PHYS 110 Exam 4

Milky Way Faint Band

The Milky Way in the sky is our view of the inward part of the Milky Way Galaxy based on our location in the galaxy. Since we are part of the disk, we see a band of diffuse light that completely encircles us.

Globular Clusters in Spiral Galaxy

Spherical haloes of spiral galaxies (older population II stars).

Star Formation in Spiral Galaxy

Occur in the spiral arms due to the abundance of dust and gas.

Main Components of Milky Way/Spiral Galaxy

Milky Way contains a barred bulge, thin disk of stars, concentrated dust in spiral arms, less substantial thick disk of stars, spheroidal halo of ancient stars and globular star clusters, supermassive black hole, and dark matter.

Center Black Hole Evidence

Recorded stellar tracks within 1 arcsecond of center, orbital periods indicate presence of a gravity sources with a mass more than 4 million suns and a radius less than 17 light-hours. Other evidence is radio and X-ray emissions from galactic center.

Long-Term Future of Our Galaxy

Milky Way and Andromeda galaxy will merge in a few billion years. The two galaxies will blend into an elliptical galaxy surrounded by a tidal stream of stars. Also, as time goes on, there will be a greater enrichment of heavier elements are more stars form and die.

Only in Spiral Arms

Open cluster, giant molecular cloud, group of O and B stars, and many planetary nebulae.

Only in Other Parts of the Galaxy

Globular cluster, many planetary nebulae.

Very Young Objects

Some open clusters, giant molecular cloud, group of O and B stars.

Very Old Objects

Globular cluster, some planetary nebulae.

Have Hottest Stars

Planetary nebula central stars are hottest stars, young open cluster, group O and B stars, and some molecular clouds contain fairly hot stars.

Star Formation in Galaxy Disk

Dust and gas needed to form new stars are now found only in the disk.

Spiral Galaxy Distinguishing Features

Have a disk, spiral arms, and central bulge.

Elliptical Galaxy Distinguishing Features

Appear as only a budge (no disk or spiral arms).

Irregular Galaxies Distinguishing Features

Do not have a well-defined or clear structure.

Measuring Distances to Spiral Galaxies

Period-luminosity relationship for cepheid variable stars, look for star that varies at consistent rate, use period to determine luminosity, camper luminosity with apparent brightness to determine distance. Look for supernova explosion and determine kind of supernova, compare peak luminosity with apparent brightness to determine distance.

Hubble’s Law

Rule that the radial velocities of remote galaxies are proportional to their distances from us. Allows us to estimate the distance for galaxies that are too far to see cepheids, describes expansion of the universe, and validates solutions to general relativity equations.

Universe Expanding

Matter inside universe is not expanding, rather empty space between galaxies is expanding.

Elliptical Galaxy Rotation

Stellar motions are randomized and of not travel in a predominant direction (variations of star motions). Spirals have a fixed direction.

Spiral Galaxy Appears Dark

Dust in concentrated in pane of disk. When seen edge on light from stars must pass through disk, which absorbs starlight, making it appear darker.

Most Useful Standard Bulb Method

Type Ia supernovae are very luminous and can be seen at greater distances, cepheid variables stars limited by distance.

Are we at Center of Galaxy

No, expansions follows proportional relationship, so all points in space could make the same observation and claim to be center.

Methods for Determining Distance of Galaxies vs. Parallaxes

Parallaxes can be measure accurately up to 100 ly. Nearest galaxies are 50,000-80,00 ly from the Sun, no perceptible change in apparent position viewed from opposite side of Earth’s orbit.

Homogeneous

Having a consistent and even distribution of matter that is the same everywhere.

Isotropic

The same in all directions.

Most Matter in Universe is Invisible

Dark matter. Stars/clusters obit centers of host galaxies faster than if only visible matter, galaxy cluster move faster than can be explained, galaxy clusters emit copious X-rays explained by fast motions of gas particles under influence of much stronger gravity than luminous matter has.

Superclusters of Galaxies Arrangement

Arranged in filaments and sheets surrounding emptier regions (voids).

Color of Galaxy and Kinds of Stars

Blue galaxy means large amount of stars are very hot and young, red galaxy means mostly old stars.

Evidence for Dark Matter

Rotation curves for galaxies, gravitational lensing (deflection of stralight by warping of spacetime), large scale structure of galaxies like clusters (visible mass insufficient to bind galaxies gravitationally).

Can Elliptical Galaxy Evolve to Spiral?

No, elliptical contains too little dust and gas to form disk and young stars like in spirals.

Can Spiral Galaxy Evolve to Elliptical?

Single spiral can’t, but some smaller spirals that collide and merge with others can be built into a giant elliptical in rich cluster.

Do Galaxies Collide?

Yes, stellar collisions are rare, galaxies influenced by gravity, shockwave created results in star formation.

Possible Futures of the Universe

Determined by fate of expansion. Expansion could come to a stop and reverse, resulting in an implosion of matter, energy, space, and time (big crunch), mass-energy density is greater than the critical density. Universe continues to expand but more slowly, coming to a stop after infinite time, mass-energy density is equal to critical density. Universe expands forever, resulting in empty universe. Universe accelerates faster rate forever, mass-energy density is less than critical density. Current measurement indicate it expands forever.

