GS107 HW #4

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Central Oregon Community College *

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107

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Astronomy

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Apr 3, 2024

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Lorraine Anderson GS107 Homework #4 Due: Wed Dec. 6 th Fall 2023 Please answer your questions on another piece of paper and include your name. 1. What is a planetary nebula? What happens to the core of a star after a planetary nebula occurs? A planetary nebula is the glowing cloud of gas ejected from a low-mass star at the end of its life. After a planetary nebula occurs, the star’s core will cool down and become a white dwarf. 2. Describe the mass, size, and density of a typical white dwarf. How does the size of the white dwarf depend on its mass? A white dwarf has the mass of the sun compressed into the volume of the earth. It is half as big as the sun, and only a little bit bigger than earth. A white dwarf is extremely dense, so dense that a teaspoon of it would weigh several tons if you brought it to earth. More massive white dwarfs are actually smaller in size than less massive ones because its greater gravity compresses it to a smaller density. 3. What processes may cause a white dwarf supernova? Observationally, how do we distinguish white dwarf and massive star supernovae? Once a white dwarf reaches 1.4Msun, its temperature will rise to the point where carbon fusion will begin. Carbon fusion will ignite instantly throughout the white dwarf, which will cause it to explode in a white dwarf supernova. This can also happen when it merges with another white dwarf: when 2 white dwarfs orbit close to each other, they emit gravitational waves, which cause them to spiral toward each other. If the mass of these 2 white dwarfs exceeds 1.4Msun, it will result in a white dwarf supernova. The difference between a white dwarf supernova and a massive star supernova is the different light they emit. The luminosities of white dwarf supernovae fade steadily, while the decline in luminosity of a massive star supernova is more complicated. In addition, white dwarf supernovae don’t have hydrogen lines, while massive star supernovae have more prominent hydrogen lines. 4. Describe the mass, size, and density of a typical neutron star. What would happen if a neutron star came to your hometown? A neutron star is 10 kilometers in radius and has a mas like that of the sun. Neutron stars resist the crush of gravity with degeneracy pressure that arises when particles are packed as close as possible. If a neutron star came to my hometown, it would destroy everything about it and the rest of its civilization. 5. In what sense is a black hole like a hole in the observable universe? Define the event horizon and the Schwarzschild radius . In the sense that a black hole is like a hole in the observable universe, once you enter a black hole, you leave our observable universe and can never return.

The event horizon is the boundary between the inside of a black hole and the universe outside, and marks “the point of no return”. In other words, nothing that passes within this boundary can ever escape. The Schwarzschild radius is a measure of the size of the event horizon of a black hole. 6. Draw a simple sketch of the galaxy as it would appear face-on and edge-on. Identify the disc, bulge, halo , and spiral arms , and indicate the Galaxy 's approximate dimensions. Also locate our sun in the sketch. *Turned in seperately* 7. Describe halo vs disk stars. Contrast their ages and stellar orbits. - Halo stars are old with a very low proportion of heavy elements, while disk stars come in all ages and contain a higher proportion of heavy elements. - Disk stars follow the orderly patterns of the disk of a spiral galaxy, while halo stars orbit the center of the galaxy with many different inclinations. - Halo stars formed when our galaxy’s protogalactic cloud was still large and wobbly, while disk stars formed after the gas had settled into a spinning disk. 8. Summarize (in your own words!) the stages of the star-gas-star cycle, in the figure below. New stars are formed when gravity causes the collapse of molecular clouds. Nuclear fusion in these stars make new energy and elements from hydrogen. The stars will return these elements into space either from stellar winds or supernovae. While the stellar winds will continue to carry more material into space, other stars will explode into supernova, resulting in a gas being ejected into space. This gas will eventually sweep up other material floating around in space, creating hot bubbles of ionized gas. At a certain point, multiple gas bubbles will merge into one giant bubble, eventually resulting in hot gas erupting from the bubble. This erupted gas will then spread throughout the galaxy, begin to cool down, and start to form atomic hydrogen clouds. Once the temperatures of these clouds drop even more, it will become a molecular cloud, and that cloud will give birth to a cluster of stars, starting the process all over again.

