Universe Topic Pictures Part 2 PDF

# Cosmology and Ancient Astronomy ## Planetary Nebulae - The planetary Nebulae phase is the final stage in the life of a low-mass star, where the star sheds its outer layers, creating an expanding, glowing shell. - This occurs towards the end of the red giant phase. - When the star becomes unstable...

# Cosmology and Ancient Astronomy ## Planetary Nebulae - The planetary Nebulae phase is the final stage in the life of a low-mass star, where the star sheds its outer layers, creating an expanding, glowing shell. - This occurs towards the end of the red giant phase. - When the star becomes unstable and pulsates, producing strong stellar winds that expel the outer layer. - The remaining core, called a white dwarf, emits ultraviolet radiation that illuminates the surrounding gas. - Over time, the material from the planetary nebula disperses into space, contributing to the formation of new stars. - This phase lasts for tens of thousands of years, a brief period compared to the star's overall lifespan. ## White Dwarfs - About 61% of stars in our part of the Milky Way are white dwarfs. - When a small star runs out of fuel, it sheds its outer layers, forming a planetary nebula and leaving behind a white dwarf. - White dwarfs are bright, hot and compact, about the size of the Earth but with the mass of the Sun. - They shine faintly with only 0.1 - 70% of the Sun's brightness and have surface temperatures over 10,000 K. - White dwarfs cool very slowly, taking billions of years to reach around 100,000°C. - They are extremely dense, with a density of about 1 million tonnes per cubic meter. - Inside, the material is in a state called neutron degeneracy, where normal physical laws don't apply. - As a white dwarf's mass increases, its radius decreases. - If it exceeds the Chandrasekhar Limit (1.44 Solar masses), it becomes unstable and can collapse into a black hole. ## Edwin Hubble - From 1919, Hubble worked as an astronomer at Mount Wilson Observatory in California. - His discoveries revolutionized ideas on the structure of the universe. - In 1924, while examining the Andromeda nebula, thought to be part of the Milky Way galaxy, Hubble discovered a pulsating star - a Cepheid variable whose brightness varied in a regular pattern. - He calculated the distance from Earth and found it to be 800,000 light years - much further than most distant stars. - He then showed that Andromeda was a separate galaxy and went on to discover 20 more galaxies. - He was the first to find that the Milky Way galaxy was a small part of the universe. ## Vesto Slipher - In 1912, Vesto Slipher, an astronomer working at Lowell Observatory in Arizona, examined the spectrum of light produced by spiral nebulae. - He found that the frequencies of light received from the nebulae were lower than a normal spectrum - each spectrum of line was closer to the red end of the spectrum than usual - they had been red-shifted. - Slipher had discovered what we now call the Doppler Effect. - He concluded that the nebulae producing the light must be moving away from us. - By measuring the amount of red-shift, he was able to calculate their speeds. ## Hubble's Law - In 1927, Hubble discovered that all the galaxies were moving away from us. - This was a prediction made by Hubble. - Then in 1932, Friedmann proved Hubble's prediction correct by calculating based on Einstein's corrected equations. - Hubble's expanding universe model implied that the matter of the universe was concentrated in one position originally and then exploded outwards. - Evidence for this event is gained by reversing the velocities of the galaxies - imagining to move backward. - This can be expressed as $v = H_0 d$, where: - $v$ is the velocity of the galaxy, - $H_0$ is Hubble's constant, - $d$ is the distance to the galaxy. - Hubble's constant has a value of approximately 70 km/s/Mpc. - Using this equation, the age of the universe can be estimated as $1/H_0$, which is approximately 13.8 billion years. ## Hertzsprung-Russell Diagram - The Hertzsprung-Russell Diagram shows the relationship between a star's temperature and its luminosity. - This diagram can be used to chart the lifecycle of a star. - If all stars were alike, all those with the same luminosity would have the same temperature. - However, we expect the hottest stars to be the brightest. - Red giant and red supergiant stars fall in the top right of the chart. - This tells us that they are brighter than the main sequence stars but also redder and cooler. - This is because they expand and cool as they reach the final stages of their lives. - However, because they are large, they remain bright. - In the bottom left corner of the chart, we find hot stars that are dimmer than the main sequence stars. - This is because they have small radii but contain a lot of mass. - These are white dwarf stars. - Stars tend to spend 90% of their lives in the main-sequence stage. - After this, they become giant stars for 10% of their remaining lives. - Finally, they either explode as a supernova or become white dwarfs. - The color of a star can be used to estimate its temperature. - As a star's temperature increases, it becomes hotter and gradually changes its color. - We can see this change from orange, through yellow, to white. - The hottest stars stay blue with temperatures of up to 40,000°C. - The coolest stars are red with surface temperatures of about 3,000°C. ## Stellar Classification - Astronomers began to categorize stars based on their mass and temperature. - Stars are grouped into 7 main categories (also called classes): O, B, A, F, G, K, and M. - Stars in the 'O' class are the most massive and hottest. - Stars in the 'M' class are the smallest and coolest. - The hotter stars are usually much less common than the cooler, redder ones. - For example, 'O' type stars make up only 1 in every 3 million stars we see. - The next hottest 'B' stars are more common, making up 1 in 800. - Then each cooler subtype gets more and more common. - The coolest 'M' type stars make up 75% of all stars we see. - We can be more accurate when we categorize stars, by splitting each class into 10 smaller sub-classes. - These sun-classes are numbered 0-9, with 0 being hotter than 1. - For example, the sun is a G2 star. - This is hotter than a G7 star, but cooler than a G0 star. - Similarly, a B2 star is cooler than a B1 star.