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Unformatted text preview: 09/07/2020 Lesson 14 – Properties and Evolution of Stars Lesson 14 – Properties and Evolution of Stars Site: UNSW Moodle Course: PHYS1160-Introduction to Astronomy T2 2020 Book: Lesson 14 – Properties and Evolution of Stars Printed by: Nathan Cheung Date: Thursday, 9 July 2020, 11 38 PM 1/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars Description In this lecture we're going to look at the properties and evolution of stars. We'll look at the important properties of stars: we'll look at spectroscopic classification of stars – how we classify them into different spectral types. We'll look at something called Hertzsprung-Russell Diagram, which is a diagram that helps to make sense of different types of stars. And we'll look at the evolution of stars – how stars change over time. 2/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars Table of contents 14. Properties and Evolution of Stars 14.1 Basic Properties of Stars 14.2 Luminosity and Distance 14.3 Colour, Temperature and Spectroscopic Classification 14.4 Spectral Classification - Some History 14.5 Mass 14.6 The Hertzprung-Russell Diagram 14.7 Binaries, Variables and Star Clusters 3/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars 14. Properties and Evolution of Stars SHOW WHOLE LESSON AS SINGLE WEB PAGE GO TO FIRST PAGE Lesson 14 Overview 4/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars 14.1 Basic Properties of Stars In this lesson we're going to look at the properties and evolution of stars. We'll look at the important properties of stars and spectroscopic classification – how we classify them into different spectral types. We'll look at something called the Hertzsprung-Russell Diagram, which is a diagram that helps to make sense of different types of stars. And we'll look at the evolution of stars – how stars change over time. Our Sun is one of billions of stars in our Galaxy. How do we make sense of this huge number of stars? Well, it helps to measure their basic properties, and to look for patterns that help us classify them. There are three important properties of stars: Mass: A key property is a star's mass – the total amount of material in the star. This turns out to be a difficult property to measure. Luminosity: Another key property is luminosity - the rate of energy production from a star or equivalently, the power the star emits. We call very luminous stars "giant" or even "supergiant" stars. Less luminous stars (like the Sun) are "dwarf" stars. Colour or Temperature: Another property is a star's colour or temperature (which turn out to be related). Red stars are very cool stars, with temperatures of about 3000 K. At hotter and hotter temperatures we go through yellow stars like the Sun (with a temperature of about 5800 K), and the hottest white and blue stars with temperatures of 10000 K to 30000 K or more. Types: Stellar types are then defined by a combination of these properties. You often hear stars referred to with labels like 'red giant' (which is a cool but luminous star); or 'white dwarf' (which is a hot, but low luminosity, star.) 5/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars 14.2 Luminosity and Distance How do we measure Luminosity? How do we measure these various properties? Let's start by looking at luminosity. The luminosity of a star is its rate of energy production, or equivalently the power (the energy produced per second) that the star emits. To measure luminosity we need two measurements: 1. We need to measure apparent brightness of the star, as seen from the earth. 2. We need to measure its distance. Both pieces of information are needed because a star can be bright because it is close to us, or because it is intrinsically bright. Stellar Magnitude For largely historical reasons, astronomers usually measure the brightness of a star in terms of something called magnitude. This is a concept that goes right back to the ancient Greek astronomer Hipparchus who made one of the first star catalogues. He described the brightest stars as being of "first magnitude" and the next brightest being "second magnitude" and so on down to the faintest stars he could see, which he called stars of the "sixth magnitude". This was a classification based on the visual (naked eye) brightness of the star. The Magnitude Scale The magnitude system has continued to be used to the present day, except that it now has a modern mathematical definition. The modern magnitude system is a logarithmic scale that is defined such that five magnitudes corresponds to a factor of 100 in brightness. So a star of magnitude 0 is 100 times brighter than a star of magnitude 5, and 10000 times brighter than a magnitude 10 star, and a million times brighter than a magnitude 15 star, and so on. The scale goes backwards, so low numbers mean brighter stars. The brightest stars can actually have negative magnitudes. So the brightest star, Sirius, has a magnitude of -1.46. We