1 2017 Div. C (High School) Astronomy Help Session Sunday, Feb. 19th, 2017 Stellar Evolution and Type 1a supernovae Scott Jackson Mt. Cuba Astronomical Observatory • SO competition on March 4th . • Resources – two computers or two 3 ring binder or one laptop plus one 3 ring binder – Programmable calculator – Connection to the internet is not allowed! – Help session before competition at Mt. Cuba Astronomical Observatory 2 3 Study aid -1 • Google each object, – Know what they look like in different parts of the spectrum. For example, the IR, optical, UV and Xray – Understand what each part of the spectrum means – Have a good qualitative feel for what the object is doing or has done within the astrophysical concepts that the student is being asked to know. 4 Study aid - 2 • Know the algebra behind the physics – Just because you think you have the right “equation” to use does not mean you know how to use it!!! – Hint for math problems: Solve equations symbolically BEFORE you put in numbers. Things tend to cancel out including parameters you do not need to have values for. – Know how to use scientific notation. 5 The test – 2 parts • Part 1 – multiple choice and a couple fill in the blanks • Part 2 – word problems for astrophysics there will be some algebra Solve the equations symbolically first then put in numbers!!!! Hint: most problems will not need a calculator if done this way Topics - 1 Stellar evolution, including - stellar classification, - spectral features and chemical composition, - H-R diagram transitions, - Accretion disks - Main sequence stars - red giants, - white dwarfs (oxygen & helium), - neutron stars, planetary nebulas Luminosity, blackbody radiation, color index, Spectral class of stars • O • B • A • F • G • K • M • L Red Dwarfs (failed stars) • T Brown Dwarfs (failed stars) 7 Categorizing stars by their spectra 1. Spectra can tell you 2. Absorption (dark) the stars approximate lines in a star’s spectra temperature give a finger print of (blackbody radiation) elements that are seen in that spectral class of stars BUT emission spectra spectra (bright lines against a dark background) are given off by nebulae – glowing gas clouds 8 Spectral class of stars He+ lines H Balmer lines (B,A & F stars) Ca+ lines (F & G stars) Fe and neural metals K & M stars) TiO2 lines 9 Spectral classification & Temperature of main sequence stars Star Surface Star Lifespan Star Mass Spectral Proportion of Stars Temperature Luminosity (Billions Example Star (Sun = 1.0) Class (°F) (Sun = 1.0) of Years) A0 1% A0 - A9 20,000 2.8 60 0.5 Vega A1 --- 18,400 2.35 22 1.0 Sirius A5 --- 15,000 2.2 20 1.0 --- F0 3% F0 - F9 13,000 1.7 6 2.0 --- F5 --- 12,000 1.25 3 4.0 Procyon A G0 9% G0 - G9 11,000 1.06 1.3 10 --- Sun G2 --- 10,600 1.00 1.0 12 Alpha Centauri A G5 --- 10,000 0.92 0.8 15 --- K0 14% K0 - K9 9,000 0.80 0.4 20 Alpha Centauri B K2 --- 8,700 0.76 0.3 24 Epsilon Eridani K5 --- 8,000 0.69 0.1 30 61 Cygni A M0 73% M0 - M9 7,000 0.48 0.02 75 --- Proxima Centauri M5 --- 5,000 0.20 0.001 200 10 (Alpha Centauri C) 11 More on stars spectral class 12 Y axis is always 13 Hertzsprung-Russell Diagram brightness or relative luinosity X axis is always temperature, color or spectral class Each dot is a star A is the location of our sun on the main sequence B are red giant stars that are fusing helium in their core C are red L supergiants with T Helium and Hydrogen buring in http://outreach.atnf.csiro.au/education/senior/cosmicengine/stars_hrdiagram.html shells and carbon in D are white dwarfs (super hot carbon stars) its core 14 Instability gaps on an H-R diagram for the pulsating class of variable stars Period of pulses scale with absolute brightness of the star “Period-luminosity relationship” • http://outreach.atnf.csiro.au/education/senior/astrophysics/variable_pulsating.html Accretion disks • Circumstellar disks • Many accretion disks seen in binary star systems when one star hass filled its “Roche” limit and is having material “sucked” away from it to a companion start (e.g., white dwarf) Disk in Orion nebulea http://planetquest.jpl.nasa.gov/documents/RdMp272.pdf Birth of a solar mass star 17 The birth of a 1 solar mass star going onto the main sequence. Before point 4, contraction of intersteller gas cloud. The cloud heats up as it contracts, causing its luminosity to increase -- we don’t see it because the protostar is hidden in dust. From point 4 to 6, -- The cloud contracts more and its luminosity drops. Point 6, hydrogen starts to