Hubble Time

Age of the universe, estimated from calculating Hubble constant and taking reciprocal (1/H). Current estimate is 14-15 billion years.

Dark Matter not in Form of Atoms (Protons/Neutrons)

Material in outer regions moving around centers too fast for gravity deduced form ordinary matter. Searches for electromagnetic ration have been fruitless, so dark matter must not be consistent with ordinary particles. Dark matter explains long-term stability of spiral galaxies and galactic clusters.

Dark Energy

Energy that is causing the expansion of the universe to accelerate; the source of this energy is not yet understood. Used Type Ia supernovae as distance indicators to explain measurements that expansion of universe is speeding up, which requires energy source. Dark energy may be new form of energy with no theoretical explanation, or a vacuum energy associated with empty space.

Local Galaxies Redshift Explanation

In general relativistic view, expansion of universe is stretching of space. As scale increases, waves of light from distant galaxy will also be stretched, their wavelengths increase (redshift). The further a galaxy, more space it travels through, the greater the increase in wavelength by time it arrives.

Evidence for Acceleration of Expansion

Comes from supernovae and other evidence that fits standard model that includes dark energy. Have only one “standard bulb” that allows us to measure large distances, observations show that distant supernovae are fainter than expected if universe expanding at constant rate. When we detect light, we are further away that if constant expansion rate, meaning rate we are moving away from supernovae has sped up.

General Timeline for the Universe

Big Bang, possible inflation at 10^-35 s (doubled in size over 90 times), times earlier than 10^-43 s unknown, 10^-45 -10^-43s = strong, weak electromagnetic fields unified, 10s fundamental particles quit forming, fusion of protons to deuterium and helium, first 1000 s primordial nucleosynthesis, 380,000 years matter cooled to combine electrons and nuclei for neutral atoms, 400 million years very first stars and galaxies created, 9 billion years our solar system created (Sun formed = 4.6 billion years old).

Type Ia Supernovae as an Essential Tool in Understanding Universe Size

Type Ia supernovae are the most accurate standard bulb, can be seen in more distant galaxies due to luminosity (can outshine host galaxies at about 4.5*10^9 Lsun), and changed understanding of evolution of universe

How Milky Way was Determined to Be a Spiral Galaxy

Was first measured using a large reflecting telescope, saw a flattened structure encircling the sky and determined the system was in the shape of a disk or wheel. First thought the Sun was in the hub of the galaxy. Now we can measure the radio and infrared emission wavelengths. We also have used a wide range of telescopes to “view” the Milky Way edge on. Radio observations indicate two major spiral arms emerging from the bar, several fainter arms, and shorter spurs. Differential rotation explains the amount of material in the disk (spiral arms). Use supercomputer calculations to model the formation and evolution of the arms.

Dark Matter Halo

The mass in the Milky Way that extends well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the center of the Galaxy; although we deduce its existence from its gravity, the composition of this matter remains a mystery.

Differential Galactic Rotation

The idea that different parts of the Galaxy turn at different rates, since the parts of the Galaxy follow Kepler’s third law: more distant objects take longer to complete one full orbit around the center of the Galaxy.

Central Bulge

The central (round) part of the Milky Way or similar galaxy

Dark Matter

Nonluminous mass, whose presence can be inferred only because if its gravitational influence on luminous matter. Composition is not known.

Milky Way Galaxy

The band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of the Milky Way Galaxy.

Population I Star

Star containing heavy elements; typically young and found in the disk.

Population II Star

Star with very low abundance of heavy elements; found throughout the Galaxy.

Redshift

When lines in the spectra are displaced toward longer wavelengths (toward the red end of the visible spectrum).

Hubble Constant

Constant of proportionality in the law relating the velocities of remote galaxies to their distances.

Hubble’s Law

Rule that the radial velocities of remote galaxies are proportional to their distances from us.

Mass-to-Light Ratio

The ratio of the total mass of a galaxy to its total luminosity, usually expressed in units of solar mass and solar luminosity; the mass-to-light ratio gives a rough indication of the types of stars contained within a galaxy and whether or not substantial quantities of dark matter are present.

Type 1a Supernova

Supernova formed by the explosion of a white dwarf in a binary system and reach a luminosity of about 4.5 × 10⁹ L_Sun; can be used to determine distances to galaxies on a large scale.

Cosmological Principle

Assumption that, on the large scale, the universe at any given time is the same everywhere—isotropic and homogeneous.

Local Group

Small cluster of galaxies to which our Galaxy belongs. 60 galaxies

Merger

Collision between galaxies (of roughly comparable size) that combine to form a single new structure.

Supercluster

Large region of space (more than 100 million light-years across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies.

Big Bang

Theory of cosmology in which the expansion of the universe began with a primeval explosion (of space, time, matter, and energy).

Closed Universe

Model in which the universe expands from a Big Bang, stops, and then contracts to a big crunch.

Open Universe

Model in which the density of the universe is not high enough to bring the expansion of the universe to a halt.

Cosmic Microwave Background

Microwave radiation coming from all directions that is the redshifted afterglow of the Big Bang.

Cosmology

Study of the organization and evolution of the universe.

Critical Density

In cosmology, the density that is just sufficient to bring the expansion of the universe to a stop after infinite time.

Fusion

Building of heavier atomic nuclei from lighter ones.