can use the magnitude scale to measure the brightness of other things as well as stars, so we can use it to measure the Sun, the moon and things like galaxies and planets. The table below shows the full range of the magnitude scale. Object Visual Magnitude Relative Brightness Sun -26.7 4.8x108 Full Moon -12.6 1.1x105 Venus -4.5 63 Sirius (Brightest star) -1.46 3.8 Vega (Bright star) 0.0 1 Faintest naked eye star 6.0 4x10-3 Faintest star in amateur size (~30 cm) telescope ~15.0 1x10-6 Faintest star in Sloan Digital Sky Survey ~22.0 1.6x10-9 Faintest objects in Hubble Ultra Deep Field ~29.0 2.5x10-12 Starting at the top with the Sun, the Sun has a visual magnitude of -26.7. The full moon has a magnitude of -12.6. The brightest of the planets is Venus and it has a magnitude of -4.5. Sirius, the brightest star has a magnitude -1.46. Vega, a typical bright star has a magnitude of 0. The faintest naked eye stars have a magnitude of 6. And then the scale goes on to fainter objects, so the faintest star you can see in a typical amateur telescope (a 30 cm telescope) in a dark sky would be a magnitude of about 15. The faintest star you can see in the Sloan digital sky survey – which is a big survey of the sky that is being carried out with modern digital techniques – has a magnitude of about 22. And the faintest objects in the Hubble Ultra Deep Field, which is the deepest image that's ever been taken, those faintest objects would mostly be galaxies, but they go down to a magnitude of 29. And they're about 2.5x10-12 times fainter than a bright star like Vega (a factor of about 1 trillion). Photometry Measurements of stellar magnitudes are called photometry. An instrument to carry out such measurements is called a photometer. These days most photometry is actually done with CCD images of a star field and software that measures the brightness of all the stars in the field. So, professional astronomers now rarely use a separate instrument called a photometer. You do sometimes see a photometers used in amateur astronomy though, and one is pictured below attached to the back of a telescope. 6/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars ! ṿ Ḏṙ Ấṙ Ṋ ǴẤǴẃǀ ẤẤǀ ǜḎǴǨ Ấṙ ǀ Ṏ ǀ Ṋ ǀ ẤǴẴẃẤǴḵǴẎǜṙ ṿ Ǵῢ Apparent and Absolute Magnitude The direct measurement we make of a star's brightness is what's called its ǀ ṿ ṿ ǀ ẃǴṎẤṊ ǀ ḅṎḙẤẴǨǴ, but we don't know if the star appears bright because it is intrinsically bright (because it is putting out a lot of energy) or because of its distance. So a magnitude corrected for the distance is called an ǀ ǘ Ẏṙ ḵẴẤǴ Ṋ ǀ ḅṎḙẤẴǨǴ. An absolute magnitude is defined as the magnitude a star would have if it was seen at a distance of 32.6 light years (10 parsecs). Distance So to determine absolute magnitude and hence luminosity we need to know the distance of a star. As we saw in lesson 1, distances can be determined by measuring stellar ṿ ǀ ẃǀ ḵḵǀ Ӂ. But parallax gets harder to measure accurately the more distant a star is. The accuracy is limited by atmospheric turbulence or seeing. ì ṙ ỹ ṿ ǀ ẃǀ ḵḵǀ ӁṊ Ǵǀ ẎẴẃǴṊ ǴṎẤẎ ỹ ṙ ẃḮ ῤẤḎǴ ǨḙḂḂǴẃǴṎǜǴ ḙṎ ẤḎǴ ṿ ṙ ẎḙẤḙṙ Ṏ ṙ Ḃǀ ẎẤǀ ẃǀ Ẏ ẎǴǴṎ ǀ ḅǀ ḙṎẎẤẤḎǴ ǘ ǀ ǜḮ ḅẃṙ ẴṎǨ ẎẤǀ ẃẎ ᾏṊ ṙ ṎẤḎẎ ǀ ṿ ǀ ẃẤẃǴỴǴǀ ḵẎ ḙẤẎ ǨḙẎẤǀ ṎǜǴῢ In the figure above we can see the parallax effect. The parallax is the apparent change in the position of a star as the Earth goes around its orbit around the Sun. For the nearest star, about 4 ly, we get a parallax of about 4 arcseconds but if we move the star out to 10 ly the parallax drops to 0.3 arcseconds, and if we move it out further still to about 20 ly we get a parallax of about 0.178 arcseconds. So the parallax gets smaller the more distant 7/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars the star is – that's how we make the measurement, but that means the more distant the star is the more difficult it is to accurately measure the star distance. ESA's Hipparcos Spacecraft The best measurements of parallax these days are obtained from measurements with spacecraft. The ESA spacecraft Hipparcos measured the parallaxes of a large number of stars (about 2.5 million stars in total) and 100000 of these were measured to particularly high precision. At the time Hipparcos was our most accurate source of stellar parallax, or distance. ḎǴ c Ď! ì ḙṿ ṿ ǀ ẃǜṙ Ẏ Ẏǀ ẤǴḵḵḙẤǴῢ ESA's Gaia Spacecraft This situation has changed markedly over the last few years. The European Space Agency has built on the success of HIPPARCOS with is latest astrometric mission, called Gaia. Launched in December 2013 is making even more accurate parallax, photometric and spectroscopic measurements of something like 1 billion astronomical objects. c Ď! ä ǀ ḙǀ Ďṿ ǀ ǜǴǜẃǀ ḂẤ Because Gaia is observing so many stars at such faint magnitudes, it is able to link its reference frame into distant quasars, producing a non-rotating reference system against which astronomer can measure tiny motions for objects as distant as nearby galaxies allowing us to make predictions for their future motions relative to our own Galaxy. The precise astrometry it is delivering is revolutionising our understanding of the Hertzsprung-Russell (H-R) diagram of stars in our Galaxy (of which we'll see more in lesson 14.6). They also have a rather nice "exaggerated" animation of the effects of parallax motion in the stars of our Galaxy as seen from the Earth. 