fuse to helium in the stars core. The heat generated from fusion balances gravity. The star’s surface heats up slightly. This is the location of T Tauri stars Point 7. The star has reached a long lived equilibrium where the heat from fusing hydrogen to helium balances gravity. The star resides on the main sequence for most of its life (~10 billion years for a 1 solar mass star). Formation of white dwarfs Death of main sequence stars Red Giant for lower mass stars Low mass star like our sun stops at carbon formation in its core... And fluffs off its outer layers to make a planetary nebulae and a white dwarf star. Red Giant for higher mass stars But a high mass star, like those in the early universe had enough mass to fuse nuclear material all the way to iron. However, once iron accumulates in its core no net energy generation can be done by fusion of iron, gravity takes over and core collapse occurs and..... Electrons are pushed into protons making neutrons and a flood of neutrinos…. It goes boom!!!!... A supernovae!!! (this is the Crab Nebulae) … Which make lots of heavy elements needed to make terrestrial (earth like) planets. This is NOT a type 1a supernovae. It is a type II supernovae. .. And it spreads heavy elements throughout space to be picked up by a new generation of stars,..... .. The shock wave either from the supernovae or from the initial star formation stage can initiate new star formation,..... Stars and planets approximate black body radiators The wavelength at maximum radiation changes with temperature λmax = 550 nm 5300 K temperature for our sun. “G” type star (subclass “2”) or G2 λmax x Temperature = constant = 2.9x106 nm-°K Or = 2.9x107 A-°K = 2.9x103 μm-K Nm[=] nanometers for wavelength Or A [=] Angstrom units for wavelength Or μm [=] microns units for wavelength °K [=] degrees Kelvin 25 Another way to look at black body radiation Plot log λ (x axis) vs log of spectral intensity at that λ Example calculation for a star’s temperature So the shorter the wavelength the hotter or colder the star???? λmax ~ 0.9 μm What it the star’s temperature? T ~ 2.9x103 μm-K / 0.9 μm = 3200 K (M type star) If λmax ~ 10 μm What it the star’s temperature? λmax x Temperature = constant = 2.9x106 nm-°K T ~ 2.9x103 μm-K / 10 μm Or = 2.9x107 A-°K = 2.9x103 μm-K = 290 K (black dwarf) Nm[=] nanometers for wavelength Or A [=] Angstrom units Or μm [=] microns units °K [=] degrees Kelvin 27 Color Index or color – color diagrams • A way to compare the apparent magnitude of stars at different wavelengths (using photometry instead of spectrometry). • Observe at narrow bands of wavelengths ( a color) and note the difference in the intensity of these different bands. • Spectrometry (measuring the entire spectrum) is more difficult than photometry (observations at a single color). • But what is U, B and V?? 28 https://en.wikipedia.org/wiki/Color_index UBV, UBVRI and JHK systems for Color-color diagrams 29 Color Index or color – color diagrams • Where is our sun on the U-B vs B-V diagram? 30 https://en.wikipedia.org/wiki/Color_index White dwarfs (oxygen & helium) -1 • White dwarfs are the end point for moderate mass stars like our sun: Mass ~0.5 to ~4+x mass of sun (Msun) the progenitor stars are not massive enough to generate neutron stars or black holes when they die. • White dwarfs do not generate any energy – they are just cooling off and will follow a well defined “cooling” curve on the H-R diagram. • Maximum mass of a white dwarf is dictated by electron degeneracy pressure ~ 1.4 x Msun– the pressure below which the electrons are not pushed into the nucleus. This is called the Chandrasekhar limit • White dwarfs will take a long time to cool off but as they do, they will become red dwarfs and then brown dwarfs as their (black body) spectra shifts to longer wavelengths of light • . 31 White dwarfs (oxygen & helium) - 2 • The more massive the white dwarf – the smaller it is(!) • Many red dwarfs or brown dwarfs were not white dwarfs to start with – they may just be failed stars that did not have enough mass to initiate fusion in their cores. • Progenitor stars of lower mass will not be able to fuse helium in their shells. When they die as white dwarfs, they will appear as helium white dwarfs. • Progenitor stars of higher mass will be able to fuse helium in their shells to carbon and oxygen and these will appear as oxygen white dwarfs. 32 Neutron stars • When higher mass stars “die” gravity takes over and the core of the star collapses.