Universe Topic Pictures Part 2 PDF

## Cosmology and Ancient Astronomy ### Planetary Nebulae - The planetary nebula phase is the final stage in the life of a low-mass star. - 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...

## Cosmology and Ancient Astronomy ### Planetary Nebulae - The planetary nebula phase is the final stage in the life of a low-mass star. - 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. - Planetary nebulae last for tens of thousands of years, a brief period compared to the star's overall lifespan. - Our sun will enter this phase in about 5,000 million years. ### White Dwarfs - About 6-1% 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 0.1-10% of the sun's brightness, and have surface temperatures over 100,000 °C. - White dwarfs cool very slowly taking billions of years to reach about 10,000 K. - They are extremely dense, with a density of about 1 million tonnes per cubic meter. - 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 and that the universe was expanding. ### Analysing Experimental Data: Hubble's Law * V = H_oD or D = V/H_o * V = velocity of galaxy away from us (kms^-1) * H_o = Hubble constant (70 kms^-1Mpc^-1) * D = Distance galaxy is from us (Mpc = 3.26 x 10⁶ly) * Average speed = distance/time * D = vt * t = D/V = 1/H_o ≈ 15.6 billion years ### Hertzsprung-Russell Diagram - The Hertzsprung-Russell Diagram shows the relationship between a star's temperature and its luminosity. - It can be used to chart the life cycle of a star. - If all stars were alike, all those with the same luminosity would have the same temperature. - We would also 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 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 have a large size, they are very bright and remain bright. - In the bottom left chart, we find hot stars that are dimmer than 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 life in the main sequence stage. - After this, they become giant stars for 10% of their remaining lives. - Finally, they explode as a supernova or become white dwarf stars. ### Colors of Stars - As a star's temperature increases, it becomes hotter and gradually changes its color. - The color changes that are evident in a star are from orange, through yellow to white. - Hottest stars stay blue, with temperatures up to 40,000°C. - 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). - These classes are 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 cooler, redder ones. - For example, 'O' type stars make up only 1 in every 3 million stars. - 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 that we see. - We can be more accurate when we categorize stars by splitting each class into 10 smaller sub-classes. - These sub-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 G9 star. - Similarly, a B9 star is cooler than a B1 star. ### Wien's Law - Stars emit all radiation, don't exist. - Stars, planets, and black holes are good approximations. - We can use Wien's Law to estimate their surface temperatures. - Let's work through an example - we will use the sun. - The λ_max of the sun is about 500 nm (5 x 10^-7m). - T = b/λ_max - T = 2.895 x 10^-3/5 x 10^-7 - T = 5800 K. - So the Sun's surface temperature is about 5800 K. - The color of a star indicates its temperature: - Hottest stars (around 12,000 K) emit most of their light in the UV range. - Cooler stars (around 8,000 K) emit in the blue part of the spectrum. - Redder stars are cooler and bluer stars are hotter. - By using telescopes with color filters, astronomers can determine a star's color and thus its temperature by measuring its apparent magnitude through two different filters. ### Variable Stars - Variable stars change in brightness over time due to internal or external factors. - The first known variable star, Mira, was discovered in 1658. - Today, over 150,000 variable stars are known. - Variable stars are studied by plotting their brightness over time on a light curve. #### Common Types of Variable Stars - **Cepheid Variables:** Yellow giant stars that pulsate, changing in brightness due to swelling and shrinking. Their period-luminosity relationship helps determine their distance. - **RR Lyrae Variables:** Dimmer than Cepheids, found in globular clusters, with short periods and distinctive light-curve shapes. - **Mira Variables:** Cool supergiants with large pulsations, varying greatly in brightness due to hydrogen layer changes. - **Eclipsing Binaries:** Systems with two stars that block each other's light, causing dips in brightness. The size of the dip helps determine the stars' relative sizes, mass, and density. ### Parallax Measurement and the Parsec - Parallax is one of the most important distance measurement methods used by astronomers. - It can only be used for nearby stars, but it is very accurate. - The method works by measuring how nearby objects appear to move against the background of more distant objects. - Parallax is measured in arcseconds using the distance to the object and the angle in the sky that it seems to have moved. - We measure the distance in parsecs, and the angle in arcseconds. - Arcseconds are used because the changes in angles are too small. - 3600 arcseconds in one degree. - One parsec is the distance to an object with a