Topic Four: Galaxies and Cosmology Flashcards
What is a galaxy?
Huge gravitationally bound collections of stars with very little else between them
What are the two small satellite galaxies that orbit the milky way?
The small and large magellanic clouds
Explain the appearance of our galaxy from Earth and why it appears this way:
Large milky band stretching across the sky
We are looking into the plain of our galaxy from one of its spiral arms
Describe the shape of the Milky Way:
A giant disk shaped Catherine Wheel rotating around a central budge
Where in the sky of the Northern Hemisphere can you find the milky way?
In a line from Cassiopeia to Cygnus
The galactic centre lies in sagittarius
Describe the three main structural components of our Galaxy:
A bulge surrounding the nucleus (black hole)
The disk
The halo
Describe the disk of the Milky Way:
Young stars, gas and dust
Four major spiral arms
Is 1-2kpc thick
30kpc in diameter
Describe the halo of the Milky Way:
Globular clusters (dense groups of relatively old stars) - spherically distributed along the galactic centre
Whereabouts is our solar system found in the Milky Way?
One of the spiral arms
10kpc from the nucleus
Apart from stars, what do the spiral arms of the Galaxy contain?
Huge clouds of gas and dust that can block light
How can we see the structure of the spiral arms if light is being blocked?
Microwave EMR can be seen by radio telescopes on Earth, this wavelength is given off by hydrogen
Doppler shifts in the wavelengths help us work out the galaxy is rotating
Who classified the different types of galaxy?
Edwin Hubble in the 1920’s
Describe the Spiral Galaxy:
Flattened disk-shaped
e.g. Andromeda
Describe the Barred Spiral Galaxy:
Like spiral, has bright bar running through nucleus through which spiral arms appear
Describe the Elliptical Galaxy:
Smooth Spheroid of
Stars
Brightest at centre
Dimmest at edge
Describe the Irregular galaxy:
Has little structure or symmetry
Describe the Lenticular Galaxy:
Class between Elliptical and Spiral
What is the local group?
Galaxies are grouped together just like stars
Our galaxy is grouped together with 50 others
Name 6 galaxies in our Local Group:
Andromeda Large Magellanic Small Magellanic The Triangulum Galaxy Pisces Dwarf Aquarius Dwarf
What is a cluster?
Many local groups of galaxies can be found in gravitational agglomerations called Clusters
Often there is a giant elliptical galaxy at the centre of a cluster
What is a super cluster?
A group of clusters held together by gravity
What is an Active Galactic Nucleus?
Region in the centre of some galaxies emitting huge amounts of radiation - not just visible light
Supermassive black holes at their centre, matter spiralling to black hole emits radiation
Name 4 types of active galaxy:
Radio galaxies
Seyfert galaxies
Quasars
Blazars
Describe Seyfert Galaxies:
IR and X-Ray emitted
Describe Quasars:
Compact Extremely Bright -One of the brightest objects in the universe Supermassive black hole at centre One of the oldest objects in universe - high redshifts
Describe Blazars:
Emit strongly in all regions of the spectrum
Optical brightness can vary over 1 or 2 days
What are absorption lines?
Black lines created when light from stare is absorbed by element in the outer layers
What is Redshift?
Absorption lines are shifted towards the red end of the spectrum when we look at the most distant galaxies
Their wavelength is longer than it should be
What can we deduce from Redshift?
That distant galaxies are moving away from us and that the universe is expanding
The universe began as an extremely small initial point in the big bang
Give an example of Blueshift and explain what this means:
Andromeda is moving towards us
What is Hubble’s law
Hubble saw that fainter galaxies showed greater redshift
The recession velocity is proportional to the distance from us
v = Hd
What are the units for Hubble’s constant?
Km/s/mpc
If we convert the km into mpc then we can express the units of the Hubble Constant as /s
What is the significance of this?
1/h gives the age of the universe in seconds
How old is the universe?
14 billion years
What is Cosmic microwave background radiation?
Microwave radiation that can be detected coming from the sky in every direction
Perfectly matches the radiation emitted by a black body with a temperature of 2.7 K
What is dark matter?
Matter which does not emit or reflect radiation but has detectable gravitational effects
Makes up 95% of our universe
What could Dark Matter be made of?
We don’t know, perhaps it’s made out of Black Holes, Neutrinos, Weakly Interacting Massive Particles.
Or maybe, just maybe, it’s all one massive conspiracy made by the government….
ok ignore that last one I’m getting bored again
Who first proposed the existence of dark matter and when?
Fritz Zwicky in 1933
Give evidence for the existence of Dark Matter:
Gravitational lensing - light bent and distorted through huge amount of matter
Rotational curve for a spiral galaxy:
- stars far away from nucleus travelling much faster than expected
- only explicable if a great deal of invisible mass was present
What is dark energy?
Mysterious repulsive force causing accelerating expansion of universe
What is the significance of Dark Energy?
Expansion of universe not slowing down as previously thought
