1. How do astronomers measure the distances to galaxies and how does that allow the sizes, luminosities and masses of galaxies to be determined?
A distance indicator is an object within a galaxy that functions as a marker to that galaxy. It may be a Cepheid star, globular cluster, H II region, planetary nebula or supernova. The distance to a galaxy, especially if it is very far away, is approximated by employing the Hubble Law which is expressed as Recession Velocity (Vr) = Hubble’s Constant (H) x distance (d).
After obtaining the distance, it is possible to determine diameter through the small angle formula and luminosity from distance and apparent magnitude where both formulas are derived equations (Garber). Meanwhile mass can be estimated in three ways. Rotation curves reveal the calculation of rotational velocities for varying distances from the galactic center so that once distance and velocity are known, mass can be obtained (Garber).
Another is through the cluster method which focuses on the motion of a galaxy within a galactic cluster. The size of the galaxy as well as range of velocity determines the total mass of the cluster (Garber). The third is through the velocity dispersion method where the spectra of the galaxy are used to approximate mass. Broad spectral lines indicate high velocity which in turns suggests a large mass (Garber).
2. Discuss how individual stars and the shapes of galaxies are affected by collisions.
As a galaxy approaches another in a collision, the gravitational fields of the stars in each of them start to interweave and the resulting powerful tidal forces disturb and radically alter the shape of the galaxy, similar to the manner in which the gravitational pull of the moon causes the tide to rise in regions of the earth nearest to it but magnified a thousand times. A collision initiates the formation of tidal tails, bars or rings and colliding gas clouds produce knots of newly formed blue stars while the nucleus of the galaxy becomes deformed (sciencedaily.com).
On the other hand, when stars collide, they merge together as one star that displays unusual brightness and heat relative to age so that they become very prominent in their globular cluster. These stars are referred to as “blue stragglers”. When stars increase in age and use up their hydrogen, they become cooler, less massive and red in color but through collisions, they obtain extra mass causing them to turn blue (Masters). This permits them to remain longer in the main sequence – the phase in a star’s life where it burns its hydrogen.
3. Explain the differences between the three types of galaxies and what happens to change their shape.
An elliptical galaxy is rounded or oval in shape, do not have visible gas and dust or bright, hot stars and consists of population II stars. Elliptical galaxies are also surrounded by globular clusters. A spiral galaxy has a disc component, consists of both populations of stars, exhibits a nucleus and may have arms with differing orientations (Garber). An irregular galaxy does not present a regular pattern and includes new and old stars alike.
Galactic interaction, collision and merging, which involve the effects of the gravitational fields of galaxies, are the primary events that change the shape of galaxies. The subsequent structure depends on both the type of the galaxies involved and the directions of their orbits (Keel). Collision, mentioned earlier, may not result in a merger if both galaxies have enough force to continue moving away from each other after the event.
Galaxies are said to be interacting when they do not collide but both their gravitational attractions cause distortion and exchange of gas and dust (astro.umd.edu). In interactions that occur at slow speeds and involve galaxies with unequal masses, spiral formations may assume irregular-lenticular shapes (Than). Gases being pulled to the central region, as a result of tidal disturbances, clear away the spiral configuration, leaving behind a disk structure.
The most drastic interaction is the merging of two galaxies and occurs when they collide but lose their momentum to slide past one another. Instead, they fall back into each other and unite into one galaxy, losing their original shapes in the process (astro.umd.edu). When a significantly more massive galaxy collides and merges with a smaller one in a type of interaction known as galactic cannibalism, the bigger galaxy does not exhibit a visible change in shape but the less massive galaxy is ripped apart, loses its shape and becomes integrated into the bigger one.
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