Swinburne Online Education Exploring the Solar System Module 19: The Sun Activity 1: The Sun: Source of Heat & Light Swinburne University of Technology
Summary: In this Activity, we will investigate (a) why the Sun is so important to astronomers... (b) the temperature at the surface of the Sun... (c) the brightness of the Sun and
(d) a few basic definitions and ideas This will include a discussion of the inverse square law, and the definitions of luminosity and flux. You will also be introduced to some of the units and conventions used in astronomy: scientific notation the Kelvin temperature scale the astronomical unit the lightyear the parsec
And now, let s have a look at the Sun carefully, because looking directly at the Sun can permanently damage your eyesight. Don t look at the Sun itself... only at a filtered, lowintensity image
(a) Why the Sun Matters The Sun is important to everything, living or non-living, in the Solar System because: it is the gravitational centre around which the planetary system moves it provides the planets with the heat and light necessary for life and many other developments
Sunrise, sunset Since the earliest times, humans have realised the importance of the Sun, venerating it as a deity or the chariot of a deity, and sharing myths about why and how the Sun continues to rise and set and what would happen if it didn t.
A Model Star Compared to most stars, the Sun is not significant by any means. However it provides a useful model for us to study when we seek information about stars in general. The Sun s place in the Milky Way Galaxy (note: this is not actually the Milky Way, but a similar spiral galaxy called NGC2997) The Sun s place in the Solar System
In particular, The Sun is much, much closer to us than any other star, and therefore is a great deal easier to study in detail. The next nearest star is about 250,000 times as far! Earth to Sun: about 15 millionths of a light year Earth to nearest star (Proxima Centauri): 4.2 light years Earth to Aldebaran: 60 light years Earth to nearby galaxies: about 3 million light years What s a lightyear?
Also, The Sun has been studied by astronomers for thousands of years, and therefore data are available for the Sun which are not available for other stars.
Physical properties of the Sun There are two very obvious things humans notice about the Sun: it is a source of heat, and it is a source of light. So the Sun s temperature and brightness make a pretty good place to start.
(b) Surface Temperature The surface temperature of the Sun is about 5780 Kelvin. What s Kelvin? Interstellar space Pluto Jupiter Mars Earth Venus Boiling iron Betelgeuse 3 40 130 250 300 700 3000 3500 The Sun Sirius A Sirius B 5780 10000 30000 0 5000 10000 15000 20000 25000 30000 Temperature (K)
How the Sun s Temperature is measured When you turn on a heater, the element will glow red until it warms up. Then the colour will become closer to yellow, or to white. low energy & temperature high energy & temperature The same thing happens with iron as it heats up. It glows orange at first, then becomes more yellow or white in colour as it warms up. Scientists say that it emits like a black body. To a good approximation, stars also emit like a black body.
We tend to associate blue with cold and red with heat, but that s only because of what our blood vessels do when the day is cold or hot. You have to forget all about that, in astronomy... low energy & temperature high energy & temperature The truth for the rest of the Universe is that cooler stars are reddish; hot stars are bluish.
(c) Luminosity The luminosity of a source of light is a measure of the power it can provide: that is, how much energy it puts out per second. 25 W Luminosity will vary from star to star... and will also vary during the life cycle of a star. Luminosity 25 W = 25 joules per second Time
The luminosity of the Sun at present is 3.863 x 10 26 Watts equivalent to about 4,000,000,000,000,000,000,000,000 light globes. What does 10 26 mean? This can be compared with the approximate luminosity of other nearby stars: Betelgeuse Antares Arcturus Procyon A Sun = 1 0.1 1 10 100 1000 10000 luminosity
Flux Flux is the power passing through a unit area, so the units of flux are watts m -2 (watts per square metre), or joules s -1 m -2 (joules per second per square metre). 1 square metre 1 square metre The power of the Sun (luminosity) is measured in watts: Power = 3.8 x 10 26 W Close to the Sun, the power passing through a square metre is high (for instance, on the surface of Mercury) Further from the Sun, the power per square metre is lower (for instance, on the surface of Neptune)
The flux of energy at the surface of the Earth depends on the luminosity of the Sun the distance between the Earth and the Sun, according to the inverse square law What s the inverse square law? How bright? How far? Luminosity = 3.8 x 10 26 W Distance = 1.5 x 10 8 km
Changes in luminosity If you alter the setting on a heater in your home, you will quickly feel the effect on your own temperature. Similarly, the surface temperature of the Earth will vary during the Sun s history, as the luminosity of the Sun varies.
Changes in distance: You get colder as you move further from the heater. In the same way, the surface temperature of planets further from the Sun is almost always lower than that of planets closer to the Sun, largely because of the decreased energy flux. * Remember that AU stands for Astronomical Units, and 1 AU is the average distance between the Earth and the Sun. Not to Scale!
Just for interest: Here is a graph showing the distance of the planets from the Sun (in AU) plotted against their average surface temperature (in degrees K). Average surface temperature (K) 800 700 600 500 400 300 200 100 0 Mercury Earth Venus is hotter than you d expect, as it is covered in thick cloud that keeps in the heat Jupiter Other than that, the further out a planet is, the cooler it is Pluto 0.1 1 10 100 Distance from Sun (AU)
This Activity focussed mainly on the temperature and brightness of the Sun. In the next Activity we will investigate more of the Sun s properties: its mass and density, and what it is made of; and we ll take a first look at how it produces energy.
