A15 - Our place in the galaxy Flashcards
Describe and explain the appearance of the Milky Way from Earth:
a fuzzy band of light stretching across the sky (the plane of the galaxy, as we are inside it)
Describe the structure of the Milky Way:
-SBb (barred spiral) galaxy
-a galactic plane, approximately 1500ly thick
-stretches around 100,000ly in diameter
-galactic nucleus seen as a bulge in the centre (in Sagittarius), and a black hole called Sagittarius A*
Where is the Sun positioned in the Milky Way?
the Sun is 30,000ly from the centre, and is on the Orion Arm
How have we managed to determine the structure and rotation of the Milky Way?
-radio waves, specifically 21cm waves (the Hydrogen line)
-allows us to see past the interstellar dust in the inner galaxy that absorbs visible light
Other than radio waves, what observational evidence suggests that the Milky Way is a spiral galaxy?
more stars are seen in a band across the sky, implying a flat and possibly spiral shape
If it were elliptical, then a band wouldn’t be observed, as their stars are more uniformly distributed
What is the group of galaxies around us called? Name the galaxies in it:
-the Local Group
-Milky Way
-Andromeda (M31)
-Large + Small Magellanic Clouds (L/SMC)
-Triangulum (M33)
How do we classify galaxies? Name the 4 categories:
-the Hubble Classification
-spiral
-barred spiral
-elliptical
-irregular
Describe the structure of a spiral/barred spiral galaxy:
a flattened disk galaxy with a nucleus, from which spiral arms extend outwards
spiral - arms wind around the nucleus
barred - spirals extend from a bar in the centre, then wind around the nucleus
Describe the structure of an elliptical galaxy:
round, spherical/ellipsoidal shape, brighter towards the centre
Usually the largest type of galaxy
Describe the structure of an irregular galaxy:
no obvious symmetry/shape, possibly due to the gravitational disturbances from other galaxies
How is the evolution of galaxies summarised?
the Hubble Tuning Fork diagram
Not considered accurate anymore, but it’s still used to classify them
Describe where the types of galaxies are placed on the Hubble Tuning Fork diagram:
-elliptical galaxies are placed in order of increasing elongation (E0-E7)
-S0 galaxies are the intermediate stage between elliptical and (barred) spiral, and have a flat disk but no spiral arms (eg Spindle Galaxy)
-the letters (A-B-C) represent how tightly wound the arms are, with A being the tightest
-the more loosely wound the arms are, the smaller the central bulge is
Irregular galaxies are not part of the Tuning Fork
What is an AGN?
Active Galactic Nucleus, an area at the centre of a galaxy that has a significantly high luminosity over the entire EM spectrum
What causes an AGN?
-powered by a supermassive black hole
-material spinning around it forms an accretion disc, and heat is emitted due to its speed and friction and the effect of it falling into the black hole
-produces enormous radiation across the EM spectrum
What is an active galaxy? Name 3 types:
-a galaxy with an AGN
-Seyfert galaxies
-quasars
-blazars
Describe the features of a Seyfert galaxy:
active galaxies with bright spectral emission lines caused by either the accretion disc, or from ionised gas around the AGN
What are blazars?
-active galaxies with a relativistic jet pointing towards Earth (has the effect of “looking down” the jet of a blazar)
-difficult to make comparisons between blazars because of the angle from which we view them
Relativistic jets are powerful jets of radiation and particles travelling close to the speed of light
