Galaxies Flashcards
What is a galaxy?
A gravitationally bound group of stars
What are the main features of a spiral galaxy?
Rotating disc, dominated by spiral arms, which contains population l stars, gas, dust and form stars.
Elliptical concentration of stars at the centre, called the bulge, which is red population ll and old population l.
Galactic halo.
Some have a bar.
What are the main features of an elliptical galaxy?
Population ll and old population l stars Smooth variation in intensity Very little gas and dust No organised rotation Come in dwarf and giant forms
What are the main features of an irregular galaxy?
No regular structure, plenty of gas and dust and blue stars, mix of population ll and l, relatively small
What are Sa galaxy classified by?
Tight arms, large bulge, small disc, little gas
What are Sc galaxy classified by?
Loose arms, small bulge, large disc, lots of gas and dust
What are S0 galaxies?
Mix between E7 and Sa/SBa. They have both elliptical and spiral components. They have a disc, bulge as well as elliptical component. It has a little gas but no spiral arms
How can we view the plane of our galaxy?
Optical light from the shows the plane, although dust lanes obscure much of the structure. Infrared is needed to show true structure.
Where is our position in the Milky Way?
8kpc from the centre, with a disc of radius 20kpc
How large is the bulge of the Milky Way?
1kpc
How large is the bar of the Milky Way?
4kpc
What is the spiral arm structure?
It is determined using tracers of massive star formation with known distances, as well as HII and CO clouds being used.
We have 2 stellar arms, and 4 star forming gaseous arms
What is the velocity of stars in the disc?
220km/s
What is the wind up problem?
Stars travel at the same speed so there is differential rotation. If the spiral arms were a fixed pattern, the arms would wind up in a few revolutions
What is the solution to the wind up problem?
The spiral arms are modeled as a density wave pattern that rotates at a slower speed, so stars and gas pass in and out of the arm. In the arm the gas gets compressed and molecular clouds form with subsequent star formation.
How can you tell where the spiral arms are?
They are dust lanes, then molecular clouds, then blue stars
What is the galactic halo?
A spherical halo around the plane of the galaxy, consisting of globular clusters and halo stars. The radius is 100kpc.
In what order did the Milky Way form?
The galactic halo formed out of metal poor material, where there was an initial burst of star formation and none since. The disc formed later, where continuous star formation lead to a metal rich population.
Bulge also has some metal rich stars as a result of mergers.
The globular clusters follow different paths, not the same rotation as the disc.
What happened when two large galaxies collide?
The galaxies get completely disrupted. Large tidal tails can form as the galaxies orbit each other in close proximity.
If both galaxies contain gas, then this gets shocked and compressed which results in a burst of star formation.
The tidal tails can just contain Hl regions not stars so need 21cm.
What are superwinds?
In starburst galaxies, there can be galactic superwinds, when large numbers of massive stars undergo supernovae explosions which blow out all the gas in the galaxy. This can remove all gas from a galaxy, stopping future star formation, leading to the formation of an elliptical galaxy.
What are the features of an active galaxy?
They have a luminous point like nucleus that looks like a star. Quasars completely outshine the rest of the galaxy. It has a non-thermal continuum spectrum, and emission lines, compared to a blackbody spectrum and absorption lines of a star. Some emission lines are very broad due to very fast Doppler motion. It is strong in the radiowaves.
Most active galaxies show interaction with other galaxies.
How can the size of the region of an active galaxy be limited?
The active galaxies vary in luminosity. The variability timescale multiplied by the speed of light, the fastest communication, gives the smallest size.
How is the high luminosity of a quasar explained?
The release of gravitational potential energy of material falling onto a supermassive black hole
What is the total energy released from a supermassive blackhole?
E=GMm/R
R is found by equating gravitational potential energy to kinetic energy.
L=0.5emc^2