parallax angle of 1 arcsecond, so 1 parsec is equal to 3.26 light years. - One parsec is the distance (d) to an object which has a parallax angle (p) of 1 arcsecond. - This is basically trigonometry - we know that the distance the Earth has moved over 6 months is equal to the diameter of our orbit. - There is a special distance in astronomy for the Earth's average distance from the Earth to the Sun, called the Astronomical Unit (AU). ### Astronomical Unit (AU) - The average distance between the Earth and the Sun is 149,600,000 km. - AU is a useful unit measuring distance in our solar system. - If you start measuring distances between stars, AU becomes less useful because our galaxy is bigger than our solar system. - The nearest star to the Sun is Proxima Centauri, which is roughly 268,000 AU from the Sun, so to measure big distances across galaxies we need bigger units like light years. ### A Light Year - A light year is a way of measuring distance. - One light year equals the distance that light travels in one year. - One light year is a very big distance, it is almost 9,500,000,000,000 km. - For comparison, it is just 17,000 km from London to Sydney. - In the vast expanse of the universe, a kilometer (km) is just too small a measurement, and a light year is much more useful when measuring huge distances between stars and galaxies. ### Star Formation - Stars form in huge clouds of gas and dust called nebulae. - These areas of space are sometimes known as 'stellar nurseries' or 'star-forming regions'. - Stars form when gravity pulls gas and dust in clouds inward, causing them to collapse into dense, hot cores. - When these cores become hot enough, nuclear fusion begins, marking the start of stellar ignition, and the star begins to shine. - Before this point, they are called protostars. - The burst of light from a new star can blow away nearby gas, but may leave enough material to form planets. - After ignition, stars become stable as gravity's inward pull balances with the outward pressure from nuclear fusion. - Stars, like the Sun, shine for billions of years and go through various stages in their life cycle. ### Stellar Evolution of the Sun - Stars are massive, glowing balls of extremely hot gas (plasma) in space, with the Sun being our closest star. - Despite appearing small in the sky, stars are actually millions of times larger than Earth, but look small due to their vast distance from us. - The nearest star after the Sun is Proxima Centauri, 4.3 light years away. - Stars generate energy in their cores through nuclear fusion, releasing it as heat and light, which makes them shine. - The balance between the outward force from fusion and the inward pull of gravity keeps stars stable. - The brightness of stars is measured in terms of luminosity. - Stars vary in size and color, which helps classify them. - They shine for millions of years, but eventually go through several stages in their life cycle before they die. ### Life Cycle of a Star - Stars form in nebulae, which are large clouds of gas and dust. - Their lifetimes depend on their size, with massive stars burning out quickly and smaller stars lasting billions of years. - During their main-sequence stage, stars shine brightly due to nuclear reactions in their cores. - As stars exhaust their hydrogen fuel, they expand and cool to become red giants. ### The Table on Page 8 - The table on page 8 lists the temperature ranges and colors for each class of star. | Class | Temperature (°C) | Color | Example Star | |:-------|:-----------------|:------------|:----------------| | O | 30,000 - 50,000 | Blue | Alnitak | | B | 15,000 - 30,000 | Blue-White | Rigel | | A | 7,500 - 10,000 | White | Vega | | F | 6,000 - 7,500 | Yellow-White | Procyon | | G | 5,200 - 6,000 | Yellow | The Sun | | K | 3,700 - 5,200 | Orange | Pollux | | M | (less than) 3,700 | Red | Betelgeuse | ### Star Size - All stars are not the same size. - They range from the size of a city to large enough to swallow half of our solar system. - Neutron stars are incredibly dense, packing the mass of one or two suns into a diameter of just 20 to 40 km. - White dwarf stars are larger, roughly the size of Earth. - Supergiant stars can be over 1500 times larger than the Sun, spanning more than 2,000 million km across. - Despite their size, they don't have thousands of times more mass than the Sun because their material is spread out due to expansion as they age. - Betelgeuse, a red giant in Orion, is about 1,000 times wider than the Sun but only has about 15 times its mass. ### Wien's Law - Stars absorb and emit light. - This light covers a wide range of wavelengths, known as a spectrum. - We can measure the light energy emitted by an object, known as its intensity. - Objects emit light at various wavelengths, with the highest intensity at a specific wavelength called λ_max. - Wien's law relates an object's temperature to this wavelength, given by the formula: Temperature (T) = b/λ_max. - Where b = Wien's displacement constant, equal to 2.897771955 x 10^-3 mK. - This formula is a true blackbody. - Although perfect blackbodies, which absorb all radiation, don't exist, stars, planets, and black holes are good approximations. - Thus, we can use Wien's Law to estimate their surface temperatures.