1 Astro 21 first lecture The H-R H R Diagram helps us study how stars are born but also helps us study how they die. Stars spend most of their lives as main sequence stars. The core does change from hydrogen to filling up with Helium. Density increases in the center of the star.
2 The core heats up and fusion The core heats up and fusion increases. Bigger stars live shorter lives, smaller stars fuse at slow rates and live longer.
3 Life does not get a chance to form Life does not get a chance to form on planets of O and B stars. Here is the next stage of life for a star. Core fills up with He. Core shrinks and begins to heat up. Outside the core there is still Hydrogen. With this new heat outside the core the outer layers expand. With this new energy the star expands getting enormous.
4 As it expands the outer layers of As it expands the outer layers of the star gets cooler and redder. The star leaves mainsequence and moves to the upper right on the H-R H R diagram as a Red giant. These super giants are much brighter than the sun and lower density, cooler surface and hotter core. They get so big they take up the orbits of the earth and mars.
5 To observe stars we look in a star To observe stars we look in a star forest called star cluster. These stars are grouped together closely so they formed at nearly the same time. The only difference in them is there mass. There are three main types of clusters: Globular, Open and stellar associations. Globular clusters are old stars the others are young.
6 One example is Omega centauri with roughly a million stars in it.
7 The brightest in Omega have The brightest in Omega have finished there main sequence stage and are pale yellow.
8 Astro 21 2 nd lecture We would not be able to see the Milky way thru the glow of all the stars near us. 150 globular clusters Most are in the galactic halo. Open clusters are in the disk of our galaxy. Having several dozen to several hundred stars.
9 We only see a few of them thru the We only see a few of them thru the dust of interstellar matter in the disk of our galaxy.
10 The Pleiades is the most famous The Pleiades is the most famous being visible to the naked eye. In association is a group of very young stars 5 to 50 very hot bright O and B stars. There is also hundreds of thousands of low mass dim stars. The H-R H R diagram predicts after a few million years massive stars have contracted to make main sequence while smaller ones are still forming.
11 Orion Nebulae is young and a good Orion Nebulae is young and a good example.
12 As clusters age the larger stars As clusters age the larger stars move off the main sequence becoming red giants. The older the cluster the less massive stars are left on the main sequence.
13 Just compare the luminosities of Just compare the luminosities of the brightest stars still on the main sequence. Globular clusters are the oldest objects in the our galaxy, some being 13 billion years old. Stars spend most lives as main sequence. Range of life depends on its mass.
14 Stars with masses of 3 solar Stars with masses of 3 solar masses or less. Red giant starts with a Helium core. Core shrinks and grows hotter. At 100 million degrees three He can fuse to make a carbon nucleus. This is the Triple alpha process. In low mass stars this starts as a quick burst of fusion called He flash.
15 Quick heating goes thru the entire Quick heating goes thru the entire core. This rapidly heats up all the He in it. After He flash the Red Giant finds balance. Surface temperature increases while luminosity decreases. In the core it will sometimes add another He to the carbon to make an Oxygen nucleus.
16 Astro 21 3 rd lecture Stable stage of making O and C does not last long. Gravity wins again. The core of O and C heats up. The core continues to heat up but lower mass stars do not get hot enough to fuse anything else.
17 Giant stars lose mass into space. Giant stars lose mass into space. Our sun will lose 25% of its mass when it is a giant. These older stars can be surrounded by shells of gas having as much as 10 to 20% of its stars mass. Fusion is now over the star collapses its surface temp goes up to 100,000K. This high temp puts out UV rays.
18 These rays sweep the gas of mass These rays sweep the gas of mass the star ejected and lights it up. This is a Planetary Nebulae
19 The current theory is that planetary The current theory is that planetary nebulae have the same shape but look differently from different angles.
20 Most nebulae are about 50,000 Most nebulae are about 50,000 years old.
21 This is the last gasp for the star. This is the last gasp for the star.
22 Each new star made from recycled Each new star made from recycled material from dying stars will have new elements in them. Bigger stars can make even bigger elements due to star cores getting hotter.
23 They can expand out to a diameter They can expand out to a diameter equal to the orbit of Jupiter.
24 If a star weighs more than 8 solar If a star weighs more than 8 solar masses its core can fuse Carbon into bigger elements O,Ne,, Na, Mg, and Si can be formed then the core collapses. This is Nucleosynthesis and will continue making bigger elements until Fe is formed. Elements bigger than Fe comes from stars dying from a supernova explosion.
25 Globular clusters have 1/10 to Globular clusters have 1/10 to 1/100 percent heavier elements. Open clusters have 1-4% 1 heavier elements. So globular clusters are older from the past when first elements were only H and He. In the end stars luminosity increases and nuclear fuel is used up fast.
26 After H runs out it fuses He and After H runs out it fuses He and other elements but the energy yield is less. The higher temp required to make bigger elements consumes it s s fuel much faster for less energy. Death is here for the star but that can happen many different ways.