Presentation on theme: "Wait … loading image The Milky Way star clouds of Sagittarius in the direction of our Galaxy’s centre The Milky Way: General Structure Swinburne Online."— Presentation transcript:
2. To use the knowledge (of only this century) that our Galaxy’s properties may be anticipated from, and compared with, properties of galaxies outside our own. Title screen image: The wide, bright, central (Sagittarius) region of our Galaxy. Although finer detail photos are available, the author included his own photo here as an example of a standard SLR camera image. 60 sec guided exposure on 1600 film. Summary 1. To assess our Milky Way Galaxy from our location within it. 3. A final visual appreciation of our Galaxy before we rely on Observatory images.
This is our last chance (as we move out into the wider Universe) to consider some naked eye views of the night sky. Introduction Galaxy studies take us from naked eye familiarity to images seen only by the world’s largest telescopes.
Naked eye views of the Milky Way To any night sky observer, especially away from city lights, the normal random distribution of stars is broken by the band of light (and increased star density) we call the Milky Way. Casual observations of the Milky Way at different times and dates give confusingly different orientations of its band of light relative to the horizon, yet it has a simple overall general ‘shape’. Let’s look at some views of the Milky Way to discover its full extent.
Milky Way near Southern Cross Star count falls off with distance away from the Milky Way Centauri, the closest star This is a tree Note other dark features The tree and the Coal Sack have something in common - they are both foreground features Note the dark ‘Coal Sack’ area below Crucis. In this image, a torrent of stars cascades from the Southern Cross down to the horizon - the Milky Way. Southeast, 10pm, early March, from southern Australia Follow this link for background material on estimating the distance to Centaurithis link
50 o field of view Here we use computer software to show Scorpius and Sagittarius rising above the eastern horizon in May from southern Australia. Note the grey contours (isophotes) indicating the brightness of the Milky Way. This region is the brightest, widest mix of stars and glow of the Milky Way. The Milky Way near Scorpius
Again, note the grey isophotes of the Milky Way. 80 o field of view The western horizon, with Sirius, through Orion to the Hyades and Pleiades clusters, setting in April from Southern USA. The Milky Way near Orion
All Sky Milky Way views All sky (fish eye) view showing the Milky Way arching across the sky (centred by the bright Sagittarius region). At a different time the fainter half of the Milky Way (centred by the Orion region) completes an evident ‘ring’.
LMC SMC Milky Way around the Horizon Sagittarius, Scorpius region Antares At the latitude of Sydney, Australia, just before midnight in early October, the Milky Way lies entirely around the horizon - a great circle on the celestial sphere. Orion region
Great Circle The Milky Way thus has the simple overall general shape of a great circle ring of faint light and increased star density, with dark features and an increased brightness and bulk in the direction of Sagittarius. We observe from our particular location, on a tilted Earth, orbiting the Sun in a different plane to that of the Milky Way. No wonder its different orientations in the sky can be initially confusing. Activity: Observe the Milky Way’s orientation and extent over a few hours.
Alternative Structures Measurement of a wide range of distances to stars established that we are looking out through a disk of stars, like ants looking out at salt grains scattered over a transparent plate. The ring of bright and faint stars we see as the Milky Way could either be a real ring or a disk of stars, with the Sun at or near the centre. Follow this linkFollow this link to background material on Magnitudes and Distances
Historical: The extent of the ‘Universe’. At the start of this century, all extended regions of misty light were called nebulae (latin - ‘clouds’). Some of these evidently were associated with star clusters whose distance could be estimated. Were all of these nebulae within our Milky Way ‘Universe’? - including those with ‘spiral’ features and detail unresolved by telescopes of the day?
The Magellanic Clouds The naked eye Large and Small Magellanic Clouds never set for observers south of latitude 25 o S. They are 33 o and 45 o (respectively) from the plane of the Milky Way. (See previous Map). They must have hinted at the fact - now known - that they are star systems, external to, but gravita- tionally bound to, the Milky Way.Map The properties of Cepheid variables in the LMC and SMC lead to their use as distance indicators for work by Hubble and Shapley. LMC SMC 47Tuc globular cluster 47Tuc globular cluster Author’s photo to show close-to-naked-eye view in relation to horizon. 35 o field, 50mm camera lens, 400ASA, 5 minute guided exposure. Follow this linkFollow this link to background material on using Cepheid variables
The term ‘galaxy’ now means an isolated assembly of millions (to billions) of stars, gas and dust. The Milky Way comprises our own Galaxy (deserving capital ‘G”). The Milky Way - One galaxy amongst many. With the 100” telescope in 1924, Edwin Hubble identified Cepheid variable stars in the ‘Great Nebula in Andromeda’ (M31*) showing it to be far outside our own star system. Which type of galaxy is our Milky Way, from the evidence from our view from within it? ???