8/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars 9/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars 14.3 Colour, Temperature and Spectroscopic Classification How do we measure Temperature? Another thing we want to measure is the temperature of stars, and one way of measuring the temperature of a star is to measure its colour. We've already seen in lesson 3 that Wien's Displacement Law means that bodies have their peak emission at different wavelengths depending on temperature, and this is how temperature effects the colour of stars. So, if we look at stars we can see there are stars of many different colours; they range from red to yellow and blue, and the colour differences are because the stars have different temperatures. Temperature in Kelvins: Colour and Temperature The colour of a star depends on its surface temperature. Red stars are relatively cool stars with temperatures around 3000 K, yellow stars are about 6000 K and blue stars about 10000 K or more. To make a more accurate measurement of a star's colour what we usually do is make photometric measurements of the star in two different colours. So, we measure the magnitude of the star in two different colours, for instance B and V and the difference between these, so B - V is a measure of the colour. Standard Photometric Filters ĎẤǀ ẃẎ Ḏǀ ỴǴ ǨḙḂḂǴẃǴṎẤǜṙ ḵṙ ẴẃẎῢ In fact, there are a number of different standard bands that we use to measure magnitudes in the visual region. They're called U, B, V, R and I. There are more bands in the infrared region called J, H, K, L and M and so on. So you can measure magnitudes in many different colour bands. The difference between two of these bands is called a colour index. 10/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars ĆḎṙ Ấṙ Ṋ ǴẤẃḙǜ ḂḙḵẤǴẃẎ ḙṎ ẤḎǴ ỴḙẎẴǀ ḵṿ ǀ ẃẤṙ ḂẤḎǴ Ẏṿ ǴǜẤẃẴṊ ῢ Spectral Type Another way of measuring a star's temperature is to take a star's spectrum and look at what spectral lines are present. The spectral lines that are present in the spectrum are an indication of temperature. Stars at different temperatures have quite different sets of spectral lines appearing in their spectrum. So if we have a very cool star where the gas in the surface layers is cool enough for molecules to exist, we will see molecular lines. A molecule often seen in the spectra of very cool stars is titanium oxide (TiO). That's an indicator of a cool star like a red giant or a red dwarf. If the star is somewhat hotter it will show mostly atomic lines, and in the very hottest stars the atoms to be ionised and show different spectral lines. So the very hottest stars show lines of ionised helium. As well as those major differences there are also differences within the energy levels of the atoms that get excited at different temperatures. The Spectral Sequence Stars at different temperatures have different spectra, and these can be assigned to different spectral types on the basis of the different spectral lines that are present. The sequence of different spectra is shown here [diagram] ranging from O at the top for the hottest stars to M at the bottom for the coolest stars. In general you can see that the cool stars have more spectral features because they have molecular features due to TiO and they also have lines of more different atoms. Whereas the hottest stars can show just a few lines of hydrogen and helium. È Mà ä » ⁄ Ẏṿ ǴǜẤẃǀ ῢ ḎǴ Ḏṙ ẤẤǴẎẤẎṿ ǴǜẤẃǀ ḵẤӑṿ ǴẎ ǀ ẃǴ ǀ ẤẤḎǴ Ấṙ ṿῡẤḎǴ ǜṙ ṙ ḵǴẎẤǀ ẤẤḎǴ ǘ ṙ ẤẤṙ Ṋ ῢ Stars are assigned a letter according to their spectral type and a sub-type which is a number from 0 to 9. The sequence of types which goes from hottest to coolest goes from O, B, A, F, G, K, M and recently new types have been added for very cool stars that we can only observe at infrared wavelengths and these are L, T and Y. 11/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars Why this strange sequence of letters? It's largely historical. The letters were assigned to the stars before the spectral sequence was understood, so basically the letters like A and B are assigned to stars that just have Hydrogen lines which turn out to be quite hot stars. The later letters were assigned to stars with more spectral lines, but as the actual meaning of the spectral sequence was realised they've ended up in a different sequence to the original alphabetical sequence and we've ended up with something that's a little strange. The order of the main spectral types are sometimes remembered using the following mnemonic: "OḎῡBǴ A FḙṎǴ Gḙẃḵᾱä ẴӑῡKḙẎẎMǴῢ" The letters for the newer spectral types (L, T, Y) have been chosen mostly to try and avoid letters used for other purposes