Image Credits AAO: Clusters and nebulae David Malin (reproduced with permission) http://www.aao.gov.au/local/www/dfm/dark_frames.html AAO: Sprial galaxy NGC2997 David Malin (used with permission) http://www.aao.gov.au/images/general/galaxy_frames.html Stonehenge (reproduced with permission) http://antwrp.gsfc.nasa.gov/apod/ap971217.html NASA: Solar flare http://antwrp.gsfc.nasa.gov/apod/ap970918.html Skylab http://nssdc.gsfc.nasa.gov/photo_gallery/photogallery-spacecraft.html Hubble Deep field http://antwrp.gsfc.nasa.gov/apod/image/hst_deep_big.gif
Now return to the Module home page, and read more about the Sun in the Textbook Readings. Hit the Esc key (escape) to return to the Module 19 Home Page
The Kelvin temperature scale 1 The Kelvin temperature scale is the same as the Celsius scale, except that the definition of zero is different. The Celsius scale specifies 0 degrees as the temperature at which water freezes. On the other hand, the Kelvin scale specifies 0 degrees as the temperature of an object in which the kinetic energy of the particles making up the object is at a minimum. This is called absolute zero (0 K).
The Kelvin temperature scale 2 Therefore, 273.15 degrees Kelvin is the freezing point of water and 373.15 degrees Kelvin is the boiling point of water. The Celsius scale is 273.15 degrees out of sync : Kelvin 0 100 200 300 400 500 600 Celsius -273-173 -73 27 127 227 327 Melting point of ice Boiling point of water
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Scientific notation 1 In order to save writing heaps of zeroes, scientists and engineers use a system of notation where very large numbers are written with the number of factors of ten as an exponent. For instance: 5 000 is written 5 x 10 3 6 000 000 000 is written 6 x 10 9 42 700 is written 4.27 x 10 4 Note that in scientific notation the aim is to present the number as a number between 1 and 10 multiplied by a power of ten: 4.27 x 10 4 On the other hand, engineering notation always presents the power of ten as a multiple of 3, e.g. 42.7 x 10 3
Scientific notation 2 Also, in order to save writing heaps of decimal places, scientists and engineers use a system of notation where very small numbers are written with the number of factors of ten as an exponent. For instance: 0.007 is written 7 x 10-3 0.00000010436 is written 1.0346 x 10-7 0.000060001 is written 6.0001 x 10-5 Note that in scientific notation the aim is to present the number as a number between 1 and 10 multiplied by a power of ten: 6.0001 x 10-5 On the other hand, engineering notation presents the power of ten as a multiple of 3, e.g. 60.001 x 10-6
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Inverse square law 1 If something is being emitted with equal intensity in all directions from a point source, it will obey the Inverse Square Law. Closer in, the intensity of light is high as the light is only spread over a small area Point source of light Further out, the intensity of light is low as the light is spread over a larger area
Inverse square law 2 Imagine that a star is emitting light equally in all directions. At planet Alpha, the light is observed as being fairly intense, as it is being shared over a small area: small radius, therefore small area, therefore high light intensity. Star At planet Beta, the light is being shared over a larger area and so the intensity of the light is far less: large radius, therefore large area, therefore low light intensity.
Inverse square law 3 Mathematically, Note that the flux is proportional to the inverse square of distance, which is where the law gets its name from!
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Units in Astronomy 1 Because astronomical distances and sizes tend to be so large, our usual (Earthly) units of length (m, km and so on) are clumsy. Instead you will frequently find astronomical measurements made in one of these units: AU pc ly AU = astronomical unit = average distance between Sun and Earth = 1.496 x 10 11 m 1 AU
Units in Astronomy 2 Another astronomical unit of measure is the parsec, which uses angle to measure the distance to other stars, galaxies and so on. Even though the unit arose from measuring angles, it is a measure of distance and not angle AU (based on distance) pc pc = parsec = distance d at which 1 AU perpendicular to the observer s line of sight subtends an angle of 1 second of arc = 3.086 x 10 16 m 1 AU d (in parsec) Angle (in seconds of arc)
Units in Astronomy 3 A third astronomical unit of measure - the lightyear - came from the knowledge that light takes a finite length of time to travel through space. The lightyear is the distance that light will travel in a year, and 3.26 lightyears = 1 pc. AU (based on distance) 1 ly (distance) pc (based on angle) ly ly = lightyear = distance that light travels in one year = 9.461 x 10 15 m An event happens here... and is seen here a year later
Look-back time 1 The lightyear is a particularly useful unit because it reminds us that what we see actually happened a while back, when the light left the star (or planet) that we are looking at. Let s imagine that three stars A, B and C are all born at about the same time. Because the stars are at different distances from Earth, and light coming from them travels at a finite speed, light which arrives at our eyes simultaneously must have been emitted from each star at a different time. C B A
Look-back time 2 Light Light from received received star B from was from star emitted star C A was was more emitted emitted recently, long very and ago. recently. so It the will pictures give we us We receive pictures will see of of B the C will close star be just of to the when about star it as was in it middle looks first formed. today. age. C looks young B looks middle-aged A looks old In this way, we can construct a series of images and ideas about the life cycle of stars, using their distances to travel in time and see similar stars at different stages of their development. Pictures from C, then B, then A, will show us how that kind of star changes with time.
Look-back time 3 For example... Distance = 1000 ly We see this star as it was 1000 years ago Distance = 10 ly We see this star as it was 10 years ago Distance = 100 ly We see this star as it was 100 years ago... when we examine these three stars, we are looking not just out into space but back in time: hence the term look-back time. In practice, look-back time is most useful when studying distant galaxies.
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