.