L3 - Galactic Astronomy Part 1 Flashcards
What is relative Brightness?
How bright a star appears to us
What is Absolute Brightness/Luminosity?
How bright the star appears (apparent magnitude) from a distance of 10 parsecs.
What is Luminosity?
radiated energy per unit time
What is the Apparent Magnitude?
the brightness of a source compared to a reference source. Magnitudes are defined for specific waveband passes, or by subtracting the apparent magnitudes as it appears in two filter, e.g. red and green.
What are Galaxies?
Extended sources - not point sources like stars
Apparent magnitude is the integrated flux up to a certain radius.
How do we measure distances close to home?
Trigonometric Parallax
What is a Parsec?
the distance corresponding to a parallax of one arc second. This is a really long distance, equal to 3.26 Lightyears.
How do we measure distance for star clusters? (or nearby galaxies)
These stars are too far away to notice any parallax shift. Instead we use ‘standard candles’. The most common type are called ‘Cepheid variables’.
What is a Cepheid Variable?
Like the Sun, which undergoes a twelve year solar cycle which affects its luminosity, Cepheid variable stars have a well-defined relationship between their pulsation period and their luminosity (Note: the Sun is not a Cepheid variable star). Once you observe the period of luminosity, you can calculate the luminosity and hence the distance to that star and the cluster of which it is a part.
These can be measured in galaxies up to 29 million parsecs away.
What is the Hubble Law?
v = Hr
This means that the further away a galaxy is, the faster it is moving away from us. So if we can measure the velocity of a galaxy (using redshift) then we can tell how far away it is.
How can type 1A supernovae be used to find the distance of distant galaxies?
These are all very similar types of supernovae and their light curves (change in luminosity before and after explosion) are very well defined. Like standard candles, measuring their apparent luminosity can provide a distance value to the supernova and hence its host galaxy. These can be measured in galaxies up to 1 billion parsecs away.
What is the Tully-Fisher relation?
The Tully-Fischer relation: relationship between the rotational velocity of spiral galaxies and its total intrinsic luminosity. Not very well understood.
Based on two assumptions: 1) There is a standard-ish ratio between the mass of a galaxy and its luminosity; 2) The contribution of dark matter and visible matter to a galaxy is well-defined.
Cite methods to determine distances at galactic scale.
Close to home: Trigonometric Parallax
Distant Stars: Standard Candles
Nearby galaxies: Cepheids, Novae
Distant galaxies: Tully-Fisher, Supernovae
Types of Galaxies? (shape)
Ellipticals: Classified according to the eccentricity of the ellipse.
Lenticulars: Look like ellipticals, but are actually spiral SO. Practically no star formation.
Spirals: subdivided into spirals and bar spirals depending on shape (see picture). Spirals are due to density waves. These arms have a high star formation rate.