Viewing a spiral galaxy face-on. M83 is some 20 million light years away and appears at magnitude 7.6 in Hydra. Its apparent size in the sky is about one-third that of the Moon. AAT 008 Note more distant galaxy Does our Galaxy have a bright central region (and can we see it)? Can we match the bright centre and spiral features of a galaxy like this to what we see in our own Galaxy? Foreground stars in our own Galaxy
The Sun was not near the centre of the distribution. Expected to share the same centre of mass as the globulars, the galactic disk was therefore not centred by the Sun. Sagittarius A third of the Milky Way’s globulars (some 150 are now known) were in fact in the direction of Sagittarius. Historical: Was the Sun near the Centre of the Milky Way system? In 1917 Harlow Shapley investigated the distribution of globular clusters* - which appear in an apparent spherical halo above and below the disk of the Milky Way. *Click here to find out about globular clustersClick here
A side-on view of a galaxy. NGC253 is a 7th magnitude galaxy in Sculptor. It is like M83 but suggestively side- on. Imagine our Sun is one of those billions of stars, at the location shown. AAT 023 Sweeping around our new celestial sphere, from the location shown, we would expect to see a great circle of higher star concen- tration; brighter toward the galactic centre or a nearby spiral arm - for us, the Sagittarius region. Away from the plane of the galaxy, we would expect fewer stars - as, indeed, is the case.
Galactic Gridlines Introduction In the next frame we are going to look at another region of the Milky Way - around Aquila. We will also use that frame to add two great circles with which you are already familiar: - the celestial equator dividing the celestial sphere, - the ecliptic near which, solar system objects are found. Corresponding approximately to the circle of the Milky Way, another great circle - the Galactic Equator - will also be shown. Try to get a feel for the angles between these three important planes.
Milky Way near Aquila From Earth’s Equator, looking east at 10pm mid June, the Milky Way at Aquila is divided by a dark rift. Galactic Equator Celestial Equator Ecliptic All great circles (including the horizon) are shown with 30 o tick intervals
It was thought that the dark rifts (as in the Aquila frame) in the Milky Way and dark patches (such as the Coal Sack) indicated an absence of stars and that, if within a disk arrangement, it would be too coincidental that they lined up, like tunnels, with our (chance) line of sight.AquilaCoal Sack The discovery of the presence of gas and dust causing dark nebulae and equatorial rifts, and the visibility of similar dark lanes in other side-on galaxies (as in NGC253) revived the disk concept; not a uniform disk, but one with a central bulge and spiral arms.NGC253 Historical: Was the Milky Way really a disk of stars? Late last century, a disk theory was nearly abandoned before the improvement in telescopes and photography.
Galactic Equator Working again with the Aquila region, we now add coordinates to the great circles shown h 19h 17h X East SENE Click for: HorizonCelestial Equator (2hrs = 30 o ) EclipticGalactic Equator X = centre Note: All coordinate values increase eastwards - toward the eastern horizon
The galactic coordinate grid Galactic longitude (l) is measured in degrees eastwards around the galactic equator from 0 o in Sagittarius. Galactic latitude (b) is measured in degrees North (+ve) and South (-ve) of the galactic equator. N (Coma Berenices) S (Sculptor) l Sagittarius b Well away from the plane of the Milky Way we have views, clear of stars (and gas and dust), to distant galaxies and clusters of galaxies - for example in Coma Berenices and Virgo (near the North Galactic Pole) and Sculptor and Fornax (near the South Galactic Pole).Fornax
Galactic Centre Features M20 TrifidM8 Lagoon Sagittarius (teapot) Tail of Scorpius M7 (naked eye cluster) M6 X Galactic longitude 0 o, a ‘spout’s’ length off the ‘teapot’ Note the dark material (but not dark matter!) hiding our view through to the Galactic centre M16 Eagle Nebula Hubble Space Telescope Image From AAT047 image Use arrow keys to step back and forth to revise Here are some of the notable features in the direction of the Galactic Centre M17 Swan
Summary There are three broad regions of our Galaxy: A disk, thin compared with its lateral extent, A brighter, wider central region or bulge, A halo of globular clusters (and, as we will see, stars and possibly dark matter.) The next Activity will look in detail at the type and distribution of stars in these regions - and the spiral structure must wait for later Activities. The next frame shows the dimensions of the Galaxy that our next Activity will reinforce.
