Astronomy: The Universe PDF

# Astronomy: The Universe ## Science core B * **The Universe** The study of the universe is called cosmology. Cosmologists study the structure and changes in the present universe. The universe contains all of the star systems, galaxies, gas, and dust, plus all the matter and energy that exists no...

# Astronomy: The Universe ## Science core B * **The Universe** The study of the universe is called cosmology. Cosmologists study the structure and changes in the present universe. The universe contains all of the star systems, galaxies, gas, and dust, plus all the matter and energy that exists now, that existed in the past, and will in the future. The universe includes all of space and time. ## Ancient Astronomy and Cosmology * Over thousands of years, humans have developed an understanding of stars, planets and twin sciences: astronomy and cosmology. ### Geocentric Models of the Universe * **The geocentric model**, in which planet Earth is in the center of the universe and circled by the sun and all the planets, had been the accepted cosmological model since ancient time. * **Ptolemy’s Geocentric Model**: posits that Earth is at the center of the universe and all other celestial bodies, including the sun, moon, planets, and stars, revolve around it. He explained that retrograde motion using a system of epicycles, where planets move in small circles (epicycles) while simultaneously orbiting Earth in larger circles (deferents). This complex motion accounted for the backward loops observed in the sky. Epicycles are small circular paths that planets follow, while deferents are the larger circular orbits around the Earth. The combination of these two motions was used to explain the observed planetary movements. Ptolemy adjusted the sizes and speeds of the epicycles and deferents to match the observed speeds of the sun and moon, allowing his model to predict their positions accurately. The Ptolemaic system was complex and required many adjustments to fit observations. It couldn’t accurately predict planetary positions over long periods and didn’t account for the phases of Venus or the apparent size changes of planets Ptolemy’s model was widely accepted and used for centuries because it provided a comprehensive framework that matched observations reasonably well. It was the dominant astronomical paradigm until the Copernican Revolution. The primary difference is that Ptolemy’s model places Earth at the center of the universe, while Copernicus’ Heliocentric model positions the sun at the center, with Earth and other planets orbiting it. Ptolemy built on Hipparchus’ work by refining the mathematical models and incorporating more accurate observations. He used Hipparchus’ star catalog and theories on planetary motion as a foundation for his own model. The Almagest was Ptolemy’s comprehensive treatise on astronomy, detailing his geocentric model concepts. It became the authoritative reference for astronomers and scholars for many centuries. The geocentric model was gradually disproven through the work of astronomers like Copernicus, Galileo and Kepler. Observations such as the phases of Venus, the moons of Jupiter, and the accurate predictions of planetary positions by Kepler’s laws of planetary motion provide strong support for a heliocentric model. ### Heliocentric Models of the Universe * **Nicolaus Copernicus** proposed the heliocentric model placing the sun at the center of the universe with Earth and other planets orbiting it, challenging the geocentric model that placed Earth at the center. He delayed publishing his theory due to fear of backlash from the church and scholars. **Galileo’s telescopic observations**, such as the phases of Venus and the moons of Jupiter supported the Heliocentric model by showing that not all celestial bodies orbited Earth. **Kepler further refined this model** with his 3 laws of planetary motion, describing elliptical orbits and varying orbital speeds. The heliocentric model faced significant opposition but eventually led to a major shift in scientific thought, promoting empirical and observational approaches paving the way for the Scientific Revolution. ## Astronomy: Stars Galaxies and Nebulae * **What is a star?** Stars are giant balls of hot gas – mostly hydrogen, with some helium and small amounts of other elements. * **4 Main ways to classify stars:** Color, temperature, size, brightness. **How color & temperature are related:** Like hot objects on Earth, a star’s color is determined by its temperature. Coolest stars appear red. Medium stars appear yellow-orange. Hottest stars appear white-blue. * **Temperature (oC) | Star color** * 2000-3600 | Antares * 3600-5000 | Arcturus * 5000-6000 | The Sun * 7500 - 11,550 | Vega * 11,000-28,000 | Spica * **5 Main sizes of stars:** | Size | Example | Diameter (km) | | -------- | -------------------------- | --------------- | | Smallest | Neutron stars | 20 | | Small | White dwarf | 12,700 | | Sun | The sun | 1,392,684 | | Medium | Giant stars | 62 million | | Largest | Hypergiants | Larger than Sun | - **Apparent Brightness** A star’s brightness as seen from Earth. - **Absolute Brigtness**: if placed at a standard distance (32.6 light years). A star’s brightness (at 32.6 light years). * Use light year to measure, 1 light year = distance travels in one year, to measure distances in the universe (space) – 1 light year = 9,500,000,000 km. ### Hertzsprung-Russell (H-R) Diagram * A - Supergiants * B - Giants * C - Main Sequence * D - White dwarfs * E - The Sun **Major steps to the Birth of Stars:** 1. - All stars form inside a giant cloud of gas and dust called a nebula. 2. - In the densest part, gas and dust get pulled together by gravity. 3. - As the nebula contracts, it forms a hot, dense sphere called a protostar. 4. - A star is born when the protostar becomes so dense that hydrogen fusion into helium begins. ### How small mass stars die: First star expands, finally cool down to a white dwarf, then a black dwarf, which it is dying, takes billions of years. ### How by Large mass stars die: Core loses mass, blowing outer layers and shrinking into a white dwarf star. ### Measuring Distance * **The cosmic distance ladder** is a series of methods used by astronomers to determine the distances to celestial objects. Each method is used for different distance ranges, building upon the previous ones. * **Radar** is used to measure distance within our solar system. By bouncing radio waves off planets or other objects and measuring the time it takes for the waves to return, we can calculate the distance based on the speed of light. * **Parallax** is the apparent shift in position of a nearby star against the background of distant stars when viewed from different positions in Earth's orbit. By measuring this shift, astronomers can calculate the distance to the star using simple trigonometry. * **Standard candles** are astronomical objects with known luminosity – by comparing their known luminosity to their observed brightness, astronomers can determine their distance using the inverse-square law of light. * **Cepheid variable stars** have a well-defined relationship between their pulsation period and intrinsic luminosity – by measuring the period of a Cepheid’s brightness variations, astronomers can determine its luminosity and thus its distance. * **Redshift** refers to the stretching of light to longer wavelengths as objects move away from us, due to the expansion of the universe. By measuring the redshift of a galaxy’s light, astronomers can estimate its distance based on the rate of expansion of the universe.