relating to spectra. Expanding the "Oh, Be A Fine Girl/Guy, Kiss Me." mnemonic for L, T and Y is left as an exercise for the reader. On this sequence the Sun is a G2 star. The bright star Sirius is an A1 star, and the brightest star in the Southern Cross is a B1 star. 12/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars 14.4 Spectral Classification - Some History The scheme we use to classify the spectra of stars, and our understanding of what that spectral classification means, is largely owing to the work of two remarkable women who worked at a time when astronomy, like most of science, was almost exclusively a male-dominated field. Both worked at the Harvard College Observatory in Boston, Massachusetts. Edward Pickering, the Director of the observatory from 1877 - 1919, was involved in a program of studying the spectra of stars using photographic methods. He used telescopes fitted with objective prisms (a prism mounted in front of the lens of the telescope) to record the spectra of many stars simultaneously. The project required the spectra to be measured and classified by assistants. When Pickering became frustrated at the work of a male assistant he reportedly yelled "My Scottish maid could do better!", and gave the job to Williamina Fleming who had been working as his maid. Fleming continued to work at the observatory for 34 years, and was the first of a large staff of women assistants employed at Harvard, sometimes known as the "Harvard Computers", as their jobs involved measurement and computing which at that time had to be done by hand. Although employed as assistants, a number of these women made substantial contributions to astronomy in their own right. Among them were Henrietta Leavitt who discovered the period-luminosity relation in Cepheid variables (see lesson 20), and Antonia Maury, who found spectral differences in stars, which led to the recognition of the division into dwarf and giant stars by Hertzprung and Russell. ! ṎṎḙǴ ẴṊ ṿ Qǀ ṎṎṙ Ṏ ᾨ ᾉ ᾒᾏᾋᾷ ᾉ ᾓᾌᾉ ᾩ But the best known of the group is Annie Jump Cannon. She was responsible for the spectral classification sequence (O, B, A, F, G, K, M) for stars that we still use today, and for completing a huge catalogue of spectral types of stars, the Henry Draper or HD catalogue. Today, astronomers still frequently refer to stars by their HD numbers, the numbers used in Cannon's catalogue. With various extensions the catalogue eventually contained around 350,000 stars, almost all classified by Annie Cannon. (The catalogue is named after Henry Draper, whose estate funded the project). The interpretation of this spectral sequence is largely due to the work of Cecilia Payne (Cecilia Payne-Gaposchkin after her marriage in 1934). QǴǜḙḵḙǀ Ćǀ ӑṎǴ ᾨ ᾉ ᾓᾇᾇ ᾷ ᾉ ᾓᾑᾓᾩ Payne was born in England and studied at Cambridge where her interest in astronomy was sparked by a lecture by Sir Arthur Eddington. Learning that there was no prospect in England for a woman to become a professional astronomer, Payne took up an offer to work at Harvard under the supervision of its new Director, Harlow Shapley. She was given the task of developing a theoretical understanding of the sequence of spectral types that Annie Cannon had developed. This she achieved in just three years and wrote up her work as a PhD thesis in 1925 (Stellar Atmospheres, A Contribution to the Observational Study of High Temperature in the Reversing Layers of Stars). It has been described as "ẴṎǨṙ Ẵǘ ẤǴǨḵӑ ẤḎǴ Ṋ ṙ ẎẤǘ ẃḙḵḵḙǀ ṎẤĆḎῢZῢẤḎǴẎḙẎǴỴǴẃỹ ẃḙẤẤǴṎ Ṏ ǀ ẎẤẃṙ Ṏṙ Ṋ ӑ ". In her work she was able to explain how the different spectral types could be produced from gas of essentially the same composition being heated to different temperatures so that different states of ionisation of various elements led to different spectral lines being observed. Importantly, she was able to determine the composition of the stellar material, leading to the conclusion that stars were made mostly of hydrogen. This was unexpected and not immediately accepted by other experts such as Henry Norris Russell and Arthur Eddington. However, in a few years, it became generally agreed that Payne's conclusion was correct. She had discovered what stars, and therefore most of the universe is made of. 13/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars You can read more about these Harvard women astronomers in the recent book: The Glass Universe by Dava Sobel 14/22 09/07/2020 Lesson 14 – Properties and Evolution of Stars 14.5 Mass How do we measure the Mass of a Star? So, how do we measure the mass of a star? This is actually one of the more difficult properties to measure. If as star is a single star it is quite difficult to measure its mass. The way we can measure mass reliably is if we have two stars in a binary system – two stars orbiting each other. In this case we can analyse the size and period of the orbit of the stars to measure the masses of one or both stars. A Spectroscopic Binary T...
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