The dimensions of our Galaxy. The Sun is located about 8000 parsecs* (8 Kpc) from the centre. Here we examine a side-on view: 2 Kpc 50 Kpc 0.6 Kpc 8 Kpc Exercise: Review these dimensions in light years, given: 1 pc = 3.26 ly Spherical halo Globular clusters, stars, dark matter (?) Thin disk Central bulge The disk and bulge contain at least 100 billion stars * (Remember: 1 parsec = million million km)
In the next Activity, we will look at the range in luminosity of stars in our region; the existence of two broad Populations of stars and how this, and the existence of interstellar extinction, confused early estimates of the size of our Galaxy and the distance to other galaxies. Hit the Esc key (escape) to return to the Index Page
Revisiting the parsec Note: Diagrams such as this give a false impression of ‘nearness’ of stars. The length of the triangle should be 206,265 times the width! What is the significance of the distance unit called the parsec? Viewing the Earth’s orbit face-on from a star one parsec away, the Earth-Sun separation* would be 1 arcsecond* (1”). * 1 astronomical unit (AU) = 149,597,870 km. 1 parsec = million million km! background *Click here to find out about arcsec (”)Click here
Distance determination (i) - by trigonometry. No star is as close as one parsec (pc). Centauri is 1.3 pc. Note: The tiny angle (a=0.76”) between Sun and Earth from even the closest star ( Cen) emphasises the difficulty in detecting planets which may be over 20 magnitudes fainter than their parent star. We can measure the same (parallax) angle a which the star appears to move against background stars, for two dates, 3 months apart, in our orbit around the Sun. A star at double the distance would halve the parallax angle. In general d (parsecs) =1 / a (arcseconds) a a d a background
The small angle formula. A frequently used formula in astronomy applications. The distance around a full circle of radius r for a sweep angle of 360 o is 2 r. The arc length s is the same part of the full circle 2 r as the angle a is to 360 ie: s/2 r = a/360 For small angles, this also gives the straight line (chord) length. or: s = ra /180 when a is in degrees or: s = ra when a is in radians or: s = ra / when a is in arcseconds a s r background
Parsecs and Light years. The relation between the two distance units. By definition of a parsec, in the formula s = ra” / , a=1” and s=1 Astronomical Unit (149,597,870 km). So one parsec (r) = million million km. One light year is the distance light travels in a year at 300,000 km per second. Multiply by 60x60x24x365 for a year, giving one light year = 9.46 million million km. So 1 parsec = 3.26 light years and 1 light year = 0.31 parsecs. background
Small angle formula applications. Using: s = ra / with a in arcseconds Parallax a=0.76”, Earth-Sun baseline s = 150 million km, so distance r = 1.5x10 8 x / 0.76 = ~41 million million km. 2. What is the angular size (a) of Centauri, assuming it is a star of similar size to the Sun? (s = 1.4x10 6 km) a=1.4x10 6 x / 4.1x10 13 = 0.007” ! Note: That’s why we don’t see ‘size’ in stars. 1. What is the actual distance (r) to Centauri? s a r s a r background Click here to return to the Activity Click here to return to the Activity
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Apparent magnitude and brightness. By its modern definition a difference of five magnitudes implies a brightness factor of exactly 100. ie: for m 2 - m 1 = 5, b 1 /b 2 = 100 or In general, b 1 /b 2 = 10 2/5 (m - m ) or (m - m ) Then: log* b 2 /b 1 = 0.4(m 1 -m 2 ) or: m 1 -m 2 = 2.5 log b 2 /b 1 The globular cluster Cen, of >100,000 stars, appears as magnitude 3.5. To a first approximation (similar stars, not masking others), what is their individual magnitude? Answer: m = 2.5 log 10 5 = 2.5x5 so m 1 = 16 Note: The log key is provided on scientific calculators 10 x background *Click here to find out about logarithmsClick here
Brightness and distance. How would a star’s brightness (and magnitude) change if we, say, doubled its distance? r A small surface area (such as a telescope mirror) collecting light from a star, is just a part of the surface area 4 r 2 of the sphere of radius r. With changing distance the light falling on the same collecting area will therefore change inversely with the square of the distance to the star. This is the inverse square law. So doubling the distance would quarter the brightness - a magnitude change of m 1 -m 2 =2.5log4 or 1.5 background
Absolute Magnitude. The inverse square law allows us to recalculate a star’s brightness if the star was at a different distance. If we could consider all stars at some fixed distance, we could compare their real brightnesses. The fixed distance is chosen to be 10 parsecs. A star at distance d with apparent magnitude m, will, if moved to distance 10 parsecs, appear at what we call its absolute magnitude M. m - M = 2.5 log b 10 /b d = 2.5 log (d/10) 2 = 5 log d - 5 inverse square law m - M is called the distance modulus. background
Absolute Magnitude example. Compare the real intensity of Sirius with that of Rigel. The brightest star Sirius, with m=-1.4 is only 8.6 light years away. Rigel appears fainter at magnitude 0.2 but is 770 light years away. (Apply 1 parsec=3.26 light years) What are their true relative brightnesses? Sirius: M=-1.4-5log8.6/ =1.5 Rigel: M=0.2-5log770/3.26+5=-6.6 b Rigel /b Sirius =10 0.4(1.5-(-6.6)) = 1,738 times brighter! Using m - M = 5 log d - 5 background
