AST1002 Exam 3
What is the Tully-Fisher relationship?
A relationship among spiral galaxies showing that the faster a spiral galaxy's rotation speed, the more luminous it is. It is important because it allows us to determine the distance to a spiral galaxy once we measure its rotation rate and apply the luminosity-distance formula.
How are the orbits of the stars in different regions of our galaxy different?
Disk stars orbit the galactic center in roughly circular paths that all go in the same direction in nearly the same plane. They bob up and down like a merry-go-round due to the localized pull of gravity within the disk. Halo stars orbit the galactic center, but the orientations of their paths are completely random, and neighboring stars may circle the galactic center in opposite directions. They swoop high above and far below the disk. Bulge stars have a complex orbit, with a range of orbital properties among its stars. Some have random orientations like halo stars, while others orbit in the same general direction as disk stars but with more elongated orbits.
Which elliptical galaxies are the most round, and which are the most elongated?
Elliptical scale is E0-E7. E0 would be the roundest and E7 would be the most elongated.
How can we know the amount of mass in our galaxy?
Orbital speeds remain about the same out to great distances from the galactic center, telling us that most of the galaxy's mass resides far from the center and is distributed throughout the halo. Because we see few stars and virtually no gas or dust in the halo, we conclude that most of the galaxy's mass must not give off any light that we can detect, and this is dark matter. By using the Sun's 27,000-light-year distance from the galactic center and its 220-km/s orbital velocity in Newton's version of Kepler's third law, we find that the total amount of mass within the Sun's orbit is about 2 * 1041 kg, or about 100 billion times the mass of the Sun.
What are the spiral arms?
Pinwheel like structures originating close to the galactic bulge and extending outward throughout the galactic disk. Spiral arms come from instabilities in gas in galactic bulge, grav effects of nearby galaxies, etc.
What does the Milky Way Galaxy look like, what type of galaxy is it, and what are the different components?
The Milky Way Galaxy consists of a thin disk ~100,000 light years in diameter, has a central bulge & a spherical "halo" that surrounds the entire disk. and contains over 100 billion stars. It is a spiral galaxy. The different components are a fairly flat disk of stars with a bright central bulge surrounded by a dimmer, rounder halo, and spiral arms. Clouds of interstellar gas and dust make up the interstellar medium.
What does the observed rotation curve (graph of rotation velocity v. distance) of our galaxy tell us about the mass in the galaxy?
The observed rotation curve allows us to determine the mass of the galaxy on larger scales by measuring the orbital motion of stars and gas at greater distances from the galactic center.
What do we know about the amount of mass in our galaxy beyond the orbit of the sun? Is it less or more, and what type?
There's more matter beyond the orbit of the sun, up to 50 kiloparsec, made of stars, star clusters, and spiral arms----dark matter
What is expected to be at the center of our galaxy, and how do we know about it?
We think a supermassive black hole is there, producing radio emissions named Sagittarius A* that is unlike any other radio source in our galaxy. Several hundred stars crowd the region and their motions indicate an extremely massive object. An object that massive (4 million solar masses) into a space just a little larger than our solar system must be a black hole.
Where are young stars, old stars, and new forming stars located in the galaxy, and why is the metallicity of very old and young stars different?
Young stars are located in the disk, replacing the old stars. They are born from the gravitational collapse of gas clamps in molecular clouds. Old, red, low-mass stars assemble in the globular clusters, and are found in the halo. There is a much smaller proportion of heavy elements than stars in the disk. New forming stars are found in the spiral arms because this is where stars die, and, therefore, are where the material for new stars is. Also, in the galactic center. Halo (older) stars contain smaller proportions of heavy elements than disk (younger) stars. This is because the halo does not contain the cold, dense molecular clouds required for star formation. In fact, it contains almost no gas at all, and the gas it has is very hot. Because of this, new stars can only be formed in the disk and not the halo. The halo stars lack of heavy elements indicates that they must have formed early in the galaxy's history.
What is the star-gas-star cycle?
atomic hydrogen clouds: interstellar gas clouds fill the galactic disk molecular clouds: gas in the disk gradually cools and forms molecules star formation: gravity makes stars from molecular hydrogen gas nuclear fusion in stars: fusion in the cores of stars makes new elements from hydrogen returning gas: supernovae and stellar winds return gas and new elements to interstellar space hot bubbles: returning gas cools and then blends into hydrogen gas clouds
What types of stars (old or young) do we find in ellipticals, what is the amount of dust and gas compared to other type of galaxies, and how are their masses?
old stars, "all halo" meaning they contain very little dust or cool gas needed for star formation but very hot gas that emits X rays,
What are the 3 major types of galaxies, and how are they different?
spiral galaxies: flat, white disks with yellowish bulges at their centers, disks are filled with cool gas and dust, interspersed with hotter ionized gas and usually display spiral arms elliptical galaxies: redder, rounder, and often elongated like a football, contain very little cool gas & dust with very hot ionized gas irregular galaxies: neither disklike nor rounded
What are standard candles, white dwarf supernovae, RR Lyrae, and cepheid variables, how are they different, how can you tell the distance based on their periods, what type of stars are they (pre-main sequence, main sequence, post-main sequence), and where are they on the HR diagram?
standard candles: an object for which we have some means of knowing its true luminosity, so that we can use its apparent brightness to determine its distance with the luminosity-distance formula white dwarf supernovae: a supernova that occurs when an accreting white dwarf reaches the white-dwarf limit, ignites runaway carbon fusion, and explodes like a bomb; often called a Type Ia supernova RR Lyrae: a pulsing variable star that is recognizable by the characteristic shapes of their light curves, all pulsate with only small differences in period between them (0.5-1 day) Cepheid variables: a pulsating variable star the is recognizable by the characteristic shapes of their light curves, pulsate in a "sawtooth" pattern, have different pulsation periods ranging from about 1-100 days You can tell their distance based on their periods is if we know its apparent brightness and luminosity, we can use the inverse square law to calculate the distance. They are all post main-sequence stars found in the instability strip of the HR diagram.