9. How can we use orbital properties to learn about the mass of the galaxy? What have we learned? If we know the orbital period and the size of the orbit, we can use Newton’s Law of Gravity to learn the mass of the galaxy. 10. Briefly summarize the different types of gas present in the disk of the galaxy and describe how they appear when we view the galaxy in different wavelengths. - Hot bubbles: Appear as pockets of gas heated by stellar winds or supernovae. - Atomic hydrogen clouds: The most common form of gas and appears to fill up a big portion of the galactic disk. - Molecular clouds: Regions of star formations. When looking through a short wavelength, we can clearly see the Milky Way’s starlight. When looking through visible light, most of the galaxy’s starlight is blocked from our view because of the dusty clouds in the disk. When looking through a long wavelength, we can see radiation from dust particles. When looking through x-rays, we can see hot gas filling the halo of the Milky Way. When looking through CO molecules, we can see that the radio waves from the molecules trace the coldest gas clouds. When looking through gamma rays, we can see the cosmic-ray collisions with gas atoms. When we look through H atoms, we can see that the radio waves from these H atoms come from warmer gas clouds. 11. What is Sgr A*? What evidence suggests that it contains a supermassive black hole? Sgr A* is a source of radio emission in the center of the Milky Way that is “unlike any other radio source in our galaxy”. There are hundreds of stars within 1 light year of Sgr A*, and the way they move indicates an extremely massive object. These stars’ orbits show that this object has a mass of about 4 million solar masses, all packed into a region of space just slightly larger than our solar system. The fact that an object that massive could fit into such a small space indicates that it must be a black hole. 12. What do we mean by a standard candle? Explain how we can use standard candles to measure distances. A standard candle is 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. In order to measure a distance with a standard candle, we must measure the apparent brightness of an object whose luminosity we already know and apply the inverse square law for light. 13. Briefly explain, in your own words, each of the techniques used to measure distances in the figure below.

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- Radar ringing: Measuring the Earth’s distance to the sun by bouncing radio waves off Venus. These radio signals will tell us Venus’s distance from Earth, and then using Kepler’s laws and a little geometry, we find the length of an AU (astronomical unit). - Parallax: Measuring the distance of nearby stars by looking at how their positions shift as Earth orbits the sun. These distances also come from geometry, as well as our understanding of the Earth-Sun distance, which was determined with radar ringing. - Cepheid variables: A cepheid variable star is an extremely luminous, pulsating variable star that follow’s Leavitt’s law. Leavitt’s law helps us to determine the different luminosities of these stars from their measured periods, allowing us to calculate their distances by using the inverse square law for light. We are therefore able to understand the relationship between Cepheid periods and luminosities by using observations of nearby Cepheids and their distances, which have been determined by using parallax. - Distant standards: Using Cepheids, we measure distances to nearby galaxies and establish standard-candle luminosities of white dwarf supernovae. These supernovae are then used as standard candles to measure large distances across the universe. - Hubble’s law: Measuring a galaxy’s distance from its speed. By measuring distances to galaxies with white dwarf supernovae, we can estimate Hubble’s constant, H0. With this, we can determine a galaxy’s distance from its redshift. 14. Explain what we mean by Big Bang theory. The Big Bang theory is known as the scientific theory of the universe’s earliest moments, and states that all matter in our observable universe came into existence at a single moment in time as an extremely hot, dense mixture of subatomic particles and radiation. In other words, it is how we believe the universe came to be. 15. Make a list of the important eras in the history of the universe, summarizing the important events thought to have occurred during each era. - Planck Era: We do not understand the physics of the universe well enough to describe the conditions of this era.

- GUT Era: A time where 2 forces operated in the universe- gravity and the GUT force. Towards the end of this era, energy that was released caused a dramatic expansion of the universe called inflation. - Electroweak Era: The splitting of the GUT force began the era in which 3 main forces operated- gravity, the strong force, and the electroweak force. In this era, elementary particles appeared spontaneously from energy, but then also transformed back into energy. - Particle Era: When elementary particles filled the universe, then quarks combined to make protons and neutrons. During this era, it is believed that protons must have slightly outnumbered antiprotons (or neutrons outnumbered antineutrons), or we would not be here today. - Era of Nucleosynthesis: When fusion produced helium from protons. At the end of this era, when fusion ceased, the chemical composition of the universe had become 75% hydrogen and 25% helium, along with trace amounts of lithium and deuterium. For the most part, this is the same chemical composition that makes up the universe today. - Era of Nuclei: When matter became fully ionized and opaque to light. In this era, fully ionized nuclei moved independently of electrons and photons bounced rapidly from one electron to the next. At the end of this era, cosmic background radiation was released. - Era of Atoms: During this era, it became cool enough for neutral atoms to form, allowing photons to travel freely through space. In this era, gravity slowly drew atoms and plasma into higher-density regions, which assembled into pro-galactic clouds. Stars formed from these clouds, and these clouds eventually merged into galaxies. - Era of Galaxies: What we refer to as the present era of the universe, beginning with the formation of galaxies when the universe was 1 billion years old.

GS107 HW #4

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