Distance determination - ii). Absolute magnitude can be inferred from a star’s intrinsic properties. We can discover the brightness of a light bulb from the wattage written on it! A star gives away its absolute Magnitude by its colour or spectrum characteristics - its place in the H-R diagram. C O L O U R Absolute Magnitude Observing a star’s apparent magnitude and colour, and deriving M, leads to its distance, using m - M = 5 log d - 5 This is called the spectroscopic parallax method of distance determination and may be accurate only to within about 50%. Distances obtained from trigonometric parallaxes are useful to about 500 parsecs - and now improved by Hipparchus satellite measures. background
C O L O U R magnitude Distance determination - iii). Stars in a cluster add weight to its distance determination. The non-evolved portion of cluster stars in the H-R diagram should fit the line of the main sequence. The apparent magnitude of the cluster is, of course, much fainter. The correction to fit the main sequence is the distance modulus m - M giving 5log d - 5 and hence the cluster distance. This distance measurement method is called main sequence fitting - a variation of spectroscopic parallax for individual stars. M m background Click here to return to the Activity Click here to return to the Activity
Distance determination - iv). Cepheid variable stars. The wonderful discovery, by Henrietta Leavitt in 1907, that the brighter variable stars in the Large Magellanic Cloud* (of the same type as Cephei) had the longer periods, provided a method to measure distances to galaxies millions of light years away - provided these very luminous stars could be detected. *The LMC is over 160,000 light years away, so relative brightness differences in its stars are true differences. Henrietta Leavitt subsequently confirmed the relationship for Cepheids in the Small Magellanic Cloud Period in days Absolute magnitude A Cepheid variable’s observed period leads to its absolute Magnitude which, with its known apparent magnitude, leads to its distance using log d=(m-M+5)/5 background Click here to return to the Activity Click here to return to the Activity
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Arcsec A full circle is defined as an angle of 360 degrees. No, you can’t see an arcsec in a diagram like this: it’s much too small a slice! Arcsec (symbol “) is short for seconds of arc and is a measure of angle. Take 1/360th of a circle Each degree ( o ) includes 60 arc minutes (‘). Take 1/60th of that Each arc minute (‘) includes 60 arc seconds (arcsec, “). Take 1/60th of that So one arcsec is one-sixtieth of one-sixtieth of one-three-hundred-and-sixtieth of a circle … a very small slice or the circle indeed. There are arcsec (“) in a full circle. Back to the Activity background
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Logarithmic scales A logarithmic view in mathematics uses a different way of making sequences of numbers. Instead of looking at what happens when you increase from 1 to 2 to 3, you consider instead what happens when you go from 1 to 10 to 100. In the 1, 2, 3 view, the increase is made by adding. The result is an arithmetic progression. Here’s another one Arithmetic Geometric 10 In the 1, 10, 100 view, the increase is made by multiplying. The result is a geometric progression. The second way of increasing can help a lot when drawing graphs. Here’s a sample. background
An example Why on Earth (or off Earth, in the case of your studies) would anyone want to use a logarithmic method? Let’s consider our tame astronomer who wants to illustrate the amount of money he owns (or hopes to own) at particular ages of his life. background
Not much good Trouble is, when he draws a graph of Money versus Age, the detail near the beginning isn’t clear. This is because the vertical scale has to go up to $100,000, so amounts like $1, $5 and $10 are too tiny to show. background
The solution The solution is to draw the graph again, but this time he uses a logarithmic vertical scale where you increase by multiplying by 10 rather than by adding $10,000. A scale that goes up by multiplying rather than by adding is called a logarithmic scale. It is particularly useful when drawing graphs in astronomy, where figures can vary so very widely. Click hereClick here to return to the Activity background
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Globular Clusters Globular Clusters are dense concentrations of stars - up to a million of them bound together by their mutual gravity GLOBULAR clusters Their total mass is so large that gravity pulls the cluster into the shape of a sphere … or globule … hence the name background
…that we might never realise there was more to the universe than our small cluster of stars. The sky would be so bright with stars at night, assuming there was a night…. (most stars would have companions) If the Sun were to have evolved within a Globular Cluster we would be surrounded by thousands of stars within the nearest parsec alone. That’s a far cry from the zero stars that share the cubic parsec surrounding our Sun today. What if we lived in a Globular Cluster? Return to the Activity